Configuring medical displays for high- and stable-image contrast in bright rooms

Abstract

Display technology is rapidly improving with increasingly brighter displays. Since the contrast perception of the human visual system improves with increasing luminance, low luminance areas in medical images should be avoided. Older displays were less bright and had a high surface reflection, so the only possible way to increase contrast was to lower the black level by reducing the room illuminance. A dark viewing room soon became a prerequisite when viewing medical images, and although not ideal, it was necessary due to technological limitations. However, brighter displays offer new possibilities.This study determined the contrast thresholds in the dark, mid-gray and bright regions of an image displayed using three different luminance ranges: 1-250, 6-500 and 12-750 cd/m 2 . All three luminance ranges covered the same number of just noticeable differences (JNDs) but had vastly different luminance ratios. The visibility of two bar-pattern frequencies were studied: 4 and 16 cycles per degree. No differences in contrast threshold could be found between the three luminance ranges, indicating that high black levels and low luminance ratios are possible to use without degrading image contrast, if the display luminance range covers enough JNDs. The high frequency patterns required ten times the contrast of the low frequency patterns to be visible.Contrast stability was also studied, and the results indicate that it is easier to maintain stable image contrast if the display minimum luminance is high. The amount of reflected luminance is irrelevant for stability if the room illumination is stable.