GB1490215A - Method and apparatus for testing lenses - Google Patents

Abstract

1490215 Measuring the power of a lens INSTITUT NATIONAL DU VERRE 10 July 1975 [15 July 1974] 31273/74 Heading G1A [Also in Division G2] Apparatus for this comprises a light source 1 and lens holder 8 and means for relatively displacing the axis of beam 6 and holder 8 at a known speed, detectors 14, 15 located to detect, at two positions spaced from holder 8, rays refracted by lens 7 and means to measure the time interval between the detection of rays. Lamp 1 in box 2 and slit 3 in plate 4 in the focal plane of lens 5 give parallel beam 6. Transport 9 supported by rollers 11 is driven at constant speed V by motor 10. Photo-cells 14, 15 are at X metres from the principal plane of lens 7 at an adjustable spacing of d metres. Preferably the photo-cells are in the image plane of the light source. On reception of rays in directions 16, 17, they respectively initiate and interrupt pulse counter 18 which emits a signal proportional to t seconds, the time between these receptions, so that calculator 19 determines the power P dioptics displayed by unit 20. Between lens 7 and the photo-cells there may be a further lens of known power which is converging if lens 7 is diverging, and diverging if it is strongly converging. The power of lens 7 is then calculated from the power of the combination. The pulses from the detectors 21, 22, Fig. 2, made square by circuits 23, 24, initiate and interrupt pulse generator 25 with integrator 26 giving a signal Xt to calculate 27, receiving a signal dependent on d and V from memory 28, so that a signal P = d/VXt is passed to de-coder 29 and display or sorting circuit 30. The power may be compared with a standard using memory 28 and the difference signal may trigger circuit 30. The embodiment of Fig. 3 (not shown) employs an oscillating mirror (33) instead of moving the lens and the single photo-resistor operates a timer for as long as it receives the refracted rays. In another embodiment the light source is on a trolley moved by a motor. In the embodiment of Fig. 4 (not shown) a diverging lens (46) being measured is moved in the focal plane of a lens (47), which is converging, between it and detectors (49, 51) and the distance X is then the focal length of lens (47). For measuring a converging lens (46) is diverging. The maximum cross-section of the beam is ¢ the maximum dimension of the lens both in directions parallel to and perpendicular to the displacement. The detector positions should be such that the rays reaching them emerge from zones of the lens, which, measured parallel to the displacement, are less than 10% of the lens dimension in that direction from the optical axis of the lens. The lamp may be quartz iodine or a helium-neon laser.