Luminaires for lighting purposes are subdivided according to several criteria.
Depending on the type of optical systems, luminaires are subdivided into
diffuser luminaires, or
luminaires with flat covers.
A further distinction is made according to the shape of the luminous intensity distribution caused by the optical system, meaning
upward-distribution (indirect distribution);
downward-upward-distribution (direct-indirect-distribution), and
diagonal-distribution (asymmetric-distribution) luminaires.
Optical systems are required to direct the light sources’ luminous flux in a way that facilitates an ideal accomplishment of the luminaire’s lighting task. They consist of photometrically/optically effective materials.
Fundamentally, there is a distinction between transmissive and non-transmissive materials with reflective surfaces. Transmissive materials include highly transparent, refractive materials whose surfaces can also be totally reflective.
Depending on surface roughness and inner material structure, there is a distinction between specular, mixed and scattered reflection or transmission.
According to the type of light direction, the materials are characterised by reflectance and degree of transmission. These material properties are defined as the ratio of reflected luminous flux (or transmitted luminous flux for certain material thicknesses) and the luminous flux incident on the surface perpendicularly.