Two Visual Systems of the Human Eye: Brightness Adaptation and Discrimination

In the human eye, there are two distinct visual systems (based on two types of retinal cells)

  1. Scotopic vision not color sensitive, more sensitive to light and mainly peripheral (rods)
  2. Photopic vision color sensitive, less sensitive to light and mainly central (cones)

The existence of these two systems result in a extraordinary dynamic range of light intensity levels to which the human visual system can adapt  about 10 exp 16   from scotopic threshold to the glare limit

Considerable experimental evidence indicates that subjective brightness is a logarithmic function of the light intensity. However the visual system cannot operate over such a range simultaneously, it accomplishes this large variation by changes in its overall sensitivity, a phenomenon known as brightness adaptation. The total range of intensity levels it can discriminate simultaneously is rather small compared with the total range

For any given set of conditions, the current sensitivity level of the visual system is called the brightness adaptation level. The ability of the eye to discriminate between changes in brightness at any specific adaptation level is also of interest: a typical observer can roughly discern a total of one to two dozen different intensities at any one point in a monochrome image. This doesn't mean that an image can be represented by such a small number of intensity values, since as the eye roams about the image the adaptation level can change and the eye is quite capable of detecting unwanted contour effects if such a small number of of levels is used

Figure  Range of Subjective brightness sensations showing a particular adaptation level (adapted from Gonzalez and Woods, 1992)

Two phenomena clearly demonstrate that perceived brightness is not a simple function of intensity:

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Last modified on April 05, 1999, G. Scott Owen,