To accurately model visual performance, the impact of imager blur and noise on human vision must be quantified. Human vision is treated as a "black box" that is characterized by its signal transfer response and detection thresholds. This appendix describes an engineering model of observer vision. It also provides the psychophysical parameters used to characterize human vision.A.1 Contrast Threshold Function
Contrast threshold function (CTF) and its inverse, the contrast sensitivity function (CSF), quantify the spatial frequency response of human vision. A sinewave pattern is presented to an observer, and a response is solicited as to whether the sine wave is visible. In Fig. A.1, the observer is viewing a sine-wave pattern. While holding constant the average luminance to the eye, the contrast of the bar pattern is lowered until the pattern is no longer visible to the observer. That is, the dark bars are lightened and the light bars darkened, holding the average constant, until the bar-space-bar pattern disappears. A decrease in contrast from left to right is shown at the top right in the figure. The goal of the experiment is to measure the amplitude of the sine wave that is almost not visible to the observer.