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Definition
A diffusive material can be characterized by multiple quantities: reflectance, transmittance, absorption, TIS measurements, bidirectional scattering distribution function (BSDF), and haze.
Haze is measured as the percentage of incident light scattered by more than 2.5 degrees from the specular from a surface or optical element. It can be measured in transmission and/or in reflection.
Haze is expressed as a percentage (%), with no angular information; in other words, we do not know where the flux is going, but we know the quantity. The higher the percentage, the more blurred the image will be.
Why is haze important to an optical design?
In optical design, to obtain accurate simulation results, you must have accurately defined optical surface and bulk properties. Geometry alone cannot determine the light distribution, because it’s the optical material and surface 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. Haze is one of the typical optical material performances to measure.
What solution does Synopsys offer?
Haze measurements are particularly difficult to perform because you need to separate the specularly reflected and transmitted incident light from the scattered light. These radiation patterns normally overlap and cannot be easily separated by the geometry of the measurement equipment.
Synopsys offers measurement services in a light- and temperature-controlled laboratory and provides two methods for obtaining measurements for haze: measuring with an integrating sphere and calculating from BSDF measurements obtained by a goniometer.
Integrating Sphere
The haze of a material sample or surface can be measured using specially designed integrating spheres. In practice, the sample is placed on the exit and then entrance port of an integrated sphere and illuminated by a laser source to measure the reflectance (specular + scattered reflectance) and the transmittance (specular + scattered transmittance power), respectively. The same measurement is performed again after opening a port that permits the specular beam out of the integrating sphere so that it is not included in the measurements of the diffuse transmittance power and the diffuse reflectance power.
Calculated from Synopsys REFLET 180S BSDF Measurements
Haze can also be calculated from BSDF measurements, as shown in the following formulas.
- θd is the scattering angle of the detector
- φd is the azimuthal angle of the detector
- θi is the scattering angle of the source
- φi is the azimuthal angle of the source
- θd Specular max and θd Specular min are the limits of the specular part of the BSDF
BSDF measurements are performed with the Synopsys REFLET 180S goniometer. The accuracy depends on the resolution of the measurements, and the different angular resolutions of these instruments allow you to extract the specular part from BSDF measurements with varying degrees of accuracy.
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