Mumbai University > Electronics > Sem 7 > Digital image processing

Marks: 5 M

Year: Dec 2010

The range of light intensity levels to which the human visual system can adapt is enormous, of the order of 1010 from the isotropic threshold to the glare limit. It simply means, see things in the dark and also when there is a lot of illumination. It has been shown that the intensity of light
perceived by the human visual system which is a logarithmic function of the light intensity incident on the human eye. The curve in the figure represents the range of intensities that the human visual system can adapt. When the illumination is low, it is scotopic vision that plays a dominant role while for high intensities of illumination, it is photopic vision which is dominant. As from the figure the transition from scotopic vision to photopic vision is gradual and a certain levels of illumination, both of them play a role. To cut a long short, the dynamic range of the human eye is enormous. But there is a catch here i.e. the eye cannot operate over the entire range simultaneously at a given point of time, the eye can only observe a small range of illuminations. This phenomenon is known as Brightness Adaptation. The fact that eyes can operate only on small range can be proved.

Stare at the sun for a couple of seconds, the eye adapts itself to the upper range of the intensity values. Now look away from the sun you cannot see anything sometime. This is because the eye takes a finite time to adapt itself to this new range. A similar phenomenon is observed when the power supply of our homes is cut-off in the night. Everything seems to be pitch dark and nothing can be seen for sometime. But gradually eyes adjust to this low level of illumination and then things start getting visible in dark.