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Table of Contents

Definition

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.

This concerns material with a rough surface where surface scattering and the total internal reflection phenomena occur together on two separate mediums.

Light Guide with VDI texture example modeled in LightTools | Synopsys

Like any BSDF, the orientation of the light must be taken into consideration. The illustration below shows how each case can be named.

Considering the orientation of light - examples | Synopsys

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.

Why is TIR important to an optical design?

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. For this reason, it’s important to know as precisely as possible the optical characteristics of the materials that will be used. The best way to obtain precise characteristics is to measure the material directly and export the data to use in an optical software tool.

Requirements:

  • Optical designers need accurate optical properties for ray tracing simulations.
  • R&D engineers need to design the right material with given optical properties.
  • The quality check in the manufacturing process must be perfectly controlled.

Solutions:

  • Angular optical scattering with BSDF (classic BSDF or TIR BSDF)
  • Amount of light propagation TIS (Reflectance, Transmittance, Absorbance ratio)

What solution does Synopsys offer for measuring TIR?

TIR is one of the most complicated scattering measurements that can be done, and Synopsys provides two solutions for measuring TIR: the Synopsys REFLET 180S and the Synopsys Mini-Diff V2.

Synopsys REFLET 180S

This requires a special measurement setup where the top polished surface has no influence on the BSDF results (Fresnel losses and scattering). Our method uses a hemispherical lens attached to the top polished surface, with a refractive index as close as possible to the sample to be measured. This hemisphere removes refraction of the incoming/exiting light over the surface that is not measured.  An index matching liquid is also used in order to create an optical contact, thus removing Fresnel reflection at the interface.

Measuring BSDF with Synopsys REFLET 180S | Synopsys

A standard BRDF or BTDF measurement is then performed using the Synopsys REFLET 180S.

Synopsys REFLET 180S | Synopsys

To fully characterize a sample, four different measurement configurations are needed. 

Four measurement configurations for a sample in the Synopsys REFLET 180S | Synopsys

The following figures show results obtained from the Synopsys REFLET 180S for a VDI finishing surface.

The following figures show results obtained from the Synopsys REFLET 180S for a VDI finishing surface.

Mini-Diff V2

TIR measurements can also be done using a Synopsys 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, which is 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 non-measured surface.

TIR measurements can also be done using a Synopsys Mini-Diff V2 instrument in combination with LightTools and its Microfacet scattering model.

After formatting the Mini-Diff V2 measurement results in Excel, the data can be imported 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 distribution is calculated according to the microfacet geometry. Therefore, the user can then use this geometry for all other simulations.

Rough Surface Modeled with Small, Oriented Facets | Synopsys

The following comparison chart validates this method, with simulation results tracking very closely to equivalent measurement results. 

Comparing simulated and measured results | Synopsys

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