GB782422A - Photomicrographic device - Google Patents

Abstract

782,422. Photo-electric exposure control. WINKEL GES., R. Sept. 6, 1954 [Sept. 11, 1953], No. 25806/54. Class 40 (3). In the control of exposure of a photographic surface sensitive to an image produced by a microscope, a photo-cell responds to a fixed proportion of the light reaching the surface to control the exposure. As shown (Fig. 2) light from the " exit pupil " 2 of a microscope falls on the photographic surface 5 to be exposed. A fixed proportion of the light passes through diaphragm 6 in the plane of the photograph, through a second diaphragm 14 and is incident on a photo-electric cell 7. This cell controls the rate of charge or discharge of a condenser to terminate exposure by closing iris diaphragm 4 through solenoid 12 when the total light flux incident on the cell reaches a pre-arranged value. The Specification shows that if the ratio of the distance of the photo-cell aperture 14 from the exit pupil of the microscope to that of the photographic surface is constant then the fraction of the total light incident on the photo-cell is always constant. Hence, if the distance to the emulsion 5 is changed, that to the photo-cell must also be changed. The Specification suggests that the photo-cell may be mounted in the plane of the emulsion and may move with it. In a modification (Fig. 3) a beam splitter 9 directs part of the light passing to the emulsion 5 through a beam splitter which diverts part of the light to a photo-electric cell 7, on which it is imaged by a lens 10. If emulsion 5 is moved, so also must be the photo-electric system. In the arrangement of Fig. 4 (not shown) the beam splitter is situated adjacent the exit pupil of the microscope. Variation of emulsion position is not possible in this embodiment. To take account of varying emulsion speeds a diaphragm 14 containing apertures of various sizes may be provided, and various graded absorption layers may be used, or different condensers charged by the photo-cell may be selected.