Optical comparator (Pdf): Parts, types, working principle, applications, advantages & disadvantages

In this article, we are going to discuss the parts and construction, working principle, their types, applications, advantages, and disadvantages of an optical comparator.

What is an optical comparator?

Mechanical optical comparator
Mechanical optical comparator

An optical comparator is a measuring device used to analyze and measure the dimensional variations of the workpiece. The optical comparator was invented by James Hartness in 1922.

Later this comparator was commercially introduced by J&L Machines. co. The optical comparator uses the reflection of light as its basic working principle.

The optical comparator projects the magnified image of the workpiece in the display screen to compare with the standard parameter. This device is contactless with a workpiece while projecting its image on the screen for magnification.

Based on the image projected by the optical comparator it is broadly classified into two variants as inverted and erect. The inverted image variant produces upside-down and left to right images.

The erect variant producers the same orientation of the workpiece as an image. While comparing each variant, the erect variant comes with some advanced internal features.

Parts & construction of an optical comparator

The optical comparator is constructed using the following parts as listed below.

  1. Plunger.
  2. Pivoted lever.
  3. Mirror.
  4. Condenser lens.
  5. Projection lens.
  6. Table.
  7. Base.
  8. Light source.
  9. Scale & Screen.


A plunger is a metal component that acts as a sensing parameter to deduct the dimensional variations in the workpiece being measured. It reciprocates, according to the irregularities present in the workpiece.

Pivoted lever

A lever is fixed at a pivot point and connects a plunger and mirror at both ends. The pivot point is placed close to the plunger. The movement of the plunger is enhanced by the pivoted lever mechanism.


In an optical comparator, a mirror acts as a reflecting medium that reflects the incoming light rays from the light source. The mirror is pivoted at a center point and hinged at one end of the pivot lever.

Condenser lens

Every optical instrument will have a condenser lens in it. Its main purpose is to convert the divergent light rays from the light source into parallel light rays. Condenser lenses are also named objective lenses.

Projection lens

The projection lens is placed next to the condenser lens and it projects the condensed parallel light rays to the reflective mirror.


The workpiece to be inspected is placed at a flat table and the plunger will be in contact with the workpiece.


The entire setup, including the table, is mounted on the base.

Optical comparator working principle

The working principle of the optical comparator is mainly based on two basic principles such as the law of reflection and refraction of light. The tilting mirror deflects the incoming light rays from the source and acts as an optical lever.

The magnification of the optical comparator depends on the optical lever principle. By using the optical lever principle, an automatic magnification is achieved in such a way that the reflected beam deflects twice the angle of the tilting mirror or the total angle of deflection is increased by 2dθ.

Construction & Working of an optical comparator

Now let’s see how an optical comparator works.

Initially, the instrument is calibrated using a standard parameter or workpiece, and the scale value is set to zero. The workpiece to be inspected is kept on the table and made contact with the plunger. The irregularities in the workpiece are sensed by the plunger and it is vertically displaced.

The vertical displacement of the plunger is amplified due to the pivoted lever fitted at its one end. The pivotal lever mechanism tilts the mirror about its hinge. The light rays from the light source are allowed to pass through the condenser lens. It converts the divergent light rays into parallel light rays.

The condensed light rays pass through the projection lens and then finally project on the reflecting mirror. Simultaneously, the mirror is tilted through the angle (α) according to the vertical displacement of the plunger.

After the mirror is tilted, the normal plane moves through an angle α and the angle between the incident ray and normal becomes θ+α. Similarly, the angle between the reflected ray and the normal also becomes α+θ. Thus, the total angle will be 2θ+2α.

In an optical comparator, the first stage of magnification is obtained using the lever principle.

The first stage of magnification = $\displaystyle \frac{l^{2}}{l^{1}}$

The second stage of magnification is achieved when the image is tilted by 2θ due to plunger movement.

The second stage of magnification = $\displaystyle 2(\frac{{{l_4}}}{{{l_3}}})$

Types of optical comparator

There are different types of optical comparators available and some of them are listed below.

  • Zeiss ultra-optimeter
  • Zeiss optimeter
  • Optical Projector (Profile Projector)
  • Eden Rolt Millionth comparator
  • Newall OMS optimeter
  • Electrical optical comparator

Zeiss ultra-optimeter

Zeiss ultra-optimeter
Zeiss ultra-optimeter

This comparator is constructed by various parts such as mirror, objective lens, condenser lens, green filter, graticule, eyepiece, and a light source (lamp). It consists of two mirrors one is fixed and the other is movable.

The monochromatic light from the light source passes through the green filter and the condenser lens. The green filter is used to filter the green light from the light source and the condenser lens converts the divergent light to a parallel beam of light.

A plunger is attached at one end of the movable mirror and it moves in both horizontal and vertical directions with respect to irregularities present in the workpiece. A fixed mirror is placed parallel to the movable mirror. This mirror is used for achieving better magnification.

The light beam from the source is reflected on the movable mirror (A) and again it is reflected on the fixed mirror (B). From fixed mirror (B), it again reflects movable mirror (A). And finally, it projects to the eyepiece.

This optical comparator gives a higher degree of magnification because it involves double reflection of light. It is very sensitive and accurate. The magnification of the Zeiss Ultra Optimeter can be calculated by the formula given below.

Let us assume,

  • $ \displaystyle x=$ Distance between plunger center to workpiece surface.
  • $ \displaystyle b=$ Plunger displacement.
  • $ \displaystyle \frac{b}{x}=$ Angular movement of mirror.
  • $ \displaystyle 2f=$ Focal length of the lens.

$$ \displaystyle Magnification\text{ = }\frac{{scale\text{ }movement}}{{plunger\text{ }movement}}$$

$$ \displaystyle Magnification\text{ = }\frac{{(\frac{{2bf}}{x})}}{b}\Rightarrow \frac{{2f}}{x}$$

$ \displaystyle Overall\text{ }magnification$

$$ \displaystyle \Rightarrow \text{ }\frac{{2f}}{x}\times \text{ }eyepiece\text{ }maginification$$

Zeiss Optimeter

In Zeiss optimeter, their optical system is constructed using a tilting mirror, condenser lens, prism, reflector, eyepiece, light source, and plunger as a mechanical component. The light from the light source is reflected by an illuminating mirror.

After the light gets reflected on the tilted mirror, the image can be seen in the scale. Due to the irregularities present in the workpiece, there will be some movement in the plunger. And this displacement of the plunger results in the movement of the scale image.

The displacement of the scale image is analyzed using the scale reading presented on the focal plane of the scale image. The Zeiss optimeter has 1μm as the scale value and ± 100μm as the measuring range.

This optical comparator can measure up to 180mm and force about 200g. This comparator is also used widely in many applications.

Optical Projector (Profile Projector)

Optical projector (profile projector)
Optical projector (profile projector)

This comparator projects the magnified image of the workpiece on the screen to compare with the standard profile or sketch. It is widely used for inspection and toolroom applications. The components used for the construction of the optical projector are the lamp, condensing lens, workpiece table, micrometer, projection lens, mirror, and screen.

At the top, an enclosure is present which consists of a lamp and condensing lens. The workpiece table along with the vernier micrometer is placed at bottom of the enclosure. The light source in the optical projector can either be xenon, mercury, or tungsten filament lamp. The light from the source is allowed to pass through the condenser lens.

The condensed beam of light days further passes through the projection lens. The workpiece on the movable table is kept perpendicular to the part of the light beam. The moment of the workpiece table can be adjusted by Knob and the distance can be measured by using the vernier micrometer.

The light from the lamp enclosure falls on the workpiece and the image of the workpiece passes through the projection lens. The projection lens enlarges the image and the enlarged image falls on the mirror. The mirror reflects the image on the transparent screen.

The standard profile or sketch of the workpiece to be inspected is printed on the tracing paper and placed over the transparent screen. The workpiece is inspected by comparing the pre-drawn standard profile and magnified image on the screen.

The sharpness of the projected image can be controlled by the distance between the projecting lens and the workpiece. The optical projector has a magnification range of 10 X to 100 X (10 – 100 times).

The applications of the optical projectors are inspecting gears, screws, pitch, circle diameter, etc. It is also used to measure tool wear.

Eden Rolt Millionth comparator

Eden Rolt Millionth comparator is a type of optical-mechanical comparator. The mechanical system of this comparator consists of a pair of blocks, plunger, pointer strips, pointer, and web. The optical system in this comparator consists of a light source, condenser lens, prism, projection lens, reflector, and scale.

Eden Rolt millionth comparator
Eden Rolt ‘Millionth’ comparator
Eden Rolt millionth comparator
Eden Rolt ‘Millionth’ Comparator

The workpiece to be inspected is placed between the fixed anvil and plunger or moving anvil. Movable block and fixed block are placed parallel and connected to each other with the help of thin strips. These two blocks are connected to the pointer arm at one end with the help of pointer strips.

At the other end of the pointer, there is a ring with stretched spider web. Due to some irregularities present in the workpiece, the measuring plunger will result in a small linear displacement. The linear displacement of the plunger further displaces the movable block and causes the pointer to be deflected.

The mechanical magnification of the Eden Rolt comparator is similar to the Reed-type mechanical comparator. Have the optical system of this comparator projects the deflection of the web. The light from the lamp passes through a condenser lens and reflects by the prism. The reflected beam passes on the web and through the projection lens.

The images are projected on the scale by the reflection caused by the reflector. The mechanical magnification of this comparator is 50X and the optical magnification is 400X. The total magnification of Eden Rolt Million comparator is 20,000X [$\displaystyle \because $ (50X) X (400X)].

The design and construction make this comparator more accurate and precise. this comparator is highly sensitive.

Newall OMS Optimeter

Newall OMS Horizontal Optimeter
Newall OMS Optimeter

The Newall oms optical comparator utilized a prism, condenser lens, mirror, scale, index, eyepiece, plunger, knife-edge, and light source for its construction. The light from the light source is reflected by the external mirror and falls on the scale by passing through the prism.

The reflected image on the scale and index can be viewed by using an eyepiece. The Newall oms Optical comparator works similar to the Zeiss optimeter. The optical comparator is used in many applications to achieve high magnification with high accuracy.

Electrical optical comparator

In an electrical optical comparator, both electrical and optical components are used for their construction and functioning. The components that play a major role in electrical optical comparators are light emitter, receiver, electronic amplifier, and optical lens.

A light emitter in an electrical optical comparator is just a light source that gives a continuous beam of light for magnification. The receiver receives the light beam and converts it to an electrical signal. These electrical signals are amplified by using an electronic amplifier.

Here the electrical signals are processed and finally give output as measurement data. This comparator comes with a lot of comparing methods such as

  • Light intensity method.
  • Shadow cast method.
  • Laser scanning gauge method.
  • Charge coupled device method (CCD).
  • Laser diffraction method.

Electrical Optical comparators are widely used for parts inspection without retooling.

Applications of an optical comparator

These are the following applications of an optical comparator.

  • Optical comparators are very useful in inspection of gears, screws, thread, cams, etc.
  • Location of pitch circle diameter in a mechanical component can be easily analysed using an optical comparator.
  • It is very helpful in analysing tool wear in many mechanical machine elements.
  • They are mostly used for inspection purpose in tool room and shop floor.
  • It is used for quality control in production line. It is used in most of the metrology laboratories for research and inspection purpose.
  • It is also used by many products manufacturing industries to analyse their Precision of their product.

Advantages of an optical comparator

The optical comparator has some advantages. The following advantages are

  • It doesn’t have many mechanical parts, so there will be no wear and tear due to friction.
  • It is lightweight.
  • These optical comparators can give very high magnification and accurate results.
  • It is easy to use and very sensitive.
  • There will be no parallax error while using this comparator.
  • Readings can be taken without influence of room lightings due to the illuminated scale used in their construction.

Disadvantages of an optical comparator

The optical comparator also has certain disadvantages. The following disadvantages are

  • It is very costly, especially the optical system setup.
  • To make the measuring process easier, it is essential to use these comparators in a darkroom.
  • Some of these optical comparators are usually larger in size.
  • There must be an external power source which is required to operate light source continuously.
  • The eyepiece of microscope is used to view the scale so it is not convenient for continuous usage.
  • The continuous heat from the light source make cause the arrangements to drift.

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