# How does an optical spatial filter work?

How does an optical spatial filter work?
You can still ask an expert for help

• Questions are typically answered in as fast as 30 minutes

Solve your problem for the price of one coffee

• Math expert for every subject
• Pay only if we can solve it

Bubbinis4
The principle behind the use of a pinhole in the way that you describe is simply to create a subresolvable source of light, thus destroying all wavefront information. You are correct that the exact diffraction pattern from a subresolvable point source is not Gaussian, but it is very nearly so and indeed Gaussian modes are the eigenfunctions of an approximation to the wave equation that holds for paraxial fields, i.e. fields of very small numerical aperture and with wave vectors all directed to within about 0.1 radians of the nominal beam propagation direction.
So in practice the beam will become more Gaussian after passage through a subresolvable pinhole, i.e. one of diameter that is smaller than approximately λ/NA, where NA is the beam's numerical aperture.