Coral fluorescence has been observed and documented for several years now. However only in the last few years have we seen significant documentation and exploration. Coral fluorescence broke onto the scene in 2004 when Dr. Charles Mazel presented his stunning photographs and discoveries with the world at IMAC. Following that presentation the number of people experimenting with coral fluorescence boomed. And here just a few years later we have dozens of hobbyists around the world documenting and testing coral fluorescence.
Fluorescence is transfer of energy that we can see. It is a phenomenon where a high energy photon (such as blue light) is transformed into a low energy photon (such as red light). In other words when an item fluoresces it is giving off a form of light, but it only happens when receiving a certain type of light. This is a major difference between fluorescence and something like luminescence. To learn more about the basics of fluorescence please read http://www.advancedaquarist.com/2005/2/lines and http://www.advancedaquarist.com/2007/11/lines. Thankfully by using special filters on the detector we can remove the light that is producing the fluorescence (excitation energy). What remains to be seen is the fluorescent energy which often times produces the vibrant colors we see in corals.
A coral shown in natural light, and with fluorescent photography
A future (and potentially current) area of interest is in fluorescence that falls below the visible light spectrum. The purpose or reason behind coral fluorescence is not known… at all. In fact I can think of a half dozen great hypothesis for why a coral may fluoresce. Unfortunately none of those reasons hold true for all fluorescent corals. A highly considered reason for fluorescence is in communication and sight. Fluorescence could very well be a mechanism to make yourself noticeable to producing a color of light different from the surrounding light. For this reason knowing if the emission light (the light given off by the coral) is visible can be of great importance. But what exactly is visible light? It all depends on the eye (detector) you are using. For this reason monitoring fluorescence in the infrared range is of great importance.
Documented here are the first attempts I know of to record fluorescence in the infrared spectrum for common reef corals. The Aquatic & Terrestrial Research Team photographed corals that were known to fluoresce, as well as those that were not believe to fluoresce. While thousands of corals have yet to be tested, it is interesting to see that after dozens were tested not one single coral was observed to produce an emission light beyond the visible light spectrum. If this is found to be a common occurrence then we can modify hypotheses. Fluorescence itself as a phenomenon would not be related to visible light, and emission energy could fall along a very long range of electromagnetic radiation. However, this isn't what has been seen. If the fluorescence is in fact far more common in the visible light range it could support beliefs that the fluorescence is being seen by something, and is therefore not just a random (yet very very cool) phenomenon.