Inspecting with Optical Comparators 351

Inspecting with Optical Comparators provides an overview of the optical comparator, which uses optics to project an enlarged, two-dimensional shadow of a part onto a glass screen for measurement of its length, width, and surface. Simple optics display the part upside down and backwards. Corrected optics display the part right side up and backwards. Fully corrected optics yield an image identical to the part orientation. Regardless of type and complexity, all optical comparators measure by comparison, screen rotation, and motion.

If optical comparators are properly maintained, measurement error is the result of the operator. By understanding the components and measurement methods of the optical comparator, operators can avoid unwanted variation. Variation in measurement can lead to faulty parts passing inspection and reaching consumers. After completing the class, users will be able to describe best practices for using the optical comparator to inspect parts.

  • Difficulty Advanced

  • Format Online

  • Number of Lessons 22

  • Language English


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Course Outline
  • The Optical Comparator
  • The Parts of an Optical Comparator
  • Optical Comparator Components
  • Using Different Optical Systems
  • Optical System Quality
  • Optical Systems
  • Illuminating the Part
  • The Viewing Screen
  • Optical Comparator Charts
  • Physical and Digital Scales
  • Edge Detection
  • Measurement by Comparison
  • Measurement by Screen Rotation
  • Measurement by Motion
  • Locating Points in Space
  • Finding the Center of a Hole
  • Reducing Variation with Optical Comparators
  • Optical Comparator Maintenance
  • Advantages of the Optical Comparator
  • Disadvantages of the Optical Comparator
  • Optical Comparator Types
  • Some Examples of Optical Comparators
  • Describe the optical comparator.
  • Identify the parts of the optical comparator.
  • Match an optical system with its resulting display type.
  • Describe factors that affect optical system quality.
  • Describe common methods for improving part illumination.
  • Describe the properties of the viewing screen.
  • Distinguish among different types of screen charts and describe their purpose.
  • Distinguish between physical and digital scales.
  • Match common methods of edge detection with their characteristics.
  • Describe methods of measurement by comparison.
  • Describe methods of measurement by screen rotation.
  • Describe methods of measurement by motion.
  • Explain how to measure "what is not there" using an optical comparator.
  • Describe ways to reduce variation when measuring with optical comparators.
  • Describe the proper maintenance of an optical comparator.
  • Describe the advantages of an optical comparator. Describe the disadvantages of an optical comparator.
  • Describe the characteristics of manually operated, digitally operated, and software-driven optical comparators.
  • Describe the characteristics of manually operated, digitally operated, and software-driven optical comparators.
Vocabulary Term

automatic edge detection

A method of edge detection that uses an electronic or fiber-optic device to locate differences between light and dark. Automatic edge detection can either be externally or internally located on an optical comparator.


Comparing and adjusting a device with unknown accuracy to a device with a known, accurate standard. A calibrated device has had its variation eliminated for increased accuracy.


An outline map that contains measurement information for comparison. Charts can quickly determine if a part fits within tolerance specifications and are available in glass, vinyl, or paper.

collimator lens

A type of lens that produces parallel beams of light. In an optical comparator, the collimator lens projects intense light to illuminate the part to be measured.

computer-aided design

CAD. A computer software program that aids in the automated design and technical precision drawing of a part, product, process, or building. CAD can be used when completing measurement by comparison on an optical comparator.

corrected optics

An optical system that projects a partially corrected image of the feature on the screen. The image displayed by corrected optics is the reverse of the part.


Either of two fine lines crossed in the center of an optical instrument that is used for calibration or sighting reference. Crosshairs are the pair of lines on the screen of an optical comparator that represent the X and Y axes.

cutting fluid

A fluid used to decrease friction and reduce the temperature of a cutting process. The presence of cutting fluid in the atmosphere adversely affects an optical comparator.

digital micrometer

A U-shaped measuring instrument with a threaded spindle that slowly advances toward a small anvil. The digital micrometer displays its measurements of length on a small screen.

digital readout

DRO. A type of readout that uses a special encoder attached to a machine that transfers information to a digital display. On a digitally controlled optical comparator, digital readout refers to the panel that includes the display and the controls.

digital scales

An electronic measuring device that uses fiber optics to detect and transmit its position to a digital or computer readout for display. Digital scales are found in digitally controlled and software-driven optical comparators.

edge detection

One of the primary means of measurement employed by optical comparators. A variety of edge detection methods may be used to find the difference between light and dark on the screen and thus, the edge of a part.


A technology that transmits light rather than electricity through a fiber made of thin, flexible glass or plastic. Automatic edge detection is done on an optical comparator using fiber-optics.

fiber-optic sensors

Part of an electronic measuring device made of glass strands that uses light to read and transmit the position of features. On an optical comparator, fiber-optic sensors are used in digital scales.


A customized workholding device that is designed to support, locate, and hold a workpiece.

fully corrected optics

An optical system that projects a fully corrected image of the feature on the screen. The image displayed by fully corrected optics has the same orientation as the part.

glass chart

A circular piece of glass that contains fine measurement markings for use on an optical comparator. The glass chart is the most precise but the least often used of charts.

go/no-go measurement

A type of measurement that uses an instrument of an established standard size to determine whether a part feature passes or fails inspection. Go/no-go measurements do not determine the degree of variation.


A machining process that uses an abrasive to machine a workpiece surface and achieve highly accurate measurements. Grinding commonly uses abrasive grains bonded into a wheel shape.

ground glass

The finely grained glass material used to make a viewing screen. Ground glass produces a fine texture that provides a bright, clear image for inspection.


The act of moving a part or machine component to another location in controlled increments. For an optical comparator, indexing is a method of part locating that translates circular motion into linear movement.

lamp enclosure

The part that covers the light source. The lamp enclosure protects an optical comparator from heat damage.

light source

The place where light originates. For an optical comparator, the light source is usually very powerful and must be protected by an enclosure.

measurement by comparison

A measurement method in which a part profile is compared to a measurement tool. Measurement by comparison either measures the part profile using direct measurement tools or using tools incorporated into the optical comparator.

measurement by motion

A measurement method in which the part is indexed, or moved linearly. Measurement by motion is usually used when the whole part or feature is too large to fit on the screen at the same time.

measurement by screen rotation

A measurement method that uses the rotation or turning of the optical comparator screen to line up chart markings with part features. Screen rotation uses the degree markings on the screen ring to measure angles.

operator error

A mistake that may be traced to the technique or practices of the operator. The purpose of automated measurement is to reduce or eliminate operator error.

optical comparator

A sophisticated measuring instrument that projects an image of a part onto a screen to compare the shape, size, and location of its features to the original. Also called optical projectors, they can measure both surface and profile features of either the length and width of a part but not the depth.

optical system

The combination of mirrors and lenses in an optical comparator that makes image projection possible. The optical system is also referred to as the optics.

overlay charts

A circular piece of transparent material that contains fine measurement markings for use on an optical comparator. Overlay charts are placed directly on top of the viewing screen.

paper chart

A circular piece of paper that contains fine measurement markings for use on an optical comparator. Usually hand-drawn at the shop, paper charts are the least precise but appropriate when go-no go measurement is acceptable.

physical scales

A tangible, visible scale that is present on the edge of the screen of an optical comparator and often on chart overlays. These vernier scales must be manually manipulated by an operator to take a measurement.

plane of sharpness

The area on the screen of an optical comparator where the projected image has the most detail. The plane of sharpness is usually in the center of the viewing screen.


The shape or outline of a part feature. An optical comparator can give an accurate measurement of a profile within the plane of sharpness.

projection magnification lens

A type of lens that magnifies an illuminated image onto the optical comparator screen. Projection magnification lenses come in a variety of different magnifications.


The fineness of detail that can be distinguished. An image's resolution on an optical comparator is the product of the mirrors and the magnification lenses.

rotary encoder

A device that translates rotary motion into a series of electronic pulses. On an optical comparator, the rotary encoder reports the screen's movement and position to the digital readout or software program.

screen protractor

Another name for the screen ring when it has a scale marked in its perimeter. A screen protractor is used in screen rotation to measure angles.

screen ring

A ring consisting of markings around its perimeter for measuring angles. The screen ring fits around the edge of the viewing screen of an optical comparator.

simple optics

An optical system that projects an uncorrected image of the feature on the screen. The image displayed by simple optics is the inverse and reverse of the part.


The part that moves on an optical comparator. Slides are the moving pieces on the staging table that allow a part to move to different positions.

staging table

The flat surface that supports the part to be measured. On an optical comparator, the staging table contains slides that move the part to position its shadow in the correct location on the screen.

surface features

A characteristic located on the exterior appearance of a part. Surface features can be measured using an optical comparator.


Linear part movement on an optical comparator. Translation requires indexing when the part being measured is too large to fit on the viewing screen.


A type of high-quality, heavy paper that is comparable to parchment. Vellum is sometimes used in the creation of paper charts.

vernier scales

A type of scale consisting of two opposing line markings with different divisions. Physical scales on an optical comparator are vernier scales.

viewing screen

A round glass surface on the optical comparator that displays a magnified image of a part. The viewing screen is also known as a projector screen.

vinyl chart

A circular piece of vinyl or plastic that contains fine measurement markings for use on an optical comparator. Vinyl charts are used as overlays and placed directly on top of the viewing screen.

X axis

The linear axis representing side-to-side movement in a device, relative to the origin. An optical comparator uses an X axis and a Y axis.

Y axis

The linear axis representing back and forth movement in a device, relative to the origin. An optical comparator uses an X axis and a Y axis.