Patent GB-225902-A (GB)
Title: Photographic exposure meter
IPC: G01J1/50
Filed: 1923-07-16
Abstract: 225,902. Gray, J., (Hewett, J. R.). July 16, 1923. Exposure meters.-A meter of the actinometer type is adapted for use on the lens mount of a camera and the exposure scale of the meter is used as the shutter time-setting scale. The casing is preferably of annular form, and in one construction, Figs. 1, 2 and 6, comprises a rear section 2 and a front section 3 detachably united as by annular spring lugs 5 and corresponding recesses 6. Within the annular housing so formed is an annular strip of sensitive paper 21 which can be stepped past an observation opening 9 in the front section 3 on either side of which is the standard tint 23. The opening 9 and standard tint' 23 are visible through an opening in a cover ring 24, secured to the front section 3 and carrying the usual actinometer-time scale, which is also the exposure scale. A cursor 35 on a ring 46 held in place bv the ring 24 moves over the exposure scale and the movement of the ring 46 is transmitted to the setting pointer 38 which moves over the usual setting-plate 36 of the shutter casing. This transmission is effected bv spaced teeth 45 on the ring 46 which move a spur wheel 44 on a shaft 43 on the plate 36; a second spur-wheel 41, Fig. 7, on the shaft 43 meshing with a pivoted plate 39 having spaced lugs 47 on either side of the pointer 38. The stop scale 26 of the meter is on a ring 27. Another ring 7 carries the plate-speed scale on the flange portion 8 behind the ring 27 and a subject scale 25 on a flange 7<1> which overhangs the stop-scale ring 27. The rings 27 and 7 are held in position bv the marginal flanges 28 of the cover plate 24. The plate-speed readings are seen through an opening 30 in the stop-scale ring 27. The subject scale 25 can be set relatively to the stop-scale ring 27 for any plate speed and the two scales coupled together by a movable coupling pin 31 on a handle portion 34 of the flange 7<1>. In using the meter, the subject scale 25 is then set to the actinometer time on the cover plate 24 and the cursor 35 moved to the stop used. The sensitive paper is stepped by a projecting finger-piece 14 of a hub 15 carrying a pawl 11 which steps a ratchet disc 10 on which a pad 20 carrying the strip 21 is secured. The hub 15 is positioned by a split ring 16 which has a finger 17 bent towards the ratchet wheel 10 to prevent backward rotation when the finger 14 and hub 15 are returned by the spring 18. The movement of the finger 14 is limited by the opening 13 in the casing and the ratchet wheel 10 has an uncut portion 19 so that the paper will not be advanced when it has been moved through one revolution. One or more strips of paper 21 may be secured to the pad 20 as by threaded loops 22 or otherwise. In a modified construction, Fig. 19 (more especially for high-speed diaphragm shutters), the time-setting pointer of the shutter mechanism itself moves over the exposure and actinometer time scale, and the diaphragm-setting pointer 49 also moves over this scale. In this construction the arrangements for supporting and moving the sensitive paper and of the meter scales are also modified. An alternative construction is also described in which the plate-speed scale is omitted, the meter being graduated for a standard plate of known speed.