Luminance is a physical and objective measure of the intensity of light. The sensation elicited by different luminances is called brightness. However brightness is a subjective measure as it is the perceived amount of light emanating from an object. It may seem logical to expect that luminance and brightness are directly proportional and that two objects that reflect the same amount of physical light into the eye will look the same brightness. However, as this essay will discuss, the apparent brightness of objects is not entirely dependent upon the amount of light received from them and other factors are influential in the way we perceive brightness.
Our perception of the brightness of objects often depends more on the luminance of adjacent objects and backgrounds than on the actual luminance of the object itself. Two surfaces reflecting the same physical amount of light to the eyes typically look differently bright if the surfaces are observed in surrounds that are themselves returning different amounts of light. This phenomenon is called simultaneous brightness contrast.
This effect can be seen when two squares with exactly the same physical brightness are each surrounded by a larger square of different brightness. The square on the dark background appears lighter than the square on the light background. This can be seen in figure 1 below.
This effect can be explained by the centre/surround organisation of retinal ganglion cells. The organisation means that the response of the ganglion cell to stimulation of one portion of its receptive field, (the area to which a ganglion cell is sensitive), can be modified by stimulation of a neighbouring area. This interaction between antagonistic regions is caller lateral inhibition. If an ON-centre receptive field, that is the centre is stimulated by light and the surround is inhibited by light, is placed over the left square, the light in the surround produces an inhibitory mechanism which reduces the neural response rate of the receptors exposed to the central square, making it appear dimmer. In the right square, the surround exposed to the dark background is less stimulated and therefore the central square appears brighter as it is undergoing less inhibition even though the amount of stimulation from the centre square is the same.
Other evidence that the perceived brightness over regions of a surface is related to activity in retinal ganglion cells is that of the Hermann Grid (see figure 2). Here you can see illusory grey spots in the intersections between the horizontal and vertical white stripes. Using an ON-centre receptive field this can be explained by the fact that the cell whose receptive field is centred on the intersection will respond less than the cell whose receptive field is centred between intersections as the cell on the intersection will have more light in its surround and therefore have reduced activity so one will experience dimming at such locales, that is to say the grey spots.
The perception of the brightness of objects is also determined by contrast boundaries the object may posses and the tendency of the visual system to emphasise borders. Figure 3 shows a uniform dark area and a uniform light area with an intermediate zone that gradually changes from dark to light. However a gradual change in brightness flanked by to uniform areas is not seen. Instead two bands are visible, one is darker than any other part of the figure and one is brighter.
This can also be explained in terms of lateral inhibition. Using on ON-response receptive field, the centre is an excitatory area and the surround an inhibitory area. The receptive fields in the uniformly white and uniformly black areas receive about the same stimulation in their excitatory centres and inhibitory surrounds. Therefore the centre excitations are in balance with the surround inhibitions. The receptive...