Synopsis
Surface Property - properties not available in TracePro database
Symptoms
The issue of obtaining accurate Surface Property data for use in TracePro can be challenging. While it may be comforting to find a property available in the TracePro database for a "mirror", for example, the actual performance of "real" mirrors can vary widely. Scatter distributions are dependent on polishing processes. There are reflectivity and polarization differences between aluminum vs silver mirror coatings. There are spectral distribution considerations for a gold mirror surface, which reflects more red than blue. Related to the surface finish, suppose the mirror was diamond turned rather than polished, then its scatter distribution might be asymmetric dependent on the direction of the machining grooves. These are just some examples of the differences in surfaces that need to be accounted for in a surface property. This is not to say that some assumptions and guesses cannot be made, provided one understands the potential impact of these assumptions on the analysis results.
Solution
1- MEASUREMENT OF AN ACTUAL SAMPLE
The most accurate (and also most expensive) approach is to have a scatter measurement made for your particular surface finish. A TracePro Surface Property can be created based on the scatter measurement. The TracePro Surface Property Generator (Define/Generate Property data from the TracePro menu) can be used to import scatter measurement files from different scatter measurement equipment/companies and convert them into TracePro Surface Properties.
Lambda Research can measure your sample and build a surface property for you Please contact sales@lambdares.com for this service.
Alternatively, you can contract with other companies that make scatter measurements. Here are the names of a few companies that make Scatter measurements of customer supplied samples:
2- DATA FROM A SPEC SHEET
If a spec sheet is available from the manufacturer, a TracePro Surface Property can be created based on the data available.
3- USE A PROPERTY THAT MIGHT BE A CLOSE MATCH TO THE ACTUAL SURFACE
You can apply a property from the TracePro Surface Properties database with a description and/or data that matches the expected behavior of your actual surface.
4- EDUCATED GUESS
The "low-cost" alternative to a scatter measurement is to take an educated guess at the parameters of the surface property.
There are 5 outcomes when light is incident on a surface (specular transmission, specular reflection, scattered transmission, scattered reflection, absorption). The first step is to take a guess at what percentage of the incident light goes into each of these 5 outcomes. If the surface does not transmit or absorb, the problem gets simpler as some of the possible outcomes can be removed from consideration.
The final part of the educated guess is the angular distribution of the scatter. Many surfaces scatter with an approximately Lambertian distribution, and this is input into the TracePro Surface Property by setting the BRDFg and/or BTDFg coefficient to 0.
Two practical suggestions to assess the Surface Property created by an Educated Guess:
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- The property can be refined over time-based on a comparison of the TracePro Analysis results with lab measurements of the actual hardware.
- It might be of value to model the extreme cases (Perfect Absorber, Perfect Mirror, Perfect Transmitter, Diffuse White. This will give an upper and lower bound to the TracePro analysis results, and suggest the significance of obtaining measured data to model the surface more accurately.