FLUORESCENCE- OR FLUO-DIVING has gained in popularity around the world in the past few years, yet it’s still considered new because so many divers and even instructors have never been exposed to it.
Fluo-diving means using a blue-light torch and mask barrier filter for viewing bio-fluorescence. Certain marine life emits light with a longer wavelength (of visible light) when illuminated with shorter-wavelength blue or excitation light, and the effect can be very dramatic, a bit like finding yourself in an underwater version of Avatar.
Many species of shelled animals, soft and hard coral structures, coral polyps, some fish and anemones emit this light, and the word “emit” is important.
This is not the sort of light you get on a night-dive when your white-light torch-beam is reflected off the reef and bounces back to your eyes. Emitted light is created by the organism and sent to you.
This is not bio-luminescence, which is a completely different process whereby the organism creates its own light through a process called chemo-luminescence. Bio-luminescence needs no excitation light – it’s all done internally through chemical reaction, as fire-flies do it on land.
The visible spectrum of light is the incredibly thin slice of the electro-magnetic spectrum that we can “see”.
It lies between the low-energy, low-frequency, extremely long-wavelength radio waves and the high-energy, high-frequency, extremely short-wavelength (and deadly) gamma rays.
The “bandwidth” of light wavelengths that the average person can see covers from approximately 400nm (nanometres) or 400 billionths of a metre, which is deep purple (almost black), to about 750nm dark red (again, almost black).
The schematic illustrates the fluorescence effect. When a high-energy, short-wavelength photon of light (blue in our case) strikes a protein (referred to generically as a Green Fluorescent Protein, or GFP), it absorbs that light energy.
This causes the electrons of its constituent atoms to make a quantum jump from one valence electron shell to a higher shell. Then this change of energy state “decays”, effectively instantly, back to its resting state or shell.
When this decay occurs, the electron gives up or “emits” a photon of light, but at a lower energy and longer wavelength. The colours emitted are determined by how many “jumps” the electron makes, and decays back down. This is where diving and quantum physics collide.
The wavelength of light used in most fluo torches is a narrow band in the blue, somewhere between 440 and 480nm, so it is not ultra-violet (UV) or black-light diving, as many people refer to it.
There are companies that produce UV torches for underwater use, but blue light is far more efficient at stimulating fluorescence of GFP and its mutations than UV light.
That’s because the only light available deeper than about 10m of water is blue, and it’s in this light that organisms such as coral have evolved over the eons.
Most UV light from the Sun bounces off the surface of the water and that which does penetrate gets no further than a few centimetres. UV light is a very inefficient light source for fluo-diving.

IT IS NOT WELL UNDERSTOOD why some corals and other sea creatures evolved to fluoresce, though there are many theories. It might serve as a form of sun-block, protecting coral in shallow water from UV energy from the Sun, or it might be a form of communication between species.
Many people think that only hard corals fluoresce, but the terms “hard” and “soft” as applied to coral can be misleading. For example, brain coral is usually considered a hard coral but is in fact part of the LPS (Long Polyp Stony) family of soft coral, because the live coral consists of tiny soft creatures that live and die, building up a large stony structure over the decades. The same applies to SPS (Small Polyp Stony) corals.
These are the coral subjects that give the most fluo effects, whereas soft corals such as those in the Alcyonacea family rarely fluoresce. As in all families there are exceptions to the rule, but the message is that many types of coral fluoresce and many do not.
That is part of the allure of fluo-diving – it’s still possible to make your own discoveries as a citizen-scientist.
Along with the blue-light torch, you need a barrier filter for your mask, and for your camera if you plan to take photos. This “blocks” the blue light that is reflected back to you from everything on which you shine it – without it, all you will see is very bright blue.
Barrier filters are designed to cut off all or most of the wavelengths in the blue part of the spectrum. Emitted light from the organism is usually so dim as to be overwhelmed by the blue light, but block the blue and all you see are the emission colours.
Fluo-dives introduce safety considerations above those on a normal night dive. With white light, all the colours of the spectrum are available to “see”, but in fluo-diving the only light you are using is a very narrow band of blue. Put on your blue barrier filter and what’s left? Almost nothing – you just eliminated your light source.
The emission light is dim and does not light up the entire reef, so you need to exercise excellent buoyancy control, be constantly aware of your surroundings and be situationally aware. If a coral head doesn’t fluoresce or “light up”, you could crash into it.
Always approach and leave a site using your back-up white-light torch, and have it handy when you enter an area in which there is little fluo activity. Alternatively you can always remove your mask-filter and you will see fine, although in blue.

DURING A RECENT TRIP TO Palau where it was dark and stormy, we decided to try fluo-diving and photography in daylight. We found that both are possible with strong excitation lights and the proper ambient lighting conditions. In daylight there are no additional safety concerns – fluo-diving in daylight is much easier, and you can still get spectacular photographic results.
It’s best to have cloudy skies, though not required. Next, look for walls or structures in the shadows. While it’s best to have the sun on the back side of the structure so that it is not illuminating it directly, you can work around this if the target is in a hollow or cavity in a wall, or under an overhang.
Using a strong excitation/focus light allows you to find subjects that fluoresce nicely and allow for plenty of light for viewing or camera focus. If it is powerful enough you can get great shots using that alone, but strobes with the appropriate excitation filters will do very well.
Many people think that fluo-diving is done simply for the wow-factor of the radiant colours, or for a different take on underwater photography, though I’ve also seen students surface after their first fluo-dive reduced to tears by the beauty of this further hidden world within the hidden underwater world.
But it is far more than that. Fluo-diving has become an indispensable tool in coral-health research and coral-propagation census (polyp bail-out) analysis. Individual almost-microscopic organisms shine in the sand like sparkles in the snow on a moonlit night.
Coral reefs are considered the rain-forests of the ocean, and many marine institutes and universities are using fluo equipment to assess the effects of ocean temperature rise, acidification and studies of coral in general.
There have even been discoveries of species that had been too small to see with white light but shine like beacons in the dark in blue light.
As global water temperatures rise, coral-bleaching occurs. Corals have a symbiosis with zooxanthellae single-cell algae, which use photosynthesis to provide food and energy to the coral.
Temperature rises cause these zooxanthellae to be ejected, giving the coral a colourless, bleached appearance and removing those nutrients the coral needs to survive. This makes them vulnerable to additional stresses that can ultimately destroy an entire reef.

OCEAN ACIDIFICATION reacts with the coral’s calcium carbonate skeleton, causing it to break down and dissolve, and while this can be seen under white-light conditions it is even more dramatic using fluorescent technologies, which are enhancing this field of marine research.
You can hire the equipment you need to fluo-dive at those dive-centres that provide it, or buy your own if you intend to pursue this style of diving.
Web-search “fluo dive gear” to find equipment-sellers as well as blogs, images and forum discussions on the subject. Or go to www.firedivegear.com for much greater detail on the science and photographic techniques used in fluo-diving – both day and night.