Definition: (1) Component of optical instrument which is used for pointing, measuring or helping focus image e.g. enlarger focusing aid, measuring microscope or loupe. * May consist of fine wires or an etched pattern of a scale or circle and is usually located at the main image plane i.e. so it appears sharply superimposed on the focused image. (2) Image, which may be etched on a glass or quartz layer, used in production of integrated circuits.

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