Creating a BRDF file and using it as a bottom boundary

It is possible to use PlanarRad to generate a BRDF file that can then be used as input as the bottom boundary condition for another run. One reason to do this is to implement a layered media. Currently PlanarRad only supports vertically homogenous media, but a two layer system (for example) can be modelled by generating BRDF for the lower layer and then using that as the bottom boundary for the upper layer.

The key points are this:

• Generating the BRDF file requires 10 runs, one for each incident theta position of 0, 10, 20, 30, 40, 50, 60, 70, 80 and 87.5 degrees.
• For layers where the refractive index does not change substantially (most likely for layers where the embedding media is the same but constituents change) the runs for generating the BRDF file should use the 'non-existent' interface option 'None'.
• The separate tool 'reflectool' must be run from the command line to generate the BRDF file from the ten runs.
• Then to model the upper layer is a standard run with normal interface settings and uses the the BRDF file for the lower boundary.
• A simple test procedure to verify everything works OK is to do a model with two layers that have same optical properties and compare that to a single run for the depth of the two layers added together, the light field in the top half of the media should be identical.

This page gives a worked example for a model of two layers and demonstrates the verification that the output is correct.