For the reaction to take place, several elements must be present, namely:
- Visible light with wavelengths in the range of 350 - 500 nm.
- Riboflavin (vitamin b2), which comes from yeast during fermentation
- Isohumulones, which are hop alpha acids isomerized during the boil
- Sulfur from cystene or methinonine, amino acids that come to beer from protein in malt
With all of these elements present, light reacts with riboflavin initiating reactions that break bonds in the isohumulones and allow combination with sulfur from the amino acid. In the end, beer contains methylbut-2-ene-1-thiol, sometimes referred to as MBT. This thiol-containing compound (also known as a mercaptan) gives beer the skunky light-struck aroma.
The reaction occurs rapidly and pale beer acquires a noticeable skunky character in a few minutes when exposed to the right light. Sunlight and flourescent light both contain high levels of the bad wavelengths. Regular tungsten filament or incandescent lights don't have enough energy in this range to worry about.
Note, however, that I specified pale beer. A clear, straw-colored beer lets light pass right through it, exposing every ounce to its various wavelengths. But dark beers are different. Have you ever smelled a skunky stout? I haven't - and many contain plenty of isohumulones. Their dark color prevents light from penetrating into the beers and exposing the IBUs to destruction.