Patent GB-276346-A (GB)
Title: Improvements relating to photographic exposure meters
IPC: G01J1/40
Filed: 1927-08-18
Abstract: 276,346. Schlichter, W. Aug. 23, 1926, [Convention date]. Photometric apparatus.-A meter of the contrast extinction type is characterized in that only a part of the whole field of view contains the contrasts to be adjusted, whilst the remainder of the field is formed by a large bright surface which causes the estimating eye to acquire rapidly a definite degree of sensitiveness. In the form shown in Fig. 3, a tube 15 encloses a hollow cylinder 16 traversed annularly by a number of channels 17 On one side of the cylinder is a matt disc 18 which receives the image from a lens 19. On the other side is an annular metal plate 20 in contact with an opal glass disc 21. The plate 20 is provided with a diminishing series of holes, Fig. 5, spaced to register with the channels 17, or with two such series one of which is covered by a grey filter 22. The eye, looking at the disc 21, observes a central bright area surrounded by a ring of smaller areas of gradually diminishing brightness. The area which just displays a glimmer of light is noted, and by means of a calculating device, which may be separate or applied to the tube 15 in the form of rings, the time of exposure may be deduced, allowing for the aperture of the objective and the sensitiveness of the plate. This form does not, however, give accurate results in dim or in very bright light unless it is used in conjunction with appropriate grey filters. In another form, which is of universal application, the front part of which shown in Fig. 7, the front end of a tube 26 is closed by a disc 27 having an opening 28 between which and a similar opening in a support 30 extends a tube 29. A matt disc 31 rests against the support 30. which has the shape shown in Fig. 10. On the front end of the tube 26 is mounted a rotary cap 34 carrying a disc 37 and an opal glass disc 38. The disc 37, Fig. 9, which rotates with the caphas, within the circle 43 a diminishing series of openings, and also a similar continuing series covered by a grey filter. The openngs are such that the area of successive stops brought before the aperture 28 in the disc 27, Fig. 8, for each turn through 1‹ of the disc 37 diminishes according to the law : 1 : 1/#2 : ¢ : 1/2#2 : “... and so on. This is the arrangement for varying the fight passing to the area 41 of the matt disc 31 through the tube 29. The remaining area 42 of this disc is illuminated by light passing through annular segments 45, 46, 47, in the disc 37 and corresponding segments in the disc 27. As the disc 37 is rotated, the segments move from coincidence to gradually diminishing coincidence, in which portions only of the segments remain open together with a series of large stops 48, and finally a series of small stops 49 alone is open. In this way the area 42 of the disc 31 is variably illuminated so that the instrument may be effective in all conditions of lighting. In use, an eye applied to the rear of the instrument, which is provided with optical viewing means, sees on the matt disc 31 the area 41 illuminated through the aperture 28 and the surrounding area 42 separated therefrom by a dark rim and illuminated through the segments 45, 46, 47, &c. After rotating the cap 34 until the area 41 is indistinguishable from the surrounding rim, the exposure may be determined by reference to scales provided on the cap and on rings surrounding the tube, which allow. as before, for the aperture of the objective and the speed of the plate. The main tube may be provided with a scale of metric candle-power viewed through a slot in the cap, to permit the instrument to be used as a photometer, for example to measure the lighting in work-rooms- Reference is also made to a meter wherein the bright field is illuminated by a standard incandescent lamp carried in the meter, in which case the ratio of darkening is equal to the ratio of increase of light value. Specification 776/15 is referred to. The Specification as open to inspection under Sect. 91 (3) (a) also describes a modification of the apparatus shown in Fig. 7 in which no provision is made for varying the illumination of the area 42 of the diso 31 surrounding the contrast area 41. This subject-matter does not appear in the Specification as accepted.