--~OAR 68-0015

AD 685400

INTRODUCTION TO HOLOGRAPHY
by CAPT MELVIN C. WATKINS, AF Res

U<APR 8 196q9

7MsdiuaA has bm B9=v

for pubhio reecma Lad se. its

OFFICE OF AEROSPACE RESEARCH *UNITED STATES AIR FORCE

IN .,P*ui

5IP;, -
WWOO

Agencies of the Department of Defense, qualified contractors and other

Government agencies may obtain copies from the
Defense Documentation Center
Cameron Station
Alexandria, Virginia 22314
This document has been released also to the
CLEARINGHC' id:
U.S. Department of Commerce
Springfield, Virginia 22151

for sale to the public.

THIS DOCUMENT HAS BEEN APPROVED FOR PUBLIC RELEASE AND SALE. ITS DISTRIBUTION IS
UNLIMITED.
 INTRODUCTION TO HOLOGRAPHY

By
Capt Melvin C. Waikins, AF Res

Office of Aerospace Research *United States Air Force

Arlington, Virginia 22209
 INTRODUCTION TO HOLOGRAPHY
by
Capt Melvin C. Watkins, AF Res*

1. INTRODUCTION

It is the intent of this paper to provide a is taken that light propagates in a wave motion and is
descriptive definition of a term Lhat is one of the thereby identified by its amplitude and phase charac-
latest contributions of science to the ever-gowing teristics. When two or more light beams intersect in
technical vocabulary. "Holography," a word whch space, the principle of superposition applies, resulting
for m y years was confined to the world of optics, is in an interference effect. The .,nciple of superposi-
rapidly becoming commonplace throughout the tion states that the resultant displacement at any
scientific community. The concept of holography and point and at any instant may be found by adding the
its nomenclature were introduced in 1948 by Dennis instantaneous displacements that would be produced
Gabor. (1) Gabor demonstrated that the amplitude at the point by the individual wave trains if each were
and phase information contained within the image of present alone. (2) For ight emitted from two sources
an illuminated object could bi, photographically generating waves in unison, there will be points in
recorded, stored, and retrieved. He called the record- space where the phases of the waves add, producing a
ing a hologram and the process holography, choosing reenforcement of the amplitudes, while at other
as a ?repx the Greek word, holo, which rreans whole, oints in space the phase relationship will produce a
to differentiate his photographic technique from cancellation of the amplitudes. This interference
conventional ones which use only amplitude informa- effect will be manifest in the appearance of bright
tion. and dark areas when viewed on a screen placed in the
A significant aspect of preserving the phase path of the light beam. The highlights of the classical
information in a photographic recording is that the demonstration and analysis by Yiung of the inter-
reconstructed image is displayed in three dimensions. ference effect illustrate the generation of fringes
A properly constructed hologram can produce in its resulting from the principle of superposition. (See
three-dimensional image not only a stereo effect, but Figure 1.)
also all of the parallactic effects of the original scene,
thereby allowing one to see behind objects by looking
around them.
To supplement my first statement, the intent S1
here is to convey, as an introduction !o hoiography,
an understanding of the holographic process by dCenter Line
discussing basic concepts and describing a few of the e
most attractive applications. Presentations of analyti- YM
cal expressions have been kept to a minimum and, it * P
is hoped, the concepts presented in a simplified R
manner. Unfortunately, the subject in depth is quite
complex and, for those individuals unfamiliar with
optical interferometry, exceedingly difficult to grasp. Figure 1. Young's Experiment.
Since it is true that the holographic technique is
intimately entwined with the principles of optical
interference and diffraction phenomena, this author Consider S, and S2 in Fib ire I as coherent light
feels that a recap of these principles would be murces generating waves which intersect in space at
appropriate and has therefore reviewed them in the the point P. For brightness to occur at this point, the
following section. light waves must be in phase; therefore, the path
lengths must be an integral number of wavelengths
Ii. OPTICAL INIERFERENCE AND DIFFRAC- !.'ng. According to this requirement and the con-
TION figuration of Figure 1, the rc'ltionship of the param-
In the study of physical optics, the point of view eter shown to the occuffenc. or spacing of bright
areas is:
* Prepand by Cept Watkins wk on active duty withRX
the Eieat
SOR1 dneDiion, DCSIIis & ?105fM*. T (I

;' I:. ... .

 where discussions, the reciprocals of the fringe spacings are
referred to as spatial frequencies; therefore, it is clear
YM = distance from the center axis from the previous discusion that the spatial fre-
quency of the diffraction pattern will be lower than
=
m integer that of the interferometric pattern. This point is
disussed below in the description of the holographic
R = distance from source to observation plane process.
X = wavelength of light
III. THE HOLOGRAPHIC PROCESS
d = distance between sources
The holographic process originally proposed by
Hence for m = 0 (zeroth fringe), a bright area will Gabor used a point source (a means of producing a
occur on the center line, as illustrated in Figure 1. spatially coherent light beam of low quality) to
The purpose of this presentation is to emphasize the illuminate a transparent object which generated a
relationship of the fringe spacing, Ym,or the spatial diffraction pattern. This diffraction pattern, when
frequency, 1/Yi, to the geometrical relationship photographed, produced a hologram. Recall that the
between the sources and the distance R to the hologram is a recording of the amplitude and phase
observation plane where the fringes are observed, information contained within the reflected energy
The second basic optical phenomenon descriptive from the object. Illuminating this transparency with a
of the holographic process is that of diffraction. point source of light reconstructed a three-
Diffraction is an eixect which appears to cause a light dimensional image. Unfortunately, due to the low
beam to bend around an opaque object. This effect purity of the light coherency, the low-quality image
can be explained on the basis of Huygens' principle inherent in the original technique, and restrictions on
which states that every point of a wave front may be those objects for which the Gabor technique was
considered the source of small secondary wavelets useful, the concept generated limited interest. The
which propagate in all directions. Thus, at the edge of advent of the laser, combined with an analysis by
an opaque object, the light wave acts as a secondary E. Leith and J. Upatnieks in 1962, (3) provided the
source directing waves in all directions and providing motivation for renewed interest in the subject. The
an illumination in areas of geometrical shadow. Since insight of these two men, apparent in their analysis,
every portion of the original wave acts like a made it possible to raise the quality of the holo-
secondary source, a very complex interference pat- graphic technique to a highly acceptable level. Their
tern is produced at observation planes in the path of approach describes the prc ,ss from a communica-
the reflected rays. The fringes generated by this type tion-theory viewpoint, thereby permitting the appli-
of interference effect produce what is referred to as a cation of electronic-system techniques to optical
diffraction pattern. A dose look at the analysis for procedures, in generating holograms, to enhance the
the spacing between the fringes of a diffraction signal-to-noise ratio of the reconstructed image. One
pattern will show an identical geometrical relation- of the major suggestions was to incorporate a
ship between sources and observation-planc distances reference beam in constructing the hologram so that
as shown by the interference effect described earlier. it would perform as does a carrier frequency in a
Although this relationship Is obvious, it is not communication system. By this process, the reference
generally emphasized, and is brought to the reader's beam acts as the second source in the optical
attention because of its significance in a later refcr- interference process, generating a high spatial fre-
ence describing the holographic process. quency. The diffraction pattern produced by the
Note that the separation between the fringes, object would have, for reasons given previnusly, a low
Ym, is inversely proportional to the "d" dimension of spatial frequency modulating the reference or carrier
the space between the wavelet sources. Since the frequency.
diffraction effect is generated from sources which are In order to understand the basic procedure in
adjacent illuminated points on an object, the value of constructing a two-beam hologram, let us cosisider a
d is small, whereas in the cases of those sources coherent source of light, such as the laser, which
generally considered In produc;.:,.q the interferometry illuminates the object and a mirror, both of which
effect, the d dimension would be several orders of redirect the light to a photographic plate, as shown in
magnitude larger. Therefore, one should recognize Figure 2. Each point on the object, acting as a source
that the fringe spacings for the diffraction pattern light, generates a diffraction pattern in the plane of
will be greater than those for the interferometric the photographic plate. The light wave reflected from
effect. the mirror, acting as a reference beam or carrier, adds
In terminology common to many holographic to the waves reflected from the object to produce an
2
interference pattern, also in the plane of the photo- the (xy) coordinates, one can readily see that the
graphic plate. Here the condition exists where the emergent light possesses the same term as that given
interference pattern producing closely spaced fringes originally for the light reflected by the object, except
is modulated or altered by the larger-spaced fringes for the constant coefficient. Therefore, looking
produced by the diffraction effect generated from the through the photographic plate, one would observe a
object. To reconstruct the image, the hologram, replica of the original subject.
which now looks like diffraction grating, is iilumi- Up to this point, the discussion has ignored the
nated by a coherent source. The image, when viewed fringe effect within the depth of the recording
through the hologram, exists as the original scene, emulsion which is permissible whenever a low spatial
frequency exists. However, this is not always the case,
and some very interesting effects occur by utilizing
the film thickness in the recording process. In these
techniques, the reference beam is directed to the rear
Laser Source of the photographic plate as opposed to exposing the
front side, as previously described. Holographic con-
struction, in this way, is referred to as a reflection
Object type, in contrast to those previously discussed, which
are commonly known as the transmission type. The
reflection-type hologram, due to the relative positions
of the light sources, generates a fringe pattern whose
spacings are less than the emulsion thickness, thereby
producing a recording in the depth of the emulsion.
Photographic Mirror These stratified layers act as resonant structures
Plate generating optical interference filter effects. A hol-
ogram of this type, by the nature of its resonant
structure, possesses extreme wavelength selectivity,
Figure 2. A Two-Beam Hologram allowing the use of white light for reconstruction. (5)

Leith (4) has presented this operation in very IV. APPLICATIONS AND LIMITATIONS
simple mathematical terms which are given below as
an additional aid in understanding the concept. The major practical applications of the holo-
The reference beam is expressed tis: graphic process have been those related to interfero-
metric analysis. To appreciate the advantages offered
U =a, cos (Wt - ex) (2) by the holographic process, a review of a conven-
tional interferometer is presented below. The particu-
lar interferometer illustrated in Figure 3 is known as
where ax represents a phase shift across the recording the Mach-Zehnder type. In the initial conditions, the
plate. The diffraction pattern generated by the object optical path lengths in the two legs are made equal,
is of the general form of: and the rays are made parallel after recombining at
the beam splitter, S2. With these conditions, perfect
u =a(x,y) os [ot + Oa, y)1 (3) interference will occur at the observation plane, and
the area at that plane will be uniformly illuminbted.
The photographic process acts as a square-law The introduction of a specimen in the test chamber
device recording the function will produce changes in the path length of the light
beam in one leg, generating an interference pattern
=
(U +u)? YAo + YA2 + oa os(ax +O) (4) from which an analysis of the specimen can be made
by a study of the fringe spacings. The requirements
In the reconstruction process, the emergent light on the optical quality of this mechanism are quite
is: stringent in order to produce a pattern at the
observation plane either void of fringes, or to produce
ao(Y.o 2 + %82 ) cOS (Wt - Cx) + V11,3a cOS (ot + 0) an orderly pattern of fringes for the initial set-up. By
(5) the use of holographic techniques, a reference pattern
+ YAo3 a cos (wt - 2mi -0) can be produced which mu-' the effects of undesira-
ble fringes generated from imperfections within the
Realizing that the a and 0 term are functions of system.
3

Ii
 s511t.r (S
Boom I this effect was reported by Upatnieks et al. (9) The
Lig-Mhrrot ( hologram is a reflectance type in which three primary
Sore ---- - colors are used as references and illuminators for the
subject. As discussed ina previous section of this
Test [[]e paper, the reflectance-type hologram produces sur-
CstmbrE faces or interfaoes within the depth of the emulsion
so as to act as an effective interference fiter. In the

Plnt onated with white light. The hologram will selectively

mio (reflect only those wavelengths used in producing the
split,, filter effect. Thus, an image in full color is viewed.
A detailed analysis of the effect concerning the
FIgu.e 3. Mach-Zehnder Interferometer emulsion thickness has been made by Leith et al. (10)
In this analysis, the authors show that the orientation
and wavelength used in the construction of the
An example of this type of problem and its hologram, combined with variations of these param-
solution by the application of holography is given by eters in the reconstruction process, establish certain
Heflinger, et al, (6) where the interference pattern conditions which must be satisfied in order for an
produced by the hot piues within an automobile image to occur. Therefore, the use of a thick
dome lamp is illustrated. The low ;uality gla of the emulsion in the holographic process can provide the
bulb in a conventional Interferometer would produce means of selectivity to store and recover data. By
such a complex contour of fringes that the thermal rotating the photographic plate between exposures, a
diffractions would be extremely difficult, if not hologram movie was produced by Leith et al (10) to
impossible, to observe. However, with the holo- demonstrate this storage capability.
graphic interferogram, these are clearly seen.
This -meapproach is applied to another tech- V. SUMMARY
nique reported by Powell and Stetson, (7) where the
diffential interference pattern is observed tor struc- The principle of holography entails the photo-
tural vibrational analysis. In this application, the graphic recording of the complex interference pattern
image produced by the hologram is a contour map of generated by the illumination of an object with a
constant vibration amplitudes. The advantage of this coherent-light source. The image produced by the
technique lies in its permitting one to analyze lap illumination of the hologram contains all of tVe
models, such as aerodynamic or hydrofoil structures, geometrical characteristics of the original scene; that
without having to be attached physically to these is, it would appear as though one were looking
structures, through a stained-glass window. This characteristic is
The principles of holographic interferometry can appealing for applications in entertainment and train-
also applied' in a dlghtly different manner in ing devices where a high degree of environmental
solving problems of patter-reoognition application. simulation is desired. As a tool for diagnostic applica-
As shown in the previous discussion, the use of a tions, the differential interferometry technique has
hologram as a spatial filter enables one to measure the amused considerable interest. Here, as a recording of
simi]ity or dissinmirity of a comparison pattern. In interference patterns produced by the geome' -al
the previous cas, the dor was to record and configuration of an object, the hologram can high-
mnmire the ngnitude of this difference whereas, in light minute deformations in the shape of the test
pattemn-ecognition application, one merely desir to qcimen. The technique can also be applied to
know whether or not a similarity exists. A very einaute the effect of optical deficiencies within
in tn se of this technique was reported by interfemmetric systems., thereby allowing increased
Houth et al (8)for fingerpint slectivity. The sensitivities with low-cost components.
authors report that experimental results have The difficulties that must be faced in implement-
shown the recogiWon to be extrmey selective, with ine the holographic proce a due to the extreme
91% of the maxiwu ulectivlty occurring with 50% mechaical rigidity required of the system duri the
of the ferprlint obscured. They also indicated the exposure pehi J,and the limitations lmpoed by the
selectivtity was reletively insenitie to lateral, vrtical, relatively short coherent length prently provided by
and qitutldinal postioning of the print in the lw sources, Needles to sy, humm ingenuity and
YMsM. dernatiom will un tedly ovrcome these bar.
Mon drmtic applications of holograms ar rim and achi the gook that theory pmphesie for
bing made wha the construction cf thue- this unque concept, thereby opening still another
i -'igad colo Images. A technique for producing door in the world of engineering technoloy.
4
 REFERENCES (6) Hefllnier. L. 0., R. F. Wuerker and R. E. Brooks,
"Holographic Interferometry," J. AppL Phys., 37, 642,
(I) Gabor, D., "A New Mlaoscopic Principle," Natu'e, 1966.
161, 777, 1948. (7) Powell, R. L and K. A. Stetson, "Interferometric
(2) Sears, F. W. and M. W. Zemanicy. Unirenfriv Aysics Vibration Analysis by Wavefront Reconstruction,"
Cambgide, Man.: Addison-Wesley Publishing Co., Inc., J. Opt. Soc. Am., 55, 12, 1965.
1955. (8) Horvath, V., J. Holeman and C. Lemmond, "Holo-
(3) Leith. E. and J. Upatnieks. "Reconstructed Wavefronts graphic Technique Recognkes Fingerprints," Low
and Communication Theory," J. Opt. Soc. Am., 52. Focus, 3, 18, June 1967.
1123.1962. (9) Upatnicks. J., J. Marks and R. Fedorowlcz, "Color
(4) Leith, E., "Holography's Piactical Dimension; Scentiflc Holograms for Whlte-Light Reconstruction," AppL
Investipation," Electronics. 39. 88, 25 July 1966. Phys Letters, 8, 286, 1966.
(5) Stroke, G. W. and A. E. Labeyrie, "White-Ight (10) Leith. E., A. Kozma, J. Upatnieks, J. Marks and
Reconst uctkin of Holographic Images Using the N. Massey, "Holographic Data Storagc in Three-
Lippmann-B g Diffraction Effect," Phys Letter, 20, Dimensional Media," AppL Opt., 5, 1303, 1966.
368, 1966.

5
 UNCLASSIFIED
AwwkvL Claaifticau
DOCUMENT CONTROL DATA - R&D
ann.eeien set be ambed nmnteee. .C. i
it"

4iiail ofRP~V aOE when. am oreral reved tohv

0840e1
[HqSietii
OfcofArsaeRsacUNSp I E

Melintn
C. kins Cat/AFRe
L REPORT TITLE a OA O FPGS jb O FR

4L. DWICRPTSR
NOPOET ofn a"hmInclsiv daobee") NOW edmt
Scie/Antiic Speci)

AC .ofVITY
N
PO P AES
.ORIN
. OtOter
rTech 1400 WSo lvd0
NT
DAPTSE N NO7S9.

bS. AISYRCTNON/NA

1.This ograp hs toe provdaodsrptbilae deini o em tsditrbtion

Hlgah,

TenfOtrmto contine wihnteWgfa lu isn oBjetcnlepotgahial

To fuarther an understanding of the holographic process, the author discusse3 basic

concepts, and describes a few of its most attractive applications.

DO 1"eo $1473 NCLASSIFIED

 UNCLASSIFIED
Security Crasslficatlon
WLINK A LINK .INK C
KEY WORDS ott RoA.- " W ROLK r
Coherent Light
Di ff'ract ion
Fringe Spacing
Ho log ram
Holography
Illumination
Image
Interferometric Analysis
Optical Interference
Optics
Paral lactic
Pattern Recognition
Superposition

&.,@ S?)M?
I. iL"Q six."I

UiLSSEL