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What Is Reflectance and Transmittance?
Definition of Reflectance and Transmittance
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.
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.
Engineers often use spherical receivers (for example, integrating spheres) for such measurements.
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How are Reflectance and Transmittance Important to an Optical Design?
During an optical design process, accurate simulation results often rely on accurate optical property definitions (in terms of surface and volume). Geometry alone cannot determine the light distribution. Optical properties determine how the energy and direction of ray changes. It is important to know as precisely as possible the optical characteristics of the materials. 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 Bidirectional Scattering Distribution Function (BSDF)
- Total integrated scattering (TIS) affecting light propagation
- Reflectance, Transmittance, Absorbance ratio
How Do You Obtain Reflectance and Transmittance Measurements?
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.
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.
Figure 1. A) Reflectance can be computed out of BRDF with this formula; B) BRDF being the BSDF measured in the reflected space.
Figure 2. BTDF being the BSDF measured in the transmitted space. 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 Keysight Offer?
Solutions to Directly Measure Reflectance:
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.
Measurement of BSDF in Order to Calculate the Reflectance or Transmittance
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.
Mini-Diff V2
Get a portable and affordable instrument for Bidirectional Reflectance Distribution Function (BRDF) and Bidirectional Transmittance Distribution Function (BTDF) measurement with a camera-based sensor and export measurement for simulation software.
- AOI: 0°/20°/40°/60°
- Dynamic range 105
- Angular accuracy: 1°
REFLET 180S
Keysight’s unique stand-alone instrument goniophotometer
- AOI: tunable 0° to 90°
- Dynamic range: 109
- Angular accuracy: 0.1°
Explore Keysight’s Optical Services and Offerings
Keysight’s optical engineering services streamline imaging, illumination, or systems engineering design. You can rely on our photometric laboratory and measurement equipment for BSDF measurements, TIS measurements, and more.
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