1- The Optical Comparator - Overview

The optical comparator is a measuring device that casts a shadow of an inspected part onto a screen. The shadow on the screen is precisely scaled through a set of lenses to become 10, 20, 50, or even 100 times actual size. The greater the enlargement the more precise is the accuracy.

Most optical comparators can measure distances accurately within .001 inch.

As the image above demonstrates the part to be measured (in red) is set on a table called a "stage". A bright lamp is turned on and some of the light streams over the part to another lens. Some of the lamp light is also blocked from streaming across. This produces a silhouette of the part for the second lens.

The second lens collects the light and magnifies the silhouette. The light is then passed through other lenses and bounced off of mirrors to arrive at the back of the viewing screen. The viewing screen has etched lines of precise scale that can be lined up with part features.

It is important to focus the part precisely to its front face so that the silhouette produced on the screen is crisp and accurate. The stage can also be raised or lowered to place the silhouette on the screen whereever desired.