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Psychophysical thresholds and digital camera sensitivity: the thousand-photon limit

In many imaging applications, there is a tradeoff between sensor spatial resolution and dynamic range. Increasing sampling density by reducing pixel size decreases the number of photons each pixel can capture before saturation. Hence, imagers with small pixels operate at levels where photon noise limits image quality. To understand the impact of these noise sources on image quality we conducted a series of psychophysical experiments. The data revealed two general principles. First, the luminance amplitude of the noise standard deviation predicts threshold, independent of color. Second, this threshold is 3-5% of the mean background luminance across a wide range of background luminance levels (ranging from 8 cd/m2 to 5594 cd/m2). The relatively constant noise threshold across a wide range of conditions has specific implications for the imaging sensor design and image process pipeline. An ideal image capture device, limited only by photon noise, must capture at least 1000 photons/pixel (1/sqrt(103) ~= 3%) to render photon noise invisible. The ideal capture device should also be able to achieve this SNR or higher across the whole dynamic range.