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REGI -B is a system incorporating a scanning microscope that automatically measures the optical microdensity of an image in conjunction with position over an 18-by 24 -inch area, with a resolution of O. 0001 inch. The high resolution of the density measurement taken during the precision scanning makes it possible to define the position of an image at any specified density level to a high degree of accuracy.Earlier positional mea suring device s could not locate images accurately because they had no accurate measurement of the optical density gradient at the edge of the image.The hardware consists of an X -Y scanner with a linear grating on one axis and a stepper motor on the other. The linear grating monitors position on a reversible digital counter. The stepper motor automatically increments a preset distance after each scan. A beam of light is reflected or transmitted from a Kohler light source. The detector is a microscope that can be fitted with a round or a rectangular aperture. A photomultiplier converts the light to an electrical signal that is amplified in a logarithmic or linear mode; the former is necessary when density is being measured directly.The system can be set to trigger on any density level; the position of that level is recorded on magnetic tape. In an alternative mode, the density information is digitized and then recorded on tape. The output tapes serve as the input to a program by which the computer can make compariscn s of the data. 29