Conceptually, the Huygens PSF is computed by converting each ray on the Spot Diagram into a small plane wave. Recall that a ray models a small piece of a plane wave, and that the ray locally is normal to the wavefront in isotropic media. The plane wave has an amplitude, phase, and direction determined from data associated with the ray from which it is generated. The total irradiance at any point on the image surface may be determined by coherently summing all of the plane waves represented by all of the rays traced. The diffraction based PSF is given directly by this integration over all the rays.

Virtually all imaging systems meet the simplyifying assumptions necessary for computing the Huygens PSF. The assumptions are:

• The F/# is large enough so that scalar diffraction theory applies
• The sampling is set high enough to accurately model the PSF

The Huygens PSF is not based upon the FFT. The net result is that the Huygens PSF is generally slower than the FFT PSF, but more accurate for those cases where the FFT PSF assumptions do not apply. The most common cases where the FFT PSF assumptions are questionable, and thus the Huygens PSF should be used are:

• The image surface is significantly tilted with respect to the chief ray
• The exit pupil is significantly distorted with respect to the entrance pupil

The Huygens PSF of an optical system is computed as follows. A grid of rays is traced from the source point to the image surface. For each ray, the amplitude, coordinates, direction cosines, and optical path difference is used to compute the complex amplitude of the plane wave incident at every point on the image space grid. A coherent sum for all rays is performed at every point in the image space grid. The intensity of each point in the image space grid is the square of the resulting complex amplitude sum. If the computation is polychromatic, the PSF's are summed incoherently.

To compute a Huygens PSF for a sequential system, choose Analysis > PSF > Huygens PSF from the main ZEMAX menu. The Huygens PSF may also be computed for Non-Sequential Component (NSC) systems and this will be discussed shortly. Note the FFT PSF cannot be computed for NSC systems.

The key user definable parameters for the Huygens PSF are the Pupil Sampling, Image Sampling, and Image Delta. These parameters can be set on the Huygens PSF settings dialog: Open the settings box and change the default settings as shown:

The Image Delta is the image point spacing in micrometers. The total size of the region where the PSF is computed is the product of the Image Delta and the Image Sampling.
Here is the Huygens PSF for the same Newtonian telescope on axis:

And off axis (change the Field to 2):



The larger the number of rays and image points, the greater the resolution and accuracy of the resulting PSF, at the expense of a longer computation time.