HOLOGRAPHIC APPLICATONS

By

Tadele Negash

A GRADUATE PROJECT PRESENTED TO THE SCHOOL OF GRADUATE STUDIES IN

PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE ATTAINMENT OF THE
DEGREE OF MASTER OF SCIENCE IN PHYSICS

AT
ADDIS ABABA UNIVERSITY
ADDIS ABABA
ETHIOPIA

JULY, 2010

0
 ADDIS ABABA UNIVERSITY

DEPARTMENT OF PHYSICS

The undersigned hereby certified that they have read and recommended
to the collage of science for acceptance a project entitled
“HOLOGRAPHIC APPLICATIONS” by Tadele Negash in partial
fulfillment of the requirements for the degree of Master of Science.

Dated: July, 2010

Advisor: __________________
Prof. A.V. Gholap

Examiner: __________________
Dr. Ghoshal

1
 ADDIS ABABA UNIVERSITY

FUCULTY OF PHYSICS

Date: July, 2010

Author: Tadele Negash

Title: HOLOGRAPHIC APPLICATIONS
Department: PHYSICS
Degree: M. Sc. Convocation: July year: 2010

Permission is herewith granted to Addis Ababa University to circulate and to

have copied for non commercial purposes, at its discretion, the above title upon
the request of individuals or institutions.

Signature of the author ____________________

2
To my family

3
Table of Contents
Table of Contents ……………………………………….…………….. 4
List of tables……………………………………………….………….. 6
Acknowledgment ……………………………………………………….7
Abstract ………………..……………………………………….…….. 8
Introduction…………………….…………………………….…………9
1 LIGHT
1-1. light wave …………………………..…….………….10
1.2. Interference………………………………………..…….….11
1.3. Diffraction…………………………..……………..…….….15
2 LASER
2.1 Introduction………………………………………..………..17
2.2. Radiation……………………………………………………18
2.3. Absorption……………………………………………..……19
2.4. Spontaneous & Stimulated Emission…………………....….19
2.5. Amplification…………………………………………..……22
2.6. Construction of laser…………………………………….…..24
2.7. Properties of laser…………………………. ………..….. …25
2.8. Types of laser………………………………. ……..…..….. .25
2.9. Laser applications……………………………….. .……......26
3 HOLOGRAPHY
3.1 Basics of holography………………………………………..28
3.2 Hologram………………………….………………….……..30
3.3 Classification of hologram………………….……….…. …..31
3.4 Main types of hologram…………………….. ..…… ..……..35
3.5 Characteristics of hologram……………… ……….…….....38
3.6 Making hologram……………………….………….………..39
3.7 Reconstruction of hologram………………………….….…..42
3.8 Electron holography ………………………………………...44
3.9Acoustic holography………………………………………….45

4
4 HOLOGRAPHIC APPLICATIONS
4.1 Data storage…………………………………….…………..46
4.2. Digital holography……………………………..… …..……46
4.3. holography in art………………………………….. …..…..47
4.4. For making diffraction grating…… …………………..….47
4.5. In health…………………………………………………….48
4.6. In military………………………………………………..…49
4.7. In Technology………………………………………………49
5 CONCLUSION
5.1 Summary and conclusion………………………..……51
Bibliography …………………………………………….………52

5
List of tables
2.1 Lasers used for holography…………………………29
3.1 properties of recording materials……………………40

6
 Acknowledgement
Next to God, I would like to thank my project advisor Professor
Ashok Gholap for his invaluable, genuine advice and guidance also for
supplying materials.
Also I would like to express my deep thanks to Dr. Mulugeta
Bekele, Ato Abebe Belay physics PhD. student, Ato Tilaye physics
PhD. student in Germany, Ato Mussa Mohammed, Ato Mulugeta
Asmamaw, Ato Samuel Birhamu for their personal advice and my
family especially my brother Tefera Negash for all your support.
My thanks pass to w/o Tsilat Adnew secretary of physics department.

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Abstract
Holography is a technique of recording the whole information of an object by using a reference
beam which is combined with the light from the object (an object or scene beam). If these two
beams are coherent, optical interference between the reference beam and the object beam, due
to the superposition of the light waves, produces a series of intensity fringes that can be
recorded on standard photographic film. The fringes form diffraction grating called hologram.
When viewed from different angles or changing orientation it seems the object is still present
and the image appears three dimensional. It is a lens less image recording technique. Hologram
and holographic products are used in various products like currency, credit cards, etc. Also it is
applied to combat counterfeit, for security purpose, for attractive product packaging and so on.

8
INTRODUCTION

The development of science helps human being to study further about

nature from the invisible to the visible huge bodies. Studying light pushed him to
concentrate on optics that improves his way of making images on two
dimensional (2D) surfaces and further studies enables him to construct three
dimensional (3D) image formations called Hologram by using LASER. That is
why when we think of holography surveying laser is very important.

Holography becomes a popular art of science and helps the world in many
ways.

The first chapter tries to recall the background knowledge about the nature
of light and its properties like interference which is caused by two or more waves
coming from different sources of waves. It also gives us basic information how to
apply diffraction gratings.

The second chapter consists of definitions and explanations of principles of

laser production, types and applications of laser.

The third chapter mainly focuses on holography, how it is formed, and

processes it, types of hologram, characteristics and so on.

The last chapter, the main objective of the author, gives the general
applications of holography in many fields of science, technologies, art,
advertisement, and so on which holography is contributing to the world.

9
1 LIGHT
1 -1 LIGHT WAVES
Properties of light can be described in terms of wave motion. As in the case of water
wave on surfaces of water bodies transverse wave motion is apparent. But light wave
propagation presents greater observational difficulties. Since the oscillation frequencies of the
electric and magnetic fields of a light wave approach 1015 Hz, there is no detector with response
rapid enough to record their instantaneous values.

In 1802 Thomas Young demonstrated that light propagates as a wave and

inferred(concluded) the wave properties from observation of interference of light coming from
separated points on wave fronts. As shown in figure 1.1 he observed the overlap, on a screen, of
the light from two secondary sources and found that there was cancellation of light intensity as
well as addition which is difficult to account for on a particle basis but easily accommodated by
a wave theory.

P1

P0

P2 S

Figure. 1.1 Young’s double slit experiment.

A hole P 0 is illuminated by collimated light, diffracts a spherical wave to an opaque

screen some distance away containing two additional holes P 1 and P 2 . These holes also diffract
spherical wave fronts of secondary, phase related spherical waves. Finally on the screen S,
parallel to the first slit and placed where the waves overlaps, an alternating bright and dark
interference patterns are observed, with linear fringes running perpendicular to the direction of
the line joining P 1 and P 2 .
10
 Observation of the wave properties of light and formation of hologram are closely
related. Both depend on recording the intensity of black and dark spatial patterns or fringes
found whenever coherent light waves intersect. The formation of the fringes pattern implies that
light has a wave character, and measurement of the spacing and contrast of the fringe pattern
can reveal the properties of the wave such as wavelength and degree of coherence.

According to Maxwell the presence of the two vector force fields, electric and magnetic,
in light which propagates in space unsupported by any known medium, and only the time
averaged effects of their(electric and magnetic) interaction with matter can be observed.

Holography is concerned with the interaction of light waves with photo sensitive matter,
e.g., silver halide grains in photographic emulsion.

As Weiner (1890) demonstrated a light standing wave pattern blackened a photographic

plate most at the electric field antinodal regions and not at all the magnetic field antinodal
regions. Therefore the electric fields are the major consequence in the forming of holograms,
and also for all the photosensitive media in which hologram have been formed.

1.2 INTERFERENCE

picture of interference

One of the most important wave properties is that interference. It is the effects of
superposition of electromagnetic fields of two or more waves that overlap and arrive at the
same place in space.

The superposition is stated as:

the resultant disturbance of a point where overlapping occurs is the algebraic

sum of the individual constituent wave at the location.

The net disturbance depends on the two quantities, amplitude and phase. If the waves
partially or completely cancel each other we call it destructive, or if a region exists where the
resultant trough and crest are more pronounced than the source waves we call it constructive.
These phenomena are very important for construction of holograms.

11
 The recording of a hologram is essentially a measurement of the intensity of an
interference pattern. If the relative phase between the interfering wave fields has some degree
of constancy in time then the spatial distributions of fringes of intensity in the interference
pattern will also have some degree of constancy.

Let us focus our attention on the interference of monochromatic waves of identical

wavelength produced from a single continuously oscillating source.

When light interacts with photosensitive emulsion (media) the darkening of a unit
volume of photographic emulsion or the bleaching of a unit volume of photographic material is
a function of the energy absorbed by that volume averaged over a time long compared to the
light vibration period. This absorbed energy can be determined by Maxwell’s theory.
According to his theory the energy u per unit volume or energy density in the electric field of
the light wave is given by
1
𝑢 = 𝜀𝑬 ∙ 𝑬 ………………………………..….1.2.1
2

Where 𝜀 is the dielectric constant and E the electric field vector.

The time average of 𝑢 is

1 𝑇
<𝑢 >= ∫ 𝑢𝑑𝑡 …………………………………1.2.2
2𝑇 −𝑇

1 𝑇 1
= ∫ 𝜀𝑬 ∙ 𝑬𝑑𝑡…………… .……………….1.2.3
2𝑇 −𝑇 2

1 1 𝑇
= ∙ 𝜀 ∫−𝑇 𝑬 ∙ 𝑬𝑑𝑡 …………………………….1.2.4
2 2𝑇

1
= 𝜀 < 𝑬 ∙ 𝑬 > ………………………………….1.2.5
2

At any point in the light wave the pointing vector may be interpreted as: magnitude and
direction of the energy flow per unit time per unit area normal to the flow. Which is classically
called Intensity of the light at a point.

The intensity at a point P is given by

𝐼𝑝 = 𝑣 < 𝑢 >……………………………………….1.2.6
1
= 𝜀 < 𝑬 ∙ 𝑬 >………………….………………….1.2.7
2

The discussion of monochromatic light waves will show that the intensity 𝐼𝑝 reduces to
the square of the amplitude of a light wave and is a very important parameter in holography.

12
 If electric field exists as a physical quantity it must be a real function of space and time,
and if it represents a truly monochromatic light wave it must be a simple harmonic function of
time represented by

𝑬 = 𝐸𝑂 cos(𝐾 ∙ 𝑟 − 𝜔𝑡 + 𝜑)……………..……….1.2.8

Or in complex form

𝑬 = 𝐸𝑂 exp 𝑖(𝐾 ∙ 𝑟 − 𝜔𝑡 + 𝜑)……………..…………………….1.2.9

When we are dealing with light illuminating a given area it is measured by the amount of
average light energy per unit area per unit time arriving at that area which is known as
IRRADIANCE (or INTENSITY of light).

In holography it is usual to define intensity in the abbreviated form such that

𝐼 = 2 < 𝑬 ∙ 𝑬 >………… …...…………………………………….1.2.10

𝑇
= 2[1/2𝑇 ∫−𝑇 𝑬 ∙ 𝑬𝑑𝑡……………………………………………….1.2.11

𝑇
= 1/𝑇 ∫−𝑇 𝐸𝑂 cos(2𝜋𝑣𝑡 + 𝜑) ∙ 𝐸𝑂 cos(2𝜋𝑣 + 𝜑)…..…………….1.2.12

𝑇
= 1/𝑇 ∫−𝑇 𝐸𝑂 ∙ 𝐸𝑂 𝑐𝑜𝑠2 (2𝜋𝑣𝑡 + 𝜑) 𝑑𝑡……..………..………..…….1.2.13

1
But, 𝑐𝑜𝑠 2 (2𝜋𝑣𝑡 + 𝜑) = (1 + cos2(2𝜋𝑣𝑡 + 𝜑)….…………….……….1.2.14
2

1
= (1 + cos(4𝜋𝑣𝑡 + 2𝜑)………..……………….1.2.15
2

1 𝑇
Hence, 𝐼= ∫ 𝐸
2𝑇 −𝑇 𝑂
∙ 𝐸𝑂 [{1 + 𝑐𝑜𝑠(4𝜋𝑣𝑡 + 𝜑)}] 𝑑𝑡……..…………….1.2.16
1 𝑇
=
2𝑇
𝐸𝑂 ∙ 𝐸𝑂 ∫−𝑇[{1 + 𝑐𝑜𝑠(4𝜋𝑣𝑡 + 𝜑)}] 𝑑𝑡…………………….1.2.17
1
=
2𝑇
𝐸𝑂 ∙ 𝐸𝑂 [2𝑇 + 0] …………..……………..………………….1.2.18

∴ 𝐼 = 𝐸𝑂 ∙ 𝐸𝑂 … … … 𝑓𝑜𝑟 𝑇 ≫ 1/𝑓……………………….………………….1.2.19

Therefore, 𝐼 = 𝐸02 ……………………………….…….……………………….1.2.20

2 2 2
= 𝐸0𝑥 + 𝐸𝑜𝑦 + 𝐸0𝑧 ………………………….…..……………….1.2.21

Thus intensity is equal to the square of amplitude of the electric field which provides no
information about the phase of the wave. According to supper position principle, the
interference of different light waves at a point yields a resultant electric field E due to the
separate fields E 1 , E 2 , ….

13
 Where 𝑬 = 𝑬1 + 𝑬2 + ⋯ …………………………………………..……….1.2.22

The resultant of the interference of two monochromatic wave sources S 1 and S 2 at positions r 1
from S 1 and r 2 from S 2 on a reference point P becomes

𝐸(𝑟, 𝑡) = 𝐸01 𝑒𝑥𝑝𝑖(𝑘 ∙ 𝑟1 − 𝑤𝑡 + 𝜑1 ) + 𝐸02 𝑒𝑥𝑝𝑖(𝑘 ∙ 𝑟2 − 𝑤𝑡 + 𝜑21 ) ……...1.2.23

According to equation 1.2.22

⇒ 𝐸 ∙ 𝐸 = ( 𝐸1 + 𝐸2 ) ∙ (𝐸1 + 𝐸2 ) ……………………………….………….1.2.24

= 𝐸1 ∙ 𝐸1 + 𝐸1 ∙ 𝐸2 + 𝐸2 ∙ 𝐸1 + 𝐸2 ∙ 𝐸2 ………………….………….1.2.25

= 𝐸1 ∙ 𝐸1 + 𝐸2 ∙ 𝐸2 + 2𝐸2 ∙ 𝐸1 ……………………………………….1.2.26

= 𝐼1 + 𝐼2 + 𝐼 ′ ……………………………………..……….………….1.2.27

Where 𝐼1 is the irradiance from S 1

𝐼2 is the irradiance from S 2 and

𝐼′ is the irradiance from combinations of S 1 and S 2

Using trigonometric relation one can find for the value

2 < 𝐸1 ∙ 𝐸2 > = 2𝐸01 𝐸02 [cos 𝑘 ∙ (𝑟1 − 𝑟2 ) + (𝜑1 − 𝜑2 )] ……….…….1.2.28

𝐸 ∙ 𝐸 = 𝐼1 + 𝐼2 + 𝐼 ′ ………………… ………………………….1.2.29

Then Equation 1.2.28 =< 𝐸1 ∙ 𝐸1 > + < 𝐸2 ∙ 𝐸2 > +2 < 𝐸1 ∙ 𝐸2 >………………….1.2.30

= 𝐼1 + 𝐼2 + 2𝐸01 𝐸02 [cos 𝑘 ∙ (𝑟1 − 𝑟2 ) + (𝜑1 − 𝜑2 )] …..……….1.2.31

Here the irradiance of the two point sources mainly depends on the third term especially on the
phase difference term k(𝑟1 − 𝑟2 ) + (𝜑1 − 𝜑2 ). The difference in phase between the
interference waves coming from S 1 and S 2 must constant for certain duration to form maximum
(bright) and minimum (dark) irradiance which are known as INTERFERENCE FRINGES.
These patterns are very useful in the formation of holograms.

14
1-3 Diffraction

Picture of diffraction

It is the bending of wave normal rays when they encounter obstacles whose optical
transmission or reflection properties change significantly in distances approaching the
wavelength of the illuminating light.

A hologram itself is a diffracting object with some peculiar properties. Holograms can
be classified as behaving like

1- plane diffraction grating

2- volume diffraction grating

The grating may consists of a set of periodically spaced transparent slits in an opaque
object (or screen).

Figure1.2 Plane diffraction due to two slits

Diffraction effect occurs when part of the wave front is removed by an aperture or
stop. The importance of diffraction effects depends on the scale of the obstacle or
aperture compared with the wave length.

15
 1- plane diffraction grating

when a plane wave is incident on a grating as shown in figure 1.2 the condition
determining the in-phase or constructive addition of diffracted light is the grating equation,

𝑑(𝑠𝑖𝑛𝑖 + 𝑠𝑖𝑛𝛿) = λ ……………………………………………….1.3.1

where 𝑑 is grating spacing, 𝑖 is angle of incidence and 𝛿 is the angle of diffraction

2- Volume diffraction

A volume diffraction grating consists of periodically space scattering planes

illuminating with a plane wave, which is shown in figure 1.3.

A C

A’

B C’

D B E

Figure 1.3. Volume diffraction grating

The same principle in phase addition of light scattered by successive planes to obtain maximum
output, is applied here with the result that

2𝑑𝑠𝑖𝑛𝜃 = λ ………………………………………..…..………….1.3.2

According to Bragg’s law,

𝐷𝐵′ + 𝐵′ 𝐸 = 𝑑𝑠𝑖𝑛𝜃 + 𝑑𝑠𝑖𝑛𝜃 ………………………………………….1.3.3

= 2𝑑𝑠𝑖𝑛θ = λ …………..……………… …..…………….1.3.4

It is this equation that determines constructive interference and diffraction of a plane wave.
Maximum diffraction occurs when the angle of incidence θ and reflection are equal as shown in
the figure.

For maximum diffraction observation, volume diffraction equation is selected. The first
Bragg’s equation has limitations of /𝑠𝑖𝑛𝑖/ and /𝑠𝑖𝑛𝛿/≤1

16
2 LASERS
2-1 Introduction
In the early 1950s a device known as the MASER ( an acronym for Microwave
Amplification by Stimulated Emission of Radiation) that produce and amplifies microwave
came in to being through the efforts of Charles Hard Townes(USA), Alexander Michailovich
Prokhorov(USSR) and Nikolai Gennadievich Basov(USSR). All of whom shared the 1964
Nobel Prize in physics. It is an extremely low noise used for low noise microwave frequency
amplifier. Having high stability of the generated frequencies, it serves in time standards in
atomic clocks.

1958 Townes and Arthur L. Schawlow set forth the general physical condition that
would have to be met in order to achieve light amplification by stimulated emission of radiation.

And then in July 1960 Theodore H. Maiman announced the first successful operation of
an Optical MASER or LASER, which is the great milestone in the history of science.

The first laser was built in 1960 and within a decade laser beams spanned the range
from infrared to ultra violet. The availability of high power coherent sources led to the
discovery of a number of new optical effects.

Laser is a device that produces and amplifies light. The light produced by laser is very
pure in color, can extremely intense, and can be directed with great accuracy. That is why it is
highly directional. Lasers can generate light from infrared through the x-ray range.

In atoms lasers generates light by storing energy by the electrons while being excited or
move to the excited state. Thus electrons are almost the source of all lights. Light is composed
of tiny packets of energy called Photons.

Laser produces coherent light which is monochromatic.

Electrons travel in orbits and exist only in certain specific energy states or level. When
the electrons move from the lower energy level to the higher absorbs energy. The photon
absorbing atom whose energy is the difference between the two energy levels. Then the atom
becomes excited. The electrons which are excited quickly jumps or return back to the lower
energy level by giving off the extra energy as light or Radiation.

Today lasers are in use everywhere: in reading video disks, cutting steels in factories, scanning
labels in supermarkets, performing surgeries in hospitals, etc.

17
2-2 Radiation
Except black body all physical bodies radiates heat to their surrounding as they received
from the outside. In the case of black bodies we can consider it as a small box having a very
small hall in it when light or heat reaches its surface entering the hole will have less chance to
come out but stays inside due to internally reflections. Since there is no radiation by the body it
seems black. The name black body is given this way.

Figure 2.1 black body

Once a wave enters the hole its probability to come out is too small. The number of
modes (which are possible standing waves in the cavity) per unit volume within the frequency
interval dν is given by

𝑣3
𝑛(ν)𝑑ν = 8𝜋 𝑑𝑣 … … … … … … … … … … … … … … … … .2.1
𝑐3

The average energy per unit volume for the total number of modes within the cavity

when thermal equilibrium is reached is given by the Rayleigh – Jeans law.

𝑣2
𝜌(𝑣 )𝑑𝑣 = 8𝜋 𝑘𝐵 𝑇𝑑𝑣 … … … … … … … … … … … … . . … … … … … … … .2.2
𝑐3

Where 𝑘𝐵 𝑇 represents the classical oscillator’s mean energy and 𝑘𝐵 is Boltzmann constant, T
P

absolute temperature.

The equation matches with experimental results in the infrared region but not with experiments
at higher frequencies. Plank reconciles this by his famous hypothesis that energy is radiated in
discrete form or quantized.

𝐸 = 𝑛ℎ𝑣 …………………………………………………………2.3
Where n is positive integer in which energy exchange requires discrete amount of energy hν for
which the above equation is the energy of a mode containing n photons. Therefore the average
energy of the mode is

ℎ𝑣
<𝐸 >= ℎ𝑣
………………………………………………2.4
𝑒𝑥𝑝� �− 1
𝑘𝐵 𝑇

18
And hence equation 2. 2 becomes

8𝜋𝑣 2
𝜌(𝑣 )𝑑𝑣 = ℎ𝑣
ℎ𝑣𝑑𝑣 …………….………………2.5
𝐶 3 exp� �− 1
𝑘𝐵 𝑇

Thus equation 2.5 is the famous Plank’s radiation law that matches with experiments.

2-3 Absorption
If molecules with energy level E 1 & E 2 are brought in to thermal radiation field they
absorb photon of energy

ℎ𝑣 = 𝐸2 − 𝐸1 …………………………………….……2.6
This means that if the atom is not isolated other effects may occur. Photons of the same energy
as the energy of the upper level may use their energy to move an electron from the lower to the
upper level. This means that the molecule becomes excited to the higher level. The process of
being excited by changing energy level from lower to higher is known as ABSORPTION.

The probability per second that a molecule will absorb a photon (dΦ/dt), is proportional to the
number of photons of energy hν per unit volume and can be expressed in terms of the spectral
energy density 𝜌(𝑣) of the radiation field as
𝑑
𝑊12 =
𝑑𝑡
Φ12 = 𝐵12 𝜌(𝑣)……………………………….2.7
The constant B 12 is Einstein coefficient of absorption.

2-4 Emission
Consider a container filled with a certain gas in equilibrium at a relatively low room
temperature. At this temperature most of the atoms will be in the ground state while some of the
atoms having enough energy will be in the excited state.

According to Maxwell-Boltzmann distribution the number of atoms in any excited state N i per
unit volume is

𝜀𝑖
𝑁𝑖 = 𝑁0 exp (− )…………………………………………….2.8
𝑘𝐵 𝑇

Where N o is constant at a given temperature.

Since our interest is in the atomic transition between arbitrary states like the j-state
where ε j > ε i , then at the j-state

19
 𝜀𝑗
𝑁𝑗 = 𝑁0 exp �− � … … … … … … … … … … … … . . … … … … … .2.9
𝑘𝐵 𝑇

The ratio of the population occupying the two states will be

𝜀𝑖
𝑁𝑖 𝑁0 exp (− )
𝑘𝐵 𝑇
= 𝜀𝑗 ………………….………………………………2.10
𝑁𝑗 𝑁0 exp (− )
𝑘𝐵 𝑇

But from equation 2.6 we have

𝜀𝑗 − 𝜀𝑖 = ℎ𝑣𝑗𝑖 ……………………………………………………..2.11
Therefore we get

ℎ𝑣𝑗𝑖
𝑁𝑗 = 𝑁0 exp (− )……………………………………2.12
𝑘𝐵 𝑇

A photon having adequate amount of energy interacts with an atom in its lowest energy
or ground state imparting that energy to the atom thereby causing the electron cloud to take on a
new configuration. The atoms jump into a high – energy exciting state.

Such an excess-energy configuration is mostly exceedingly short lived (10nanosecond

or so). Without any external influence the atom will emit its overload of energy as a photon and
back to its stable state in the process called SPONTANEOUS EMISSION.

2-4.1 Spontaneous Emission

It is dependent on the electron being in the upper level. Let us consider an idealized
atom with two energy levels and one electron as shown in figure 2.2.

E2 N2 , g2

E1 N1 , g1

Figure 2.2 the two energy level model

The electron may be in either of the two energy levels. If the electron is in the higher level it
may fall down to the lower level. If it does it must give up a certain amount of energy equal to
the energy difference between the two levels. This is the law of conservation of energy being
applied. This energy is given up in the form of light.

20
 Light is also quantized. It may be represented as a group of photons. Each photon
carries one quantum of light energy. The amount of this energy in the quantum depends on the
wavelength (color) of light
𝑐
𝐸=ℎ …………………………………………………2.13
λ

Where E is the energy of the photon λ is the wavelength, h & c are fundamental constants.

Thus from equation 2.13 we can see that a short wavelength wave such as blue at
470nm has a high energy, and red light at 670nm has low energy per photon. Here the
important point is that the wavelength of light is linked to the energy of a photon in a defined
way. Thus our electron in the idealized atom, which has given out a photon of defined energy,
emits light of a certain defined wavelength or color.

Thus without external influence the atom emits light spontaneously in the process called
SPONTANEOUS EMISSION. In this process the photon may travel in any direction and can
be emitted at any time.

The probability per second that a photon spontaneously is emitted by a molecule

depends on the molecule and the selected transition but is independent of the external radiation
field.

𝑑
𝑤21 =
𝑑𝑡
Φ = 𝐴21 … … … … … … … … … … … … … … … … . .2.14
Where A 21 is Einstein’s coefficient of spontaneous emission and is often called the spontaneous
transition probability.

2-4.2 Stimulated Emission

The radiation field can also induce molecules in the excited state E 2 to make transition
to the lower state E 1 with simultaneous emission of photon energyℎ𝑣. This process is known as
STIMULATED OR INDUCED EMISSION. The induced photon energy is emitted to the same
mode which caused the emission. This means that the number of photons in the mode is
increased by one. The probability 𝑑/𝑑𝑡 that one molecule emits one induced photon per second is

𝑑
𝑤21 = Φ = 𝐴21 = B21 … … … … … … … . . … … … … … . .2.15
𝑑𝑡 21
So for the stimulated emission to be greater than the spontaneous emission we need many
photons in the laser. For the stimulated emission to be greater than stimulated emission N 2
should be greater than N 1 which means more atoms have their electrons in the upper level than
in the lower level. This is known as INVERSION. This may be seen from the fact that in the

21
absence of external influence, i.e., no photons(n=0) the only process which can occur is
spontaneous emission which allows any electron that began in the upper level to fall to the
lower level but not vice versa.

At thermal equilibrium the ground level is far more populated than the excited level as
shown in the figure 2.3(a) below. While an inversion population is a situation where the exited
level is the most populated as shown in figure2.3(b)

E1 N1 E1 N1

E0 N0 E2 N0
a) Thermal equilibrium b) population inversion

Figure 2.3 Energy level of an atom at different situations.

By an external action on the collection of atoms it is possible to obtain an out of

equilibrium situation where the ratio between the populations is different from the equilibrium.
It is more populated than the ground level, the collection of atoms is then said to contain
inversion of population. The problem in producing a laser is creating inversion in the
populations of the two levels

2-5 Amplification
Assuming now we have an inversion, N 2 > N 1 and considering a single photon entering
a region with the atoms in. the photon will pass by an atom with its electron in the upper level
and cause it to emit a second photon traveling in the same direction, by the process of
stimulated emission in two more atoms to give four photons and so on.

photon

atom

Figure 2.4. Amplification of photons

22
 Thus we have amplification which is also known as GAIN. The region containing the
atoms is known as the GAIN MEDIUM. The final stage in a laser is to get the first photon to
amplify. This is done by placing the gain medium between two mirrors that forms the so called
a laser CAVITY shown in figure 2.5.

Mirror 1 Gain medium Mirror 2

Figure 2.5 laser cavity. Gain medium with reflection mirrors

Initially there is no light in the cavity. The only possible process for the atoms to
undergo is therefore spontaneous emission, and this duly occurs. As stated earlier this may
travel in any direction out of the gain medium and most will lost from the cavity. However out
of the millions of photons emitted by the millions of atoms in any real medium there is bound
to be at least one which travels directly to one of the mirrors and is reflected back to the gain
medium. This is now our first photon. As it passes through the gain medium it causes
stimulated emission as described earlier and by the end of the gain medium there are, say 10
photons. Now the important part is that these are all travel in the same direction as the first
photon, so it will be reflected back to the gain region by the other mirror. This 10 photons now
each cause stimulated emission, and when they get out of the medium to the first mirror again
there are one hundred which are reflected back to the gain medium again and are amplified to
1000 etc…

Thus we very rapidly get very many photons traveling back and forth in the cavity. In an
idealized atom case where no photons are lost from the steady amplified beam, the photon
number just goes on increasing. But in any real laser some photons are lost, for many different
reasons. One of this is quite deliberate. One of the mirrors is made to reflect only part of the
light, and to allow the rest through. This is then the output beam of the laser and the leaky
mirror is referred to as the output coupler. A steady state may then be reached where the gain
exactly replaces the photons lost from the cavity by the output coupler. There is then a constant
number of photons in the cavity any time.

The output beam thus has photons which are traveling in a fixed direction and also have
fixed wavelength (or color) defined by the energy levels of the electrons in the atoms of the
gain medium. Laser produces special images in holography which includes Transmission
hologram, rainbow hologram and reflection holograms.

23
2-6 Construction of laser
As shown in figure 2.6 a laser has two parallel mirrors, of which one of them is nearly perfect
reflector and the other partially, facing each other to form resonator to allow light reflecting
back and forth along the optical axis. Active medium which amplifies the stimulated light is
placed between the mirrors. The active medium is pumped using pumping mechanism so that it
can be excited from its lower energy level. Photons can move in any direction but those photons
travelling along the optical axis will oscillate. The other photons will either be absorbed or
scattered. Finally most of the photons will oscillate along the optical axis. Thus for every
stimulated emission the photons number increases. Hence under proper conditions the light
density is amplified. Eventually the partially reflecting mirror will transmit laser light either in
continuous wave (cw) or in pulsed way.

Cooling system

R≈100% gain medium laser beam

Rare mirror Front mirror R≈95%

Pumping system

Figure 2.6 Basic laser system

The basic three parts of laser source are

a) The active medium
b) The pumping energy mechanism
c) Optical resonator

Thus laser light is coherent because it is radiated by a homogenous collection of atoms under
precisely the same conditions. The mirrors at both ends make the small percentage of photons
that hit the mirrors return in a straight line. This develops a cascade of light along the horizontal
line of the tube. If you were to remove the laser casing you would see the same monochromatic,
saturated light but the straight beam, so distinctive of laser light, would only be emitted from
the end with the partially coated mirror

24
2-7 Properties of laser beam
Coherency:- Laser light differs from all other light sources, man-made or natural, in one basic
way which leads to several characteristics. Laser light can be coherent light. Ideally, this means
that the light being emitted by the laser is of the same wavelength, and is in phase. Thus laser
light is coherent because it is radiated by a homogenous collection of atoms under precisely the
same conditions. The mirrors at both ends make the small percentage of photons that hit the
mirrors return in a straight line. This develops a cascade of light along the horizontal line of the
tube. Other properties of laser beam are monochromatic, highly directionality, brightness and
less divergence makes it special.

2-8 Types of lasers

Lasers are generally classified according to the material, called the gain medium use to produce
the laser light. Solid-state, gas, liquid, semiconductor, and free electron are all common types of
lasers.

A. Solid state laser

Solid-state lasers produce light by means of a solid medium. The most common solid laser
media are rods of ruby crystals and neodymium-doped glasses and crystals. The ends of the
rods are fashioned into two parallel surfaces coated with a highly reflecting nonmetallic film.
Solid-state lasers offer the highest power output. They are usually pulsed to generate a very
brief burst of light. Bursts as short as 12 × 10-15 sec have been achieved. These short bursts are
useful for studying physical phenomena of very brief duration.

One method of exciting the atoms in lasers is to illuminate the solid laser material with higher-
energy light than the laser produces. This procedure, called pumping, is achieved with brilliant
strobe light from xenon flash tubes, arc lamps, or metal-vapor lamps.

B. Gas lasers

The lasing medium of a gas laser can be a pure gas, a mixture of gases, or even metal vapor.
The medium is usually contained in a cylindrical glass or quartz tube. Two mirrors are located
outside the ends of the tube to form the laser cavity. Gas lasers can be pumped by ultraviolet
light, electron beams, and electric current or chemical reactions. The helium-neon laser is
known for its color purity and minimal beam spread. Carbon dioxide lasers are very efficient at
turning the energy used to excite their atoms into laser light. Consequently, they are the most
powerful continuous wave (cw) that is, lasers that emit light continuously rather than in pulses.

25
 C. Liquid lasers

The most common liquid laser media are inorganic dyes contained in glass vessels. They are
pumped by intense flash lamps in a pulse mode or by a separate gas laser in the continuous
wave mode. Some dye lasers are tunable, meaning that the color of the laser light they emit can
be adjusted with the help of a prism located inside the laser cavity.

D. Semiconductor lasers

Semiconductor lasers are the most compact lasers. Gallium arsenide is the most common
semiconductor used. A typical semiconductor laser consists of a junction between two flat
layers of gallium arsenide. One layer is treated with an impurity whose atoms provide an extra
electron, and the other with an impurity whose atoms are one electron short. Semiconductor
lasers are pumped by the direct application of electric current across the junction. They can be
operated in the continuous wave mode with better than 50 percent efficiency. Only a small
percentage of the energy used to excite most other lasers is converted into light.

Scientists have developed extremely tiny semiconductor lasers, called quantum-dot vertical-
cavity surface-emitting lasers. These lasers are so tiny that more than a million of them can fit
on a chip the size of a fingernail.

Common uses for semiconductor lasers include compact disc (CD) players and laser printers.
Semiconductor lasers also form the heart of fiber-optics communication systems (see Fiber
Optics).

E. Free electron lasers

Free electron lasers employ an array of magnets to excite free electrons (electrons not bound to
atoms). First developed in 1977, they are now becoming important research instruments. Free
electron lasers are tunable over a broader range of energies than dye lasers. The devices become
more difficult to operate at higher energies but generally work successfully from infrared
through ultraviolet wavelengths. Theoretically, electron lasers can function even in the X-ray
range.

2-9 Lasers Applications

The characteristics of laser light makes lasers a valuable tool in many areas, such as
communication, industry, medicine, military and scientific research. And also a laser is applied
in holography, which is our main concern. Some of the areas are:

Communication: - laser working in the infrared area are right now revolutionizing the
communication industry. A laser transmits voice or data via fiber optic cables at much
improved speed and capacity. These lasers are part of the broadband revolution we hear
about daily.

26
Industry: - lasers are used to cut, drill, weld, guide and measure with high accuracy.
Medicine: - surgeons use lasers to remove deceased body tissues, with little damage to
surrounding area. In addition laser seal off blood vessels served during the surgery.
Thus reduce the amount of bleeding.
Laser eye surgery is becoming common for correcting near sightedness as well as for
reattaching retinas.
Military: - lasers are used in military applications both as weapons and for guidance
systems for weapons. Future application may include very powerful lasers that can
down planes and missiles.

27
3 HOLOGRAPHY

DANIS GABOR The father of this topic

3-1 Basics of Holography
"What is a hologram? And how does holography work?" Note that: the process is
referred to as holography while the plate or film itself is referred to as a hologram. The terms
holograms and holography were coined by the father of holography Dennis Gabor in 1947. The
word hologram is derived from the Greek words "holos" meaning whole or complete and
"gram" meaning message. Older English dictionaries define a hologram as a document (such as
a last will and testament) handwritten by the person whose signature is attached.

The theory of holography was developed by Dennis Gabor, a Hungarian physicist, in the year
1947. His theory was originally intended to increase the resolving power of electron
microscopes. Gabor proved his theory not with an electron beam, but with a light beam. The
result was the first hologram ever made. At that time there was no source of coherent radiation
with sufficient intensity.

What was the light source he needed? The LASER was first made to operate in 1960.

Now that we know a little something about light in general, we may consider the light source
needed to perform holography: the laser. The understanding of the stimulated emission of
light, or how a laser works, will greatly aid in conceptualizing the holographic process.

28
Without the laser, the unique three dimensional imaging characteristics and light phase
recreation properties of holography would not exist as we know them today. Two years after
the advent of the continuous wave laser, c.1959-1960, Leith & Upatnieks (at the University of
Michigan) reproduced Gabor's 1947 experiments with the laser, and launched modern
holography. By 1964 they proved that holography was practical and that the use of laser light
was an important factor. They were able to generate the 3D images by illuminating a
photographic plate with light from a laser. Their work excited others to focus on holography.

The light emitted from a laser is all exactly the same type, or make, depending upon
the characteristics of the substance which is lasing. I will explain in the next chapter what the
term laser means, and how the laser works to give coherent light. Right now it is important to
remember that the frequency of laser light is unvarying and that in the same medium, all light,
i.e., light of different wavelengths of frequency, travels at the same speed.

It's true that all electromagnetic radiation, including the very small portion we call
visible light, travels in a vacuum at the approximate finite speed of 186,000 miles per second.
(Note, the velocity of light in a vacuum is one of nature's constants and is referred to by the
letter c). Light waves, can oscillate at different frequencies and with correspondingly different
wavelengths so that for any given amount of time, say one second, a greater number of shorter
wavelengths of (blue) light would be emitted from a laser than longer wavelengths of (red) light.
This does not mean that different wavelengths travel at different speeds.

Why we need laser for holography?

In order to maximize the visibility of fringes formed by the object and reference beams,
while recording a hologram it is essential to use coherent illumination. In addition to
being spatially coherent, the coherent length of the light must be much greater than the
maximum value of the optical path difference between the object and the reference
beams in the recording system. Lasers are therefore employed almost universally as
light sources for recording holograms. We can use different lasers for holography. Here
some of laser characteristics are listed in the table.

Laser output Wavelength(nm) power

Ar+ cw 514,488 1w
He – Cd cw 442 25mw
He –Ne cw 633 2-50mw
Kr+ cw 647 500mw
Diode cw 670-650 5mw
Diode-YAG cw 532 100mw
Dye cw Tunable 200mw
Ruby pulsed 694 1-10J
Table 2.1 lasers used for Holography.

But for a simple holographic system the (He-Ne) laser is the usual choice. It is
inexpensive and operates on a single spectral line at 633nm which is well matched to the

29
 peak sensitivity of many photographic emulsions. In addition it does not require water
cooling and has a long life.

3-2 THE BASIC HOLOGRAM

The hologram, that is, the medium which contains all the information, is nothing more than a
high contrast, very fine grain, black and white photographic film. Silver halide emulsion much
like the black and white film you can buy in your neighborhood drug store. The film designed
especially for holography is capable of very high resolution. One way of judging resolution of
film or lenses is to see how many distinguishable lines can be resolved within a certain width,
in this case it's a millimeter. a good film designed for holography is able to resolve up to 3000
lines/mm. Holographic film is also especially prepared to be sensitive to a certain wave length
of light Why the need for such special resolving power? The answer is that the hologram is not
a recording of a focused image as in photography, but the recording of the interference of laser
light waves that are bouncing off the object with another coherent laser beam, i.e., a reference
beam which will be described later. The wavelengths of light from a He-Ne laser are
approximately 24 micro-inches or twenty four millionths of an inch long, thus the need for such
fine grain or high resolving power.

A hologram is an image recorded on a photosensitive plate or flat sensitive film. In a hologram

a three dimensional (3D) image is recorded which is different from the ordinary or conventional
photography.

What happens when you take a photograph, and what happens when you
make a hologram?

A photograph is basically the recording of the differing intensities of the light reflected by the
object and imaged by a lens. The light is incoherent, therefore, there are many different
wavelengths of light reflecting from the object and even the light of the same wavelength is out
of phase. There is a point to point correspondence between the object and the emulsion.

- In the ordinary photography imaging technique the intensity distribution in the

original scene is recorded. Therefore all the information about the optical paths to
different parts of the scene is lost.

Any object to be recorded can be thought of as the sum of billions of points on the object which
are reflecting more or less light. The lens of the camera focuses each object point to a
corresponding point on the film and there it exposes a proportional amount of silver halide.
Thus, your record is of the intensity differences on the object which form a pattern that one may
ultimately recognize as the object photographed,

In holography we are working with light waves and with, most likely, a silver halide film, yet,
beyond that it is very difficult to compare the two. If we were to simply illuminate our object
with laser light and take a photograph, we would still only be recording the different light
intensities of the object; we would not have captured any information about the phase of the
light waves after bouncing off the object.

30
 - In holography technique case
a) both the phase and the amplitude waves coming from the
object are recorded.
b) it uses coherent light illumination and using reference
beam to convert the phase information into variation of
intensity

We need a standard or reference. In the same way that a surveyor needs a reference point in
order to make his measurements, we need a standard or a reference source in order to record the
phase difference of the light waves and thus capture the information which supplies the vital
dimensions and depth, to the holographic presentation. This standard we call a reference beam
and it is supplied by the laser light itself.

The reference light is emitted in what we call a plane wave. By enlisting the aid of a beam
splitter we are able to form two beams. The reference beam is allowed to hit the film directly. It
might be spread with a lens and aimed at the film by a mirror, but for all practical purposes this
does not affect the light waves.

The other beam which we will refer to as the object or scene beam is also usually spread by a
lens and guided by a mirror but it is directed at the object being holographed.

As soon as object beam hits the object it is changed, or modulated according to the physical
characteristics and dimensions of the object. So that the light which ultimately reaches the film
plane after being reflected by the object now deviates in intensity and phase, from the virtually
unhampered reference beam. That difference is a function of the object. What once began as a
plane wave is now bouncing off the object in a complex wave front which consists of the
summation of the multitude of infinitesimal object points reflecting light.

3-3 CLASSIFICATION OF HOLOGRAMS

Holograms may be classified in a number of different ways depending on their thickness,
method of recording, method of reconstruction etc.

Amplitude and Phase Holograms

A hologram may be of an absorption type which produces a change in the amplitude of
the reconstruction beam. The phase type hologram
produces phase changes in the reconstruction beam due to a variation in the refractive index or
thickness of the Roughness, Film medium. Phase holograms have the advantage over amplitude
holograms of no energy dissipation within the hologram medium and higher diffraction
efficiency. Holograms recorded in photographic emulsions change both the amplitude and the
phase of the illuminating wave. The shape of the recorded fringe planes depend on the relative
phase of the interfering beams. Consequently the reconstructed wave is reflected from the
hologram according to the density of the silver deposited with the amplitude variation

31
proportional to the amplitude of the object. Similarly the phase of the reconstruction wave is
modulated in proportional to the products and services phase of the object wave. Thus both
amplitude and phase of the object wave are reproduced.

Classification based on Hologram Thickness

Thin Holograms or Plane Holograms

Holograms may be thin (plane) or thick (volume). A hologram may also be regarded as
thin if its emulsion thickness is much less than the fringe spacing.
As the angle difference between the object beam (or the wave fronts bouncing off the object)
and the reference beam changes, so does the spacing of the patterns in the emulsion. As long as
the angle difference remains less than 90 degrees the hologram is called a plane hologram.
If you imagine your film in a fixed plane and your object in a stationary position, as you rotate
the incidence angle of the reference beam, you can determine whether you are making a plane
or volume holograms. If your angle is less than 90 degrees it's plane, from 90 degrees - 180
degrees it's volume.
A volume (thick) hologram may be regarded as a superposition of three dimensional
gratings recorded in the depth of the emulsion each satisfying the Bragg law. The grating planes
in a volume hologram produce maximum change in refractive index and/or absorption index. A
consequence of Bragg condition is that the volume hologram reconstructs the virtual image at
the original position of the object if the reconstruction beam exactly coincides with the
reference beam. However, the conjugate image and higher order diffractions are absent.

The spatial distribution of fringes recorded by the photo emulsion throughout its
entire emulsion forms volume hologram.

In-line and Off-axis holograms

Figure 3.1making In-line and Off-axis hologram

32
The in-line hologram
We consider the optical system shown in fig. 1.5 in which the object (a trans parency containing
small opaque details on a clear background) is illuminated by a collimated beam of
monochromatic light along an axis normal to the photographic plate.
The light incident on the photographic plate then contains two components. The first
one is the directly transmitted wave, which is a plane wave whose amplitude and phase do not
vary across the photographic plate. Its complex amplitude can, therefore, be written as a real
constant r.

Fig. 3.2. Optical system used to record an in-line hologram.

Finally, the hologram is illuminated, as shown in fig. 3.2, with the same collimated beam of
monochromatic light used to make the original recording.

Fig. 3.3 Optical system used to reconstruct the image with an in-line hologram, showing the formation of the twin
images.

With an in-line hologram, an observer viewing one image sees it superimposed on the out-of-
focus twin image as well as a strong coherent background. Finally, the hologram must be a
‘positive’ transparency.

33
 Off-axis holograms
The term off-axis means that the reference beam and object beam are not coming from the
same direction.

To understand the formation of an image by an off - axis hologram, we consider the recording
arrangement shown infig. 3.4, in which (for simplicity) the reference beam is a collimated
beam of uniform intensity, derived from the same source as that used to illuminate the object.

Fig. 3.4. The off-axis hologram: recording.

Figure 3.5 reconstruction of off-axis hologram.

If the offset angle of the reference beam is made large enough, the virtual image can be
separated from the directly transmitted beam and the conjugate image.
In this arrangement, corresponding points on the real and virtual images are located at equal
distances from the hologram, but on opposite sides of it. Since the depth of the real image is
inverted, it is called a pseudoscopic image, as opposed to the normal, or orthoscopic, virtual
image.

34
3-4 Main Types of Holograms

A hologram is a recording in a two- or three-dimensional medium of the interference

pattern formed when a point source of light (the reference beam) of fixed wavelength
encounters light of the same fixed wavelength arriving from an object (the object beam).
When the hologram is illuminated by the reference beam alone, the diffraction pattern
recreates the wave fronts of light from the original object. Thus, the viewer sees an
image indistinguishable from the original object.
There are many types of holograms, and there are varying ways of classifying them. The
basic types are listed as follows.

THE REFLECTION HOLOGRAM

The reflection hologram, in which a truly three-dimensional image is seen near its surface,
is the most common type shown in galleries. The hologram is illuminated by a “spot” of
white incandescent light, held at a specific angle and distance and located on the viewer’s
side of the hologram. Thus, the image consists of light reflected by the hologram.
Recently, these holograms have been made and displayed in color their images optically
indistinguishable from the original objects. If a mirror is the object, the holographic
image of the mirror reflects white light; if a diamond is the object, the holographic image
of the diamond is seen to “sparkle.”

Although mass-produced holograms such as the eagle on the VISA card are viewed
with reflected light, they are actually transmission holograms “mirrorized” with a
layer of aluminum on the back.

3. THE TRANSMISSION HOLOGRAM

The typical transmission hologram is viewed with laser light, usually of the same type
used to make the recording. This light is directed from behind the hologram and the
image is transmitted to the observer’s side. The virtual image can be very sharp and deep.
For example, through a small hologram, a full-size room with people in it can be seen as
if the hologram were a window. If this hologram is broken into small pieces (to be less
wasteful, the hologram can be covered by a piece of paper with a hole in it), one can still
see the entire scene through each piece. Depending on the location of the piece (hole), a
different perspective is observed. Furthermore, if an undiverged laser beam is directed
backward (relative to the direction of the reference beam) through the hologram, a real
image can be projected onto a screen located at the original position of the object.

35
3. HYBRID HOLOGRAMS

Between the reflection and transmission types of holograms, many variations can be
made.

Embossed holograms: To mass produce cheap holograms for security application such as
the eagle on VISA cards, a two-dimensional interference pattern is pressed onto thin
plastic foils. The original hologram is usually recorded on a photosensitive material
called photoresist. When developed, the hologram consists of grooves on the surface. A
layer of nickel is deposited on this hologram and then peeled off, resulting in a metallic
“shim.” More secondary shims can be produced from the first one. The shim is placed on
a roller. Under high temperature and pressure, the shim presses (embosses) the hologram
onto a roll of composite material similar to Mylar. Embossed holograms are used in the
security industry because they are difficult to counterfeit.

Integral holograms: A transmission or reflection hologram can be made from a series of

photographs (usually transparencies) of an object—which can be a live person, an
outdoor scene, a computer graphic, or an X-ray picture. Usually, the object is “scanned”
by a camera, thus recording many discrete views. Each view is shown on an LCD screen
illuminated with laser light and is used as the object beam to record a hologram on a
narrow vertical strip of holographic plate (holoplate). The next view is similarly recorded
on an adjacent strip, until all the views are recorded. When viewing the finished
composite hologram, the left and right eyes see images from different narrow holograms;
thus, a stereoscopic image is observed.

Multichannel holograms: With changes in the angle of the viewing light on the same
hologram, completely different scenes can be observed. This concept has enormous
potential for massive computer memories.
Computer-generated holograms: The mathematics of holography is now well understood.
Therefore, we can dream up any pattern we want to see. After we decide what wavelength we
will use for observation, the hologram can be designed by a computer. This computer-generated
holography (CGH) has become a sub-branch that is growing rapidly. For example, CGH is used
to make holographic optical elements (HOE) for scanning, splitting, focusing, and, in general,
controlling laser light in many optical devices such as a common CD player.

36
120° Integral Stereogram (Multiplex)
A type of white light transmission hologram which is formed by recording multiple photographs
onto a single hologram. The resulting image usually only provides horizontal parallax, and often
provides the effect of an animated three dimensional image. 120° integral stereograms are not
complete cylinders

360° Integral Stereogram (Multiplex)

A type of white light transmission hologram which is formed by recording multiple photographs
onto a single hologram. the resulting image usually only provides horizontal parallax, and often
provides the effect of an animated three dimensional image. 360° integral stereograms are
complete cylinders, and are often mounted on a motor-driven base which allows them to rotate at a
constant speed.

Computer Generated Stereogram

Hologram produced from multiple 2-d perspective recordings of computer-generated images.
Images can be analog, animated, reduced or enlarged. This is an alternative to the analog hologram
process, in which the subject is imaged directly onto the film with a laser exposure.

Dichromated Gelatin (reflection)

Dichromated Gelatin (DCG) is a chemical-gelatin mix that produces very bright images in a
golden-yellow color. The images have the least range of depth, but they are viewable in normal
room light without special spotlights.

Holographic Stereogram
Hologram produced from movie footage of a rotating subject. Images can be computer generated,
animated, reduced or enlarged, or photographed on site. This is an alternative to the original
hologram process, in which the subject is imaged directly onto the film with a laser exposure.

Rainbow Holograms
Reflection Holograms
Reflection Holograms are lit from the front, reflecting the light to you as you view it, like a
painting or photograph hung on a wall. Different film emulsions produce images with different
characteristics. (Silver Halide, Dichromated Gelatin, Photo Polymer)

White Light Transmission Holograms

White light transmission holograms are illuminated with incandescent light (white light) and
produce images that contain the rainbow spectrum of colors. The colors change as the viewer moves
up and down and are often called "rainbow" holograms.. Transmission holograms are lit from the
rear (like a photographic transparency) and bend light as it passes through the hologram to your
eyes to form the image.

Primary Support Options Film

Most holographic film is just like photographic film in many ways. It often contains
photosensitive silver halide crystals. The major difference in holographic film is that it is
capable of very high resolution. Holographic film is also specially designed to be sensitive to
a particular wavelength of light.

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Film (photopolymer)
Photopolymer is the newest of the recording materials. They have a plastic backing and are
suitable for long production runs. The image depth of photo polymers is slightly less than that of
silver halide; however, the images are brighter, with a wider angle of view.

Foil
Foil is often the support material for embossed holograms.

Glass
Sometimes emulsion is applied to glass, which provides greater stability than film during the
exposure process.

Hard Plastic
Sometimes used as a support material for embossed holograms (such as record albums).
Holographers occasionally apply emulsion to thick plastic just as they would to glass. Film can be
made sturdier after being developed if it is laminated onto plastic sheets. This technique is most
often used in large-format holography, since heavy glass plates would be difficult to safely manage.

Metal
Anything that is solid enough to retain an imprint image can be used to record a hologram. Metal is
often used as a master shim(wedge shape), from which other holograms are embossed onto plastic
or other material.

3-5 CHARACTERISTICS OF A HOLOGRAM

Holograms have certain unique characteristics. These are given below:

Hologram Aberrations: One of the basic characteristics of holograms is that they suffer from
aberrations which are caused by a change in the wavelength from construction to reconstruction.
This is also caused by a difference in the reference and reconstruction beams. There are two
types of aberrations-chromatic and non-chromatic-which are important even when there is a
small difference between the reference and reconstruction geometry. One simple way to
eliminate all the aberrations simultaneously is to copy exactly one construction beam in the
reconstruction process.

Orthoscopic and Pseudoscopic Images: A hologram produces two images, one which is real
and the other a virtual image which is an exact replica of the object. However, to the
appearance of the observer, the two images differ in appearance. The virtual image has the
same appearance of depth and the parallax and produced at the same position as the original
object. It appears that the observer is viewing the original object through a window defined by
the size of the hologram. This virtual image is known as orthoscopic image. The real image is
also formed in front of the hologram at the same time and at the same distance from the
hologram. This real image is called pseudoscopic image where the scene depth is inverted.

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Some Other Characteristics

It is possible to reconstruct the hologram of a diffuse object by a small portion of the

hologram. In other words, if a hologram breaks into pieces, the entire image can be
produced by each piece. However, as the size of the hologram reduces, a loss of image
perspective, brightness and resolution result in the constructed image.

Another characteristic of hologram is that a contact print of a hologram will reconstruct a

positive image which is not distinguishable from the image produced by the original.

A cylindrical hologram makes a 360 degree view of the object.

Without any cross-talk, more than one independent scenes can be stored in the same
photographic plate and these can be viewed one at a time.

3- MAKING HOLOGRAMS
Holographic recording process

Figure 3.6 Arrangement for holographic recording.

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 To produce a recording of the phase of the light wave at each point in an image, holography
uses a reference beam which is combined with the light from the scene or object (the object
beam). If these two beams are coherent, optical interference between the reference beam and
the object beam, due to the superposition of the light waves, produces a series of intensity
fringes that can be recorded on standard photographic film. These fringes form a type
of diffraction grating on the film, which is called the hologram..

General properties of recording materials for holography.

Reusable Processing Type of Max. Required Resolution

Material hologram efficiency exposure limit [mm-1]
[mJ/cm²]
Photographic No Wet Amplitude 6% 0.001–0.1 1,000–10,000
emulsions Phase 60%
(bleached)
Dichromated gelatin No Wet Phase 100% 10 10,000
Photoresists No Wet Phase 33% 10 3,000
Photothermoplastics Yes Charge and Phase 33% 0.01 500–1,200
heat
Photopolymers No Post Phase 100% 1–1,000 2,000–5,000
exposure
Photochromics Yes None Amplitude 2% 10–100 >5,000
Photorefractives Yes None Phase 100% 0.1–50,000 2,000–10,000

Table 3.1 properties of recording materials.

This is possible because during holographic recording, each point on the hologram's
surface is affected by light waves reflected from all points in the scene, rather than from just
one point. It's as if, during recording, each point on the hologram's surface were an eye that
could record everything it sees in any direction. After the hologram has been recorded, looking
at a point in that hologram is like looking "through" one of those eyes.
In table 3.1 the principal materials for holographic recording are shown. The required exposure
is for a long exposure. Short exposure times (less than 1/1000th of second, such as with a
pulsed laser) require a higher exposure.

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To demonstrate this concept, you could cut out and look at a small section of a recorded
hologram; from the same distance you see less than before, but you can still see the entire scene
by shifting your viewpoint laterally or by going very near to the hologram, the same way you
could look outside in any direction from a small window in your house. What you lose is the
ability to see the objects from many directions, as you are forced to stay behind the small
window.
The first holograms were recorded already prior to the invention of the laser, and used other
(much less convenient) coherent light sources such as mercury-arc lamps.
In simple holograms the coherence length of the beam determines the maximum depth the
image can have. A good holography laser will typically have a coherence length of several
meters, ample for a deep hologram.

Experimental Set up

One should follow the following steps before, while and after recording a hologram.

Precautions

 Chemicals should be treated with respect

 No need of talking
 Do not move (be still)
 Do not touch the table while exposure
 Turn off your mobile and other vibrating systems
 Use rubber gloves while developing
 Dispose the used developers and bleach properly

Steps to follow

 Choose a sturdy table or counter in a dark room that is free of noise,

vibration, air currents, and small movements (creaky floors etc)
 Keep the table feet in sand filled cans to avoid vibrations
 Arrange the object, He-Ne laser, diverging lenses to spread the beams,
mirrors according to the figure
 Position the laser till the subject is fully illuminated
 Make ready the developing chemicals
 Turn off dark room light; block any direct light from reaching the
holography system.
 Open the box, take one film (plate) and immediately close the box.
 Place the plate on the plate holder
 Wait 10 – 20sec for the plate to settle
 Illuminate the hologram by removing the shutter that blocks the laser
light from the source
 Block again the light
 Remove the plate from its holder
 Immediately start the developing process

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 Process of developing

• Mix the dried powder photo chemicals with distilled water to form two
solutions, the developer and bleach.
• Dip and wiggle plate in developer for 20sec.
• Rinse the plate in water for 30sec.
• Dip and wiggle the plate in bleach for 20sec.
• Rinse in water for 30sec

3-7 RECONSTRUCTION OF HOLOGRAM

When the processed holographic film is illuminated once again with the reference
beam, diffraction from the fringe pattern on the film reconstructs the original object beam in
both intensity and phase.
Because many viewpoints are stored, each of the viewer's eyes sees the image from a slightly
different angle, so the image appears three-dimensional. This is known as stereopsis. The
viewer can move his or her viewpoint and see the image rotate exactly as the original object
would.
The central miracle of holography is that when the recorded grating is later illuminated by a
substitute reference beam, the original object beam is reconstructed, producing a 3D image

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 a) interference patterns

b) reconstruction of picture a)
Pictures made on a transmission hologram

Mass replication

An existing hologram can be replicated, either in an optical way similar to holographic

recording, or in the case of surface relief holograms, by embossing. Surface relief holograms
are recorded in photoresists or photothermoplastics, and allow cheap mass reproduction. Such
embossed holograms are now widely used, for instance as security features on credit cards or

43
quality merchandise. The Royal Canadian Mint even produces holographic gold and silver
coinage through a complex stamping process[2].
The first step in the embossing process is to make a stamper by electrodeposition of nickel on
the relief image recorded on the photoresist or photothermoplastic. When the nickel layer is
thick enough, it is separated from the master hologram and mounted on a metal backing plate.
The material used to make embossed copies consists of a polyester base film, a resin separation
layer and a thermoplastic film constituting the holographic layer.
The embossing process can be carried out with a simple heated press. The bottom layer of the
duplicating film (the thermoplastic layer) is heated above its softening point and pressed against
the stamper so that it takes up its shape. This shape is retained when the film is cooled and
removed from the press. In order to permit the viewing of embossed holograms in reflection, an
additional reflecting layer of aluminium is usually added on the hologram recording layer.

Dynamic holography

The discussion above describes static holography, in which recording, developing and
reconstructing occur sequentially and a permanent hologram is produced.
There exist also holographic materials which don't need the developing process and can record
a hologram in a very short time. This allows to use holography to perform some simple
operations in an all-optical way. Examples of applications of such real-time holograms
include phase-conjugate mirrors ("time-reversal" of light), optical cache memories, image
processing (pattern recognition of time-varying images), and optical computing.
The amount of processed information can be very high (terabit/s), since the operation is
performed in parallel on a whole image. This compensates the fact that the recording time,
which is in the order of a µs, is still very long compared to the processing time of an electronic
computer. The optical processing performed by a dynamic hologram is also much less flexible
than electronic processing. On one side one has to perform the operation always on the whole
image, and on the other side the operation a hologram can perform is basically either a
multiplication or a phase conjugation. But remember that in optics, addition and Fourier
transform are already easily performed in linear materials, the second simply by a lens. This
enables some applications like a device that compares images in an optical way [3].
The search for novel nonlinear optical materials for dynamic holography is an active area of
research. The most common materials are photorefractive crystals, but also in semiconductors
or semiconductor heterostructures (such as quantum wells), atomic vapors and gases, plasmas
and even liquids it was possible to generate holograms.

A particularly promising application is optical phase conjugation. It allows to remove the

wavefront distortions a light beam receives when passing through an aberrating medium, by
sending it back through the same aberrating medium with a conjugated phase. This is useful for
example in free-space optical communications to compensate the atmospheric turbulence (the
phenomenon that gives rise to the twinkling of starlight).

3-8 Electron holography

Electron holography is the application of holography techniques to electron waves rather than
light waves. Electron holography was invented by Dennis Gabor to improve the resolution and

44
avoid the aberrations of the transmission electron microscope. Today it is commonly used to
study electric and magnetic fields in thin films, as magnetic and electric fields can shift the
phase of the interfering wave passing through the sample.

3.9 Acoustical holography

Ultra-high-frequency sound wave (Ultra sound) is used to create the hologram. Laser
beam is the used to reconstruct the image. The acoustical hologram helps to record images in
dense liquids and solids where light cannot. It can record diverse things under water submarine
and internal body organ.

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4 APPLICATIONS OF HOLOGRAMS

Holography has today emerged as an important tool in science and technology. It is a

well used method to produce pictures and represents one of the most prominent examples of
recombining of scattered radiation to produce pictures. This process of producing holograms is
now spreading from the research laboratory to various industries, and holograms find wider
employment in communication and other engineering problems. A hologram is not only a
three-dimensional image but also can store numerous quantities of information. In the computer
technology, holograms can be used to store memories which are much larger and faster.
Hologram has today become a very well known concept in credit cards, tickets or original
covers on software computer programs or any objects to prevent falsification. An important
area of application of hologram is bar-code readers in shops, warehouses, libraries etc. A code
reader is based on the use of holographic components like optical gratings. Some other
examples of the use of holographic technology is in the aircraft industry's head-up displays
(HUD) or for making holographic optical elements (HOE) and so on. Let’s see some of them.

4-1 Holographic data storage

Holography can be applied to a variety of uses other than recording images. Holographic data
storage is a technique that can store information at high density inside crystals or
photopolymers. Holographic storage has the potential to become the next generation of popular
storage media. The advantage of this type of data storage is that the volume of the recording
media is used instead of just the surface.
In 2005, companies such as Optware and Maxell have produced a 120mm disc that uses a
holographic layer to store data to a potential 3.9 TB (terabyte), which they plan to market under
the name Holographic Versatile Disc.
Holographic Optical Element (HOE)
- Can be used for non pictorial purposes like making diffraction gratings
- Consists of fringe system
- Are used inside supermarket checkout scanners that automatically read the bar
patterns of the universal product code (UPC) on merchandise
- Are used in heads up display in airplane cockpits, in office copy machines and solar
converters.

4-2 Digital holography

An alternate method to record holograms is to use a digital device like a CCD camera instead of
a conventional photographic film. This approach is often called digital holography. In this case,
the reconstruction process can be carried out by digital processing of the recorded hologram by
a standard computer. A 3D image of the object can later be visualized on the computer screen
or TV set.

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4-3 Holography in art
In London, Dalí assembled his models by hanging objects with wires inside of wooden frames.
This technique allowed for overlapping and differences in depth. Since then the quality of the
holograms has increased dramatically, mainly due to better holographic emulsions. As of 2007
there are many artists who use holograms in their creations.

Holographic interferometry: Microscopic changes on an object can be quantitatively

measured by making two exposures on a changing object. The two images interfere with
each other and fringes can be seen on the object that reveals the vector displacement. In
real-time holographic interferometry, the virtual image of the object is compared directly
with the real object. Even invisible objects, such as heat or shock waves, can be rendered
visible. There are countless engineering applications in this field of holometry.
Today holographic testing of mechanical systems is a well established practice in
industry, serving from noise reduction in automobile transmission to routine jet engine
inspections.
It has three approaches:
• Double exposure technique
• Real –time method
• Time average approach
Micro inch distortions in an object resulting from strain, vibration, heat and so on can be
studied. It also indicates displacements suffered by an object

4-4 Holograms as diffraction gratings

When holograms are constructed, the reference beam and the object beam interfere with one
another, and the dark and light fringes of the interference pattern are recorded. The clear, light
parts become like the slits of a diffraction grating, and the angle at which they bend incoming
light (the reconstruction beam) is determined by the spacing between them, which in turn was
determined originally by the object beam and reference beam, when the hologram's interference
pattern was made.

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 Fig. 4.1 a diffraction grating receives incoming light from the left which is diffracted by the slits

The destructive interference leaves behind slits that become a diffraction grating. The incoming light is
bent by them.

All objects that we see, we see as a collection of point sources. Each point on the object radiates
out light as a point source and the collection of points our eyes see becomes a whole object. It is
the same with holograms: every single point on the object records its own private interference
pattern, which gets individually reconstructed, and our eyes see all these points reconstructed
together to see the whole picture of the hologram all at once.
This explains why our view on the object in the hologram changes with our position; each time
we move we are seeing a different ray emitted from each point source. With
normal photography, the camera records just one view, so when you move you are in effect
seeing the same ray again and your view doesn't change. (You are seeing different rays from
each dropet of ink, but each droplet of ink is one ray of the picture.) The hologram, in
comparison, records every possible view there is to see, all at once.

4-5 In health sector

Recent improvements in hologram recording techniques and the availability of tools for the
interpretation of holographic interferograms and the success of holographic techniques in
imaging through tissues, ophthalmology, dentistry, urology, otology, pathology, and
orthopedics shows a strong promise for holography to emerge as a powerful tool for medical
applications. Holographic 3D images of eyes and interferometric testing of human teeth and
chest motion during respiration were carried out quite early.

X-ray holography can be applied for imaging of internal parts of the body and living
biological specimens with very high resolution without the need for sample preparation.
Endoscopic holography has opened up the possibility of noncontact high resolution 3D
imaging and nondestructive measurements inside the natural cavities of internal organs.

Endoscopic Holography Endoscopic holography has potential of providing a powerful tool for non

48
contact high resolution 3D imaging and nondestructive measurements inside natural cavities of
human body or in any difficult to access environment. It combines the features of holography
and endoscopy.

4-6 In military
Battle Simulation and Scenarios can be played in advance so that every possible contingency
can be calculated.
Holograms are a valuable tool on the battlefield itself also, consider Holographic Decoys and
Deception Applications - deception tactics are extremely important in wartime. Better yet just
the fact that you have these technologies makes the enemy second guess you and hesitate and
the way that wars are fought now at light speed, that is an extreme advantage.
Many new soldiers are not quite prepared for the reality of war and the gruesome sights they
will see, which often leave psychological and emotional scares. With Holographic Imaging the
soldier can be toughened up prior to battle using hologram Virtual Reality Training and Mind
Conditioning equipment.
Tele-Presence in Command and Control Communication also will be a major military
application of holographic technology. Instead of mere, voice or video, specially coded
holographic communication will rule the day.

In technology
- Holographic TV is not a very used product today but with advancement of
technology it is no doubt that 3D hologram TV is sure to be an essential commodity
in every one’s home.
- Holographic glasses have digital computer generated hologram lenses. With 3D
effect make 3D films work worthy. Generally have the ability to generate diffractive
elements in the form of binary amplitude and binary phase.

We can generalized the uses of holograms in the following way that holograms:

 Combat counterfeiting (forgery)

 Cannot be reproduced except from their original master.
 Provide authentication (many manufacturers use embossed holographic seals to
identify authenticity)
 Enhance packaging appeal and increase brand sales.
 Minimize document tampering(from being damage)

Holographic elements are now utilizes in packaging products such as sporting goods and
merchandises.

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Holography has been increasingly used for brochures and magazines.

Here are some pictures that shows application of holography.

Seal on passports 3D TV

Holographic eye glass stickers

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CONCLUSION
Summary and Conclusion
A typical holographic system consists of laser source, diverging senses to increase
the beam size, mirrors to change the direction of the source beam and photosensitive film or
plate to record the image in the form of interference patterns. Once the patterns are formed on
the transmission hologram one cannot see the image which needs reconstruction by
illuminating the plate from the back with the reference beam.

In this project basic concepts and applications of holography are presented. Models
and results are described in a simple form. The reader can find more details about theoretical
and experimental parts. Holographic system or technique requires patience of the user.

The author recommends to the ministry of Education curriculum developers that since
nowadays holography id applicable in all fields and contributing a lot for the advancement of
science and technology, in research areas for security purpose etc. it should be included as a sub
topic in preparatory level physics curriculum and can be exercise in high school physics
laboratories.

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BIBLIOGRAPHY
Germain Chartier “ Introduction to optics©2005

P. Hariharan “ Basics of Holography” ©2002

Tung H Jong Fundamentals of Photonics. Module 1.10

Basic principles and applications of holography

Sr. Hosseini “ Holography and 3D Imaging technology(PPt)

K.K . Sharma “ Optics Principles and applications” ©2006

William S.C Chang principles of lasers & Optics©2005

Eugene Hecht: Optics 4th edition ©2002

Robert J. Collier, Christoph B, Burkhardt, Lawrence H. Lin “Optical

Holography”©1971

F.G. Smith, J.H. Thomson: OPTICS©1971

Mussa mohammod “Measurement of wavelength of laser by making

hologram”,AAU,2007

http://www.integraf.com

http://www.hologramsuppliers.com/threed-holorgam-html

http://www.wikipedia.org/wiki/hologram

info@integraf.com

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