mmw Optical Design

I have been using Zemax to evaluate various mmw optical designs presuming
that millimeter wave light behaves in a similar fashion to optical (or at
least quasi-optical) wavelengths.  Does anyone know if this a reasonable
assumption?

Is there an optical tool that provides a more accurate method of predicting
the behavior of millimeter and sub-millimeter light when used with optical
lenses?

I have used Zemax and added the proper refractive indices for the mmw
optical materials into a new materials library.   I read of one tool called
"MODTool" that was developed at JPL for mmw optical design.  Is anyone
familiar with it?  It may only be for reflective surfaces, not for
refractive.  I am interested in both.

The empirical data I have collected seems to be in reasonable agreement (to
a first order) with the Zemax predictions, but because the wavelengths are
so large (relatively speaking), I cannot confirm exact agreement.

Any assistance would be appreciated.

Thanks,

 - Russ in Santa Barbara