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.
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.
