GB797513A - Improvements in and relating to the testing of optical lenses - Google Patents

Abstract

797,513. Testing lenses photo-electrically. BRITISH BROADCASTING CORPORATION. May 31, 1955 [June 14, 1954], No. 17473/54. Class 40 (3). Apparatus for examining the resolution of optical lenses comprises a test chart having one or more bars thereon the width of each bar decreasing progressively in direction parallel to the length of the test chart so as to give a progressive change of grade of detail, means including the lens to be tested for examining an area of the test chart, means for causing relative displacement of the area and the test chart in a direction parallel to the length of the chart, a photoelectric device for receiving at least part of the light from said area and producing signals representative of the intensity variations of the received light and visual display means responsive to these signals so as to produce a display indicative of the resolution of the lens. As shown, the test chart 10 consists of opaque bars on a light transmitting carrier fitted on a rotatable drum 11, which houses a light source 12 and diffusing screen 13. Lens 16 to be tested is positioned in the optical path between source 12 and a photo-multiplier 20, this path including narrow slits 14, 18 defining an area on the test chart 10. The photo-multiplier output is applied to the Y-plates 22, 22a of a C.R.T., the X-plates of which are fed with a signal, representative of the grade of detail being examined, by a variable resistor 25 operated by a motor 28 which also rotates the drum 11. The width of the several spaced opaque bars of which the chart consists decrease linearly or logarithmically in the direction parallel to the longitudinal centre line of the bar, so that, as the drum rotates, the area scanned presents to lens 16 a progressive change of grade of detail. The width of the scanned region of the test pattern is less than the smallest width of any of the test chart zones and the scanned region lies wholly within one of the zones. If a light transmitting zone is being scanned, as long as the lens 16 can resolve the detail, the output of cell 20 is constant. As the detail becomes finer the lens can no longer resolve and the output of the cell decreases until finally the output of the cell becomes constant again at a lower level. The arrangement may be adapted to examine offaxis resolution.