Human spatial processing accounts for dynamic range and color

Abstract

Silver halide photography has a fixed response to scenes. Films count photons, and that results in a count-dependent optical density. The range of the film’s sensitivity to light, as well as its optical density, is determined in the factory where the film is made. However, digital imaging makes it possible to render wider ranges of light intensity, therebymore closely reproducing the variations in illumination that are found within scenes. With better imaging hardware, it is now possible to improve the range of light sensors and displays, and increase the number of digital quantization levels. It should also be possible to make more accurate reproductions over a greater range of light intensities. Raw formats in today’s commercial cameras enable more bits per pixel.1 A sensor’s dynamic range is the ratio of the amount of light that causes its maximum response to the light that causes its minimum response. Active pixel processing has led to techniques that can increase the sensor’s dynamic range to a ratio of 1010 to 1.2–4 For displays, Seetzen et al. used an ‘unsharp mask’ technique to illuminate a high-resolution liquid-crystal display with an out-of-focus, modulated array of LED illuminators,5 and make digital high-dynamic-range (HDR) displays with the ratio of maximum light emission tominimum emission of 105 to 1 and a 1500cd/m2 maximum screen luminance.6 As we start to incorporate these techniques in imaging, the question becomes, how much of the 1010 range of the sensor response and the 105 range of the displays can we actually use? In 1941, Jones and Condit measured the range of light in 128 outdoor scenes and the camera responses to them.7 They also measured the range on each camera’s film plane, and showed that the optical veiling glare (scattered light), not the sensor signal-to-noise, sets the usable dynamic range of cameras. They Figure 1. Two opposing spatial mechanisms control the appearance of gray patches.