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What Is Total Internal Reflection (TIR) Bidirectional Scattering Distribution (BSDF)?
Definition of Total Internal Reflection (TIR) Bidirectional Scattering Distribution (BSDF)
In an optical design, total internal reflection (TIR) bidirectional scattering distribution function (BSDF), also called internal BSDF, corresponds to a bidirectional scattering distribution function measured at an interface that separates two different mediums. The aim is to measure surface scattering from the inside of a material. It’s especially relevant for materials with a rough surface where surface scattering and total internal reflection phenomena occur together on two separate mediums. See the image for an example of a light guide with VDI texture modeled in LightTools.
Like any BSDF, you must take the orientation of the light into consideration. This illustration shows how you can name each case. Such a measurement can be very useful when the scattering element can’t be considered as a thin diffuser. A light pipe, for example, if combined with grained surface finishes, will have to consider a TIR BSDF measurement.
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Why Is TIR Important to an Optical Design?
Along with classic BSDF, TIR BSDF is an essential tool for measuring angular optical scattering. It’s one of several methods that allow you to know as precisely as possible the optical characteristics of the materials you will be using.
In an optical design, accurate simulation results rely on accurate optical properties. Indeed, geometry alone does not determine light distribution; it’s the optical properties that determine how the energy and direction of the rays change. The best way to obtain precise characteristics is to measure the material directly and export the data to use in an optical software tool.
Accurate measurements benefit the entire design process:
- Optical designers need accurate optical properties for ray tracing simulations.
- R&D engineers need to design the right material with the given optical properties.
- Quality assurance personnel must perfectly control the quality check in the manufacturing process.
What Solution Does Keysight Offer for Measuring TIR?
TIR is one of the most complicated scattering measurements. Keysight provides two solutions for measuring TIR: the Keysight REFLET 180S and the Keysight Mini-Diff V2.
Keysight REFLET 180S
This requires a special measurement setup where the top polished surface has no influence on the BSDF results (Fresnel losses and scattering). The Keysight method uses a hemispherical lens attached to the top polished surface, with a refractive index as close as possible to the sample.
This hemisphere removes refraction of the incoming/exiting light over the surface that is not measured. The Keysight method also uses an index matching liquid to create an optical contact, thus removing Fresnel reflection at the interface.
You can then perform a standard BRDF or BTDF measurement using the Keysight REFLET 180S.
To fully characterize a sample, you’ll need four different measurement configurations.
This figure shows results obtained from the Keysight REFLET 180S for a VDI finishing surface.
Mini-Diff V2
You can also perform TIR measurements using a Keysight Mini-Diff V2 instrument in combination with LightTools and its Microfacet scattering model.
The solution combines the LightTools Microfacet scattering model and one BTDF measurement done with the Mini-Diff V2: the Front BTDF with an angle of incidence of 0° measured at one wavelength. This setup presents the advantage that no refraction will occur over the nonmeasured surface.
After formatting the Mini-Diff V2 measurement results in Excel, you can import the data into LightTools for the Microfacet model.
The Microfacet scattering model constructs a virtual surface finish with small facets. The slope of those facets is set so that the exiting rays match with one distribution when a beam hits the surface from one angle of incidence. For any other angles of incidence, the model calculates distribution according to the microfacet geometry. You can then use this geometry for all other simulations.
The following comparison chart validates this method, with simulation results tracking very closely to equivalent measurement results.
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