Optical Comparator Calibration

Optical Comparators are simple pieces of equipment that use optical principles and illumination sources to project a magnified image of a part onto a glass screen.  Using a variety of lenses for magnification, Optical Comparators can make precise two-dimensional measurements for a wide range of inspection applications. Their ease of use has allowed the Optical Comparators to be a primary piece of inspection equipment found in manufacturing environments, machine shops, and inspection laboratories.

Although the technology and physics inside an Optical Comparator haven’t changed much in the last 60 years, Optical Comparators have evolved into important machines that serve many functions. Improvements in optical quality, edge detection technology and the addition of computer readouts that perform mathematical computations make taking measurements easier for the user and minimize inspection time and improve cost savings.

There are 2 main types of illumination used for Optical Comparators. Profile illumination is the standard illumination that projects a silhouette of the part onto the comparator screen. It is useful for checking edge contours, through holes and other features. Surface illumination is used to inspect dimensions that can’t be viewed in profile illumination since they are on the part’s surface rather than along its edges, such as blind holes or angled surfaces.

Chart overlays are typically used on Optical Comparator screens to quickly determine if a part measurement is within its specifications. Common overlays contain crosshair configurations, circular radius lines, screw thread outlines, angles, and grid patterns.

Optical Comparator Calibration Process

The calibration of Optical Comparators uses general length measurement standards that are available to most calibration laboratories. Typically, Gage Blocks or an etched glass scale are used for the length standard. Some manufacturers make Optical Comparator Calibration Kits that contain calibrated steel balls of various diameters to use for the length and magnification measurements.

X and Y Axis Scale Accuracy

Calibration of the X-Axis and Y-Axis scales involves comparing the scale readout with a length standard. Test points should be taken every 1 inch across the measuring range of Optical Comparator for each axis. The measured value on the scale readout should be within the stated manufacturer’s specifications.


The magnification of the different lenses is typically checked by the use of precision balls and a special chart overlay used to measure the changes in the magnification of the balls. Other grid or radius overlays can also be used to determine the magnification specifications.

Squareness of the Measurement Stage

The X-Axis and Y-Axis squareness of the measurement stage is calibrated using a precision square or etched glass scale. One axis is aligned to one edge of the square and then the measurement stage is moved along the other axis. The squareness error is the amount of linear error from the standard edge at a point near the end of the stage travel.

Calculated Functions

Many Optical Comparators have functions that perform calculations to determine lengths and widths, diameters and radii, and angles. Using Gage Blocks, Precision Steel Balls and angle blocks, the standards can be measured with the computerized systems and the calculations checked to ensure that they read the appropriate dimensions.

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