Optical measuring instruments

Metrology subject-Mechanical engineering

1. Unit III
OPTICAL MEASURING
INSTRUMENTS
P.Srihari,
Associate Professor,
Dept. of ME, AITAM

2. Engineering microscope
•Optically assisted instruments.
•Measuring Geometric dimensions, forms of small and
medium sized technical parts.
•Present enlarged view of the object.
•Index lines are used for determining reference positions
for specific surface elements
•Index lines are used for alignment and comparison with
the observed parts.

4. Toolmakers Microscope
•Worksontheprincipleofoptics.
•Consistsofheavyandhollowbase.
•Accommodatesilluminatingunitunderneath.
•Abovethisonthetopsurfaceofbaseworktablecarriageis
supportedonballsandcontrolledbymicrometerscrews.
•Projectingupfromtherearsideofthebaseisacolumnwhich
carriesthemicroscopeunitandvariousinterchangeableeyepieces.
•Thetoolmaker'smicroscopehascoordinatemeasuringsystemand
widelyusedformeasuringcenter-to-centerdistancesofholes,as
wellasthecoordinatesofacomplexparts.

5. •Opticalheadwhichcanbeadjustedverticallyalongthewaysof
supportingcolumn.
•Opticalheadcanbeclampedatanypositionbyascrew.
•Thetablehasacompoundslidebymeansofwhichthemeasured
partcanhavelongitudinalandlateralmovement.
•Backofthebasethereisalightsource,whichprovideshorizontal
beamoflight.
•Whichreflectedfromamirrorby90deg.Upwardstowardsthe
table.
•Beamoflightpassesthroughatransparentglassplateonwhich
jobisplaced.
•Ashadowimageoftheobjectpassesthroughtheobjectiveofthe
opticalhead.

6. •Imageisthenprojectedbyasystemof3prismstoagroundglass
screen.
•Crosslinesareengravedonthegroundglassscreen,whichcan
berotatedthrough360deg.
•Measurementsaremadebycrosslines.
•Leastcountis1minute.
•Opticalheadtubeisadjustedinheightforfocusingpurpose.
•Magnificationarefrom10Xto100Xontheprojectionscreen.
•Attachmentsarefittedtotheworktable.
•Microscopecanberevolvedonitsmounting.
•Arecessedholeforholdingtheglassonwhichjobiskept.
•Linearmovementoftablearecontrolledbymicrometerscrews
(0.0025mmaccuracy).

7. Schematic view of Tool Makers Microscope

8. Toolmaker's microscope

9. •Theopticalhead1canbeadjustedverticallyalongthe
supportingcolumn2andisclampedinpositionby
screw3.
•Thetable5,securedonthebase4,hasacompound
slidebymeansofwhichthemeasuredpartcanhave
longitudinalandlateralmovements.
•Thetablepositioniscontrolledbyaccurate
micrometerscrewshavingthimblescalesandverniers
6and7.
•The optical system is shown in Fig,
•Light source 8 provides a horizontal beam which is
reflected by the mirror 9 (see optical system) through a
90°angle.
•The beam of light passes through a transparent glass
plate or stage 10on which flat parts may be placed.

10. •A shadow image or contour of the part passes through the
objective 11 of the tube, and is projected by a system of
three prisms to a ground-glass screen 12.
•Observations are made through the eyepiece 13.
•Measurements are made by means of cross-lines engraved
on the ground-glass screen.
•This screen can be rotated through 360°;
•the angle of rotation is read through the auxiliary eyepiece
14.
•The eyepiece field of view contains an illuminated circular
scale with a scale
•division value of 1' shown in Fig.

11. Optical system

12. Illumination in tool maker’s microscope

13. Applicationsoftoolroommicroscope:
•Determinationofrelativepositionofvariouspointsonwork.
•Measurementofanglebyusingaprotractoreyepiece.
•Comparisonofthreadformswithmasterprofilesengravedinthe
eyepiece,measurementofpitchandeffectivediameter.
•Comparisonofanenlarged,projectedimagewithatracingfixed
totheprojectingscreen.
•Measurementofcomplexformslikeprofileofexternalthreads,
tools,templatesandgauges.
•Measuringcentertocenterdistanceofholesinanyplanes.

14. •Toolmaker’smicroscopesareabletoviewandmeasurehole
diameters,lineardistances,threadangles,threadpitch,tool
edges,toolwearsurfaces,andmore
•Toolmaker’smicroscopeinstrumentsgettheirnamefromtheir
mainapplicationofmeasuringandviewingtooledgesandwear
surfacesinthetoolingindustry.
•However,thesemicroscopesaregreatfordoinggeneralmicro
measurements.
•Thecrosshairreticleintheeyepiecegivesaprecisepointof
referenceasthemicroscope’sstageismovedandthestage
micrometersareusedtoprovidereadoutofdistancetraveled.
•Precisemeasurementoflengths,diameters,anddistancesis
importantformanyapplicationsinindustry.

15. •Commontoolsandequipmentare
measuringmicroscopesandvernier
calipers.
•Amanualordigitalverniercaliperis
abletogivebasicmeasurements.
•Theleastcountmeasurement(smallest
incrementaldivisibleresolution)ofthe
micrometerheadsontheshown
toolmaker’smicroscopesis0.01mm,
whichis10microns.
•Themostcommonstagemicrometeris
1mmsubdividedinto100divisionsof
0.01mmresolution.

16. •Observationtube:Monocularinclinedat30°stand.Largeand
heavybaseprovidesextraoverallrigiditytotheinstrument.
•MeasuringStage:150mmx150mmassembledonballbearing
guidestoprovideaccurateandsmoothtravelupto50mmineach
directionwiththeuseofgaugeblockshavingmicro-head
standard0-25mm,leastcount0.01mm.
•RotaryStage:Circularstageisfittedonmeasuringstage,
graduatedinto360°withvernierandlock.
•EyepieceProtractor:Graduatedin360°withadjustablevernier
readingto6Min.iscoupledwithmonoculartubeforsmooth
anglemeasurement.

17. •Illumination :
(i) Sub-stage lamp provides transmitted light from a bottom
source providing collimated green filter Halogen light.
(ii)Oblique illuminator with adjustable inclination for surface
illumination of sample with relief structure, is also provided.
Three separate knobs for different illumination is provided with
variable light control on the front panel.

18. Revolving screen

19. Measurementofscrewthreadpitch
•Imageofscrewthreadprofileisset.
•Suchthatsomepointoftheimagecoincideswiththecross
hairsasseenonthegroundglassscreen.
•Thethimblereadingoflongitudinalmicrometerisnoteddown.
•Partistraversedbymicrometerscrewtolocatecorresponding
pointontheprofileofthenextthreadcoincideswiththecross
hair.
•Thimblereadingisagainnotedandthedifferencegivesthe
pitchofthescrewthread.

20. Measurementofangleofthread
•Determinedbyrotatingthescreenuntilalineonthescreen
coincideswithoneflankofthethreadprofile.
•Angleofrotationisnotedandthescreenisfurtherrotatedtill
thesamelinecoincideswiththeotherflankofthread.
•Thedifferencebetweentwoangularreadingsgivestheactual
angleofthethreadonthescrew.

21. Measuring distance between two holes
•Tomeasureadistancebetweentwoholesinthepart
placedonthetable5,forexamplewefirsthavetoadjust
theheightoftube1.
•Asharpfocusedimageoftheprojectedcontourshould
befinallyseenontheground-glassscreen12.
•Usingmicrometer6wemovethisimagesothatsome
pointonthecontourcoincideswiththecrosshairsas
seenontheground-glassscreen.
•Thereafterthereadingonthimble6ofthelongitudinal
micrometerscrewshouldbenoted.

22. •Then,thepartistraversedbythesamescrewuntila
correspondingpointontheimageofthesecondhole
coincideswiththecross-hairsontheground-glassscreen.
•Thereadingonthimble6isagainnoted.Thedifference
betweenthesetworeadingsistheactualdistancebetween
thesetwonotedpointsonthepart.
•Asimilarprocedurecanbeusedtomeasurethediameteror
otherdetailsofthepartgeometry.Theactualanglebetween
thestraight-lineboundariesisdeterminedbyrotatingthe
screen.
•Whenalineonthescreencoincideswithonestraightimage
oftheprofileboundarytheangleofscreenrotationisnoted.

23. •Thenthescreenisrotatedfurther,untilthesameline
coincideswiththeotherboundaryoftheprofile.The
differenceinangularreadingsgivestheactualangle.
•Differenttypesofgraduatedandengravedscreensand
correspondingeyepiecesareusedformeasuringdifferent
elements.
•A revolving screen, of the type shown in Fig, is used for
measurement of standard threads.
•The basic profiles of all standard metric thread in a pitch
range from 0.25 to 4 mm, inclusive, are engraved on the
screen. It also has angles of 120°, 60°, 55°, and 53°8'.

24. •Check the zero of the vernier
protractor (under the
eyepiece) as follows
•Set the protractor scale to
zero.
•Then verify that the
horizontal cross line is
parallel to the cross travel of
the stage.
•To do this rotate the side
micrometer knob and note
that any object viewed
through the eyepiece runs
parallel to the cross line.

25. Optical projector
•Optical comparators which make use of the enlarged image principle
are commonly known as optical projectors.
•Determination of an unknown value by comparison with a known
value.
•Used to compare the shape or profile of a relatively small engineering
component with an accurate standard or drawing much enlarged.
•The optical projector throws onto a screen an enlarged image of the
component under test.
•The rays of light from lamp L are collected by the condenser lens C.
•They are transmitted as a straight beam.

26. •Theobjecttobetestedisplacedontheworktable.
•Theworktablemaybeeitherstationaryormovingtype.
•Sometablesarealsoequippedwithanangularadjustmentforpositioningtothe
helixofthreadsandworms.
•Thesetablesusuallyhaveinandoutmovementparalleltotheaxisofthebeam
forfocussingpurposes;andalsoprovisionformovementsinothertwoplanes.
•Thelightbeamafterpassingtheobjecttobeprojectedpassesintotheprojection
systemhavinglensesandmirrorswhichmustbeheldinaccuratealignmenton
rigidsupports.
•Thelensesareusedtoobtainthedesiredmagnificationmirrorstodirectthe
beamoflightonscreen.
•Thescreensareusuallymadeofglasswiththesurfacefacingtheoperator
groundtoveryfinegrade.
•Magnification vary from 10 to 100.

27. The basic elements of an optical projector

28. Light source interferometry
•To obtain interference over large path difference, it is essential
to use a source with very narrow lines.
•A wide variety of light sources is available for interferometry.
•Selection of source depend upon application, cost and
convenience.
•Light sources like Mercury 198, cadmium, krypton 86,
thallium, helium, hydrogen, neon, sodium, potassium, zinc.

29. OpticalFlat
•Anopticalflatisaglass,fusedsilica,Zerodurorsapphiredisk
polishedtoahighdegreeofflatness
•Typicallywithinafewmillionthsofaninch,andisusedasa
referencetoevaluatetheaccuracyofflatsurfaces.
•Thesearecylindricalpieces25to300mmindiameter.
•Thicknessabout1/6
th
ofdiameter.
•Measuringflatnessusinganopticalflatentailsdirectcontact
betweenthespecimentobemeasuredandtheopticalflatitself.
•Holdingthesurfaceofahighprecisionopticalflatagainstthe
testspecimenundermonochromaticlightcreatesvisiblelight
bands,

30. •Monochromatic light source is required.
•Sometimes coated with thin film of titanium oxide to reduce
the loss of light due to reflection.
•Use lintfree paper for cleaning optical flat and the surface of
checked part.
•Which are formed by the air gaps where the two surfaces
are not in perfect contact.
•These interference fringes show the contour of the surface
under test.
•The light and dark patterns visually represent the flatness of
the surface being tested
•It is the curve and spacing between these fringes which
indicate the surface accuracy.

31. Opticalflatsareoftwotypes
TypeA
•Singleflatworkingsurface.
•Usedfortestingflatnessofprecisionmeasuringsurfacesofflats,
slipgauges,measuringtables.
TypeB
•Doubleflatworkingsurfaceparalleltoeachother.
•Usedfortestingsurfaceofmicrometer,measuringanvils,meters.
Eachofthesearetwogrades. ReferencegradeorGradeI
(toleranceonflatness0.05,parallelismB0.15,thicknessB0.20
micrometer)
WorkinggradeGradeII(toleranceonflatness0.10,parallelismB
0.20,thicknessB0.30micrometer)

32. Precautions
•The optical flat is a precision tool and great care should be
taken when handling it.
•One side of the glass is guaranteed to be flat to better than a
quarter of a wavelength
•(wavelength is nominally 600nm).
•The faces of the flat must be kept clean. Only hold the flat
by the edges, touching the
•faces will cause the build up of grease and give incorrect
results.
•Never drag the flat over the sample to be measured. This
can cause scratching and
•scoring which will make the flat useless.

33. Principles
•Optical flats use the principles of interferometry to make
measurements of the surface.
•The deformations in the surface must not be large with
respect to the wavelength of light being used.
•The incident light passing through the optical flat is either
reflected off the rear surface of the flat or transmitted
through the air gap between the flat and the test piece.
•Some of the transmitted light is then reflected of the surface
of the test piece.
•The amount of light reflected from the test piece needs to be
similar to the light reflected from the back surface of the
optical flat.

34. •When θ is considered small the difference in the path
length for the two reflected beams is measured as 2d.
This is twice the distance of the gap between the
optical flat and the test piece.
•The light reflected from the front surface of the test
piece undergoes a phase reversal.
•This is due to the Test Piece having a higher
refractive index than the air gap.
Therefore it can be shown that constructive interference will
occur when:
2d
c= (N + ½) λ
Where:
dc is the distance between the test piece and the optical flat to
create constructive interference and,
λ is the wavelength of light used.

35. Similarly it can be shown that destructive interference occurs
when:
2 d
d
= N λ
Where:
d
d
is the distance between the test piece and the optical flat to
create destructive interference.
From these two formulae we are able to deduce the change in
distance between the
Optical Flat and the test piece from one fringe to the next.
This can be expressed as:
2Δ d = d
c
-d
d
= λ/2
Δ d = λ/4
This can be represented pictorially as shown in Figure.

36. Ray diagram of the interference created when using
an optical flat.

37. Fringe measurements with an optical
flat.

38. Some simple Interference Pattern Generated by Contact
Measurement with an Optical Flat.

39. Disadvantages of Surface Flatness Measurements with Optical
Flats
•The optical flat is in intimate contact with the test sample,
causing scratches in both.
•Test samples are placed on top of an optical flat for viewing
with the help of a mirror. When placed horizontally, large
diameter, thin parts can conform to the surface of the optical
flat, causing an inaccurate reading.
•Classic Fizeau interferometers cannot measure flatness of
thin, transparent glass 0.5mm (.020") and thinner in a free-
state, without applying an opaque coating to the opposite
side of the thin part to be measured. This also distorts the
specimen, and will cause an inaccurate reading.

40. Testing of parallelism of any surface w.r.t. standard optical flat

41. Testing of optical flat
Flatness test

42. Parallelism test

43. Interferometer
•Incorporates the extended application of optical flat.
•By refined arrangement it overcome the disadvantage of
optical flat.
•Optical instrument used for measuring flatness and
determining the length of slip gauges.
•Works on the principles of interference.
•Wavelength is the measure of length.
•It uses the beam divider that splits the incoming ray into
two parts.
•These two parts moves in two different paths until they
recombined.

44. Types of Interferometer
•Michelson Interferometer
•Fabry-perot Interferometer
•Fringe counting Interferometer
•N.P.L. Flatness Interferometer
•Pitter N.P.L. Gauge Interferometer
•Zeiss gauge block Interferometer
•Multiple beam Interferometer
•Laser Interferometer

45. Michelson Interferometer

46. Michelson Interferometer
•Uses monochromatic light from an extended
source.
•Light falls on beam divider (BD) consisting of
semi reflecting layer.
•Light ray is divided into two paths.
•One is transmitted through compensating plate
(CP) to mirror M
1.
•Other is reflected through beam divider (BD) to
mirror M
2..
•From both these mirrors the rays are reflected
back.
•Reunite at semi reflecting layer.
•From where they are transmitted to the observers
eye.
•Thus the fringes can be observed.
•Mirror M
2.is fixed andmirrorM
1is movable.

47. N.P.L. Flatness Interferometer

48. A
B

49. N.P.L. Flatness Interferometer
•Used to check the flatness of flat surface.
•Mercury vapour lamp is used, whose radiations are passed
through a green filter.
•Leaving a green monochromatic light.
•The wavelength of monochromatic radiation is of the order
of 0.5 micrometer.
•Radiation is then brought to focus on pin hole in order to
obtain an intense point of source of monochromatic light.
•Then collimating lens provides parallel beam of light.
•This beam is directed on to the gauge to be tested which is
wrung on the base plate via an optical flat.

50. •Optical fringes are formed across the face of the gauge.
•Fringes are viewed from directly above by means of a
thick glass plate semi reflected set at 45
0
to the optical
axis.
•If the gauge face is flat and parallel to the base plate,
fringe pattern produced will be straight, parallel and
equally spaced.
•If taper is present the fringe pattern is shown in fig A.
•If gauge surface is convex or concave then fringe pattern
as shown in fig B

51. Pitter N.P.L. Gauge Interferometer

52. Pitter N.P.L. Gauge Interferometer
•Used for determining absolute length of the gauges.
•Light from source falls on slit I through lens B.
•After collimation by lens D it goes through constant
deviation prism E.
•Whose rotation determines wavelength passed through flat
F to upper surface of gauge block G and base plate H.
•Light is reflected in mirror E and its patterns are observed
through a telescope.
•This instrument should be used in standard conditions of
temperature and pressure.

53. FLAT SURFACE MEASUREMENT

55. Collimators
A collimatoris a device that narrows a beam of particles
or waves.
To "narrow" can mean either to cause the directions of
motion to become more aligned in a specific direction
(i.e. collimated or parallel) or to cause the spatial cross
section of the beam to become smaller.

56. Autocollimator
An autocollimator is an optical instrument
This instrument uses the principle of optical reflection where a
beam of light is projected on a mirror.
The beam reflects with an angle as that of the mirror surface
and as such the deflection can be measured visually or by an
electronic detector.
Used to measure small angles with very high sensitivity.
As such, the autocollimator has a wide variety of applications
including precision alignment, detection of angular movement,
verification of angle standards, and angular monitoring over
long periods.

57. •Tube mounted
objective lens
•Beam splitter mount
which contains two
reticles
•Eyepiece
•Illumination device
The main components of a standard autocollimator i.e.
focused at infinity are

58. •The Autocollimator is a single instrument combining the
functions of a collimator and a telescope to detect small
angular displacements of a mirror by means of its own
collimated light.
•The two reticles are positioned in the focal plane of the
corrected objective lens, so that the emerging beam is
parallel.
•This usual configuration is known as infinity setting, i.e
the autocollimators are focused at infinity.
•When moving the reticles out of the focal plane of the
objective lens, the autocollimator can be focused at finite
distances, and the beam becomes divergent (producing a
virtual image) or convergent (real image).
•This results in a focusing autocollimator

59. •The shape of the beam -convergent or divergent-depend
on the direction in which the reticles are moved.
•The illuminated reticle projected over the beam splitter
towards the lens is known as collimator reticle.
•The second reticle placed in the focus of the eyepiece is
the eyepiece reticle.
•The beamsplitter mount together with the eyepiece and the
illumination device form a main unit called:
Autocollimator head.
•A focusing autocollimator (finite distance setting) is
similary built. The autocollimator head containing the two
reticles is now mounted on a draw out tube for focusing
adjustment.

61. Principle
•Autocollimation is an optical technique of projecting an
illuminated reticle to infinity and receiving the reticle image
after reflection on a flat mirror.
•The reflected image is brought to the focus of the objective
lens in which the eyepiece reticle is located.
•Thus the reflected image of the collimator (illuminated)
reticle and the eyepiece reticle can be simultaneously
observed.
•When the collimated beam falls on a mirror which is
perpendicular to beam axis, the light is reflected along the
same path. Between the reflected image and the eyepiece
reticle -which are seen superimposed -no displacement
occures.

62. •If the reflector is tilted by an angle a, the reflected beam is
deflected by twice that angle i.e. 2a.
•The reflected image is now laterally displaced with respect
to the eyepiece reticle.
•The amount of this displacement "d" is a function of the
focal length of the autocollimator and the tilt angle of the
reflector:
•d = 2 a ƒ. (a in radians)
•The tilt angle can be ascertained with the formula:
•a = d / 2ƒ
•where ƒ is the effective focal length EFL of the
autocollimator.
•Since the ƒ is a constant of the autocollimator, the eyepiece
reticle can be graduated in angle units and the tilt angle can
be directly read off.

63. Working principle of autocollimator

64. Surface plates
For testing the flatness of surfaces, surface plates shown in
Fig. are widelyused
Surface plates (a), angle plates (b) and V-blocks (c)

65. •Surface plates are often made with longitudinal T-slots
for clamping center stocks or dividing heads and thus
must be massive and highly rigid in design.
•Two surface plates with deep ribbing for reinforcement,
shown in Fig. a, are being fitted together by a marking
compound test.
•Angle plates, with two working surfaces square with each
other, are shown in Fig. b. They are used for checking
perpendicular surfaces. Sets of V-blocks (Fig. c) have
either 60°or 90°V-slots and are used in setting up shafts
•not having center-holes or bushing.

66. •Surface plates are mostly rectangular having 4:3
length to width ratio.
•These plates are rigid in design & generally ribbed at
the bottom to carry heavy load without deflection.
•The top surface of the plate is scraped to true flatness.
•For big surface plates, four leveling screws are
provided for adjusting their top surface truly horizontal.
•The standard available sizes of the plates vary from
100 x 100 mm to 2000 x 1000 mm in about 13 ranges.
•The four edges of the plates are finished, straight &
are square to each other.
•According to IS-2285-1963, the CI surface plates are
classified into two grades as GRADE –I & GRADE–II.

67. •Surface plate is used to provide datum or a reference
surface for measurement in workshop & laboratories.
•It is also used to check flatness of any surface.

68. •The majority of our surface plates are produced with
charcoal black granite.
•Upon request, we can also manufacture black dia base,
pink or gray plates as well.
•Plates are produced to meet or exceed federal
specification GGG-P-463c.
•Final inspection of our surface plates is done with an
autocollimator or electronic levels and Repeat-O-Meter.
•Black and pink plates are engineered to support a loading
weight of 100 pounds per square foot, loaded in the
center of the plate; which is twice that required by federal
specification.
•This means that the designated load may be placed in the
center of the plate without deflecting the overall accuracy
more than 50%. Special loading and size.

69. •In surface plates, flatness is very important; however, repeat
reading is equally important.
•The flatness and repeat readings of Tru-Stone plates are
unilateral, not bilateral.
•The term unilateral accuracy means that all points on the work
surface are contained between two parallel planes, separated
by a distance greater than the distance specified for each size
and grade.
•The term bilateral accuracy means twice (+ or –the accuracy
stated) as much flatness deviation may exist.
•The values specified on Tru-Stone calibration certificates are
the TIR (total indicator reading).
•In addition to our standard surface plate sizes, we will quote
any other size requirements you may have.
•We can also make modifications to your plate, which includes
t-slots, inserts, holes, etc.

70. SURFACE PLATE SUPPORT
•To insure accurate readings, you need to support your
granite surface plate properly on three or four points.
•Tru-Stone offers several options for supporting your plate.
•The standard work height for all stands is 36” unless
otherwise specified.
•Floor locks are available for all castered stands.

71. Straight edge
•Straight edge is rectangular or ‘I’ shaped in section
with beveled edge.
•Steel straight edges are available up to 2 meter
length & CI straight edges are available up to 3
meter length & are widely used for testing machine
tool slide ways.
•It is used in conjunction with surface plate & spirit
level for measurement of straightness and flatness
of parts.
•For checking the straightness of the part, the straight
edge is placed along the full length of the surface
against the bright light.
•The absence of the light between straight & surface
indicates the straightness of the element.

72. •Similarly the flatness of the surface can be tested by
placing the straight edge in different directions at
different places on the surface.
•By using Prussian blue & straight edge, the
irregularities on the surface can also be found out.
According
•to IS-2220-1962, straight edges are provided into two
grades.
•Grade A –for inspection purpose
•Grade B –for workshop purpose