Thanks to popular demand, here goes a little "how to" guide of how to capture underwater fluorescence: How it works
First of all you will need specialised equipment in order to trigger the effect of and to observe underwater fluorescence. This you will need in any case, whether you want to just watch or capture the fluorescence on camera, because in the latter case you have to be able to find suitable subjects first.
For maximum colour details, the best torches are the ones emitting blue light around 450-480 nm and equipped with an additional excitation filter (an interference or "dichroic" filter, for precision and light efficiency), which will "trim" the light output for better contrast and colour saturation (see also Comparison without and with dichroic filter). You recognise these torches by their golden shimmering front glass, which is an effect of this excitation filter.
Near UV (around 400 nm) and true UV (below 400 nm) generally gives inferior results and is therefore not recommended (see also Ultraviolet versus blue excitation light).
You will need at least one such torch as described above to find your subjects and as a focus light for your camera. This can be a relatively weak torch (e.g. with just a single high-power LED).
If you want to make videos, you will need very strong torches, if possible at least two, for better illumination.
For underwater fluorescence photography you have the choice between using strong torches (and be able to make videos as well) or strobes.
Note that using white light torches with filters is possible, but not recommended. If you use a simple dichroic filter to filter your torch's light, the results will be poor. If you use the strobe filters mentioned above, the results will be better, but only if the torch is powerful enough. However, in any case, you will lose about 80 percent of your torch's light output. This also applies to strobes, but with strobes this is less problematic, because they usually have an abundance of light. A video light which loses 80 percent of its output is pretty much useless, on the other hand. See also White light with filter versus dedicated blue light for more information.
To complete your equipment, you will need a yellow perspex (Plexiglas) mask filter, and a similar filter for your camera. These filters separate the blue excitation light (which you don't want to see) from the fluorescence light (which is what you do want to see).
Beware that mixing equipment from different manufacturers may give unexpected and disappointing results, because the torches, excitation filter and barrier (mask and camera) filters need to be fine-tuned to harmonise with each other for best results. See Mixing equipment from different vendors for some example images.
The next important piece of equipment is your camera. Unfortunately not all cameras give equally satisfying results, probably due to their own internal optimisations and filtering. See Influence and importance of the camera for more information.
The problem is of course that no manufacturer optimises their camera's performance for capturing underwater fluorescence. Likewise it is impossible to know beforehand from the usual reviews whether any given camera will perform well in capturing underwater fluorescence, since this is not (yet?) something being tested by reviewers.
One series of cameras that appears to consistently have given good results is the Canon PowerShot G... series. See for example Coral Project RSEC Dahab 2012 for fluorescence images taken with several different cameras, among which a Canon PowerShot G7.
A camera that has been reported not to give good results is the Olympus E-PM1 PEN.
I use two compact digital cameras, a Nikon Coolpix P300 and an Olympus Tough TG-1. Both give good results, the Nikon a little better than the Olympus.
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 or strobes 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.
To be continued.
When moving at night under water with your yellow fluo mask filter on, it can get extremely dark if there happens to be nothing fluorescent around you - MUCH darker even than on a normal night dive with your lights off, because the yellow filter swallows that tiny little remnant of light (starlight, moonlight, light pollution) you would still see otherwise.
Therefore you need to make extreme efforts to stay aware of your surroundings at all times, and exert extremely good buoyancy control, in order not to bump into something, both for your own sake (imagine your inadvertently bumping into a fire coral or a scorpion fish, or cutting or bruising yourself) and for the sake of not harming the coral reef.
Moreover, experience shows that chances are you'll get disoriented every time you stop to see something better, to take a picture or to film. Therefore you should ALWAYS look at your compass EVERY TIME after stopping. I mean this LITERALLY. Don't take this lightly, DO IT! You'll eventually see the benefit!
There are several ways for you to have enough light to navigate around safely. One way is to shove your mask filter up to your forehead when moving from one place to another (which is why some people prefer flippable mask filters such as Flippable Mask Filter, Flippable Mask Filter Aqualung LOOK 2 and Flippable Mask Filter XS Scuba GoPro, for example).
Another way is to carry a white light, just for the purpose of navigation. However, it is somewhat of a hassle to carry several lights and having to switch between them constantly.
By the way, as a side note, blue light is much better to read your instruments (at least if they are phosphorescent) than white light. Contrary to white light, which also tends to glare, the blue light charges the phosphorescent dye of your instruments, which allows you to read them with much more leisure for a couple of seconds after shining your blue light torch on them.
In order not to have to carry a separate white light, several solutions exist:
Some people have built special torches which can switch between white and blue light. However, this is costly and laborious.
Another possibility is to use phosphor filters (see also Phosphor Filters) in the form of a cap which can simply be tucked over the torch's head whenever you need white light (otherwise the phosphor filter is left dangling from a cord, string, cable tie or wire etc. attached to the torch).
This is much more efficient than it sounds. Actually all white light LEDs nowadays do exactly the same, they are actually blue light emitters with a phosphor layer on top of the LED chip (below the transparent lens). This is MUCH more efficient than the other way around, namely to use a blue filter on a white light torch, which would in theory also allow you to switch between white and blue light under water (however, you'll probably remember from the first part of this series that you would lose about 80 percent of your torch's light output that way!).
The last and simplest method is to have very strong blue lights (video lights). These lights are usually so strong that it is possible to navigate with their blue light only, and without even flipping up your mask filter (see also the photos of fluorescent markers and a tarpon hunting to see what that looks like). This also has the added benefit that the eyes do not have to constantly adapt to changing light conditions.
To be continued.
You may wonder what camera settings you will need in order to capture underwater fluorescence.
This is both easier and harder than daytime underwater photography:
The easier part is that you don't have to worry about white balance. Since you do not depend on daylight filtered by several (tens of) meters of water column, which distorts the colour balance, you can simply use "auto" (automatic) white balance.
Dr. Charles Mazel, the inventor of the modern form of fluo diving with blue light and yellow filters, prefers to use the setting "clouded" instead.
If in doubt, just experiment a bit in order to see what you like best!
Most cameras also have a "fluorescent light" option for white balance. This you should NOT use, because this is intended for indoor use with light coming from gas discharge tubes or energy-saving light bulbs. Unless you want to deliberately create an artistic effect, or to reduce the blues and enhance the reds, that is, for example.
The harder part is that photographing at night means very low light conditions. And since fluorescence is a rather weak effect (only a portion of the light shone on the fluorescent pigments will be converted and re-emitted as fluorescent light), this means that you are dealing with even less light.
Under these conditions you should have a camera with a sensor (and lens) with outstanding low-light properties.
Even if you have, the correct focus, motion blur and depth of field become real challenges, because the camera will have difficulty focusing, and because it will likely use the widest aperture it has at its disposal, and still use very long shutter times - it may also use ISO settings so high that the images become visibly very noisy and grainy.
For better focusing, you can either switch to manual focus, if the camera permits, or try to use a stronger (blue!) focus light.
Using a white focus light would either ruin your photo (taint it with white light, possibly outshining the fluorescence) or would make the handling almost impossible: you would have to switch off the focus light while keeping the shutter release button half-pressed, after focusing.
In order to alleviate the problem of motion blur and depth of field, there is a trick: deliberately underexpose by two or three stops (many thanks to Markus Laube for this tip!).
First of all this will reduce the background to black and therefore nicely separate and emphasise your subject.
Next it will allow shorter exposure times and therefore reduce motion blur. It may also give you some leeway to improve the depth of field.
And thirdly, you can always correct the brightness afterwards in Photoshop or the like, if necessary (of course a camera with the capability of storing images in "raw" format will significantly increase your correction possibilities).
Most cameras tend to optimise the brightness of the WHOLE picture, even when configured to use "spot metering" mode, which is completely unnecessary in underwater fluorescence photography, where the background SHOULD preferentially be completely black, and not a standard grey, as the camera thinks.
See Seeing natural fluorescence underwater in the daytime, Fluorescence Photography without Darkness and Underwater fluorescence photography in the presence of ambient light (in increasing order of sophistication) for articles about how to observe/capture underwater fluorescence during daytime.
On my digital compact camera Nikon Coolpix P300 I use a fixed shutter speed of 1/60s in order to avoid motion blur, and I limit the ISO to the range 160-400 (because the camera's small sensor will produce too much noise at higher ISO settings). In addition to the ISO, the camera automatically chooses the aperture, usually one of the widest, such as 1.8f, 2.0f or 2.8f. I often set the exposure to -1, plus I often use the small (10 x 5 mm) built-in flash with a double dichroic filter attached to it.
To be continued.
Part four of the "how to" guide of how to capture underwater fluorescence: Frequently asked questions
Today's contribution will be organised as a set of "frequently asked questions" and answers.
To be continued.
Part five of the "how to" guide of how to capture underwater fluorescence: Light intensity specifications (Lumen)
Why it makes no sense to indicate "Lumens" of blue or UV light torches for fluo diving:
"Lumen" is a measure of how much light the human eye will perceive, weighted according to the specific sensitivity of the human eye to different wavelengths of light (see also https://en.wikipedia.org/wiki/Lumen_(unit)).
The light of our blue (or UV) light torches however is not meant to eventually reach the human eye, on the contrary, the blue light is meant to be filtered out by the yellow mask and camera filters (true UV light < 400 nm is invisible anyway). Instead, it is meant to pump energy into the fluorescent pigments of the underwater organisms of interest, so the most appropriate measure for our torches is the light energy or "radiant flux" they emit.
What reaches the human eye (or the camera) is the fluorescent light emitted by the fluorescent pigments, whose efficiency in doing so (for any given amount of light energy absorbed) varies greatly depending on the organism (and the pigment) in question.
To be continued.