Hi, everyone,

I have a problem trying to understand the FFT PSF. The user manual and related knowledge base articles say that the FFT PSF algorithm exploits the fact that the diffraction PSF is related to the Fourier transform of the wavefront complex amplitude at the exit pupil of the optical system.

Here's how it works. First, the amplitude and phase of the ray grid are calculated at the exit pupil, then the FFT is performed, and then the intensity of the diffraction image is calculated.

I am well aware of the principle of FFT PSF, but my question now is: How does Zemax calculate the complex amplitude distribution at the exit pupil? As for the phase, the OPTH operand can be used to obtain the phase of each ray falling on the exit pupil, and then the phase distribution of the exit pupil plane can be obtained by smooth fitting. But as for amplitudes, how does Zemax get them? My current idea is to divide the exit pupil into small grids and calculate the amplitude by counting the number of rays falling in each grid. I don't know if that's correct?

Thanks!