Abstract
An instrument is described which is capable of recording automatically both the amplitude and the phase of the O. T. F., as would be required by a test laboratory. A simplified version permits a continuous display on a C. R. T. and this is suitable for rapid workshop assessment of lens performance. The basic system consists of an object generator, an image analyser, and an electronic processing unit. By imaging part of a rotating radial grating on to a slit and then varying the orientation of the lines with respect to the slit it is possible to generate a periodic test pattern of constant contrast but in which the spatial frequency varies over a range of 100 to 1 and includes zero c/mm. The image of this object produced by the lens under test drifts at a constant rate over an analysing slit and photo-multiplier. A single low pass filter selects the fundamental of the photo-multiplier signal and this is proportional to the amplitude of O. T. F. of the lens under test. A reference optical system and photo-multiplier is used to measure phase and to provide a facility for null measurement by phase coherent detection.
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