The Reflectance describes how much light is reflected from a surface or optical element. It is equal to the ratio of reflected power and incident power when light is shot onto a surface.

The Transmittance describes how much light is transmitted from a surface or optical element. It is equal to the ratio of transmitted power and incident power when light is shot onto a surface.

Spherical receivers (e.g. Integrating spheres) are often used for such measurements.

Reflectance vs. Transmittance | Synopsys

How are reflectance and transmittance important to an optical design?

During an optical design process, accurate simulation results often rely upon accurate optical property definitions (in terms of surface and volume). Geometry alone often cannot determine the light distribution. Optical properties determines how the energy and direction of ray changes. 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 directly measure how light interacts with the material and export the results to an optical software.

Having accurate measurements affects: 

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

Optical measurement solutions: 

  • Angular optical scattering with Bidirectionnal Scattering Distribution Function (BSDF)
  • Amount of light propagation TIS 
  • Reflectance, Transmittance, Absorbance ratio

How do you obtain reflectance and transmittance measurements?

1. Direct measurement of the Reflectance and Transmittance

The Reflectance (Transmittance) measurement consists in the integration of all the reflected (transmitted) signal (scattering + specular) into a hemisphere: the reflected (transmitted) light from the sample is integrated into a reflective hemisphere then normalized by the total reflected (transmitted) power of the incident beam. 

In practice, for Reflectance the sample is placed on the exit port of an integrating sphere and lighted up by a laser source. Then, the reflected intensity by a Lambertian Spectralon is measured. Knowing the calibrated reference reflectance, reflectance of the sample is calculated.

For Transmittance measurement the sample is placed at the entrance port of an integrating sphere. Then, the transmitted intensity of the incident light without anything at the entrance of the sphere is measured. From this, the transmittance of the sample is calculated. 

Direct measurement of the Reflectance and Transmittance | Synopsys

2. Reflectance and Transmittance from BSDF

Reflectance and Transmittance can be calculated directly from BSDF measurements. 

Indeed, it exists a direct correlation between BSDF and Reflectance or Transmittance.

Reflectance can be computed out of BRDF with following formula: 

Reflectance can be computed out of BRDF with following formula | Synopsys

BRDF being the BSDF measured in the reflected space. 

BRDF being the BSDF measured in the reflected space | Synopsys

BTDF being the BSDF measured in the transmitted space. 

Reflectance vs. Transmittance | Synopsys

With θd and φd respectively the scattering angle and the azimuthal angle of the detector.

θi and φi respectively the scattering angle and the azimuthal angle of the source.

The Reflectance or Transmittance computation accuracy relies on the BSDF accuracy and BSDF sampling.

The higher the resolution, the more accurate the Reflectance or Transmittance evaluation.

What solutions does Synopsys offer?

1. To directly measure reflectance

TIS Pro is a standalone optical scattering instrument that measures the total reflectance, transmittance, and absorption. ​

This portable device features an integrated sphere and spectral detector assembled in a housing that controls stray light to ensure accurate measurement results. ​

It provides measurements over the entire visible spectrum at various angles of incidence.

Synopsys TIS Pro | Synopsys

Measurement services: We offer reflectance and transmittance measurements at different angles of incidence and with a wide range of wavelengths in our measurement lab. We also offer spectral reflectance measurements. The setup is the same as the one presented in the previous section.

2. Measurement of BSDF in order to calculate the reflectance or transmittance

Synopsys offers high-end scattering measurement instruments for your lab:

  • Synopsys Mini-Diff V2: Portable and affordable instrument for BRDF and BTDF measurement. Camera-based sensor and export measurement for simulation software. (AOI: 0°/20°/40°/60°; Dynamic range 105; Angular accuracy: 1°).
Synopsys Mini-Diff V2 | Synopsys

  • Synopsys Mini-Diff VPro: Laboratory version of the Mini-Diff V2 with a dark box included and tunable AOI from 0° to 60°. (AOI: tunable from 0° to 60°; Dynamic range: 106; Angular accuracy: 0.5°)
Synopsys Mini-Diff VPro

  • Synopsys REFLET 180S: Unique stand-alone instrument goniophotometer (AOI: tunable 0° to 90°; Dynamic range: 109; Angular accuracy: 0.1°).
Synopsys REFLET 180S | Synopsys

All these devices are able to measure BSDF in order to extract/calculate reflectance or transmittance from these measurements. The accuracy depends on the resolution of the measurements.

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