The Huygens calculator feature of LensForge can plot line-spread function (LSF) or optical transfer function (OTF) as calculated using the Huygens-Fresnel wavelet theory.

The goal of the Huygens-Fresnel wavelet theory
is to calculate the spot intensity distribution
*I(x,y)* on the image surface, taking into account the
effect of diffraction. The method is to trace a set
of rays to the image surface using geometrical optics. The
centroid of these rays locates the patch on the surface where we
will calculate the intensity distribution.
For each ray we set up
a spherical wavelet source at the point
where the ray intersects the exit pupil.
The phase of each wavelet source is determined by the optical path length,
along the ray, from the object to the wavelet source.
Then, for each point of interest on the image surface, we
calculate the wavelet amplitudes at that point.
The intensity distribution
*I(x,y)* is given by squared magnitude of the summed
amplitudes.

The Huygens calculator window shows a plot of LSF or MTF, and a thumbnail (small image) of the PSF. The thumbnail is useful for troubleshooting your settings. A separate Huygens PSF window is available for a closer look at the PSF image.