Intrinsic contrast vs perceived contrast

The detection of a displayed message is the first step of the cognitive process to identify its meaning. It can be psychophysically evaluated through measurement of contrast. The intrinsic contrast (Ci) is a physical parameter depending on display characteristics and environmental light distribution. The perceived contrast (Cp) is our real parameter of interest and it is obtained from Ci considering the veiling luminance (LVeil), evaluated with a mathematical model of the eyes properties. Disability glare is due to the scatter of light within the eye, and it results in veiling light superimposed over the normal scene, effectively reducing contrast of objects observed. On equal conditions of vision, it reduces perceived contrast and its value can descends under the threshold of perception making not legible the information displayed otherwise perfectly detectable. We calculate Cp adding LVeil both on target and background luminance, to Ci: As LVeil increase, Rc (Cp /Ci) goes from 1 to 0, Cp decreases and can reach a value under the contrast threshold rendering the character illegible. To evaluate the veiling luminance we used the CIE general disability glare equation, where glaring sources can be ‘single points’ or ‘large surround sources’. Simulations of worst cases have been done with the method described.