Fluorescence night dives (or „fluo dives“ for short, also known as „fluoro dives“, „UV dives“ or „glow dives“) have grown increasingly popular recently. More and more dive centres offer fluo diving , and more and more vendors for the necessary equipment appear on the market . A better understanding of the background and basics allows to make more of this exhilarating experience (some people say that fluo diving is like being in the movie „Avatar“ from director James Cameron), and also allows to separate the wheat from the chaff between vendors.
James Cameron , who also directed the famous movies such as „Terminator“, „Aliens“, „Abyss“ and „Titanic“, is a seasoned diver himself: He personally went down to the wreck of the Titanic many times, and on the 26th of March 2012 he reached the bottom of the Challenger Deep in the Mariana Trench, about 11 km (almost 36,000 feet) below sea level, as the third person in history ever to accomplish such a feat. It is therefore conceivable that James Cameron was familiar with and inspired by pictures and footage of underwater fluorescence when he created his movie „Avatar“.
Fluorescence is the physical effect shown by some materials to absorb light of high energy (and therefore short wavelengths) and to re-emit light (usually after a few nanoseconds) of lower energy (and therefore longer wavelengths), see the schematic in Fig. 1. Other sources of energy may also cause the same effect, such as for example high-speed electrons (for instance in cathode ray tubes).
Fig. 1 - Schematic of fluorescence
This phenomenon not only occurs in living organisms, but also in minerals (the mineral „fluorite“ or „fluorspar“ or calcium fluoride CaF2 gave this effect its name, by the way), and interestingly also in petrified fossils .
We will also have to talk more about the optimal wavelengths to stimulate fluorescence in underwater organisms later in this article.
Fluorescence should not be confused with phosphorescence (which is the capability to store light, which is then re-emitted over a longer period of time, as you will probably be familiar with from your dive instruments or glow-in-the-dark toys) or with bio-luminescence (which is the active production of light by living organisms under consumption of energy with the help of bio-„fuels“ and enzymes, called Luciferin and Luciferase, respectively).
The phosphor coating on the inside of cathode ray tubes and fluorescent tubes („neon lamps“) provides for both fluorescence (the energy of high-speed electrons or of UV light is transformed into visible light) as well as phosphorescence (the familiar afterglow). This is also why you will find an option „fluorescent light“ under the white balance menu in your digital camera (note that this is not the correct setting for underwater fluorescence photography and filming, though! More on that later!). In the following we will occasionally talk about „fluorescent tubes“, but when we talk about „fluorescence lamps“ or torches, then we mean dive lamps with ultraviolet (UV) or blue light, exclusively.
The capability of marine organisms to fluoresce was first discovered in 1927, as far as we know , by a certain Mr. Charles E.S. Phillips, at the beach of Torbay, England. He noticed some anemones in a tidal pool which had a particularly bright green colour. He took some samples to his laboratory and examined them with the help of a light source and a filter called „Wood's Glass“ , which absorbs visible light and only lets UV light pass through, and thereby proved that these anemones were in fact fluorescent.
In the 1930s the Japanese marine biologist Siro Kawaguti examined the pigments of corals and discovered that the most common pigment fluoresced in green.
In the 1950s divers started to explore fluorescence under water:
In 1951-1961 Dr. Richard G. Woodbridge III (also famous for sound recordings recovered from ancient pottery) built some underwater blacklight torches and wrote articles in „Skin Diver“ magazine about his discoveries in the cold waters of the Atlantic near the coast of Maine.
Luis Marden, a photographer for National Geographic magazine and discoverer of the wreck of the famous „H.M.S. Bounty“, reported in 1956 that he had found red anemones at a depth of 18 meters (60 feet), although there should not have been any red light at that depth. The red colour disappeared in flash photographs, and Marden concluded correctly that the effect was due to fluorescence. Fig. 2 shows such a red anemone at the same depth.
Fig. 2 - Red fluorescing anemone during daylight
In 1963 Sir Arthur C. Clarke , a renowned diver and science fiction author (e.g. of the movie „2001: A Space Odyssey“), bought some fluorescence torches from Dr. Richard G. Woodbridge III and described his experiences with them for instance in his SF novel „Dolphin Island“ .
In 1955 the fluorescent pigment later baptised „Green Fluorescent Protein“ (GFP) was first described and recognised as a protein. In 1962 GFP was extracted from 10.000 jellyfish of the species „Aequorea victoria“. In 2008 Osamu Shimomura, Martin Chalfie and Roger Tsien, who had worked independently in this field, jointly received the Nobel Prize in chemistry „for the discovery and development of the green fluorescent protein, GFP“ .
GFP and its variants (they exist in all colours of the rainbow, nowadays) have many applications in molecular biology, genetics and medicine. In reproductive medicine for instance there is a method called „Fluorescence In-Situ Hybridization“ (or FISH for short, coincidentally), which allows to detect missing or surplus chromosomes in embryos (so-called „aneuploidies“ such as Trisomy 21 or „Down Syndrome“) in what is called pre-implantation genetic diagnostics or PGD.
In 1959 Rene Catala, Director of the Noumea Aquarium in New Caledonia, was the first to systematically test corals for fluorescence with UV light in his aquarium. He built displays with fluorescent corals at his own aquarium as well as in Antwerp, Belgium. Aquarium displays of fluorescent corals using what is known as „actinic lighting“ have been an indispensable attraction ever since, both in zoos all over the world as well as among amateur fish and reef tank owners. This will yet be of some importance later on in this article.
In the 1970s Dr. Charles H. Mazel started to conduct scientific research in the area of underwater fluorescence. In one - rather unusual - project he investigated how to allow Navy divers to see under water at night without being seen from the surface, with the help of strong UV torches. Unfortunately this did not work out as expected, because of the fluorescence of omnipresent organic matter dissolved in the water (mostly fulvic and humic acids from decaying organic matter, runoff from land, etc.). It should be noted by the way that silt will generally not fluoresce to any significant degree. Dr. Mazel wrote: „There was a very weak visible glow from water itself (not the dissolved components) that comes from Raman scattering. This is not fluorescence and in itself would not produce the kind of distinct »water-glow« you show in your excellent photo. Rather, it would be a very weak purple, not visible from much of a distance“. See „The inherent visible light signature of an intense underwater ultraviolet light source due to combined Raman and fluorescence effects“  for details, and see Fig. 3 below.
Fig. 3 - Intense UV-A light beam under water (365 nm)
By measuring excitation and emission spectrographs, Dr. Mazel discovered around 1990-1992 that contrary to standard practise and common belief (which inseparably associated ultraviolet light to fluorescence) ultraviolet light was not the most effective wavelength for stimulating fluorescence in GFP, but blue light was, of about 450-470 nm wavelength. Based on these findings Dr. Mazel subsequently developed the modern form of fluo diving with blue light torches and yellow filters .
In 1999 he founded NightSea in order to make this technology available, not only to enthusiasts and research institutes, but also to industry e.g. for Non-Destructive Testing or NDT (BlueLine NDT).
Starting in 2007 Prof. Dr. Horst A. Grunz built so-called HiTec fluorescence torches using blue high power LEDs, based on the principles established by Dr. Mazel, which allow to illuminate a bigger section of a coral reef at once (see for instance DiveMaster Nr. 64 issue 2010/02 p. 17-22). This is of special interest in the scope of measures to protect coral reefs - more on this later.
Inspired by Sir Arthur C. Clarke's SF novel „Dolphin Island“ , in autumn of 2010 the author began building his own fluorescence torches, because he could not find any commercially available UV dive lights. In order to get as close to the experience described in that book, initially the author exclusively used UV LEDs and built torches of increasing power, first with a single LED of about 395-410 nm and 1 Watt, then a torch with two LEDs from market leader Nichia with 365 nm and about 6 Watt, and finally a torch with 4 quadruple LEDs (equivalent to 16 single LEDs) from Nichia with 365 nm and together about 46 Watt.
Later on the author also bought and built blue light torches with up to 90 Watt. His web page http://guest.engelschall.com/~sb/fluolinks/ documents all these do-it-yourself projects and their results. Additionally this page contains the most comprehensive collection of links and information about fluo diving and DIY fluo torches on the Internet.
Together with the dive instructor and physicist Lynn Miner, the author founded www.FireDiveGear.com in May 2012, and (for practical reasons) www.FluoMedia.org in December 2013, in order to be able to offer high quality yet affordable equipment for fluo diving, in order to lower the entry barrier to this unique and overwhelming experience.
As mentioned earlier, in the old days „fluorescence“ automatically meant ultraviolet light. In order to get a sufficiently strong and pure beam of UV light, a white light source with a high UV content was used, such as an arc lamp, which was then transformed into a UV light source thanks to a special filter, so-called „Wood's Glass“ . Divers are familiar with such arc lamps under the name of High Intensity Discharge or HID lights.
Alternatively mercury vapour lamps were used . Note that fluorescent tubes and energy-saving lights are in fact mercury vapour lamps with an additional phosphor coating on the inside of their glass tubes which converts the UV light of the gas discharge into visible light (an effect which will be of special interest to us later on in this article). Blacklight tubes, such as used in discos, are also mercury vapour lamps but without such a phosphor coating (they have a different coating, though, in order to block any remaining visible light and to restrict the UV light output to long wave UV-A).
„Wood's Glass“ is a conventional absorption filter, made of a special barium-sodium-silicate glass incorporating about 9% nickel oxide . Absorption filters are characterised by the fact that they absorb the unwanted (blocked) wavelengths of light and that they transform the energy of the absorbed light into heat. This is why absorption filters (e.g. for stage projectors) are susceptible to breaking e.g. through sudden temperature changes (if made from glass) or melting (if made from plastic).
Modern filters however do not absorb the unwanted wavelengths of light, but instead reflect them, like a mirror. They are called interference filters or dichroic filters (see Fig. 4). These filters consist of a transparent substrate (usually glass), onto which several layers of varying thickness and optical refraction indexes are deposited in a vacuum chamber - similar to antireflection coating on eyeglass lenses. The same principle also produces the iridescent colours of thin films of oil or gasoline on water puddles. When carefully chosen, these different layers cause certain wavelengths of light to be reflected, while other wavelengths of light can pass through unobstructedly.
The big advantage of dichroic filters is that their spectral properties can be controlled very precisely, and that these filters do not heat up during operation. Their disadvantage is their higher price, compared to conventional absorption filters.
Fig. 4 - Dichroic or interference filters of different shapes and sizes
By the way, there are also dichroic filters with the same spectral properties as the original „Wood's Glass“, which also carry the same name. This often leads to confusion.
Dichroic filters are coated on one side only, but it doesn't matter which side faces the light source. Only for mechanical reasons (to protect the soft coating, e.g. from scratching) it may be advisable to be able to differentiate the coated from the non-coated side. This is done very easily by holding a soft and rounded object against the filter. If the object and its mirror image appear to touch, you are looking at the coated side. If there is a gap between them (equal to the filter's thickness), you are looking at the non-coated side.
Using filters, preferentially dichroic filters, white light sources (such as dive lights and underwater strobes) can be operated as fluorescence stimulating light sources, either with a „Wood's Glass“ filter for UV light, or with other filters for blue light.
Since professor Shuji Nakamura (then working for Nichia Corporation in Japan) developed blue and ultraviolet LEDs (and blue lasers) in 1993 , the use of filters to make fluorescence stimulating lights is not necessary anymore (except for strobes), especially since high power blue and ultraviolet LEDs have become available on the market. Prices for these LEDs have also dropped considerably during the last couple of years.
Using blue or ultraviolet LEDs is much more efficient than transforming white into blue or ultraviolet light, in which case there is always a certain loss of light output.
As mentioned earlier, however, Dr. Mazel realised during his investigations that UV light is in fact not very efficient to stimulate fluorescence. His measurements  indicate that blue light of about 450-470 nm wavelength is approximately four times more effective to stimulate fluorescence in GFP than the near UV range (UV-A).
The disadvantage of blue light, however, as opposed to UV light, is that it is visible to the human eye, and that it usually by far outshines the relatively weak effect of fluorescence.
This is why a yellow mask filter (and possibly a yellow camera filter) is indispensable for fluo diving, in order to filter out any blue excitation light and to make it invisible to the human eye (see Fig. 5).
Fig. 5 - Author's dive mask with yellow mask filter and neoprene mask strap wrapper
It is important that the yellow mask filter largely covers the dive mask, in order to avoid blue light leaking in through any gaps and fissures, which would spoil the experience. The relatively narrow cut-out for the mask's nose pocket prevents the mask filter from wiggling around, by centring it automatically. The neoprene mask strap wrapper prevents losing the yellow mask filter when you push it up to your forehead, for instance for better orientation and navigation. It can also be used to protect the filter for transportation and storage. When the mask strap wrapper is made of fluorescent neoprene as in Fig. 5, it can be used to easily identify divers during a fluo-dive when different colours are assigned to different divers or buddy teams.
Fluorescence consists of longer wavelengths (in the range from green over yellow and orange to red) than the (blue) light used to stimulate it, the former of which is therefore not affected by the yellow filter, while the latter (blue) excitation light is blocked. At least in theory - surprisingly, a very tiny little bit of remaining blue light even enhances the aesthetic impression , see Fig. 6. It seems that the blue coloration of the coral visible in Fig. 6 does not solely stem from unfiltered blue excitation light (filter cross-talk), but may actually be blue fluorescence. This is corroborated by the fact that the same blue tips of this coral can be seen under (invisible) UV excitation light.
Fig. 6 - Fluorescent coral with blue coloration
It is beyond the scope of this article to describe the exact spectral properties of the necessary yellow filters, which are usually kept a trade secret by the various manufacturers anyway. Suffice it to say that the emitters (i.e., LEDs) used should have a peak wavelength of about 450-470 nm, and the yellow filter should block all light (or almost all light, see above) with a shorter wavelength than about 500 nm, and should let pass all wavelengths longer than 500 nm, approximately.
Any yellow filter material which resembles the one shown in Fig. 5 will probably allow you to observe fluorescence, but only filters precisely attuned to the fluorescence torch used will likely give you optimal results. When the fluorescence you see only comprises dull reds and greens instead of a whole rainbow of colours, then this is likely caused by the yellow filter, and you should probably change it to a more suitable filter material.
In addition to the mandatory yellow filter for mask and camera, the technique developed by Dr. Mazel also uses a blue dichroic filter in front of the blue light torch (in front of strobes anyway), see also Fig. 7.
Fig. 7 - Author's torch with 18 blue Cree LEDs and a dichroic blue filter
It may seem superfluous and a complete waste of resources to use a blue filter in front of a blue light source, but the rationale behind this is that blue light sources, just as any light source by the way, actually produce a broad, bell-shaped spectrum of wavelengths around a central peak, i.e., their light is neither monochromatic nor coherent, as the light of a laser would be. In order to get a better separation between excitation and emission light despite this fuzziness of wavelengths from the excitation light source this additional filter is very important.
Just as with the yellow filter, the cut-off wavelength between blocking and passing of this dichroic blue filter should be around 500 nm, but exactly opposite to the yellow filter: the dichroic blue filter should block all light with a longer wavelength than about 500 nm, and should let through all wavelengths shorter than 500 nm, approximately (i.e., blue and ultraviolet). The blue filter and the yellow filter are thus complementary; together they block the whole spectrum of visible light. However, the effect of fluorescence transforms the light from the passing range of the blue filter into the passing range of the yellow filter, allowing it to become visible, but nothing else.
The presence of a dichroic filter on a fluorescence torch can be considered a sign of high quality, since only very few vendors bother to put one in, preferring not to give you the best possible results in exchange of slightly higher profits and less hassle for them. Dichroic filters can easily be recognized by their mirroring properties (see Fig. 4). Dichroic filters always shimmer in the complementary colour of the wavelengths they let through, since they reflect the wavelengths to be blocked by them. The complementary colour of blue is orange; a blue dichroic filter therefore shimmers golden. A „Wood's Glass“ dichroic filter only lets pass UV light, while it reflects the whole spectrum of visible light, which gives white, and the filter therefore shimmers silvery.
The following pictures (Figs. 8a to 11b) demonstrate the effect of the additional blue dichroic excitation filter . Without the excitation filter the pictures lack contrast and look pale, and red fluorescence only shows to advantage with the filter in place:
|Fig. 8a - Fluorescent corals #1 without blue filter||Fig. 8b - Fluorescent corals #1 with blue filter|
|Fig. 9a - Fluorescent corals #2 without blue filter||Fig. 9b - Fluorescent corals #2 with blue filter|
|Fig. 10a - Fluorescent corals #3 without blue filter||Fig. 10b - Fluorescent corals #3 with blue filter|
|Fig. 11a - Fluorescent corals #4 without blue filter||Fig. 11b - Fluorescent corals #4 with blue filter|
Some vendors do not use blue or ultraviolet excitation light, but violet light, somewhere between 400 nm and 450 nm, and consequentially also use different filters, but also achieve very remarkable results (e.g. the French manufacturer www.Dyron.fr / www.Plongimage.com). There is no „absolute truth“ in fluo diving and there are no „right“ or „wrong“ torches and filters, but there are torches and filters which are carefully chosen to harmonise well, while others are not, and your results may vary accordingly.
However, there are corals for instance which do not fluoresce under UV light (of 365 nm) at all, while the same coral does fluoresce under blue light (of 450-470 nm).
Moreover, ultraviolet LEDs (with 365 nm) with the same electrical power rating are about four times more expensive than blue LEDs, while the ultraviolet LEDs have a four times lower light output (radiant flux), approximately. This is true for all LEDs: The shorter the wavelength, the higher the price and the lower the efficiency. As mentioned earlier, Dr. Mazel discovered  that ultraviolet light is about four times less effective to stimulate fluorescence in GFP than blue light of equal energy. This means that UV LEDs are about sixteen times less efficient to stimulate fluorescence in GFP than blue LEDs with the same electrical power rating, for four times the price. In other words, it would cost you about sixty-four times as much to achieve the same effect with UV LEDs as with blue LEDs.
Interestingly the most effective wavelength to stimulate fluorescence in GFP, approximately 450-470 nm, coincides with the wavelength of greatest optical permeability of water, which is more transparent to blue light than it is to any other wavelength, UV included. This is certainly no accident, but very likely an evolutionary adaptation of marine organisms to the properties of their aquatic environment, since below a certain depth blue light is the only light available to them.
Fluorescence is not only observed in corals, but also in many other marine organisms such as e.g. tunicates, barnacles, sponges, anemones, jellyfish, clams, nudibranchs, cephalopods, shrimp, crabs, worms and fish, to name just a few. There are also fluorescent organisms in fresh water, such as for instance cave olms in Eastern Europe . The green chlorophyll responsible for photosynthesis in plants is also fluorescent, it fluoresces in red or - in some algae - in orange.
Given the enormous spread of unrelated species showing the capability to fluoresce, it is extremely unlikely that this phenomenon is a mere by-product (as it appears to be in the case of chlorophyll). This capability must provide some benefit to all these animals. Unfortunately research into what benefit this might be is still in its infancy.
However, there are some preliminary hypotheses and findings:
There are studies which suggest that fluorescence in corals may act as a sunscreen, protecting the corals (especially in shallow parts of the reef) and in particular their symbiotic algae (which live inside the coral's tissues) against harmful UV radiation.
Another hypothesis states that fluorescence allows corals to transform the only light available to them, namely blue light, into such wavelengths as can be used by their symbiotic algae for photosynthesis, allowing the corals to dwell successfully at greater depths than their competitors without such a capability can, giving them an evolutionary advantage to survival.
It is also speculated that fluorescence in fish might help them to visually merge with the background of e.g. fluorescent corals, in order to make these fish less conspicuous to predators.
Some new findings by Prof. Nico K. Michiels at al. („Red fluorescence in reef fish: A novel signalling mechanism?“ ) suggest that some fish are using red fluorescence on their bodies for communication among themselves. It was already known that fish can wilfully change their body's coloration (similar to octopuses, squid and cuttlefish), and that they use this ability for communication, e.g. to attract mates, intimidate competitors and discourage or hide from predators. However, it is a new discovery that they can also wilfully change their fluorescence. It is also surprising that they use red fluorescence, because it is the colour most quickly absorbed by water, and because many fish do not even have the ability to see red light. It seems that these fish have developed a secret and private communication channel of their own, allowing them to be conspicuous towards their peers and unconspicuous towards predators at the same time. The video Fluorescent Goatfish by Liquid Motion Film demonstrates such a change in fluorescence, which takes place in the matter of only a few seconds.
To learn more about this fascinating underwater communication through colours, you should see the (multiple award-winning) TV series „Water Colours“ by Anita & Guy Chaumette/Liquid Motion Film produced for National Geographic . The three DVDs „Fisheye Illusion“, „A Colourful Language“ and „A Touch of Fluorescence“ (3x50 minutes) can be ordered directly from the producers for a total of €45 plus shipping (from the UK). See also the previews (about 8 minutes each) of these 3 DVDs Fisheye Fantasea, Colour Talks and Beyond The Blue, respectively, as well as the preview The Significant Others of a new series to come. The German version of the middle part „A Colourful Language“ can be found on YouTube (subdivided into three parts): . The third part of the series is mainly dedicated to fluorescence, as the title „A Touch of Fluorescence“ suggests, in particular to fluorescence found in the deep sea, comprising some extraordinarily spectacular footage .
Another recent finding is a little sensation: according to Lukyanov et al. , fluorescent proteins such as GFP are capable to act as electron donors when excited by light - exactly as chlorophyll does in plants during the crucial first step of photosynthesis.
Could this mean that the old schoolbook definition and conventional wisdom of what differentiates animals and plants, namely the ability for photosynthesis, is about to become obsolete?!
At least one thing will not change: even if animals should in fact be able to perform some kind of photosynthesis, they will continue to be unable to live self-sufficiently and exclusively from it, as plants do. Moreover it should not be forgotten that other forms of photosynthesis in animals are in fact already known; for instance the human body produces vitamin D in the skin with the help of sunlight.
As mentioned earlier in this article, fluorescence torches using high power blue LEDs (as those built by Prof. Dr. Horst Grunz and the author, see Fig. 7) allow to illuminate greater areas of a coral reef at once. This has the potential to replace or at least complement the traditional, tedious and time-consuming method called „Reef Check“  of evaluating the health of part of a coral reef, which method consists of laying down one or more transect lines and manually counting and identifying species of corals along with recording their state of health by specially trained marine biologists, while swimming along the transect line or lines.
Thanks to these strong fluorescence torches, corals which are affected by disease or which fell victim to predation, dead or broken off corals as well as zones of conflict between competing corals can be detected from a great distance, usually through colour changes of their fluorescence, the lack of fluorescence altogether (of dead corals) or intense red fluorescence from algal overgrow.
A recently published study shows that the health state of corals does indeed correlate with their fluorescence, which means that the latter can be used as a measure of the former: „Effects of cold stress and heat stress on coral fluorescence in reef-building corals“ .
Another important benefit of using high power fluorescence torches is that they make finding coral recruits , i.e., freely swimming coral larvae which have settled down to become coral adults, much easier and allow to do so from a much greater distance, provided the coral recruits in question are fluorescent, in which case they will stick out like little beacons - see Fig. 12.
Otherwise detecting coral recruits is hard even for heavily trained specialists, because coral recruits are usually transparent and extremely small (one to a few millimetres), which is a bit like the famous search for a needle in a haystack.
Coral recruits are essential to assess the dynamic balance between destruction and construction of a coral reef, in order to determine whether the coral reef is able to recover from present damages or succumbing to them, and in order to measure the effectiveness of conservation efforts. Factors of destruction are predation, such as from parrot fish, crown-of-thorns starfish and snails such as Drupella and Coralliophila, detrimental environmental factors such as global warming, pollution, careless snorkelers and divers, diseases, such as coral bleaching, etc.
Fig. 12 shows several coral recruits of varying stages of development (and hence, different sizes) and various species:
Fig. 12 - Coral recruits of varying stages of development and various species
It is worth mentioning that the „Red Sea Environmental Centre“ or RSEC (www.redsea-ec.org) in Dahab/Sinai/Egypt offers the possibility to participate in reef monitoring and reef conservation projects as a volunteer or as part of the practical work for a diploma thesis, for instance, which includes introductory courses (both theoretical and practical) into reef ecology, and fluorescence night dives.
There are however concerns among many divers and even among specialists that strong UV or blue light torches might harm underwater organisms.
Generally all forms of nightly illumination carry the risk of disturbing the underwater creatures or even of disrupting their circadian rhythms. Some people go as far as to suspect that nocturnal lighting might disrupt the reproductive cycles of certain organisms. In any case it is known that some fish (e.g. parrot fish) build a protective sheath around their bodies at nightfall made of mucus, which is secreted from their mouths. If disturbed and caused to flee, they are unable to produce another such sheath, which means that they will have to spend the rest of the night without their usual protection from parasites and predators, with all the possible consequences thereof.
However, as mentioned previously in the history section of this article, reef tanks with „actinic lighting“, i.e., lamps with UV and/or blue light, have been a great attraction ever since 1959.
Reef tank owners use powerful lamps which produce light comprising the entire visible spectrum, plus some UV. These lamps usually use several of the most powerful UV and blue LEDs currently available on the market. One of these lamps has a total power (including its fan) of up to 170 Watt  (many thanks to Joey Ruberti from Nano News for this information). The power of this lamp in the blue and UV range of the spectrum is comparable with or superior to the strongest fluorescence torches used for fluo diving. Moreover it has to be considered that these lamps are usually mounted at shortest possible distance above the reef tank, thus illuminating the fish and corals below at close range, and that these lamps are functioning many hours per day.
It is therefore highly unlikely that fluorescence torches can cause any significant harm under water, especially since they do not usually shine on one and the same spot for more than a few seconds, at most a few minutes. Should such torches in fact be able to cause detectable damage, reef tank owners should have noticed this a long time ago.
Not to mention that the energy levels given off by these torches are lower by several orders of magnitude than those from natural sunlight in the tropics shining onto underwater life every day, at least up to those depths usually reached by recreational divers (max. 40 meters or 130 feet).
However, it is not even necessary to dive that deep in order to observe fluorescence, since even in the shallow (2-10 meters or 6-33 feet) and on house reefs one can find an abundant variety of fluorescent life forms, which allows for easier and pleasingly longer dives.
Some people may think that fluo diving is just the old phenomenon of bio-luminescence, that is, the flashes of light emitted by microscopic plankton in the water when the latter is stirred, or night diving after consuming hallucinogenic mushrooms  (an extremely dangerous idea, yet maybe the most preferable way to drown!), but nothing could be further from the truth.
The experience of fluorescence night diving is very difficult to describe adequately with words. As mentioned at the top of this article, many people feel reminded of the movie „Avatar“. Some call it a psychedelic underwater disco with neon lights, because the corals and many other underwater critters light up in many different colours under the light of the fluorescence torches like neon signs, some even at great distance.
Others point out that normal night dives already let you see things usually concealed during the day, while fluorescence night dives reveal things that even during a normal night dive remain invisible, and therefore call it a hidden world behind a hidden world.
It is true that many creatures which are hard to see, no matter whether during the day or at night, for example because they are transparent or because they are too well hidden or too well camouflaged, stick out spectacularly due to their fluorescence.
If you have never experienced or seen it yourself, then the following video by the author might give you a reasonably good idea: http://www.fluomedia.org/gallery/videos/?6 (9:11)
Don't be fooled into thinking that fluorescence can only be found in tropical waters, though. There are also plenty of fluorescent organisms in cold waters, such as for instance in the North Sea: http://www.fluomedia.org/gallery/videos/?5 (2:49)
To prevent blurring of your photographs from camera shake due to slow shutter speeds due to the faint fluorescence light, it may help to deliberately underexpose by two or three stops, to make faster shutter speeds possible. The underexposure can easily be corrected afterwards with an image editor, if necessary (if necessary at all - often this has the desirable effect of letting the background disappear). Thanks a lot to Markus Laube for this trick.
In order to avoid to have to carry and handle several, possibly heavy, torches, or for cost reasons, some people prefer to dispense with efficiency and use a white dive light with a blue dichroic filter (or only strobes with blue dichroic filters), which can be taken off under water thanks to a removable mount.
However, thanks to phosphor filters which have become available on the market, in order not to lose any of the precious (and faint!) fluorescence light, also the contrary is possible, namely to use a phosphor filter fitted to a fluorescence torch with a removable mount, which transforms the blue light of the torch into white light, when needed. This is the same principle as that of fluorescence tubes described earlier in this article. There is of course always a certain loss of luminosity, and additionally the phosphor filter acts as a diffuser (it changes a spotlight into a floodlight), but for navigation or white light photography this should still be sufficient, if the blue light torch is sufficiently strong in the first place (which is always advisable for night time photography anyway). See also Fig. 13.
Note that for reading your instruments, provided their gauges are phosphorescent, blue light torches are even better than white light torches, by far.
Fig. 13 - Blue light torch with removable phosphor filter
The only possible disadvantage of very strong fluorescence torches might be that the strong fluorescence they stimulate could illuminate other objects which are not fluorescent themselves. This will undoubtedly make photography easier due to higher levels of general luminosity, but could lead to wrong impressions as to which objects are in fact fluorescent.
The following Fig. 14 shows the author's current camera set-up, consisting of a digital compact camera Nikon CoolPix P300, a corresponding Ikelite underwater housing, an Ikelite tray with arms, and torches built by the author, based on the system and batteries from www.TillyTec.de. The Swiss drinking bottles serve as non-compressible buoyancy devices (as opposed to e.g. foam material), whose buoyancy can easily be adapted to changing camera configurations by filling in different amounts of air and water - even under water, if need be. This idea goes back to Prof. Dr. Horst Grunz .
Fig. 14 - The author's current camera set-up
Since any reflected blue light is absorbed by the yellow camera filter, fluorescence photography is not as susceptible to backscatter as white light photography (where any light reflecting off silt suspended in the water appears as „snow“ in the resulting pictures), unless the silt is itself fluorescent to a significant degree. Therefore blue light torches do not need to be positioned as far away from the optical axis of the camera as for instance the strobes in white light photography, which are usually attached to widely projecting and cumbersome arms.
Some readers may wonder (at least the author did, in the beginning) what might be the „correct“ setting for the white balance of the camera. As said earlier, the option „fluorescent light“ is not the correct setting for underwater fluorescence photography and filming. Since subjects are usually photographed at short range, colour absorption from the water is usually negligible (with white light photography using strobes or video lights as well, by the way), which means that the „automatic WB“ setting should be used. Dr. Charles Mazel for instance prefers the setting „clouded“, though, because he likes the results with this setting better.
To continue reading and for more in-depth details about underwater fluorescence see the article The Biology of Underwater Fluorescence.
The author will gladly answer any questions. You can contact him by email via "Steffen Beyer" <firstname.lastname@example.org>.
The author, born 1964, married, already preferred diving to learning to swim at the age of six, because it was so much easier and more convenient (for example no choking and no stiff neck from desperately gasping for air). The books by Hans Hass and Jacques-Yves Cousteau, which he found in his grandfather's library, only reinforced his fascination for marine life (in particular for dolphins) and diving. During his studies of Computer Science and Biology (with focus on Ecology and Ethology) at the RWTH Aachen university he got his first diving certificate in 1988, and made his first open water dives in 1989 in Key West, Florida. After graduating he worked as a software engineer for industry (software development and consulting) and in Free Software projects. He authored some of the most widely used Perl modules (such as for date calculations) and worked as an expert proof-reader and translator of technical books (about Perl). Since 2004 he has been working as a patent examiner in the field of „computer-implemented inventions“ (vulgo known as „software patents“) at the European Patent Office in The Hague, Netherlands. In 2009 he started diving regularly again. Other hobbies are music, skiing, reading, programming, administrating a home network (e.g. with TV reception possible on every computer in every room), volleyball, sailing, traveling, languages (3 Germanic, 3 Romanic, about 10 programming languages), photography, and more.
All photos © 2012 by Steffen Beyer, except figures 1, 4, 8a to 11b and 13 © 2012-2013 by Lynn Miner, used with kind permission.
Fig. 15 - Scorpion fish
Fig. 16 - Baby scorpion fish
Fig. 17 - Hermit crab
Fig. 18 - Anemones
Fig. 19 - Feather star
Fig. 20 - Pipefish
Fig. 21 - Sea urchin
Fig. 22 - Fire coral with crinoid
Fig. 23 - Coral
Fig. 24 - Coral
Fig. 25 - Coral
Fig. 26 - Brain coral
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