What is Reflectance and Transmittance?

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

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. 

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.

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.