Fimbriated Moray Fluor opt

Everything you need to know about fluoresence

A guide to the wonderful world of fluo diving - what causes it, when was it discovered, why some marine animals go with the glow and where to see it 

It's a psychedelic disco, with neon lights. It's like being in James Cameron's movie Avatar. It's a night dive like no other, discovering a hidden world within the already elusive world of the reef at night.

It's fluo diving – using special torches and filters to reveal the fluorescence of corals and other critters on the reef at night. And the practice is growing, as more dive centres around the world become turned on to its possibilities and the necessary equipment becomes easily available.


What is it?

Fluorescence is the absorption of excitation light or energy by a material (such as the living tissues of corals) and its emission with a longer wavelength just nanoseconds later. It shouldn't be confused with phosphorescence, where the energy from light is stored then re-emitted over a longer period, such as happens on your dive gauge, nor with bioluminescence – which is light produced by living organisms, for example by phytoplankton glowing in the waves on a beach.

The ability of marine organisms to fluoresce was discovered in 1927 by Charles Phillips, who noticed bright green anemones in a rock pool in Torbay, Devon. He took samples to his laboratory and examined them using a lamp and a Wood's glass – a filter that only lets ultraviolet light through. In this way he proved they were fluorescent. 


In the 1950s, divers started taking black light torches underwater and Louis Marden, a photographer for National Geographic, reported finding, during daytime, a red anemone at 18m – a depth at which red light is filtered out. He concluded its colour was due to fluorescence. 

In 1962, author Arthur C Clarke – a keen diver – was among the first to take fluorescence torches diving. He used his experiences to write a passage in his sci-fi novel Dolphin Island. His characters take an ultraviolet torch diving, and see corals: they 'burst into fire, blazing with fluorescent blue and golds and greens in the darkness. The invisible beam was a magic wand, revealing objects that were otherwise hidden and that could not be seen even by ordinary light.' This book later inspired me to devise my own torches and see fluorescence for myself. 

Dr Charles Mazel started researching underwater fluorescence in the 1970s. In one rather unusual project he investigated giving navy divers UV torches to see underwater at night without being seen from the surface. It was unsuccessful, however, because omnipresent organic matter dissolved in the water is fluorescent. 

However, Mazel did discover that, contrary to popular belief, ultraviolet light was not the best wavelength for viewing fluorescence; blue light, with wavelengths between 450-470nm, was much better. In fact, he found blue light to be approximately four times as effective in stimulating fluorescence in particular corals as UV light at the same energy. Based on these findings, he developed the modern form of fluo diving using blue light torches and yellow filters. 

Why do corals do it?

The most effective wavelength for stimulating fluorescence coincides with the wavelength that permeates best through water. This is almost certainly not a coincidence, though, but likely to be an evolutionary adaptation of marine organisms to the properties of their aquatic environment – below a certain depth, blue light is the only light available to them. 

Fluorescence is not only found in corals, but in barnacles, sponges, anemones, jellyfish, clams, nudibranchs, cephalopods, shrimp, crabs and fish too. Given this range of species, it's unlikely that this phenomenon is a mere by-product – it must provide some benefit for the animals. 


As yet, not much scientific work has been done into the area. However, some studies suggest that fluorescence in corals could act as a sunscreen, protecting them, particularly in shallow water, from harmful UV radiation. Another hypothesis is that fluorescence allows corals to transform the only light available to them – blue light – into wavelengths that can be used by their symbiotic algae to photosynthesise. This would enable them to live at greater depths than their competitors (other corals that don't fluoresce). 

It's possible that fish use fluorescence in order to camouflage themselves and merge into their background – such as all those fluorescing corals. But there's also some interesting, recent research led by professor Nico K Michiels that suggests fish use red fluorescence for communication. It's widely accepted that fish change their colour for communication – to attract mates, intimidate competitors or hide from predators for example. You'll have seen this yourself on dives, with octopus, squid and cuttlefish. But it's a new idea that animals use fluorescence to do this. It seems some fish have a secret and private communication channel of their own!

So are there any applications for this phenomenon? Well, the fact corals fluoresce could be used to evaluate the health of reefs. The traditional method of doing this is called a reef check and is done by laying a line or a transect on the reef and counting and identifying the coral species found there. But researchers may also be able to see how healthy a reef is by using fluorescence torches. Dead corals wouldn't fluoresce, algal overgrowth would fluoresce red, and disease or areas of conflict could be spotted too. A recent study found corals fluoresce less if they are under stress. Researchers at the Scripps Institution of Oceanography, at the University of California, reported that corals exposed to both heat and cold showed a decline in fluorescence. 

Also, new colonies of corals would be incredibly easy to find as they shine like beacons. With the naked eye, these incredibly tiny and usually transparent coral 'recruits' are almost impossible to spot. They're a key indicator of the health of a reef, however – they can tell researchers if the reef is overcoming damage or succumbing to it, and also indicate whether conservation efforts are working.

How do you do a fluo dive?

In the early days, divers used ultraviolet light to see fluorescence. This meant taking a white light source, such as an arc lamp, and using a Wood's glass filter. Since 1993, however, blue and ultraviolet LEDs have been developed, and their prices have been dropping in the last few years – plus they're much more efficient and easier to work with than white light with a filter. There is a disadvantage to blue light as opposed to UV light, however – it's visible to the human eye. Blue light easily outshines the weak effect of fluorescence. To counter this, a yellow mask filter is used by the diver. This blocks the blue light from the torch while letting the fluorescence (which has longer wavelengths) through.


In September 2012, my colleague Lynn Miner and I travelled to the Red Sea to test various set-ups of dive gear for our company Immediately I noticed that I wasn't capturing with my camera all the colours of fluorescence that I was seeing. At first I suspected that this was due to the camera –different rendering of colours under these low-light conditions – but by testing various combinations I could rule out the camera and I realised it was the filter. After systematic tests both in the field and the laboratory, we found some filters allowed a tiny amount of crosstalk, that is they 'leaked' some blue light, allowing us to see blue fluorescence. From the many materials we tested, our current filters have been selected for reproducing the widest range of colours and enabling most subtle rendition of the different shades of fluorescence. 

We've also perfected blue light torches, adding a dichroic (aka interference) blue-pass filter in order to limit the range of wavelengths these torches produce. As we're enthusiasts and do this for leisure, we spent the necessary time, money, effort and materials to get the best possible results we can.

There are some concerns among the diving community that strong UV or blue light torches might harm underwater organisms. In fact, all night dives carry this risk – any illumination could disturb marine life, disrupt their behaviour, or put them at risk of predators. But it's unlikely that fluorescence torches can cause significant harm underwater – they do not shine on one spot for more than a few seconds at most. Plus the energy levels given off by underwater torches are lower by several orders of magnitude than those from natural sunlight in the tropics. In other words, reefs are subject to much more light than divers can possibly create.

You should really have some night diving experience before embarking on a fluorescence dive. Good navigation skills are essential too, as it's very dark behind the mask filter, and if there's nothing fluorescing around you, or if you take time to take photos, it's easy to get disorientated. Keep checking your compass frequently.

Many creatures that stay concealed and are hard to spot during the day or the night are revealed in a fluo dive. They might be transparent, or masters of camouflage, but many will shine spectacularly under a blue light. There's no need to dive deep to see fluorescence – the greatest variety of marine life can often be seen between 2-10m. And the phenomenon is not limited to warm water either. There's plenty to see in the North Sea if you team up your fluo torches with your drysuit.


Where to go

There are dive centres all over the world that offer fluo diving. Head to one of these for a night dive with a difference…



Bonaire Dive & Adventure 

Buddy Dive

Dive Friends Bonaire 

Divi Flamingo Beach Resort


Great Adventures Bonaire 

VIP Diving

Grand Cayman 

Ocean Frontiers


Aquanauts Grenada

Indian Ocean


Wakatobi Dive Resort


Diving Centres Werner Lau 

Euro Divers 

Pro Divers


Glow Scuba

UV Dive Koh Tao



Atlantis Resorts

Blue Ribbon Dive Resort

Magic Island Dive Resort

Prana Bohol

Red Sea


Extra Divers, Dahab 

INMO Divers, Dahab 

Red Sea Environmental Centre, Dahab

Reef 2000, Dahab

Sea Dancer Dive Centre, Dahab

Sinai Divers, Sharm El Sheikh and Dahab


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