Characteristics of materials

When light hits material, such as a window pane, there are three different phenomena that come into play. Some of the light is reflected, some is absorbed by the material and the rest is transmitted. The resultant luminous flux components are known as F_r (reflected luminous flux), F_a (absorbed luminous flux) and F_t (transmitted luminous flux).

The quantitative relationship between these parameters is as follows: degree of reflectancer= Fr / F degree of absorptiona= Fa / F degree of transmittancet= Ft / F(where r + a + t = 1 and Fr + Fa + Ft = F) The luminous flux absorbed by material is converted into heat, which increases its temperature. The darker the material, the more luminous flux is absorbed. A single glass sheet with a thickness of 4 mm, for example, reflects 8% of the luminous flux that falls on it, 90% is transmitted and 2% is absorbed. The diagram below shows the extreme forms, namely fully directed and fully dispersed reflectance and transmittance.

Reflectance and transmittance