It is important to note that the image must be sufficiently sampled to generate accurate partial coherence results. To verify that the sampling is sufficient, you can look at the PSF that is being used in the partially coherent calculations. Change the “Data Type” to Partially Coherent Test:  PSF. Zoom-in on the non-zero portion of the PSF:

DIA window showing partially coherent PSF

Adequate sampling is achieved when there are at least 10 points displayed over the non-zero portion of the PSF, which is the case here. If the sampling is not adequate, try increasing the “Oversampling”.

 

In general, if the sampling is not sufficient, ZEMAX will issue a “Sampling Too Low, Data Inaccurate!” message when you try to generate the PSF or partially coherent image:

DIA window showing "Sampling Too Low, Data Inaccurate!" error