Patent GB-538696-A (GB)
Title: A calculating device for use in photographic printing operations
IPC: G03B27/72
Filed: 1940-04-24
Abstract: 538,696. Photo-electric densitometers. SANGAMO WESTON, Ltd. April 24, 1940, No. 7421. Convention date, April 24, 1939. [Class 40 (iii)] [Also in Group XX] A special form of calculator is described for evaluating the exposure from the various data in a printing process comprising (1) measuring the maximum and minimum densities of the negative and the intensity of the printing light, (2) selecting a printing paper having an exposure range determined by the difference between said densities, and (3) exposing the paper for a time determined by the intensity of the printing light, the sensitivity of the paper, and the maximum density of the negative. The form shown in Fig. 1 is mounted over the double-ended pointer 25, 26 of a photo-electric densitometer for determining the densities and the light intensity. The pointers 25, 26 move over fixed scales 55, 56 graduated non-uniformly to indicate equal increments of cell current, and the graduations are connected by lead lines to a pair of uniformly graduated scales 52, 53. Pivoted to move independently of each other round an axis coinciding with the pointer axis are two segmental dials. 27, 28, of which 27 carries a scale 54 co-operating with scale 52 and graduated to indicate the contrast range of the paper required to print the negative under test, and 28 carries a scale 34 co-operating with scale 53 and graduated to indicate the exposure time necessary to print the negative on the indicated type of paper when illuminated by the printing light in use. In operation, the densitometer is placed to measure the maximum density of the negative, and the indication of the pointer 25 is noted on the scale 55-e.g. that shown' at 25a. The dial 27 is turned so that a mark 33 thereon registers with the graduation on the scale 52 which is connected to the graduation 25a. The densitometer is then placed to measure the minimum density of the negative, and the reading of the scale 54 corresponding to the new position of the pointer 25 is noted. This reading indicates the contrast range of printing paper required. From known data, the sensitivity of such paper (i.e. the exposure necessary to yield a faint grey on the paper at the bottom bend of its characteristic curve) is determined, and the dial 28 is then set in a position corresponding to this sensitivity. For this purpose, dial 27 bears a scale 35 of paper sensitivities, and dial 28 is turned until the marking on the scale corresponding to the already-determined paper sensitivity appears in a window in dial 28. Finally, the densitometer is placed to measure the intensity of the printing light beam in the position which will be occupied by the paper, and the indication on scale 34 corresponding to the position of the other pointer 26 then gives the necessary printing exposure time in seconds. When a densitometer is used in which a single pointer moves over two contiguous scales corresponding to scales 55, 56 graduated respectively to indicate negative densities and intensities of illumination from the printing light, the indications from these scales are transferred by the operator to a separate calculator comprising three relatively rotatable dials corresponding to the support of the scales 22, 23, and the segmental dials 27, 28. The Specification discusses the calibration of the various scales. ' The Specification as open to inspection under Sect. 91 describes also a densitometer, Fig. 1 (Cancelled), for use in carrying out the process, comprising an opaque box having a small aperture through which the negative is illuminated by a light source within the box, and a photoelectric cell mounted on the box so as to receive the light from the aperture passing through the negative and also so as to be removable without disturbing its electrical connections to the measuring instrument in order that the same cell may be placed in the 'position to be occupied by the printing material during its exposure, to measure the intensity of the printing light. The instrument c:omprises an opaque box vented at 6, having a small hole 3 illuminated by a lamp-condenser system 2, 50, preferably through a diffusing screen 13, and a blue filter 4 which absorbs infra-red rays and may be matched with the positive emulsion to be used. The hole 3 may be formed in a removable plug 12 so that, if desired, it may be replaced by a plug having a hole of different size. The lamp support is universally adjustable by three screws 7, and the condenser is mounted on heat-insulated supports 5 to protect the negative when placed over the aperture 3. On an arm 18, hinged at 19 so as to rest in any position to which it is swung, is mounted a casing 15 carrying in an easily releasable manner, a photoelectric cell 14 which is held at a short distance from the negative when a felt ring 20 on the arm rests on the negative. The cell has flexible connections 11a with a measuring instrument mounted in the box, and a rheostat is included in the lamp circuit to permit a constant intensity of illumination to be maintained. The measuring instrument (not shown) is supported beneath the cover of the box, and has a double-ended pointer arm co-operating with a set of scales mounted on the outside of the cover. This subject-matter does not appear in the Specification as accepted.