Hi John,

I know a lot less about measurement systems than you, but I see no fundamental reason for any difference. I’m assuming the OS model includes all filters and windows, as these can provide aberration. One place to look is in the definitions of what the software is computing, versus what your experiment is measuring. Anything in the software with a paraxial reference would be suspect IMHO, and even real rays can be misleading as many definitions are based on a single, infinitely thin ray. Experimentally you have to measure a finite cone, there are no infinitesimal lines.

That said, I wonder how well what you measure experimentally matches up to the definition of the RMS spot size/wavefront error. Just make sure you’re comparing equivalent things.

The last thing I would suggest is simplifying the system or measuring as early in the beam as you can, so you can see at what point any difference occurs.

Hope that helps,

• Mark

Real life is so much harder than software