Colour
How we perceive the color of an object or material depends upon the wavelength content of the light reflected off it as well as the physiological structure of our eyes and the nature of sensory processing in the visual cortex of our brains. The human visual system is quite an imprecise measurement tool, however it is an enormously forgiving and flexible one that can adapt to a huge range of light conditions and automatically compensate for all sorts of environmental phenomenon.
Light Absorptance
Light absorptance is a measure of a material's capacity to absorb light that is incident upon it or passing through it. In opaque materials, light absorptance is very high and directly related to surface color, with black the most absorbant and white the least. In transparent materials, light absorbance is very low and affects the transmitted component only as it passes through.
Light Behaviour
Understanding the behaviour of light within materials and spaces is a key skill for any competent architect or building designer. Without this knowledge, a huge amount of the amenity available from natural light and solar control is at risk of being under-utilised or worse, seriously compromised. The concepts themselves, though many, are relatively straightforward. However, once you understand them, you will very likely use them everyday in your design work and specification writing.
Light Reflectance
Reflectance is a property that describes how well incident light and other electromagnetic radiation is reflected from the boundary between two disparate materials. Unless otherwise specified, any reflectance value given for a material assumes an air/material boundary and, in this context, reflected simply means bounced back somewhere on the same side as the incident light as opposed to being transmitted through the boundary.
Light Refraction
Refraction is the bending of light waves that happens when light travels at anything other than normal incidence between two materials with different refractive indexes. The refractive index of a material is basically the ratio of the speed that light travels in a vacuum to the speed that light travels within that material. Refraction effects occur all the time around us and explain how items encased in glass or underwater appear in slightly different positions and also why we find diamonds so attractive.
Light Transmittance
Light transmittance is a property of transparent and translucent materials, measuring their capacity to transmit light all the way through themselves. Opaque materials obviously have a transmittance of zero as no light at all gets through.
Reflectivity and Reflectance
Many opaque materials have the same values for reflectance and reflectivity, so people tend to use the terms interchangeably. However, there are many other materials for which the value can be very different so it is important to properly understand what reflectance and reflectivity really mean.
Refractive Index
The refractive index of a material is basically the ratio of the speed of light in a vacuum to the apparent speed of light within that material. Whilst opaque materials can have a refractive index, it is of minimal interest as light energy transmitted into such materials is quickly absorbed before it travels very far. However, the refractive index of transparent materials is of great interest as it governs both the refraction and reflection that occurs when light passes into or out of a material and is fundamental to the design of lenses and other optical systems.
Specularity
Specularity is a measure of the angular distribution of reflected light. Materials with a low specularity, usually termed matte, reflect light diffusely in all directions. Materials with a high specularity, usually termed gloss or shiny, reflect light like a mirror where the angle of reflection equals the angle of incidence reflected about the surface normal.