1. A perceptual difference model using a pair of human vision systems, one for a reference video signal and the other for an impaired version of the reference video signal, each human vision system having a spatio-temporal filter receiving the respective video signal, the outputs of which are differenced to obtain an initial perceptual difference map, further comprising means for noise masking the output of each spatio-temporal filter to account for photon noise based on the relation of pupil size to luminance prior to obtaining the initial perceptual difference map.