GB490464A - Improvements in or relating exposure meters for photographic enlargements - Google Patents

Abstract

490,464. Photographic esposure - meters. SCHLICHTER, W. Jan. 12, 1937, No. 985. Convention date, July 25, 1936. [A Specification was laid open to inspection under Sect. 91 of the Acts, July 14, 1937] [Class 98 (i)] A meter for enlarging purposes includes a photo-electric cell large enough to receive the whole of the light passing through the enlarging objective when held close thereto, and has a scale arrangement allowing the exposure to be read off in accordance with this factor and also the sensitivity of the printing paper and the degree of enlargement. In the form shown, Figs. 1 and 2, a circular photo-electric cell 62 is used at least as large as the lens 63 of the enlarger. The pointer 65 of the meter moves over a plate 66 carrying a scale with divisions connected by straight lines to the equidistant graduations of a logarithmic scale 70 of light values. The logarithmic scale 74 of enlargement sizes also has equidistant graduations. Between these two scales are situated two others 75, 76 on an endless band 69 carried by rollers 68, one of which may be rotated by a knob 78. This knob is rotated until a number on the scale 76 which indicates the sensitivity of the printing paper is opposite the required size of enlargement on the scale 76. The required exposure time is then read off from the scale 75. The sensitivity number of the printing paper is determined by a test exposure. By means of a knob 77, a shunt may be switched out of the meter circuit to increase the sensitivity. The change is such that the exposure is then read directly from the scale 76. A dial lamp is fed from a battery 80 and controlled by the switch 79. In a modification, the photo-electric cell is of sufficient area to be incorporated beneath the masking frame which holds the printing paper. The Specification as open to inspection under Sect. 91 describes also other forms of apparatus adapted to be held close to the lens of the enlarger and to receive the whole of the light emerging therefrom. In the form shown in Fig. 1 (Cancelled), the light from the lens 14 is received through a milk glass plate 12 and reflected from a mirror 16 to a ground glass plate 20 and so through a number of passages 22 in a member 21 on to a sheet 23. This sheet has a number of areas of graded opacity, one opposite each passage, and each carrying a number. Tables are drawn up enabling the required exposure to be read off from the size of enlargement, the sensitivity of the printing paper and the largest number which is just visible in the instrument. An iris diaphragm may be provided in the path of the rays to take account of the sensitivity factor. In another form of similar shape, a system of lenses enables the projected image to be viewed in the instrument and compared with the brightness of a comparison lamp, which brightness is variable by a variable resistance. A meter in the circuit enables the exposure to be read off from tables. This subject-matter does not appear in the Specification as accepted.