Chapter three

OPTICAL INSTRUMENTS FOR MEASUREMENT

3.1 Introduction

The measurement of geometrically defined dimensions with “light touch” of light is appealing,

as well as positively advantageous for many metrological applications. The optical measuring

instruments which provide either virtual or projected images of magnified portions of the test piece

are the engineering microscopes and the optical projectors.

Engineering microscopes are optically assisted instruments for measuring geometric

dimensions and forms of small and medium sized technical parts.

These instruments provide the means for carrying out the following basic functions:

a. Magnification, the primary function of microscopes in general, used for presenting the

enlarged view of the observed object area, either in its contours, or a surface image;

b. Referencing or aiming, accomplished by providing index lines on a transparent graticule

inside the microscope tube, observed concurrently with the magnified image. The index

lines guide in determining reference positions for specific surface elements or, when

representing nominal contours, the alignment and comparison with the observed part

section; and

c. Staging, this involves holding the object and displacing it along controlled tracks

translationally or, as an option, rotationally, over measured distances.

Iluminations, although indispensable for any optical process, is an auxiliary function. The

source, as well as the means of transmission for the light, are generally integral elements of the

engineering microscope, although ilumination can, and is occasionally provided by members attached

to the instrument, or even independently arranged.

The measurement of geometric conditions by a microscope is based on the observation of the

object, or of one of its parts, at an appropriate magnification. This observation is carried out in a plane

normal to the direction of viewing. It follows that, to be adaptable to dimensional measurements by

microscope, the critical parameters of the object feature must be associated with a common plane of

observation.

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3.2 Basic Concepts in Optical Microscopy

Modern compound microscopes feature a two-stage magnifying design built around separate

lens systems, the objective and the eyepiece (commonly termed an ocular), mounted at opposite ends

of a tube, known as the body tube. The objective is composed of several lens elements that together

form a magnified real image (the intermediate image) of the specimen being examined. The

intermediate image is further magnified by the eyepiece. The microscopist is able to observe a

greatly enlarged virtual image of the specimen by peering through the eyepieces.

The total magnification of a microscope is determined by multiplying the individual

magnifications of the objective and eyepiece. This section discusses the basic concepts associated with

optical microscopy, including objectives, eyepieces, condensers, stages, magnification, numerical

aperture, optical aberrations, and a variety of related topics.

3.3 Introduction to Microscopy

Microscopes are instruments designed to produce magnified visual or photographic images of

objects too small to be seen with the naked eye. The microscope must accomplish three tasks:

produce a magnified image of the specimen, separate the details in the image, and render the details

visible to the human eye or camera. This group of instruments includes not only multiple-lens

(compound microscopes) designs with objectives and condensers, but also very simple single lens

instruments that are often hand-held, such as a loupe or magnifying glass.

Modern compound microscopes are designed to provide a magnified two-dimensional image

that can be focused axially in successive focal planes, thus enabling a thorough examination of

specimen fine structural detail in both two and three dimensions.

Most microscopes provide a translation mechanism attached to the stage that allows the

microscopist to accurately position, orient, and focus the specimen to optimize visualization and

recording of images. The intensity of illumination and orientation of light pathways throughout the

microscope can be controlled with strategically placed diaphragms, mirrors, prisms, beam splitters,

and other optical elements to achieve the desired degree of brightness and contrast in the specimen.

Presented in Fig.3.1 is a typical microscope equipped with a trinocular head and 35-millimeter

camera system for recording photomicrographs. Illumination is provided by a tungsten-halogen lamp

positioned in the lamp house, which emits light that first passes through a collector lens and then into

an optical pathway in the microscope base.

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Fig.3.1 Microscope component configuration.

Also stationed in the microscope base is a series of filters that condition the light emitted by

the incandescent lamp before it is reflected by a mirror and passed through the field diaphragm and

into the substage condenser. The condenser forms a cone of illumination that bathes the specimen,

located on the microscope stage, and subsequently enters the objective.

Light leaving the objective is diverted by a beam splitter/prism combination either into the

eyepieces to form a virtual image, or straight through to the projection lens mounted in the trinocular

extension tube, where it can then form a latent image on film housed in the camera system.

The optical components contained within modern microscopes are mounted on a stable,

ergonomically designed base that allows rapid exchange, precision centering, and careful alignment

between those assemblies that are optically interdependent.

Together, the optical and mechanical components of the microscope, including the mounted

specimen on a glass micro slide and cover slip, form an optical train with a central axis that traverses

the microscope base and stand.

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The intermediate image plane is usually located about 10 millimeters below the top of the

microscope body tube at a specific location within the fixed internal diaphragm of the eyepiece

(Fig.3.2). The distance between the back focal plane of the objective and the intermediate image is

termed the optical tube length. Note that this value is different from the mechanical tube length of a

microscope, which is the distance between the nosepiece (where the objective is mounted) to the top

edge of the observation tubes where the eyepieces (oculars) are inserted.

Fig.3.2 The optical system of an engineering microscope.

The eyepiece or ocular, which fits into the

body tube at the upper end, is the farthest optical

component from the specimen. In modern

microscopes, the eyepiece is held into place by a

shoulder on the top of the microscope observation

tube, which keeps it from falling into the tube.

The placement of the eyepiece is such that its eye

(upper) lens further magnifies the real image

projected by the objective. The eye of the

observer sees this secondarily magnified image as

if it were at a distance of 25 centimeters from the

eye; hence this virtual image appears as if it

were near the base of the microscope. The

distance from the top of the microscope

observation tube to the shoulder of the objective

(where it fits into the nosepiece) is usually 160

mm in a finite tube length system.

This is known as the mechanical tube length as

discussed above.

The eyepiece has several major functions:

The eyepiece serves to further magnify the real image projected by the objective;

In visual observation, the eyepiece produces a secondarily enlarged virtual image;

In photomicrography, it produces a secondarily enlarged real image projected by the

objective. This augmented real image can be projected on the photographic film in a camera

or upon a screen held above the eyepiece;

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The eyepiece can be fitted with scales, markers or crosshairs (often referred to as graticules,

reticules or reticles) in such a way that the image of these inserts can be superimposed on the

image of the specimen.

The factor that determines the amount of image magnification is the objective magnifying

power, which is predetermined during construction of the objective optical elements. Objectives

typically have magnifying powers that range from 1:1 (1X) to 100:1 (100X), with the most common

powers being 4X (or 5X), 10X, 20X, 40X (or 50X), and 100X. An important feature of microscope

objectives is their very short focal lengths that allow increased magnification at a given distance when

compared to an ordinary hand lens. The primary reason that microscopes are so efficient at

magnification is the two-stage enlargement that is achieved over such a short optical path, due to the

short focal lengths of the optical components.

Eyepieces, like objectives, are classified in terms of their ability to magnify the intermediate

image. Their magnification factors vary between 5X and 30X with the most commonly used

eyepieces having a value of 10X-15X. Total visual magnification of the microscope is derived by

multiplying the magnification values of the objective and the eyepiece. For instance, using a 5X

objective with a 10X eyepiece yields a total visual magnification of 50X and likewise, at the top end

of the scale, using a 100X objective with a 30X eyepiece gives a visual magnification of 3000X.

Total magnification is also dependent upon the tube length of the microscope. Most standard

fixed tube length microscopes have a tube length of 160, 170, 200, or 210 millimeters. Many

industrial microscopes, designed for use in the semiconductor industry, have a tube length of 210

millimeters. The objectives and eyepieces of these microscopes have optical properties designed for a

specific tube length, and using an objective or eyepiece in a microscope of different tube length will

lead to changes in the magnification factor (and may also lead to an increase in optical aberration lens

errors). Infinity-corrected microscopes also have eyepieces and objectives that are optically-tuned to

the design of the microscope, and these should not be interchanged between microscopes with different

infinity tube lengths.

In Fig.3.3 is shown a Tool Maker Microscope. The Tool Maker’s Microscope (TMM) essentially

consists of the cast base, the main lighting unit, the upright with carrying arm and the sighting

microscope. The rigid cast base is resting on three foots screws by means of which the equipment

can be leveled with reference to the build-in box level.

The base carries the co-ordinate measuring table, consists of two measuring slides; one each

for directions X and Y and a rotary circular table provided with the glass plate. The slides are

running on precision balls in hardened guide ways warranting reliable travel.

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Two micrometer screws each of them measuring range of 0 to 25 mm permit the measuring

table to be displaced in the directions X and Y. The range of movements of the carriage can be

widened up to 150 mm in the X direction and up to 50 mm in the Y direction with the use of gage

blocks.

The rotary table has been provided with 360 degrees graduation and with a three minute

vernier. The rotary motion is initiated by activation of knurled knob and locked with star handle screw.

Fig.3.3 Tool Maker Microscope.

Slots in the rotary table serve for fastening different accessories and completing elements.

The sighting microscope has been fastened with a carrier arm to column. The carrier arm can be

adjusted in height by means of a rack and locked with star handle screw.

Thread measuring according to the shadow image permits the column to be tilted in X

direction to either side about an axis on centre plane level. The corresponding swivel can be adjusted

with a knurled knob with a graduation cellar.

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The main lighting unit has been arranged in the rear of the cast base and equipped with

projection lamp where rays are directed via stationary mounted mirror through table glass plate into

the sighting microscope.

Measuring principle: The work piece to be checked is arranged in the path of the rays of the

lighting equipment. It produces a shadow image, which is viewed with the microscope eyepiece

having either a suitable mark for aiming at the next points of the objects or in case of often occurring

profiles. e.g. Threads or rounding – standard line pattern for comparison with the shadow image of

the text object is projected to a ground glass screen. The text object is shifted or turned on the

measuring in addition to the comparison of shapes.

The addition to this method (shadow image method), measuring operations are also possible by

use of the axial reaction method, which can be recommended especially for thread measuring. This

involves approached measuring knife edges and measurement in axial section of thread according to

definition. This method permits higher precision than shadow image method for special measuring

operations.

Applications: The large tool maker’s microscope is suitable for the following fields of

applications:

- Length measurement in cartesian and polar co-ordinates;

- Angle measurements of tools; threading tools punches and gauges, templates etc.

- Thread measurements i.e., profile major and minor diameters, height of lead, thread angle,

profile position with respect to the thread axis and the shape of thread. (rounding, flattering,

straightness of flanks)

- Single point lathe tool angle measurements.

Procedure of measurement with TMM. Place the tool bit on the glass stage so as to obtain a

clear image on which angular measurements are done (Fig.3.4). Focus the microscope to get a real

image super imposed on the graticule pattern of the eyepiece. Tilt the graticule pattern so as to align

the shank edge with the reference hair line. Read microscope angle scale. Tilt the angle so as to

bring the cutting edge of the tool to align with the reference hairline. If necessary X, Y movements

may be made to retain the edge in the field of view. A typical field of vision before and after

adjustment is shown in Fig. 3.5

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Fig.3.4 Orientation of face and flank surfaces with respect to machine reference system.

(a) before adjustement; (b) after adjustement.

Fig.3.5 Field of vision of sighting microscope of TMM.

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3.4 Optical Projectors

Observation and measurement of objects with the aid of optical magnification is not limited

to viewing through an ocular, such as used in the microscope. The magnified image of the object can

also be projected on a glass screen where it may be observed from a convenient distance. By the

projection of the magnified image, the visual impression becomes a physical reality insofar as the

dimensions and geometric forms, as they appear on the screen, can be directly compared to physical

masters – graduated rulers, templates, etc. – made to the scale of the magnified specimen image.

Optical projectors, in their applications as inspection instruments, present many favorable

properties, several of which are listed below:

a) The projected image can be observed by several people simultaneously; thus this method of

inspection lends itself to analyses by group;

b) Several dimensions and form characteristics of a specimen can be observed, compared and

evaluated in a single setting;

c) The number of dimensions to be inspected on the part, whether individually, or in their

interrelations with other dimensions of the same part, can be increased without needing additional

instruments or tooling, as long as these dimensions are contained in a common observational plane;

d) Standard comparator charts, particularly for such repetitive forms as circular arcs with different

radii, angles, thread forms, gear contours etc., can be used on optical projector screens. Such

standard charts are made of plate glass, precisely fitting the chart ring of the screen, and permit the

rapid, yet precise comparison of the projected image with the basic design form.

3.4.1 Operating principles of optical projectors

To produce an undistorted and magnified shadow or reflected image of an object on a

screen, where it can be conveniently observed, is the primary purpose of an optical projector. To

accomplish this objective, the following basic elements are needed:

1. Light source, usually of great intensity, to produce a clearly defined projection even at a

high rate of magnification;

2. Collimating lens, whose role is to refract the light into a beam with parallel rays of

practically uniform intensity on the entire area of object illumination. Sometimes this

element is also called the light condenser;

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3. Projection lens system which magnifies and transmits the object contour or image

resulting from the collimated parallel light rays; and

4. Viewing screen, on which the projected contour or image of the object appears and is

displayed for inspection.

Fig.3.6 Schematic view of the optical system in typical

projector, designed to produce a magnified shadow of the

object on the screen.

The main elements schematic arrangement is

shown in Fig.3.6, where the light travel path is

indicated by arrows. Light rays originating in the

light source hit the object, whose physical body

creates a shadow bounded by the actual contour

of the object when viewed in the direction of the

light rays. This shadow is then magnified by the

lens system and projected on the viewing screen.

In the particular system, an auxiliary element, a

relay lens is used to transfer the shadow on the

projecting lenses.

3.4.2 The optical system – magnifications

When choosing the most favorable magnification for the viewing of the object, two opposing

aspects must be brought into proper balance:

a. The higher the magnification, the better definition may be obtained of the intricate details

of the object;

b. The lower the magnification, the larger will be the field diameter, that is, that area of the

object which can be projected onto the screen.

This relationship may be expressed by the following simple formula:

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3.4.3 Applications of optical projectors

Microscopes are instruments primarily intended for tool room and gage-room applications

and require a certain degree of skill and experience for their efficient operation.

Optical projectors are basically production-oriented instruments and are, in fact, often used

on the shop floor by machine tool operators or trained inspectors. Optical projectors are generally

sturdy instruments, less sensitive than microscopes, easier to operate and even unskilled personnel

can quickly be trained to carry out simple inspection processes on these instruments.

On the other hand, optical projectors provide application advantages in many other respects,

in comparison to the capabilities of engineering microscopes. Examples of such characteristics and

operating conditions are listed below, also with the purpose of pointing out the preferential areas of

optical projector usage.

Volume and weight of the test piece - Optical projectors are available in sizes which can

accept for staging and inspection heavy parts of considerable outside dimensions.

Field view on the object - Even for medium size optical projectors this exceeds the comparable

capacity of microscopes in general. A larger object field permits the synoptic viewing of extended

areas on the work surface, reduces the need for object displacement or indexing, and offers wider

applications for inspection by contour comparison without actual measurements.

The open screen - commonly at eye level, permits group viewing and the observation of the

image in unrestricted position under more natural conditions than the viewing through a microscope

eyepiece.

Machine tool applications – used for the continued observation of the work progress, guided

by screen charts, without impeding the movement of machine tool members or the handling of the

controls by the operator.

Individual screen charts - for purely visual inspection of toleranced part features can be

prepared and mounted on the screen according to the requirements of the scheduled inspection

operation. On such charts the tolerance ranges appear as graphically laid out zones which must contain

the pertinent contour of the part being inspected.

Reproduction by photography - requires only the exchange of the screen against a plate

frame, for preparing a silhouette photo exactly in the size in which the image appeared on the screen.

The silhouette image of the part can also be reproduced superimposed on the pertinent screen chart,

resulting in photos of value for subsequent analysis or for reference purposes.

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The most common application of the optical projector is based on projecting the shadow

image of the object on the viewing screen. The shadow, or silhouette, represents the contour of the

object in that plane which is brought to coincide with the focal plane of the optical system. For round

objects, this should be identical with the diametral plane, where the projection of the silhouette

faithfully represents the cross sectional contour of the part. For flat pieces of essentially uniform

thickness the projected image is, for most practical purposes, also a true replica of the object contour.

This claim is particularly valid for relatively thin objects; however, in the case of flat parts with greater

thickness the projected shadow could suffer in sharpness of the contour. Regular bodies of revolution,

or substantially round objects whose surface contour is either continuous or repetitive, e.g., the teeth of

thread cutting taps, gears, milling cutters etc., are primarily suited for the inspection of the

geometric characteristics by means of a projected shadow.

a) b)

Fig.3.7 a) Digital Profile Projector; b) Digital comparator.

This Digital Profile Projector (Fig.3.7a) can effectively test several of work-pieces with

complex shapes, such as templates punching pieces, cams, gears, threads by profile comparative and

coordinates measuring ways.

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It is widely used in different measuring departments and inspection stations of following

industries: machinery, watch-making, electron, instrument & meter, science & research institute, etc,

including space-flight & aviation industry.

Characteristics:

Size of worktable (mm): 400 x 150

← X axis travel (mm): 0 ~ 200

← Resolution: 0.001mm

← Y axis travel (mm): 0~80 (focusing)

← Z axis travel (mm): 0~150

← Accuracy of X.Y axis: (3+L/75) µm

← Screen diameter (mm): Ø350

← Rotary range: 0°-360°

← Rotary angle resolution: 1'

← Rotary angle accuracy: 4'

Digital comparator (Fig.3.7b) can test effectively profile, section and surface shapes of

various complicated work-pieces, such as templates, punching pieces, cams, gears, forming cutters,

mercerizes etc. it is widely used in workshops and measuring stations of those industries: machinery,

instrument & meter, watch-making, mould and electron etc.

Characteristics:

← Size of worktable (mm): 400 x 150

← X axis travel (mm): 0 ~ 200

← Resolution: 0.001mm

← Y axis travel (mm): 0~80 (focusing)

← Z axis travel (mm): 0~150

← Accuracy of X.Y axis: (3+L/75) µm

← Screen diameter (mm): Ø350

← Rotary range: 0°-360°

← Rotary angle resolution: 1'

← Rotary angle accuracy: 4'

Magnification 10X (Standard) 20X (Optional) 30X (Optional) 100X (Optional)View-field on object Ø35mm Ø17.5mm Ø7mm Ø3.5mmWorking distance 88.376mm 81.375mm 54mm 44.9mm

Magnification error 0.08%

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