Patent GB-561242-A (GB)
Title: Improvements in or relating to instruments for measuring photographic gamma
IPC: G01J1/42
Filed: 1942-11-05
Abstract: 561,242. Measuring photographic "gamma" and speed. KODAK, Ltd. Nov. 5, 1942, No. 15627. Convention date, Nov. 6, 1941. [Classes 97 (i) and 98 (i)] A meter for determining photographic gamma from a test sensiometric strip of graded density comprises means for illuminating a viewing field by light reflected from or transmitted by the strip and also by a standard light beam of graded intensity, the standard beam being produced by a continuously - graded density wedge which is adjustable in the direction of the' rays from a small source of light so as to vary the rate of graduation of the intensity of the beam, and there being a scale coupled to the wedge which indicates the gamma of the strip when the said rate is adjusted to match that of the strip. In a modification, the test strip is ,similarly moved, in place of the density wedge. The light from the test strip and the standard beam may respectively illuminate adjacent parts of the field, the density wedge being adjusted to secure a match in the gradation of the two areas, or the wedge may be reversed to produce a gradation in the opposite direction and the illuminated areas be superposed, in which case it is adjusted to secure a uniform density. In this latter case, an adjacent part of the field is preferably illuminated with light of uniform intensity, for comparison purposes. Speed indications may be obtained in the first case by shifting the 'wedge or strip transversely to the light path, or by varying the intensities of the light sources, until the densities as well as the density graduations of the two areas are equal. In the second case, the adjustment of a device which varies the relative intensities of the superposed and comparison areas of the field gives a measure of the speed. A number of forms of apparatus for use with test strips, the density of which is graded either continuously or in steps, are described, together with exposure devices for making the test strips. In the form shown in Figure 5 the test strip, which is graduated in steps, is held in grooves 55 against one face of a Lummer-Brodhun cube, and is illuminated by light reflected by a mirror 58 from a lamp 57 through a ground glass screen 22. Light from the same lamp is reflected by a second mirror 59 through a continuously-graded density wedge 32 to illuminate the field at either side of the centralportion, which is illuminated through the test strip. The light from the continuouslygraduated density wedge 32 passes through a pile of integrating plates 51, preferably having diffusing end surfaces, which form steps, corresponding to those of the test strip, of graduated intensities each equal to the average intensity of the light incident on it as controlled by the wedge. The wedge is mounted on a base 60 movable by a rack and pinion, so that the rate of density-gradation of the parts of the field illuminated through it may be decreased or increased by moving it nearer to or farther from the lamp 57 respectively, a scale 76 indicating the adjustment, from which the value of gamma is determined. The intensity of the field illuminated by the wedge is made equal to that corresponding to the test strip by adjusting the wedge vertically, the vertical adjustment being shown on a scale 78 and indicating the speed of the test strip. Figure 6 shows a form intended particularly for measuring gamma only, in which the density wedge 81, integrating plates 51, and Lummer- Brodhun cube are contained in a tubular housing provided with an eyepiece 80. The sensitometric strip 50 is adjusted as indicated by the arrow 90 until the average intensity of the field illuminated through it is approximately equal to that of the comparison parts of the field. The wedge 81 is then adjusted axially until the rates of gradation in both parts of the field are substantially equal. Accurate matching of the average intensities of the two parts'of the field is obtained by adjusting a rheostat controlling the lamp 21 or 33, or by shifting the pile of plates 51 by means of adjusting screws 91 laterally through a distance less than the thickness of a plate, after which final adjustments may be made. Figures 12-15 (not shown) show simple instruments intended for gamma measurements only, and with or without integrating plates. In One form the density wedge is between the sensitometer strip and light source, and is optically inverted-in relation to the strip, that is, the density gradation is in the opposite direction. It is adjusted to and from the strip until the latter is of uniform brightness. In another form, adjacent parts of the viewing field are illuminated by light from the same source passing respectively through the density wedge and the sensitometer strip, one of which is adjustable towards and from the light source. Figure 16 shows an apparatus for measuring gamma and speed, in which the latter is indicated on a scale 176 'by a pointer carried by a lever 170 which is operated through a pin- and-slot connection by lateral movement of the density wedge 32. The pivotal point 174 of the lever is positioned so that the' lever is parallel to a line 171 joining the light source. 33 to the step of the wedge having the density at which the speed is to be measured. For example, for inertia speed measurements, the lever is parallel to a line 181 joining thedight. source to the zero density end of the wedge. The operation of the device is explained in the Specification, and a modification using an optical lever formed by a light beam is described. Figure 11 shows a device for making a stepped test strip, in which light from a source 100 passes through a density wedge and pile of integrating plates 108 to the sensitive material, which is held by a platen 110 against the face of the pile. The: exposure is controlled by a gravity-operated shutter comprising an apertured pivoted circular plate 122 carrying an adjustably mounted pendulum weight 125 and released by operation of a detent 130. At the end of its swing the plate is prevented from returning by a second detent 132.