Patent GB-1100260-A (GB)
Title: Improvements in and relating to photometric devices
IPC: G01J1/12
Filed: 1965-04-12
Abstract: 1,100,260. Comparison photo-meters. INTERNATIONAL POLAROID CORPORATION. April 12, 1965 [April 13, 1964], No. 15483/65. Heading G1P. [Also in Division G2] A comparison photo-meter, particularly for use in a camera, comprises means defining a field of view within which two light-source areas, independently illuminated by an unknown source and a reference source, are viewed simultaneously, together with means for causing an index symbol to be uniquely intelligible in the field of view only for a predetermined ratio between the values of light flux falling on the two source areas from the two sources respectively. In the embodiment of Fig. 15 three index symbols "darken", "yes", "lighten" are rendered in terms of a low density tone on a transparent support 147. Immediately in front of the support 147, there is arranged a second transparent support 146 carrying a reflective pattern 149. Light from the scene to be photographed passes through a diffuser 142 and is then transmitted through the transparent areas between the pattern 149 to illuminate the index symbols on the support 147. A lamp 12 arranged above and behind the supports 146, 147 illuminates the reflective pattern 149 through the support 147, providing a uniformly decreasing brightness over the pattern 149. When light from the scene passing between the areas of the pattern 149 has a brightness equal to that in a top zone R D  of the pattern 149, the word "darken" will appear uniquely intelligible to a viewer 16. Similarly, when equality of brightnesses is obtained in a bottom zone R L , the word "lighten" is observed. Blades 120, 121 which define the aperture 125 of the camera extend into the path of light passing to the diffuser 142 and are shaped to define a variable aperture 126. Thus on adjustment of the diaphragm blades 120, 121 the light beam incident on the diffuser 142 is attenuated or increased as required until equal brightnesses are obtained in a middle zone R Y  when the word "yes" is uniquely intelligible. In a modification the lamp 12 is arranged symmetrically with respect to the supports 146, 147 and the zones R D , R Y , R L   of different intensities are defined by an unpatterned density sheet having three density bands. In another embodiment diffused light from the scene is passed through a support carrying a first checkerboard pattern in terms of three optical densities and diffused light from the source 12 is passed through a second different checkerboard pattern in terms of the same three densities. The transmitted beams are combined in the field of view by a semi-transparent mirror. The two checkerboard patterns are such that when the diffused light beams incident thereon are of equal intensity the superimposed pattern define the index symbol "yes" against a uniform background. In a further embodiment diffused light from the scene and the source is polarized in two perpendicular directions, combined by means of a semi-transparent mirror and then passed through two superposed vectographs which define two checkerboard patterns in terms of degree and direction of polarization. The resultant beam is then passed through a rotatable polarizer which is coupled to control mechanism for the exposure aperture. This polarizer is rotated until the two patterns define the index symbol "yes" against a uniform background.