Patent GB-570429-A (GB)
Title: Improvements in or relating to photoelectric exposure meters
IPC: G01J1/42
Filed: 1943-06-29
Abstract: 570,429. Photographic exposure meters. SANGAMO WESTON, Ltd., and SNELL, M.S. June 29, 1943, No. 10488. [Class 98 (i)] [Also in Groups XXXVI and XL] In a combined photo-electric exposure meter and calculator, an adjustable member of a calculator is manually operable when the photo-electric cell is exposed to the view the light intensity of which is to be measured with the indicator in full view of the user to control a shutter in front of the cell to bring the pointer of the indicator to a fixed mark, whereby the aperture number to be used for taking a photograph for a given exposure time can be read off directly from the calculator. The photo-electric cell may be arranged as described in Specification 531,996. The scale may be provided with three marks only, a normal central mark, an upper mark for very bright lights and a lower mark for very dim lights. The meter comprises a photo-electric cell, an instrument sensitive to the current therefrom, a shutter for controlling the amount of light reaching the cell, a calculator, and a. link mechanism between the calculator and the shutter. As shown in Fig. 6, a photo-electric cell 20 is provided with a multiple lens 19, a fixed apertured shutter plate 22 and a movable apertured shutter plate 23. The arrangement is preferably such that the light falling on each individual cell is never entirely cut off, but is reduced to a pin-point. In a modification, the multiple lens 19 is replaced by a plain glass sheet, and two movable apertured plates are successively positioned in front of the photoelectric cell. In a further modification, the shutter may be of venetian blind form. The various apertures for the successive stages controlled by the calculator are each one-half the size of the one above it. As shown in Fig. 5, the calculator 15 comprises a moulded disc 27 with a serrated rim, an outer ring 28 marked with exposure times, an inner ring 29 marked with apertures, and a disc 30 marked with emulsion speeds which can be seen one at a time through the window 31. All except the disc 30 and ring 29 move round with the disc 27, which is provided with a cam-shaped slot 32 in which a pin 33 is adapted to move as the disc 27 turns. The edge serrations form locating cogs for a spring-pressed pawl 35 serving to align the calculator so that every aperture number comes exactly opposite an exposure time. The ring 28 normally moves with the disc 27 due to the coupling effected by the spring-controlled bead 36, which allows the ring 28 to be located in three different positions. When the disc 29 is moved so that a different emulsion speed value appears through the window 31, all the aperture numbers are shifted relatively to the meter body. The spring bead 36 is one of three beads each having a different colour, red, yellow and blue, the yellow middle bead being normally used and corresponding to the middle yellow mark 16 of the dial. The red and blue beads correspond to the red and blue dial marks 17 and 18 for extremely bright or extremely dull scenes and are used when the pointer cannot be brought to the yellow mark. The pin 33 is carried by a lever 34 having a slot 37 engaged by a pin 38 on the rack 39, which engages pinions 40 carrying shutter leaves 41. Movement of the disc 27 causes all the shutter leaves 41 to turn like the slats of a Venetian blind, the minimum position being represented by passage of light only through notches 43. The slot 32 may be shaped to cause the rate of shutting to follow a logarithmic law. In a modification, the shutter comprises a number of slidable vanes between fixed top and bottom vanes, and the coupling between the disc 27 and the vanes comprises a bellcrank lever having pins engaging a slot in disc 27 and slots in the vanes. The slots in the vanes are arranged so that the upper movable vanes move oppositely to the lower movable vanes. In a further modification, the coloured balls are replaced by letters appearing through a window in the time ring 28. The meter movement may have a diamond-shaped permanent magnet armature core and a mild steel yoke is provided. A non-linear characteristic to provide even spacing of the marks is obtained with an ordinary moving coil instrument normally giving a linear characteristic by adding a pair of auxiliary springs to the movement, part of the pointer catching the tail of one spring when moving between the first and second marks and catching the tail of the second spring when moving between the second and third marks. With a circular soft iron armature core, one of the polepieces of the permanent magnet may be provided with an adjustable auxiliary polepiece to achieve the same result.