Interferometers, types of Interferometers, Principle of Interference, NPL Flatness, Pitter–NPL Gauge, Laser Interferometers, Auto-collimator

1. Interferometers
By
Prof N D Sadaphal
Assistant Professor
Sanjivani College of Engineering,
Kopargaon (Maharashtra State) 423601
Mechanical Engineering

2. 7/17/2016
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INTERFEROMETRY
TE Mechanical
Metrology & Quality Control
Prof. N.D.Sadaphal
Prof N D Sadaphal
Principle of Interference
If two rays of same wavelength meet at
some point, mutual interference occurs &
natural interference depends on Phase of two
waves at their meeting point.
Prof N D Sadaphal

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•If two rays are in same
phase, then resulting
intensity will be the sum of
two intensity.
•If two rays are out of phase,
then resulting intensity will be
the difference of two
intensity.
•If two rays having same
amplitude are out of phase,
then resultant will be zero &
result will be Dark spot.
•If two rays having same
amplitude are in same
phase, then resultant will be
twice & result will be
Bright spot.
Prof N D Sadaphal
NPL Flatness Interferometer
The NPL flatness interferometer is used
for checking flatness between gauge
surfaces.
(a) Equal fringes on
parallel
(b) Unequal fringes
due to flatness error
Prof N D Sadaphal

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•The light from a mercury vapour lamp is condensed and passed through
a green filter, resulting in a green monochromatic light source.
•The light will now pass through a pinhole, giving an intense point
source of monochromatic light.
•Therefore, the collimating lens projects a parallel beam of light onto the
face of the gauge to be tested via an optical flat.
•This results in the formation of interference fringes.
•The light beam, which carries an image of the fringes, is reflected back
and directed by 90° using a glass plate reflector.
•In Fig. (a), the fringes are parallel and equal in number on the two
surfaces. Obviously, the two surfaces are parallel, which means that the
gauge surface is perfectly flat.
•On the other hand, in Fig. (b), the number of fringes is unequal and,
since the base plate surface is ensured to be perfectly flat, the work piece
surface has a flatness error.
Working
Prof N D Sadaphal
Pitter–NPL Gauge Interferometer
•This interferometer is used for determining
actual lengths of slip gauges.
•Illuminator provides a concentrated light source
Prof N D Sadaphal

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•Light from a monochromatic source (the preferred light source is a cadmium
lamp) is condensed by a condensing lens and focused onto an illuminating aperture.
•This provides a concentrated light source. Thus, a parallel beam of light falls on a
constant deviation prism.
•This prism splits the incident light into light rays of different wavelengths and
hence different colors. The user can select a desired color by varying the angle of
the reflecting faces of the prism relative to the plane of the base plate.
•The prism turns the light by 90° and directs it onto the optical flat.
•The optical flat can be positioned at a desired angle by means of a simple
arrangement. The slip gauge that is to be checked is kept right below the optical
flat on top of the highly flat surface of the base plate.
•The lower portion of the optical flat is coated with a film of alluminium, which
transmits and reflects equal proportions of the incident light.
• The light is reflected from three surfaces, namely the surface of the optical flat,
the upper surface of the slip gauge, and the surface of the base plate.
•Light rays reflected from all the three surfaces pass through the optical system
again; however, the axis is slightly deviated due to the inclination of the optical
flat. This slightly shifted light is captured by another prism, so that the fringe
pattern can be observed and recorded by the user.
Working:-
Prof N D Sadaphal
Field of view of fringe pattern
It can be seen that the two sets of
fringes are displaced by an amount
a with respect to each other. The
value of a varies depending on the
color of the incident light. The
displacement a is expressed as a
fraction of the fringe spacing b,
which is as follows:
f = a/b
Prof N D Sadaphal

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Laser Interferometers
•Laser interferometers can be
used for measurements of small
diameters as well as large
displacements.Prof N D Sadaphal
•Laser light first falls on the semi-reflector P, is partially reflected by
90° and falls on the other reflector S.
•A portion of light passes through P and strikes the corner cube. Light
is turned through 180° by the corner cube and recombines at the semi-
reflector S.
•If the difference between these two paths of light (PQRS − PS) is an
odd number of half wavelengths, then interference will occur at S and
the diode output will be at a minimum. On the other hand, if the path
difference is an even number of half wavelengths, then the
photodiodes will register maximum output.
Each time, the moving slide is displaced by a quarter wavelength,
the path difference (i.e., PQRS − PS) becomes half a wavelength
and the output from the photodiode also changes from maximum
to minimum or vice versa.
Working:-
Prof N D Sadaphal

7. 7/17/2016
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Auto-collimator is an optical instrument used
for the measurement of small angular
differences.
For small angular measurements it provides a
very sensitive and accurate approach.
L.C.= 2''

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….
 When a beam of rays coming out from a
monochromatic source of light are made parallel by
placing a collimating lens(source should be at focus
point of lens), and if these parallel rays are made to fall
on reflector ,the rays reflect back and travels in the
same path and converges at the source through the
lens (fig A).
 If the reflector is tilted by an angle “” as shown (fig B)
, then the reflected rays makes an angle “2* ”(twice
the angle ) and converges at point O’.
 OO’ = (2* )*(f)…….where f is focal length(distance
between source and lens).