OPTICAL COMPARATOR: WHAT IT IS FOR AND PARTS - PHYSICAL - 2025

The optical comparator is a metrological instrument designed to inspect the measurements of manufactured parts using a simple optical principle. These pieces, components of machinery that operate in various fields, are illuminated in such a way that the analyst has an amplified view of their profile and surface.

Its operation is analogous to that of the overhead projector that works with acetate sheets; except that the projected shadow does not end on a wall, but on a screen at the height of the observer or analyst. The image of the illuminated piece or object, through a set of mirrors, allows direct measurements of its dimensions, geometry or designs.

In optical comparators, the profiles of objects in any field can be analyzed and measured, such as those necessary for naval forces. Source: US Navy photo by Marshall Fukuki

The metrologists make use of the optical comparator to guarantee that the pieces fulfill the necessary requirements that the quality standards demand; otherwise, they could jeopardize the functionality of the machine or equipment to which they conform.

Although its technology is considered primitive and inadequate for complex pieces, it continues to be very useful due to its simplicity, and its ability to be coupled to a computer that facilitates calculations of the dimensions of the piece; such as area, length, angles and width.

What is an optical comparator for?

Study profiles and surface of parts

The optical comparator is an optical metrology tool, so its imminent function is to allow measurements to be obtained directly from the projected and amplified image of a part.

It could be said that it is a mix between an overhead projector and an optical microscope: the piece is illuminated from below by a light source, but without the need to look through an eyepiece to appreciate the image obtained.

Because this image is large enough, points in it can be processed and, with units of measurement of inches or centimeters, the length or curvature of their profiles can be measured; that is, the measurements of its projected edges (as in the first image).

By having the profile (contour or silhouette) of the piece, an imperfection in it may become evident that at first glance, because it is very small, would go unnoticed. Likewise, by modifying the way in which said piece is illuminated, especially if it is flat, the quality and characteristics of its surface can be studied.

Metrological quality analysis

The manufactured parts undergo a series of quality analyzes to discriminate which are defective and which are ready to integrate the corresponding machine. These parts are usually screws, stampings, gears, gaskets, grids, propellers, chips; all of them small and difficult to inspect with the naked eye.

Starting from their profiles and a measurement pattern, dimensional inspection can be immediate if these two when compared do not match in their shapes. On the other hand, using the optical comparator, measurement strategies can be devised for a new part, so that it can be reproduced on future occasions.

The platform where the piece rests can be moved to cover different areas of the profile or surface, as well as rotated in order to measure the radii of its curvatures or the diameters of its internal circles.

Areas of use

This metrological and optical instrument offers high precision in its measurements, ergonomic comfort, and time savings in numerous aeronautical, aviation, automotive, electronic research laboratories, as well as in their mechanical workshops, watchmakers, and in the inspection lines of their factories.

Optical comparators have been used to study the surface of the coins, thus verifying their condition and characteristics; if they have suffered deterioration, bumps, scrapes, or if they have lost color.

Parts

Vertical optical comparator. Source: Sonny Leung

In the image above we have a vertical optical comparator on which a kind of template is projected. The difference between vertical and horizontal optical comparators is the direction from which the piece is illuminated; the former are used especially for flat pieces, while the latter for those with elongated shapes.

The profiles or contours of the template are observed with the naked eye, with sufficient resolution and amplitude so that more than one analyst can evaluate it and make their measurements, in such a way that they can discuss their results at the same time. This makes inspections quick and easy.

Projection screen

Unlike other artifacts, the optical comparator makes it possible to measure the dimensions of objects from their two-dimensional images.

Said 2D images can be seen on the projection screen, which is large enough for the analyst to comfortably perform his measurements without straining his vision; as long as you reduce the intensity of the light.

Lamp and mobile platform

The piece is illuminated by a light source that can be of different colors. Through a set of mirrors and lenses, the image ends up being projected on the screen. Thanks to the mobile platform on which the part is placed, the profiles can be adjusted while marking the points according to their shapes to measure lengths or widths.

Reference template

On the same platform where the piece is placed, there is a circular reference template with the Cartesian axes, so that the relative positions of the profile obtained with respect to the origin can be known.

Thus, the origin would become the center of the screen, and the piece can be placed in a convenient position on the template so that it coincides with a specific part that facilitates its measurement.

Control Panel

In the lower part of the optical comparator is the control panel, with the switches and knobs for its power, light intensity or image resolution.

Accountant

A smaller screen allows the reading of the X and Y axes, which counts the selected units of measure while traversing the profiles of the part. This counter also determines the angles at which the screen is rotated according to curvature; for example, with it you can measure the diameters of the circles in the template above.

Shaft knobs

And finally, on the sides there are knobs that allow you to move along the X and Y axes, raise or lower the projected image, move it left or right, bring it closer to or further away from the light source.

References

1. Tad A. Davis. (2001). Measure for measure: Optical comparators measure more than simple dimensions. Recovered from: qualitydigest.com
2. Wikipedia. (2019). Optical comparator. Recovered from: en.wikipedia.org
3. PCSI. (2019). What is an Optical Comparator? Recovered from: precisioncalibration.com
4. Hexagon. (2019). All About Optical Comparators, and why you should avoid them. Recovered from: hexagonmi.com
5. Martinez Jessica. (sf). Objective Optical Comparator. Recovered from: academia.edu
6. GR Metrology. (2019). What is an optical comparator. Recovered from: grmetrology.com