Light distribution by transmission

Figure 2.17: Types of transmission

Transparent materials (such as glass and plastics, see figure a) are used e.g. in prismatic covers for directing light, utilising the prisms’ refraction and total reflection of light. When a ray of light coming from an optically thinner medium (e.g. air) penetrates an optically denser medium (e.g. highly transparent plastic such as polymethyl methacrylate (PMMA)), it changes its direction depending on the angle of incidence and the plastic’s refractive index. The result is a direction of light which influences the luminaire's luminous intensity distribution.

Transparent materials are characterised by high degrees of transmission.

Light scattering materials contain pigments which scatter and expand the ray of light penetrating the material. This entails diffuse transmission as well as diffuse reflection (backscatter) of part of the luminous flux (see figure b). The fraction of the luminous flux transmitted in a scattered way creates a homogeneous brightness impression of the luminaire. The backscattered fraction of the luminous flux is reflected yet again inside of the luminaire and in turn transmits partially. Due to this multiple reflection, losses are greater when compared to complete transmission by transparent material. Opal plastics are examples for materials which scatter due to added pigments.

Translucent materials (see figure c) have higher degrees of transmission compared to (fully) scattering materials such as opal plastics. Number and type of the spherical additives encased in the material are optimised to achieve sufficient diffusivity of transmission and the desired elevated degree of transmission.