Optical Comparator

Overview

An optical comparator, also called a profile projector, is a metrology bench that measures a part by looking at its enlarged shadow. The part sits on a stage; a lamp shines through it; and a lens system throws a silhouette of its outline, magnified ten to a hundred times, onto a large frosted screen. An inspector then compares that outline against reference lines or a printed chart, or measures features directly by moving the stage and reading the travel.

Light begins at the Illumination System system, where a halogen lamp and condenser form an even beam through the part. The Projection Optics magnify and fold that beam onto the Screen Assembly, whose ground glass carries a cross-hair reticle and a rotary protractor for reading angles. The part itself rides on the Measuring Stage, a precision X-Y cross-slide whose travel is captured by linear encoders and shown by the Digital Readout. Everything is carried on a heavy Cast Base that keeps the long optical path stable.

How it works

For profile measurement the Illumination System back-lights the part: a halogen lamp passes through a heat filter and a telecentric condenser so that rays travel nearly parallel, which keeps the silhouette the same size regardless of how the part is positioned in the beam. Opaque surface detail instead uses an oblique surface lamp that lights the front face.

The shadow enters the Projection Optics. A projection objective on an indexing turret sets the magnification, and a chain of front-surface folding mirrors bends the long path back and forth inside the cabinet so a 100× system fits in a benchtop body. A relay lens re-erects the image and casts it onto the Screen Assembly, where the inspector aligns edges to the reticle and reads angles on the protractor, or lays a Mylar overlay chart for go/no-go form checks.

To measure a dimension, the operator drives the Measuring Stage with its ball-screw axes, by micrometer dial or by motorized servo, until a feature edge crosses the reticle, then repeats at the next edge. Glass-scale linear encoders track the travel between the two points, and the Digital Readout subtracts the readings to display the distance, computing diameters, angles, and positions from the captured coordinates.