Patent GB-654250-A (GB)
Title: Improvements in or relating to devices for measuring the light cast by a photographic enlarger toward a surface
IPC: G01J1/12
Filed: 1947-07-11
Abstract: 654,250. Exposure meters for photographic projection printing; diaphragms. HALL, J. M. July 11, 1947, No. 18468. Convention date, Dec. 4, 1944. Addition to 653,539. [Classes 97 (i) and 98 (i)] A densitometer for evaluating a light-beam projected from an enlarger upon a photographic printing surface comprises a base adapted to rest on said surface and extending on opposite sides of said beam while having an aperture leaving a portion of the image formed thereon unmasked, a casing mounted on said base to lie to one side of the projected beam and carrying a mirror substantially parallel to the axis of the beam and normal to the printing surface, said mirror having a light-transmitting spot, and variable means within the casing to illuminate said spot to known light intensities for matching with said image as viewed in the mirror. Preferably the image is viewed by reflection by the mirror across the projected beam with the aid of an eyepiece mounted on the base and inclined downwardly towards the casing and the mirror is shielded from direct light from the enlarger. As shown, light projected by the enlarger 11 is diffusely reflected from a portion 31 of the image receiving surface 12 selected by the apertured plate 27 attached to the densitometer casing 15, which is slidable upon the surface 12, and is reflected by a mirror forming part of one wall of the casing to an eyepiece 25 mounted on the plate 27 beyond the aperture 30. The mirror is apertured at 17 and light from a lamp within the casing 15 falls on a flash opal or ground-glass disc behind the mirror in contact therewith, the translucent illuminated spot thus formed being also viewed in the eyepiece. The intensity of the spot 17 is adjusted by turning a knob 18 coaxial with a pinion 63 which is engaged on opposite sides by racks 57, 61 carried by the two leaves 55, 56 of a diaphragm until equality of the two fields viewed in the eyepiece is achieved, a pointer 20 carried by the knob 18 then indicating on a logarithmic scale 22 the density of the portion of the negative corresponding to the selected image portion. The diaphragm edges conform to logarithmic curves so that equal successivemovements of the two leaves result in successive areas which are in geometrical progression, and, to ensure that the light flux passing through the diaphragm is uniformly distributed, a diffusing disc is placed between the diaphragm and the lamp. A slide 23 having notches 52 by which it is held set in the densitometer 15 by a spring catch and carrying four apertures 50, one of which holds a transparent disc for black-andwhite work and the others hold red, green and blue filters for use in colour photography, is arranged between the diffusing disc and the diaphragm and close to the latter. For each batch of printing paper a test strip is made by exposure through a stepped grey scale for a chosen printing time at a chosen aperture. The strip is developed and fixed, and the steps of the scale which produced dead black and dead white noted. The projected images of these two steps are then measured with the densitometer and the readings noted, thus determining the contrast of the paper. Alternatively, assuming that the density ranges of several grades of paper are already known, the pointer 20 is set to a chosen value and the intensity of one step of the projected grey scale image is brought to equality with the comparison spot 17 by adjusting the intensity of the enlarger lamp, whereupon all the grey scale steps correspond to known values on the scale 22. The choice of paper is determined by the difference between the density readings obtained by comparing with the comparison spot 17 the lightest and darkest parts of the image of the negative to be printed.