Abstract
The instrument described measures alternatively the distribution of intensity in the image of a narrow slit (the spread function), or the frequency response function, which latter is the Fourier transform of the spread function. For the former use a slow scan with an exploring slit is used while for the direct determination of the frequency response curve a continuous scanning with a grating of slits arranged around the periphery of a variable speed drum is employed with a single frequency electrical detector. The in-focus aberration-free image of a line source has been measured together with the axial variation of central intensity and good agreement with the theoretical curves has been obtained. The frequency response curves obtained by taking the Fourier transforms of four out-of-focus spread functions numerically agree well with the direct determinations of the frequency response with the scanning drum. Further the frequency response curves for a well corrected lens in different focal planes have been compared with results obtained by Hopkins theoretically.
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