Detectability decreases with off-normal viewing in medical liquid crystal displays

To quantify the reduction in detection performance of subtle signals at off-normal viewing directions in medical active-matrix liquid crystal displays (AMLCDs).

Fifty synthetic image pairs per viewing condition (a total of 350) were used in a two-alternative forced-choice experiment in which 11 trained observers viewed images at 0, 30, and 45 degrees from the display normal, along the diagonal axis of a 5 million pixel in-plane switching monochrome AMLCD. The images were generated using white-noise backgrounds. A Gaussian signal was added to the signal-present set with three different signal amplitudes (4, 8, and 12 gray levels in a 10-bit scale).

The average percent correct achieved for a signal of 4 gray levels was 79.6 (95% confidence intervals based on reader and case variability: 71.6-86.9), 63.4 (CI 56.0-71.3), and 55.3 (CI 48.4-62.0), for 0, 30 and 45 degrees from the display normal, respectively. When the signal amplitude was increased by a factor of two, the performance was 76.9 and 57.0 for 30 and 45 degrees, respectively, and 95.3 and 85.3 when the amplitude was increased by a factor of three. The observers took on average about twice as long and as much as seven times as long to reach decisions in off-normal viewing.

Off-normal viewing of diagnostic images in AMLCDs significantly reduces the detection of low-contrast abnormalities. Increased off-normal signal amplitudes were required to regain the detection performance measured for normal viewing. We observed this decrease in detection performance for off-normal viewing even when measured decision times were about twice as long as for normal viewing.