Photo © 2013 S. Beyer


On this page we explain the science behind underwater fluorescence in general as well as the science behind our products in particular.


The photo at the top right shows a fluorescent brain coral.


Many underwater organisms are capable of fluorescence, a physical effect which will be further explained below.

Underwater fluorescence is best observed at night, during night dives, using specialized equipment. Such dives are then called "fluorescence dives", "fluo dives", "fluoro dives", "uv dives" or "glow dives".

Underwater fluorescence can exceptionally also be observed during the day, which either doesn't need any specialized equipment at all, or needs more sophisticated equipment than during night dives. However, the effect is most spectacular and dramatic during darkness.

A few words about physics:

There are many luminous phenomena in nature, some of which are of a chemical nature (e.g. fire, glow sticks), some are biological (e.g. the bioluminescence of fireflies, some species of mushrooms, deep sea creatures, some jellyfish and plankton), and some are physical (phosphorescence, fluorescence, lightning, St. Elmo's fire, through radioactivity, e.g. from the sun, from Radium dials or Cherenkov radiation). See this page for a detailed overview.

You already know phosphorescence, the capability of phosphor (hence the name!) and other materials (such as e.g. "glow-in-the-dark" toys) to store the energy of light and to release it again as light slowly over time, e.g. from the phosphor coating of compact fluorescent lamps and fluorescent tubes, from cathod ray tubes of old-fashioned TV screens and oscilloscopes, and from the gauges of your dive instruments.

Coincidentally, the blue light torches used to see fluorescence under water are also great for "charging" the phosphor of your dive instruments, which makes reading your instruments much easier than with white light torches!

Bioluminescence is an active process of the corresponding living organisms, which costs them energy, therefore called "active". You probably have experienced this phenomenon during night dives in tropical waters; the water lights up when you stir it, i.e., when you upset the plankton which is present in the water.

Fluorescence of marine life on the other hand, which is what we are interested in here on this website, is a completely passive physical process, i.e., it does not change these organisms and it does not cost them any energy. On the contrary, energy has to be provided from an external source, e.g. in the form of light, in order for this phenomenon to occur:


When light of higher energy (and therefore of shorter wavelengths, e.g. of ultraviolet or blue colour) hits a suitable (fluorescent) target, electrons in the material absorb the energy of the light and are thereby kicked up to a higher energy level. Within nanoseconds these electrons fall back to a lower energy level, thereby each emitting a photon of light of lower energy (and longer wavelengths, e.g. of green, yellow, orange and red colour) than the light initially absorbed.

The difference in wavelength of the absorbed and the emitted light is called "Stokes shift" after Sir George Gabriel Stokes, who was among the first to scientifically investigate fluorescence, and who also coined the term "fluorescence" in the first place, after the mineral "fluorite" or "fluorspar" or calcium fluoride CaF2, which has fluorescent properties.

The light (or energy) used to stimulate the fluorescence is called "excitation light" (or energy), whereas the fluorescence itself is also called "emission light".

Compact fluorescent lamps and fluorescent tubes are called "fluorescent" because in addition to reducing flicker through phosphorescence, their internal phosphor coating transforms the invisible ultraviolet light of their mercury vapor gas discharge into visible white light (of lower energy and longer wavelengths than ultraviolet).

Underwater fluorescence:

Traditionally, fluorescence has always been inextricably linked to ultraviolet light as THE excitation light source for stimulating fluorescence. However, it has been found that blue light is a much better choice for observing underwater fluorescence than ultraviolet, for several reasons.

This makes total sense, because due to water absorption, blue is the only remaining wavelength of light available below a certain depth, and obviously the marine organisms capable of fluorescence have had to adapt to this fact during their evolution - assuming that their ability to fluoresce gives them an evolutionary advantage over their competitors in their daily struggle for survival. Given the large number of biologically unrelated organisms under water capable of fluorescence, it is a valid hypothesis that fluorescence must provide them with some benefit, the nature of which is the subject of ongoing scientific research.

Therefore the state-of-the-art technique for observing underwater fluorescence uses blue light, not ultraviolet (UV) light (also known as "black light").

The minimum equipment you will need in order to observe underwater fluorescence is a blue light dive torch and a blue-block yellow mask filter (also called a "barrier filter").

You need this barrier filter in order to filter out any reflected blue light from your torch, which would otherwise outshine the much weaker fluorescence and would therefore prevent you from seeing the fluorescence.

[UW Fluorescence]

However, state-of-the-art torches for observing underwater fluorescence not only use blue light, but additionally come with an internal blue-pass filter (called an "excitation filter") for "trimming" the spectrum of the torch's light output in order to match and complement the properties of the yellow mask filter (barrier filter) used more precisely.

See the page excitation filters for images which demonstrate the difference this excitation filter makes, and see the page barrier filters for images which demonstrate how carefully the properties of both filters need to be tuned to each other in order to achieve optimal results.

The excitation filter is an interference filter called a "dichroic" filter, easily recognizable by its reflective golden shine. This is how you can tell the better torches for observing underwater fluorescence apart; by verifying that the torch's lens shimmers golden, because many vendors shun the additional cost and work of putting such a filter in, or they use UV light instead of blue, in which case usually no filters are used at all (neither excitation nor barrier filter), but UV is inferior to blue light anyway, as explained above.

In case you want to capture underwater fluorescence on camera (photo and/or video), you will additionally need a barrier filter (B) for your camera and either one or more very strong blue light torches as video lights, or excitation filters (A) for each of your strobes, and preferably a small blue light torch as focus light.


Underwater fluorescence has important applications not only in scientific research, but also in marine conservation projects, where fluorescence helps to assess the state of health of coral reefs, by facilitating the identification of damages and of so-called "coral recruits", from a larger distance.

Coral recruits are larvae of corals which have settled recently to found a new colony, and whose abundance helps to estimate the regeneration (self-healing) capacity of a given coral reef.

Lighting technology:

See the page technology for a deeper explanation of the lighting technology needed to observe underwater fluorescence.

Our publications:

See also our publications for even more detailed information.



Excitation Filters

Barrier Filters

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