Physical Properties of light Psychological Properties of light 1. Illuminance: 1. Brightness -amount of light falling on a surface -perceived luminance (or illuminance, if perceiving a source) 2. Luminance: -amount of light reflected from a surface 2. Lightness 3. Reflectance: -proportion of light being reflected from a surface -perceived reflectance Already seen simplistic example of lightness not being directly related to luminance (mach bands).

Physical Properties of light Psychological Properties of light 1. Illuminance: 1. Brightness -amount of light falling on a surface -perceived luminance (or illuminance, if perceiving a source) 2. Luminance: -amount of light reflected from a surface 2. Lightness 3. Reflectance: -proportion of light being reflected from a surface -perceived reflectance Already seen simplistic example of lightness not being directly related to luminance (mach bands). But lateral inhibition can’t account for all the examples. The problem of perception is to decide whether a boundary difference in shade is a result of the gray paint, a shadow, or both. That is, is the difference a result of changes in illuminance , reflectance or both?

A more fundamental question: how does the brain determine shadows? The simple answer: by detecting the penumbra

Fuzzy contours are more likely to be seen as a consequence of shadows (illuminance differences) and tend to be ‘ignored’ by the perceptual system -in part, because they are physiologically less noticeable, thanks to microsaccades/ocular microtremors.

Lightness Constancy -lightness, again, is the perception of reflectance (i.e. answers the question “what shade of grey is this paper?”) Generally speaking, if >80%, tend to call it “white” if <10%, tend to call it “black” -lightness constancy: tend to see an object as having a static lightness, despite vast differences in the raw luminance values across illuminance settings (a piece of coal is always seen as “black”, regardless of lighting conditions) Illustrated by Burzlaff’s easel study

Lightness Constancy

Lightness Constancy -How is this accomplished? -A comparative process, where lights from various surfaces are compared to one another, accounting for source lights. -Called the ratio principle Two areas will be perceived as having equal lightnesses if the ratios of their luminances to the luminances of surrounding objects is kept constant -in general, brain must compare the amount of light coming off each surface to each other, factoring in where it thinks lighting (illuminance) differences exist, in arriving at perceived lightnesses -not clear exactly how this calculation takes place (reflectance comparisons?) We will, however, tend to anchor our perception of “white” to the surface of the greatest reflectance in a room, though (“highest luminance rule”) -Gilchrist & Li: present a ganzfeld of ‘grey’ and ‘black’, participants will tend see white and grey. -so, perception is a consequence of comparisons across objects.

Lightness Constancy Gelb: demonstrated the ratio principle in action by (secretly) changing it

Lightness Constancy Gilchrist and associates took this a step further by changing what the perceived illuminance (changed the atmosphere) on various surfaces was:

The Koffka Ring A continuous ring with uniform reflectance, and lightness Useful to demonstrate the comparative and computational nature of lightness perception The reason we see it as uniform is the continuous nature of the ring allows for direct comparison between halves. Under these circumstances, the background is seen as lightness (reflectance) differences. But by altering the ‘continuations’ and ‘atmosphere’ we can influence the perceived lightness of the ring. Prediction 1: If we remove the ‘good continuation’ cue by simply pulling it apart, each background will now be seen as a distinct atmosphere (i.e. shadows or a filtre)

The Koffka Ring A continuous ring with uniform reflectance, and lightness But by altering the ‘continuations’ and ‘atmosphere’ we can influence the perceived lightness of the ring. Prediction 2: if we use good continuation, we can alter what part of the ring ‘goes with’ what part and induce two atmospheres (combination of gestalt good continuation assigning figures, and apparent illuminance changes across the boundary)