Fraunhofer Diffraction

Consider the Fresnel diffraction expression. If Δz is very large, then q( r, Δz ) may be neglected. This yields the Fraunhofer diffraction expression, which is

Or

where phase and amplitude factors are omitted. The far field distribution is just a scaled version of the Fourier transform of the near field distribution. Fraunhofer diffraction is only valid if the Fresnel number is nearly zero.

The ray based diffraction features in OpticStudio, such as the diffraction MTF and PSF assume Fraunhofer diffraction. This is why OpticStudio cannot compute the ray based diffraction MTF or PSF if the beam is too much out of focus. The Fraunhofer assumption is never used by the physical optics propagation algorithm in OpticStudio, it is presented here for completeness.

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• The Pilot Beam