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Bidirectional Scattering Distribution Function (BSDF)

Every material surface is reflecting incident light. This effect may vary considerably and such reflections are therefore considered as disturbing or not to different degrees. Especially in the case of solar installations, this topic is currently often the subject of debates.

To have comparable figures available for such discussions, SPF operates a measuring facility for determining the optical scattering behaviour of any material surface.

For this purpose the so-called Bidirectional Scattering Distribution Function (BSDF) is determined. This provides a three-dimensional image of the reflection effect at a selectable, defined angle of incidence on the surface. Three different glass samples are shown below, which are exposed to the same solar radiation under identical conditions. Due to their different surface structure, the reflection is also clearly different.

Foto BSDF Reflections depending on the surface structure

The glass on the left is prismatic, it broadly diffuses the light, but the intensity of the reflection is very weak at any viewing angle.
The glass on the right is clear float glass, it hardly scatters at all, acting like a mirror. The reflection is very strong, but only at a narrow angle.
The glass in the middle has a matt surface and shows a moderately strong reflection with moderate scattering.

The BSDF measurements on these glasses give the following results:
(upper row: in polar coordinates, lower row: three-dimensional; angle of incidence 30° for each sample)

A) prismatic glass, broad scattering, peak = 18'000 Cd/m²
B) matt glass, medium scattering, peak = 280'000 Cd/m²
C) clear glass, virtually no scattering, peak = 950'000 Cd/m²
Figure BSDF measurements

The term Bidirectional Scattering Distribution Function (BSDF) stands for scattering in general. The measurement differentiates between reflection effects (Bidirectional Reflectance Distribution Function, BRDF) and transmission effects (Bidirectional Transmittance Distribution Function, BTDF). We can offer both at SPF, for angles of incidence in the range of 10 to 70°.

Samples ranging in size from 100 x 100 mm to approx. 2500 x 2000 mm can be measured. For BRDF measurements on transparent samples, a light trap can be used as an option. This eliminates the reflection at the second layer, quasi the «inner» material surface. This corresponds to a glass pane, for example, which is laminated together with PV cells using EVA to form a module.

Figure BSDF report specimen

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