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Effect of viewing angle on visual detection in liquid crystal displays

Display devices for medical diagnostic workstations should have a diffuse emission with apparent luminance independent of viewing angle. Such displays are called Lambertian, or they obey Lambert's law. Actual display devices are never truly Lambertian; the luminance of a pixel depends on the viewing angle. In active-matrix liquid crystal displays (AMLCD), the departure from the Lambertian profile depends on the gray level and complex pixel designs having multiple domains, in-plain switching or vertically-aligned technology. Our previous measurements established that the largest deviation from the desired Lambertian distribution occurs in the low luminance range for the diagonal viewing direction. Our purpose in this work is to determine the effect that non-uniform changes of the angular emission have on the detection of low-contrast signals in noisy backgrounds. We used a sequential two-alternative forced choice (2AFC) approach with test images displayed at the center of the screen. The observer location was fixed at different viewing angles: on-axis and off-axis. The results are expressed in terms of percent-correct for each observer and for each experimental condition (viewing angle and luminance). Our results show that for the test images used in this experiment with human observers, the changes in detectability between on-axis and off-axis viewing are smaller than the observer variability. Model observers are consistent with these results but also indicate that different background and signal levels can lead to meaningful performance differences between on-axis and off-axis viewing.