Reticle

A planar, or flat, element located at the image plane of an optical system designed to measure the object imaged. The element is typically made of glass, and can have a wide variety of lines, circles, or patterns either etched or chrome deposited on it. Microscope eyepieces accept reticles while magnifiers or comparators accept contact reticles; the former must be placed close to the eye or sensor, whereas, the latter close to the object under inspection.