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Diffuse reflection and transmission, the cosine law, and inaccessibility

When the light encounters a surface whose unevenness is about the same as or slightly greater than the wavelength of the light, there is no longer a single reflected or refracted light, but the energy of the light is dispersed in all directions from the point of incidence, just as in the case of scattering. The light that returns to the medium in which the incident light is located is called diffuse reflected light, and the light that is transmitted into the second medium is called diffuse transmitted light. In general, the exact angular distribution of reflected and transmitted light depends on the angle of incidence of the light on the surface and the nature of the surface roughness. For surfaces with very fine particles, when the angle of incidence is close to 90°, the reflection is almost mirror reflection.

For the convenience of calculation, the concept of uniform diffuser is often used. The so-called uniform diffuser means that its reflected light distribution is independent of the angle of incidence of the light, and in the direction of the angle θ with the surface normal, the intensity of the reflected light is proportional to cosθ. This cosine law also applies to uniform diffuse transmission. However, no actual surface can fully meet the conditions of a uniform diffuser, but some surfaces, such as the surface of magnesium oxide powder coating, are very close to uniform diffusers.

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