An Optical Comparator is a device that uses optical components and light sources to project a magnified image of a part or component onto a screen for inspection and measurement purposes. An SPI Optical Comparator is a specific type of optical comparator that is designed and manufactured by the company known as SPI.

SPI Optical Comparators are used in manufacturing and quality control to inspect and measure the dimensions of parts and components with high accuracy and precision. They use a high-intensity light source to illuminate the part being inspected and a series of lenses and mirrors to project an enlarged image of the part onto a screen. The operator can then use a measuring scale or a digital readout system to take accurate measurements of the part's features.