To see this integration, open the sample file HPSF_Integration, and then choose Analysis > Detectors > Ray Trace / Detector Control. Select “Auto Update” on the Detector Control. Now, click on “Trace”. Because the source defines only 1 analysis ray, a single random ray is traced and the detector is updated. Click on “Trace” again, without pressing “Clear Detectors” first, to trace a second ray. The two rays now traced will coherently interfere like two plane waves making an angle with respect to each other, resulting in a fringe pattern on the detector. Because the rays are random, the fringe pattern will be different every time, and so will not look exactly like the picture below.

Each time “Trace” is pressed, another ray is added to the sum. After 10 rays have been traced the diffraction PSF begins to emerge.

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 After about 40 rays, features such as the Airy ring can be seen forming.

It takes a few hundred rays before the PSF reasonably converges to the final result.