### 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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