Optical Flats

• Optical flat are flat lenses, used in interferometers,

for testing plane surfaces.
• They are cylindrical in form with diameters ranging
from 50 to 250mm
• Thickness varies from 12 to 25 mm.
• Generally made from quartz, having a very
accurate surface to transmit light.
• They are available with a coated surface. The
coating is a thin film, usually titanium oxide,
applied to reduce the light lost by reflection.
 Making of Optical Flats
• Optical flats are made of
• Fused quartz
• Coefficient of linear expansion < 0.6*10 -6 /degree at standard
temperature of 200 C
• Borosilicate glass
• Coefficient of linear expansion < 3.6*10 -6 /degree at standard
temperature of 200 C
• This glass is clear and colourless.
• The cylindrical surfaces of the optical flats are
finished by grinding and the working surfaces are
finished by lapping and polishing process.
 Types of Optical Flats
Type A
• It has only one surface flat.
• The flat working surface is indicated by an arrow head on the
cylindrical surface pointing towards the working surface.
• Used for testing the flatness of precision measuring surfaces of
flats, slips etc.
Type B
• Both surfaces are flat and parallel to each other.
• used for testing measuring surfaces of micrometers, measuring
anvils.
Optical Flats Interference Bands
• A beam of monochromatic light (AB) at point C is:
• Partly reflected by the bottom of the optical flat along CDE
• Partly reflected at F by the surface under test and goes further along FGHJ.
• The beams DE & HJ differ in phase because of the extra distance CFG travels.
• If the air gap between the bottom of the optical flat and the test surface be 'h'
h = CF = FG = (λ /4) (since θ is very small)
where λ = wavelength of source
• The beam HJ will lag behind DE by 2h.
• Thus when h is an odd multiple of λ / 2 a dark band will be created
 Precautions in the use of optical flats
• Before using, it should be ensured that the
workpiece and flat are clean and free from dirt,
dust and oil.
• Do not slide the optical flat around on the surface
as this may scratch the flat.
• The bands should be viewed from a distance of at
least 10 times the diameter of the optical flat.
• Line of vision must be as nearly perpendicular to
the flat as possible.
 Checking of flatness using optical flat
• A perfectly flat surface will show
straight and evenly spaced
bands.
• A convex surface displays a bright
patch in the center, while a
concave surface displays a dark
patch in the center of the
pattern.
• Testing Using Finger Pressure:
• Convex: If and edge is subjected to
pressure, the flat will roll about the apex
• Concave: If light pressure is applied at
the center, fringes move outward and
reduce in number. Convex Surface Concave Surface

Flat Surface
 No Bands!!
• Bands may fail to appear due to one of the following reasons:
• Dust, burrs or nicks are still holding the optical flat away from the
surface.
• The wedge between the surface of the work and the optical flat may
be:
• Too thick: Press down on the optical flat with a uniform pressure to
squeeze out the air film.
• Too thin: Clean the surfaces to remove any moisture or oil that may
cause wringing
• Too angular: Apply pressure at different points around the edge of the
optical flat.
• Too nearly parallel: In this case the bands would be too far apart to be
distinguishable as bands.
• If interference bands are not good flat should be lifted and set
down again, applying vertical finger pressure at various locations on
the upper surface to obtain satisfactory bands
 Concave and Convex Surface
•The line of contact (AB)may
be determined by the color of
the substrate.
• If the band curve is around the
line of contact the surface is
convex
• If the band curve is in the
opposite direction then the
surface is concave.
• Magnitude of convexity/concavity
is determined by the curvature
 Flatness of Optical Flat
• The working surface of optical
flats is tested by comparing its
flatness with that of a master
flat.
• Interference pattern formed
under monochromatic light is
observed.
• A perfectly flat surface will
show straight, parallel and
equally spaced fringes.
 Parallelism of Optical Flat
• The parallelism of working surfaces of type B optical
flats can be tested by Fizeau Interferometer method.
• In this method light from mercury vapour lamp is
focussed on to an opening in the eyepiece and is partly
reflected by beam splitter .
• The reflected light strikes the collimating lens which
collimates it and throws it further on the flat under
test. The flat under the test is placed on the table
provided.
 Parallelism of Optical Flat (Contd.)
• The table is arranged such that beam strikes the
surfaces of the flats perpendicularly.
• After reflection from the two surfaces of the flat
under test , the beam retrace its path.
• Due to interference of the light reflected at the
two parallel surfaces of the optical flat,
interference fringes are formed which can be
observed from the eyepiece of the auto-
collimator
• These interference fringes helps in determining
the degree of flatness of two surfaces of the flat.
 Checking Height of Gauge Blocks
• A standard reference gauge (A) and the gauge block
under test (B) are wrung on a flat reference plane surface
and an optical flat is then placed on the top of both the
blocks.
• In case A and B are perfectly flat, but different in height;
equal number of fringes will be seen on both
• In case the surfaces are not parallel, then number of
fringes per unit width on the two gauges will be different
• When two gauges are flat and parallel to each other, the
difference in height of two gauges is given by:
H = (L N λ) / (2l)
Where,
N = no. of fringes on the reference gauge over a width of l mm
L = Distance between the two gauges
l= Width of reference gauges
 Error in Parallelism
• The gauge is placed on a rotary base
plate that can be rotated at any angle.
• The under-side of the gauge is lapped
truly parallel to the working surface of
the base plate.
• A non parallel gauge makes an angle with
the optical flat.
• If the number of fringes formed in 1st
position are N1 and when the table is
turned through 180°, N2.
• Then,
• δ1 = N1*λ/2
• δ 2 = N2 * λ/2
• Error in parallelism
δ 2 - δ1 = (N2 – N1)* λ/4
2
 Interferometers
• Optical instruments used for measuring flatness and
length of slip gauges by direct reference to the
wavelength of light.
• It is an extension of the application of the optical flat.
• It overcomes some disadvantages of the optical flats like:
• The lay of the optical flat can be controlled
• Fringes can be oriented to the best advantage.
• There is arrangement to view the fringes directly from above
the fringes thus avoiding any distortion due to incorrect
viewing.
 Michelson Interferometer
• Oldest type of interferometer
• Michelson using this interferometer, established
exact relationship between meter and red
wavelengths of cadmium lamp.
• Consists of a monochromatic light source, a beam
splitter and two mirrors.
• Relies on the principle of constructive and
destructive interference
 Michelson Interferometer
• A beam splitter splits monochromatic light into two rays of equal intensity at right angles.
• One ray is transmitted to fixed Mirror M2 and the other is reflected through beam splitter
to movable Mirror M1.
• Reflected ray from M2 serves as reference beam.
• Mirror M1 is movable, i.e. it is attached to the object whose dimension is to be measured.
• The reference beam and the beam reflected from mirror M1 rejoin at the beam splitter.
• Thus,
• If both mirrors are at same distance from beam splitter, then light will arrive in phase
and observer will see bright spot due to constructive interference.
• If movable mirror shifts by quarter wavelength, then beam will return to observer
180° out of phase and darkness will be observed due to destructive interference.
• Each cycle of intensity represents λ/2 of mirror travel.
 Michelson Interferometer
• With a monochromatic light source fringes can be seen over a range
of path difference that may vary from a few to a million wavelengths.
• When white light is used, fringes can be seen only if both ray paths
are exactly equal to a few wavelengths in total length in glass and air.
• Some other sophistications :
(i) Use of laser as light source, enables accurate measurements over
longer distances.
(ii) Mirrors are replaced by cube-corner reflectors (retro-reflectors)
which reflect light parallel to its angle of incidence regardless of
retroreflector alignment accuracy.
(iii) Photocells are employed to convert light-intensity variations in
voltage pulses which are processed by electronic instruments to give the
amount and direction of position change.
 N.P.L. Flatness Interferometer
• Used for checking the flatness of flat surfaces.
• Designed by National Physical Laboratory
• Flatness is compared with an optically flat surface (generally
the base plate of the instrument)
• Radiations from a mercury lamp pass through a green filter
and then a pinhole (Wavelength is of the order of 0.0005
mm).
• The pinhole is placed in the focal plane of a collimating lens
resulting in a parallel beam of light.
• This beam is directed on the gauge to be tested via an optical
flat.
• The fringes formed are viewed directly above by means of a
thick glass plate semi-reflector set at 45° to the optical axis.
• Interference fringes are formed between
– Rays reflected from the under surface of the optical flat and those
reflected from the surface of the gauge,
– Rays reflected from the under surface of the optical flat and those
reflected from the base plate.
 N.P.L. Flatness Interferometer:
Fringe Pattern

Gauge Surface Surface of Gauge

Slight Rounding Off Concave/Convex Inclined to Base Plate
at Corners

Gauges are Flat and parallel to base plate

 The Pitter—N.P.L. Gauge
Interferometer
• Also called the gauge length interferometer
• Used for determining actual dimensions or absolute
length of the gauges.
• Measurements are carried out at temperature of
20°C, barometric pressure of 760 mm of mercury
with water vapour at a pressure of 7 mm and
containing 0.33% by volume of carbon dioxide.
• In case conditions are different, then correction
factors have to be applied.
 The Pitter: Construction
• Monochromatic light (from a cadmium lamp)passing through a condensing lens is focused on an
illuminating aperture.
• It acts as a point source of light at the focus of the collimating lens
• A parallel beam of light passing through a constant deviation prism disperses the light into its
constituent colors.
• The beams of different colors are reflected downwards by the prism.
• Any one of these colored beams is directed vertically downward by rotating the constant deviation
prism about a certain axis.
• Light is then reflected from
– Surface of optical flat
– Upper surface of slip gauges
– Surface of the base plate
• Reflected light (deviated due to the inclination of optical flat) falls on prism
• Reflecting prism then reflects the rays into the eyepiece at normal to these rays so that the fringe
pattern may be observed
 The Pitter—N.P.L. Gauge
Interferometer
• Two interference systems are produced.
– Due to the upper surface of the gauge
– Due to the base plate’s reflecting surface.
• Gauges being calibrated by this method
must possess a very high degree of
flatness and parallelism
• The two fringe patterns will be displaced
as shown in fig.
• The displacement (a), is expressed as a
fraction of the fringe spacing b, i.e. f = a/b.
• Height of slip gauge
H = n(λ/2) + (a/b) (λ/2)
where n is the number of fringes on the slip
gauge surface
 Laser Interferometer
• Based on interferometry principles.
• Used for high precision measurements
• Unit of measurement: Very small, stable and
accurately defined wavelength of laser.
• He-Ne lasers are most common
• Light intensity is 1000 times that of a
monochromatic source
 Laser Interferometer (Contd.)
• Problems
– Frequencies of lasers are not very stable
– Extremely costly and require many accessories
– Laser beam is small and a high degree of
collimations needs a limited spread. So additional
optical devices are required to spread the beam
over a large area
 Components of Laser Interferometer
• Consists of:
• Fixed Unit: consist of laser head, a air of semi reflectors
and 2 photodiodes
• Moving Unit: Corner cube reflects light at 180 deg
• Photodiodes to electronically measure the fringe intensity
 Laser Interferometer: Working
• Laser light falling on P splits into
two beams towards
– Reflector S
– Corner cube
• The two beams recombine at the
semi-reflector S to create a fringe
pattern
• Sinusoidal output from the
photodiode is amplified and fed to
a high speed counter calibrated to
measure in mm
• Second photodiode measures the
direction of movement of the slide
 Measurement Systems
• Scales
• Gratings
• Reticles
 Scales
• Used in optical systems
• Rulings are spaced far apart
• Needs interpolation device for accurate reading e.g.
eyepiece, projection system etc.
• Common materials: Stainless steel, Glass, Etching
photo-resistive materials
 Gratings
• Rulings are closely spaced typically of the
order of 50 -1000/mm
• Cannot be read manually
• Require special readout system
• Photo-detectors: As the scale moves across a
detector it measures:
• Speed of movement: Rate of pulse
• Distance moved: No. of pulses
 Reticles
• Provides a reference in the
form of cross-wires
• Usually etched on glass
• Types of Reticles
• Type A: Most common, low
accuracy, cross-wire 1-5μm thick
• Type B: Used when line on
feature is narrower than reticle line
• Type C: Parallel lines spaced wider
than the scale lines enable precise
readings
• Type D: Highest Accuracy, cross-
wires are at 30 0 to each other