Stray light comes from internal reflections, dirt on optics, and so on. Stray light causes small, completely opaque objects to display a definite amount of transmittance. The presence of stray light in the optical path, aside from the principal beam, which reaches the photosensing element, can be directly assessed by "measuring" (with a stationary spot) opaque objects such as small specks of activated charcoal mounted in immersion oil between a slide and a cover slip (12,38,51,64). Any value for point transmittance appearing on the transmission meter demonstrates the extent of glare in the optic train. The Vickers M86 microdensitometer provides a convenient electronic offset correction for stray light or glare, which is determined as the transmittance of an opaque object mounted in immersion oil, the transmittance value of which is then subtracted by positioning the set infinity knob to a value of zero on the transmittance meter. This offset then defines total darkness for the system.
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