Model of the Human Eye
Based on the anatomy of the eye, a model can be constructed as shown in Figure(2.2).Its first part is a simple optical system consisting of the cornea, the opening of iris, the lens and the fluids inside the eye. Its second part consists of the retina, which performs the photo electrical transduction, followed by the visual pathway (nerve) which performs simple image processing operations and carries the information to the brain.
Fig (2.2): A model of the human eye. Image Formation in the Eye.
The image formation in the human eye is not a simple phenomenon. It is only partially understood and only some of the visual phenomena have been measured and understood. Most of them are proven to have non-linear characteristics.
Two examples of visual phenomena are:Contrast sensitivity , Spatial Frequency Sensitivity
Contrast sensitivity
Figure( 2.3) : The Weber ratio without background
Let us consider a spot of intensity I+dI in a background having intensity I, as is shown in Figure (2.3) ; dI is increased from 0 until it becomes noticeable. The ratio dI/I, called Weber ratio, is nearly constant at about 2% over a wide range of illumination levels, except for very low or very high illuminations, as it is seen in Figure (2.3). The range over which the Weber ratio remains constant is reduced considerably, when the experiment of Figure (2.4) is considered. In this case, the background has intensity I0 and two adjacent spots have intensities I and I+dI, respectively. The Weber ratio is plotted as a function of the background intensity in Figure (2.4). The envelope of the lower limits is the same with that of Figure (2.3). The derivative of the logarithm of the intensity I is the Weber ratio:
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Thus equal changes in the logarithm of the intensity result in equal noticeable changes in the intensity for a wide range of intensities. This fact suggests that the human eye performs a pointwise logarithm operation on the input image.
Figure (2.4): The Weber ratio with background
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