Glossary of Optics Words

This glossary is written simply, with as little confusing jargon as possible, to aid the layman's understanding. If there are more words you need explained here, please drop us a note and we'll add them!

Abbe Number

Typically pronounced "abbey" (like where monks live). Also called "V-number".

​This value describes how differently a transparent material will bend different colors of light that pass through it, aka the amount of dispersion. High Abbe numbers represent materials that have low dispersion. Lower Abbe numbers represent higher dispersion, or materials that bend red light a lot differently than than they bend blue light (and all the colors in between).

So, if you want to make a prism that separates white light into the many colors it's made of, you're better off creating that prism with a low Abbe number material. Usually, though, high dispersion is not a desirable characteristic when designing an optical system - especially in imaging optics. If Abbe number is an important factor in your system, usually you'd be looking for materials with a high Abbe number, or low dispersion.

BRDF

BRDF stands for "Bidirectional Reflectance Distribution Function". It is a mathematical description of a surface finish, or rather, how light reflects off a surface.
If a surface is pretty evenly rough, you can usually get a BRDF measurement of it and create an optical model from that. Every optical simulation software has its own method of importing and formatting these models and usually, you'll have to have a custom model made for each optical software being used and for each material. Once you have a working model in your optical software, you can simulate the surface texture in your simulations, which is often super important.

CCT - Correlated Color Temperature

This is a description of light color in units of Kelvin. Lower Kelvin means yellower or more red/orangey color. Higher Kelvin means bluer color. See longer article on CCT HERE.

Diffuse

This term can refer to the surface finish of a material. If it's not specular (see below), it's probably diffuse. Diffuse means a rougher surface than, say, a mirror and a rough surface means that light coming it to hit that surface will encounter lots of different incident angles. So then, when it bounces off the surface, it will be leaving at all sorts of angles.

If you think about a small, red laser beam hitting a specular mirror surface, that beam is still going to be a small dot of light when it leaves. On the other hand, if your small, red laser beam hits a diffuse surface like sand paper, that small red dot is going to spread out into a bigger blob. The light rays going onto the sandpaper would be closely similar, and the light rays leaving the sandpaper would be shooting in all sorts of directions.

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