Question: ZEMAX is well known for modeling visible, infrared, and ultraviolet applications, but is it accurate when using radio, x-ray, or other "non-optical" wavelengths?

As long as components are larger than say 10*wavelengths, and accurate dispersion data is available, ZEMAX can be used to model propagation at any part of the electromagnetic spectrum.

ZEMAX is an optical design program that uses ray tracing to calculate various data of interest to the user. For more information on rays, see this article: What is a ray? The accuracy of using rays to simulate the interaction of electro-magnetic (EM) radiation with optical components is based upon a few assumptions. As long as these assumptions are not violated, the exact wavelength used for the simulation is arbitrary.

1) As stated in the article above, a ray represents some finite portion of a wavefront. A ray itself is infinitely thin; it has no associated area. This ray approximation is valid when the transverse dimensions of the optical elements through which the ray is propagating are at least several times the wavelength of the ray. When the element size begins to approach the wavelength, the wave nature of light begins to dominate and rays will not accurately predict system behavior. In this domain, optical components act as waveguides rather than lenses.

2) Any simulation requires accurate material data to determine how EM radiation interacts with a given medium. ZEMAX includes data for many materials, mainly glasses and plastics that transmit in the visible, infrared and ultraviolet regions of the EM spectrum. All of these materials in the glass catalog are defined with a minimum and maximum wavelength. These represent the spectrum of wavelengths over which the data are considered accurate. When modeling any system, care must be taken to ensure the accuracy of dispersion and thermal data. This is especially true when working with wavelengths far from the visible region. The same consideration should be applied to materials used in coatings.

In summary, as long as components are larger than say 10*wavelengths, and accurate dispersion data is available, ZEMAX can be used to model propagation at any part of the electromagnetic spectrum.