Fluoro Diving: The Art and Science behind it (Part 2)

Fluoro Diving: The Art and Science behind it (Part 2)
By Lynn Miner

The first installment in this series dealt with the physics and biology of bio-fluorescence.  This installment will cover the required equipment.

 Lights and Filters

A blue light torch is required equipment for fluo diving with a bulb or LED that emits light in the 450-470nm wavelength range.  This is usually referred to as Royal Blue or Actinic light.

Additionally, some fluo torches include a dichroic filter to further enhance the imaging experience.   Both images below are taken with the same torch, same camera configuration, same settings.  The image on the left had no filter, the one on the right had a dichroic filter installed on the torch.  The addition of the dichroic filter has a dramatic effect on color saturation.


Figure 1 Figure 2

 

These torches are commercially available at some dive shops but since this is such a specialized product line, your best source is the internet.  Simply search on “fluo dive gear”.

Prices can range from several tens of dollars into the thousands for commercial videographer units. 

Below are examples of a blue light torch courtesy of www.firedivegear.com

 

 Excitation Filters

Excitation filters are required for strobes and white light sources to make the required blue light.  It’s usually a bad idea to try to use an excitation filter on a white light torch because you lose about 80% of the output power.  The reason why that is becomes a bit complicated for a short series like this but for the technically minded go to:

http://firedivegear.com/wp-content/uploads/2015/01/White-Paper-Why-a-Dedicated-Blue-Source-is-better-than-a-White-Source-with-Excitation-Filters-for-Fluo-Diving.pdf 

and

http://firedivegear.com/wp-content/uploads/2016/03/White-Paper-Why-We-Use-A-Dichroic-Filter.pdf 

This is less of a problem with a strobe because they produce so much output light.  They still lose about 80% but there’s still plenty of light to get the job done in most cases (especially macro work).

 

 Barrier Filters

When using blue light, you need a barrier filter to go over your face mask.  This is an essential piece of equipment.  It serves two purposes:

1.  It blocks the blue light that causes the fluorescence effects in the marine creature from entering into your eyes.  This blue light can cause your eyes to burn and become irritated after a long period of time making the dive very unpleasant.

2.  The blue light is also much brighter than the emitted fluorescence given off by the organism so the colors you are looking to observe are overwhelmed and washed out by the blue light.

 

The barrier filter solves both of these problems by “cutting off” the wavelengths (colors) of light just above the blue portion of the visible spectrum.

 

If you were to overlay figure 4 and figure 6 you would see that the resultant spectrum is just the wavelengths above about 480nm.   This type of filter will leave a tiny bit of blue that most photographers find aesthetically pleasing.

  

The images below illustrate the effect of the barrier filter.  The first image is of a fluorescent “slate” that was photographed under white light.  The second image is the same slate under blue excitation light and the final image was taken under blue excitation light with a yellow barrier filter placed over the camera lens.  It can readily be seen how much visual acuity is reduced because a barrier filter over your mask will have the exact same effect as that over the camera’s lens.  The barrier filter is required however because as noted earlier, the blue light completely overwhelms the fluorescence emission of marine organisms which don’t “light up” like this slate does.

 

An extremely detailed discussion of barrier filters can be found here:

http://firedivegear.com/science/barrier-filters/ 

o   Blue Light vs. UV Light

Generally, there are two ways of experiencing underwater fluorescence, either with invisible (ultraviolet) or with visible (usually blue) excitation light.

In the case of ultraviolet excitation light (with wavelengths <400 nm), no filters are needed for your mask and camera.  However, it is not as efficient as using a torch which has the desired range of wavelengths in the first place.  An extremely detailed discussion of why that is can be found here:

http://firedivegear.com/wp-content/uploads/2015/01/White-Paper-Why-a-Dedicated-Blue-Source-is-better-than-a-White-Source-with-Excitation-Filters-for-Fluo-Diving.pdf 

Even though using invisible (UV) excitation light has the advantage that usually no filters are needed, it has been discovered that visible light (most prominently blue light, with wavelengths around 450-470 nm) is MUCH more effective in stimulating fluorescence of Green Fluorescent Protein (GFP) than ultraviolet light (by about a factor of four, with light of the same energy).  An extremely detailed discussion of why that is can be found here:

http://firedivegear.com/blue-light-vs-uv-light/ 

If you decide to use UV, be aware however that shining powerful UV radiation into your eyes (or your buddy's) may cause severe harm to you or your buddy's eyesight, especially because this radiation is invisible and therefore the corneal reflex (blink reflex) and the pupillary reflex do not work.  Powerful ultraviolet lights are not something you want to give to inexperienced divers to handle.

Strong blue light (as any strong light) can also cause eye discomfort, but since you will immediately feel that discomfort this is somewhat less likely to occur. 

There are actually corals which do not fluoresce under UV light at all, while they do under blue light.  This is presumably due to the fact that through evolution, underwater organisms have adapted to the properties of water, which is more transparent to blue light than it is to UV (or to any other color than blue, for that matter).   95% of the UV from the sun bounces off the surface of the water.  The 5% that penetrates the surfaces only goes 1-2 meters deep before it’s complexly absorbed.

This, by the way is the very reason why you get a bad sunburn when you’re on the surface waiting for the boat to pick you up.  You’re getting a direct dose of sunlight on your face and at the same time getting another near full dose reflected off the water.

Compare the two images below:  The one on the left is a target exposed to a UV torch.  The image on the right is the exact same torch only configured with blue LEDs rather than UV LEDs (no dichroic filter).

 Why Lumens can’t be used to rate torch output brightness

Most white light torches are rated in lumens.  A blue light torch can’t be rated in lumens and a lumen meter will give erroneous results when trying to measure a blue light torch. 

Here again, we have an extremely technical topic that can’t be adequately discussed in a short series like this.  Go to:

http://firedivegear.com/wp-content/uploads/2015/12/White-Paper-Lumens.pdf 

Now that we’ve gotten the science and equipment issues under our belts, our next part of the series will get to the heart of the photography of fluorescence diving.

 

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