A method has been developed for measuring the contrast rendition of optical systems for a square wave line test object whose angular spacings vary from 1 sec to 129 sec in 2% increments. Another test object covers the range from 1 sec to 620 sec in 10% increments. These ranges cover the region of interest for practically all optical aids and photographic systems. Values of contrast rendition as high as 99.7% have been measured. The maximum value occurs for the coarsest target elements and degrades smoothly to zero at the limit of resolution. This occurs at the contrast threshold for the photomultiplier tube used as the receptor in a device referred to as the recognition contrast rendition (RCR) apparatus which is described in this paper.
RCR data might be described as the frequency response to a spatial square wave input signal. For well-corrected optical systems, changes in RCR can be measured when changes in OPD of the order of one-fortieth of a wavelength occur. In special cases, this sensitivity is maintained in the presence of as much as ten Rayleigh Limits of aberration, but in most cases the sensitivity is approximately one-twentieth of the maximum OPD in the system.
Uses for RCR data include inspection, field performance evaluation, and investigation of the vision of the human observer. Since the RCR Apparatus is impersonal, and the data obtained using it may be expressed numerically, it has certain advantages over visual inspection processes. Field performance evaluation of optical aids to be used for recognition tasks should be possible when nomographs for recognition are available. By making simultaneous visual and photoelectric measurements, the visual response to actual diffraction images may be compared to the measured contrast.
© 1955 Optical Society of AmericaFull Article | PDF Article
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