How do we get the different colors of light?

[dafunkyman]

In the Lighting section of how to set up a tank my understanding of what WaterKeeper says is that photons emitted by the mercury vapor strike phosphors that coat the glass tube and these phosphors then glow the light that we see. Does that sound right? How do the coatings differ to change the light?

[atreis]

My understanding is that the color/temperature of the light depends on the excitation energy for the materials used to coat the tubes, or added to the blob of metal in MH bulbs. (Excitation energy is the energy needed to force an electron into a higher orbit - when that electron falls back to its normal orbit it releases a photon.) To get the different colors, the manufacturers add impurities to the coatings, usually very small amounts of rare earths with different excitation energies. This is also why less expensive bulbs don't tend to work as well - they're made less expensive by incorporating less of the expensive rare earth materials needed to get the color levels we want.

BTW - I really don't know how they get different color temperatures from LEDs - I'm less familiar with the technology. At heart, it would have to be similar - incorporating materials with different excitation energies, but I think is also sometimes done using filters instead - this is done for residential "warm white" LED bulbs. I don't know if that's true for reef LEDs though.