A spatial filter assembly for a laser beam is shown
schematically in Figure 1.

At its source, a laser beam is coherent and exhibits
a clean, smooth intensity profile, close to the bell shaped
Gaussian curve in Figure 2.

As the beam passes through air and objective lens,
it becomes scattered by particles in the air and
defects in the objective. Its actual intensity profile
varies from the ideal as shown in Figure 2. The
beam is said to be contaminated with spatial noise.
Spatial noise is random and varies rapidly over any
distance. An average “wavelength” of noise, dn, is
much smaller than the laser beam size. As a beam
with wavelength λ is focused through an objective
lens with focal length F, the noise appears as a concentric
annulus with radius Fλ/dn, as shown in
Figure 3.

A spatial filter assembly consisting of an objective
lens, pinhole, alignment, and focusing axes can be
used to remove the undesirable noise while transmitting
most of the beam’s energy. For a laser beam
diameter of DL and pinhole diameter of DP, the ratio
of energy passed to total energy is graphed in
Figure 4.