Hi Sasha,

there is no standard approach available for this evaluation, but several options to get the efficiency and peak intensity. One of them is as you have mentioned, using the GIA (and its IMAE operand).

The process is launching many rays into the entrance pupil and computing the fraction of rays that pass through all surface apertures to any surface. The percent efficiency is defined by:

where the sum i is over all rays which were unvignetted, and the sum over j is over all rays which were launched. The efficiency calculation considers vignetting, source distribution, wavelength weights, and reflection and transmission losses in the optical system if the "Use Polarization" checkbox is selected. If "spot diagram" is selected for the "show" option, then the percent efficiency includes all unvignetted rays. The other display options vignette rays that are beyond the extent of the detector size. Therefore, the percent efficiency will be different between spot diagram displays and other displays if rays fall outside the region of the detector.

Please find here some further information:

OS tips & discussion – Operand to report spot size in X or Y direction separately | Zemax Community

Optimizing with the IMAE Operand

How to model multi-mode fiber coupling – Knowledgebase

You may also want to consider analyzing the Vignetting Plot for calculating fractional vignetting as a function of field angle or the Footprint Diagram to get the % number of rays going through the system.

Also for evaluating peak intensity you can move forward as you have suggested. The Huygens PSF computes the diffraction PSF using direct integration of Huygens wavelets method. The Strehl ratio is also computed, defined as the peak intensity of the diffraction point spread function (PSF) divided by the peak intensity of the diffraction point spread function (PSF) in the absence of aberrations.

Huygens PSF

For spot size, you also have the possibility to check the Spot Diagram.

Best Regards,

Sabrina