| 16 PRELIMINARIES
For an imaging system, the impulse response is the image in the output plane due to
an ideal point source in the input plane. In this case, the impulse may be taken to be an
infinitesimally small white dot upon a black background. Due to the limited accuracy of
the imaging system, that dot will be resolved into a broader region. This impulse
response is usually referred to as the point spread function (PSF) of the imaging system.
Since the inputs and outputs represent a positive quantity (e.g., light intensity), the PSF is
restricted to be positive. The term impulse response, on the other hand, is more general
and is allowed to take on negative and complex values.
As its name suggests, the PSF is taken to be a bandlimiting filter having blurring
characteristics. It reflects the physical limitations of a lens to accurately resolve each
input point without the influence of neighbering points. Consequently, the PSF is typi-
cally modeled as a low-pass filter given by a bell-shaped weighting function over a finite
aperture area. A PSF profile is depicted in Fig. 2.2.
h(x)
Figure 2.2: PSFprofile.
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