# Fluorescence

Warning! Some information on this page is older than 5 years now. I keep it for reference, but it probably doesn't reflect my current knowledge and beliefs.

Mon
05
May 2014

The main and general formula in computer graphics is Rendering Equation. It can be simplified to say that perceived color on an opaque surface is: LightColor * MaterialColor. The variables are are (R, G, B) vectors and (*) is per-component multiplication. According to this formula:

• When the light is black (there is no light), the surface remains black, because: (0, 0, 0) * MaterialColor = (0, 0, 0).
• Similarly, when the material is black, it stays black regardless of the light color.
• When light is white, you can see original color of the material, because (1, 1, 1) * MaterialColor = MaterialColor.
• Similarly, when the material is white, you can see the original light color on the surface.
• Green material lit with red light looks black, because: (1, 0, 0) * (0, 1, 0) = (0, 0, 0).

There are many phenomena that go beyond this model. One of them is subsurface scattering (SSS), where light penetrates object and goes out from different place on the surface. Another one is fluorescence - a property of a material which absorbs some light wavelength and emits different wavelength in return. One particularly interesting kind of it is UV-activity - when material absorbs UV light (also called blacklight, which is invisible to people) and emits some visible color. This way an object, when lit with UV light, looks like it's glowing in the dark, despite it has no LED-s or power source.

I've never seen a need to simulate fluorescence in computer graphics, but in real life it is used e.g. in decorations for psytrance parties, like this installation on main stage on Tree of Life 2012 festival in Turkey:

So what types of materials are fluorescent? It's not so simple that you can take any vividly colored object and it will glow in the  UV. Some of them do, some don't. You can take a very colourful T-shirt and it may be not visible under UV at all. On the other hand, some substances glow while they would better not (like dandruff :) But there are some materials that are specially designed and sold to be fluorescent, like the Fluor series of Montana MNT 94 paints I used to paint my origami decorations.