Abstract

Studies of image quality with photographic and TV systems suggest that visual performance depends on the effective contrast of the image (the difference between image contrast and the observer’s contrast threshold integrated across the spatial-frequency spectrum of the image). This implies that an increase in image contrast will compensate for a reduction in spatial-frequency range due to low-pass filtering. To test this hypothesis, we measured reading rates for low-pass filtered text. Letters were digitized to 256 grey levels on a 32- × 32-pixel array and low pass filtered. The filters had cutoff frequencies (at half-amplitude) of 1.34, 1.02, or 0.60 cycles/letter. Paragraphs composed of letters subtending 0.5° were displayed for 20 s on a monochrome monitor with maximum luminance of 120 cd/m². Observers read the text aloud, and the reading rate (number of words read correctly per minute) was computed. The process of low-pass filtering also reduces the contrast of the text. We compared reading rates for the filtered text with reading rates for filtered text in which the contrast was restored (stretched) to the maximum attainable with our display (0.94). Reading rates for the filtered text decreased with reduced cutoff frequency (down 81 % for the lowest cutoff frequency, 0.60 cycle/letter). However, reading rates for the stretched filtered text were less affected by filtering (down only 20% at 0.60 cycle/letter). The results indicate that the increased image contrast can compensate for a reduction in spatial-frequency range due to low-pass filtering.

© 1988 Optical Society of America