Point and Line Spread Functions

Another concept that may be new to neophyte vision people is that of point spread function.

Most lenses including the human lens are not perfect optical systems. As a result when visual stimuli are passed through the cornea and lens the stimuli undergo a certain degree of degradation. The question is how can this degradation be represented? Well suppose you have an exceedingly small dot of light, a point, and project it through a lens. The image of this point will not be the same as the original. The lens will introduce a small amount of blur. Click on point spread function (PSF) so see a graphical representation.

If you clicked on the graphical representation you saw a diagram representing the light distribution of a point after passing through a lens, for example a human lens. But this plot only represented light distribution along one axis of a plane. Click on spread functions to see a three dimensional representation. The point spread function you saw if you clicked on the above figure was a two dimensional cross section of this three dimensional solid. In this figure you also saw an image of a line.

The cross section of the line image is called a line spread function (LSF). A LSF is derived by integrating the point solid along sections parallel to the direction of the line. This works because a line image is the summation of an infinite number of image points along its length.

Ok, so PSF represents a tool for describing the visual stimulus. But you are interested in visual perception and want to know how that helps you to understand what you are looking at.

Suppose you have two points of lights and when you plot the energy as a function of space you have two distinct distributions. The question is how would these two distributions be perceived? The quick answer is that depends on how close the two points are. If you have already clicked on two distinct distributions. you know what is involved. If not click on it now.

Now you are probably thinking, yes, ok but the world is not made up of just a bunch of points. For example, sometimes I see lines. You can readily imagine that if a point undergoes a certain amount of degradation so does a line. If one has a series of parallel lines this is called a square wave grating.

Square wave gratings are often used to determine the modulation transfer function of an optical system.

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