GB234147A - Improvements in or relating to photographic photometers - Google Patents

Abstract

234,147. Schlichter, W. Dec. 17, 1923. Exposure meters.-Relates to a meter of the type in which the objective of an optical actinometer projects a real image of the subject to be photographed on a portion of a plane of which the remaining parts contain reference fields which are illuminated by dispersed daylight from the neighbourhood of the subject, the actinic power of which is measured by means of sensitized paper darkened to standard tints, in such a manner that from the adjustment of the image to the reference fields, the time taken for the exposure of the sensitive paper, the sensitiveness of the negative and the lens aperture, the required time of exposure can be determined by means of a scale of calculation arranged on the actinometer, and consists in providing the meter with a member formed in sections corresponding to the reference fields, each section having a group of openings of different aggregate area so that the gradation of brightness of the reference fields is determined by the aggregate areas of the openings. The meter comprises a tube h containing a translucent glass plate d, a tube c, an objective a, an iris stop b, an eye-piece e and an actinic filter g e.g. of cobalt glass through which the image produced on the plate d may be inspected. The reference light passes through the annular chamber between the tubes c, h, bounded at the front end by the translucent annular part of a glass plate k and a perforated flange or filter i. The annular chamber is divided into four sectional chambers by partitions 15, corresponding to which the filter is divided into four sections, as shown in Fig. 10, one completely cut out, one having holes 16, one having small holes 17, and one inperforate, so that reference fields of different intensity of illumination are produced on the plate d. By this means a numerical gradation of the reference fields is obtained. An actinometer at the rear of the instrument comprises an internal tube 20, adapted to receive a strip of sensitized paper 21, and a rotary sleeve 22 with peep-hole 23 and adapted to display a portion of the paper which is inspected through a slot 24 in the tube h. Two standard colour fields 25 are disposed laterally of the slot. The intensity gradation of the reference fields measured logarithmically is preferably made commensurate with the logarithmic unit of the calculating system (usually log @2) and so arranged that the logarithmic area of contrast between the brightest and darkest field includes a whole number of scale divisions and that the medium field halves logarithmically the whole area. In this way the scale arrangement provides for an accurate subdivision of the coarse grading supplied by the reference fields. To determine the exposure, the image and reference scale are balanced by adjustment of the iris stop b, and the actinic value of the light is measured by the sensitized paper 21. The ring w is then set so that the time in seconds obtained bv the second operation coincides with the value taken from the iris scale u designed with logarithmically equivalent scale sections and carried over to the scale v of the calculating arrangement. By means of the ring x which carries a scale indicating the lens aperture and an arrow which is set to point to the correct number of the plate speed scale y, the exposure is determined from the scale w, the correct number of the series being that corresponding to the particular division on the scale x which indicates the lens aperture.