Transmission: light passes through a medium

In physics, transmission refers to the passage of rays (light) through a medium. The measure for permeability is transmittance: this is the ratio of the transmitted luminous flux to the incident luminous flux. Similar to reflection, transmission can also be either directional or diffuse. Translucent materials are used in lighting technology to direct light, but also to model the spectrum.

How does transmission work?

In transmission, the light falling on a body passes through depending on the transmittance au of this body. The degree of scattering of the transmitted light plays a role. With perfectly transparent materials such as clear glass there is no scattering whatsoever. The light continues to penetrate the material in a directed manner. As the scattering power increases, the directional component of the transmitted light becomes increasingly smaller until, in the case of complete scattering, only diffuse light is emitted. When transmitted through frosted glass, directional light becomes diffuse. The light scatters many times due to multiple reflection and a uniform distribution of brightness is achieved.

Transmitting materials in luminaires can be transparent. This applies to simple cover glasses as well as to filters which absorb specific spectral ranges but transmit the others, thus creating coloured light or lowering the UV or infrared component. Occasionally, diffusing materials – for example opal glass or opal plastics – are also used as luminaire covers to avoid the effects of glare by lowering the lamp luminance.