Technology Discussion

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. You may need a “Wood’s Glass” type of excitation filter on your torch, though, in case you use a torch with white light (or a strobe), which should have lots of output in the ultraviolet range of the spectrum, such as HID (high intensity discharge) lights.

 

Photo © 2012 L. Miner

Photo:

The photo above shows a laser cutter (e.g. used to make all our acrylic barrier filters).

The same can be done with a white light (or strobe) and a blue excitation filter in order to obtain a blue excitation light, by the way.

However, using a white light in combination with an excitation filter that will throw away most of the torch’s light output is not as efficient as using a torch which has the desired range of wavelengths in the first place.

On the other hand, such a setup may have the advantage to be able to switch between white light and excitation light for fluorescence very quickly, during the dive, obviating the need to carry and to handle another torch with white light.

Note however that this can also easily be solved the other way round, by using detachable phosphor filters which transform blue light into white light; see phosphor filters.

Ultraviolet Versus Blue Excitation Light

Filters

Two types of filters are needed for experiencing fluorescence with visible excitation light (usually blue): excitation filters and barrier filters.

Excitation filters are needed to discard all light from your torch above a certain wavelength, usually with a cutoff wavelength around 500 nm for blue excitation light (which typically has 450-470 nm). All of our fluorescence torches come equipped with such a filter.

This filter is usually a “dichroic” filter, i.e., a transparent substrate (usually glass) onto which several layers with different refractive indexes of predetermined thicknesses have been deposited in a vacuum chamber. These layers cause a certain range of wavelengths to pass the filter, whereas all other wavelengths are reflected. This is also the major difference and advantage compared to conventional filters, which absorb the unwanted wavelengths of light. This can cause conventional filters to heat up considerably, or even to melt. Dichroic filters on the other hand do not suffer from this drawback.

It may seem unnecessary to use a blue-pass dichroic filter on a torch with blue LEDs, but the filter is necessary in order to filter out remnants of the blue light that would otherwise pass the yellow barrier filter (see below for more on these). The reason is that otherwise, very faint fluorescence would be masked or out-shined. The advantage is that overall contrast is enhanced and red fluorescence is much more vivid with the filter than without. Without the filter, images look pale and lack brightness.

See also the special page about excitation filters for several pictures which will demonstrate the difference between blue light with and without excitation filter.

Barrier filters are needed to discard all light below a certain wavelength, usually also with a cutoff wavelength around 500 nm, in the case of blue excitation light (which typically has 450-470 nm).

Barrier filters are used in front of your dive mask and/or in front of your camera in order to filter out any light coming from your torch, either directly or through reflection. At the same time the barrier filter needs to let pass any fluorescent light coming from any fluorescent objects. Coincidentally, fluorescent light always has a longer wavelength than the excitation light used to excite the fluorescence, so this all works out very nicely to enable you to view fluorescence without being blinded by the excitation light.

In a certain way of speaking these filters make the blue excitation light invisible to your eyes, similar to invisible ultraviolet light, which can be used to excite fluorescence without using any filters, because the human eye is insensitive to it. The filters make your eyes (or the camera’s sensor) insensitive to blue light as well.

Barrier filters are usually conventional filters of a predetermined color (usually yellow for blue excitation light) usually made from an acrylic material, or glass.

The advantage of acrylic is that it is easier to cut into arbitrary shapes, and that there is less risk of injury from breaking, whereas the disadvantage is that this material is more prone to scratching. However, this latter problem is alleviated by water, which usually fills the scratches under water and renders them imperceptible.

Barrier filters made from glass are therefore usually used for camera filters only.

See also the special page about barrier filters for several pictures which will demonstrate the importance of choosing the right barrier filter.

Influence and importance of the camera

As you can see on the pages excitation filters and barrier filters, we have optimized torches, excitation filters and barrier filters for best possible results.

Our conclusion from these efforts is that the torch used, the excitation filter and the barrier filter all together form an intimately interconnected system whose components have to be carefully tuned to match and complement each other’s optical properties, in order to achieve optimal results. For this reason, mixing equipment from different manufacturers is likely to give very disappointing results; see also Mixing equipment from different vendors located at the very bottom of the excitation filter page for some illustrative images.

We therefore wondered about the importance and influence of the other remaining, very important component of the system, the camera.

We wondered, everything else kept the same, how big is the influence of the camera on the end result when trying to capture underwater fluorescence?

To answer this question we filmed fluorescent corals in Bonaire (September 2013) with three cameras at the same time, synchronously.

The cameras were an Intova SP1, a GoPro HD Hero3 Black Edition and a Nikon CoolPix P300, which used identical yellow camera filters (barrier filters).

Here are our results:

Comparison Intova SP1 (top left), GoPro HD Hero3 Black Edition (top right), Nikon CoolPix P300 (bottom)

As it turns out, the camera has about as much influence on the results as the filters and torches used!

White light with filter vs. dedicated blue light

White light with filter versus dedicated blue light:

See also the White Paper on Why a Dedicated Blue Light Source is better than a White Light Source with an Excitation Filter for Fluorescence Night Diving (PDF 492KB).  This is an in-depth discussion of this topic.

Many people believe that a white light torch donned with a dichroic filter is sufficient to observe underwater fluorescence.
While it is certainly true that you will probably be able to see SOMETHING with such a setup, there is more to fluorescence diving than meets the casual eye.

First of all, you throw away about 80% of the light output of your white light torch
(see also this video for illustration).
And since fluorescence is a relatively weak effect, you want to have as much power to excite the fluorescence as you possibly can.
This is why throwing away about 80% of your light output is such a disaster,
and something that might well make the difference between a nice crispy photo or a completely blurred one.

Secondly, and even more so with very strong white light torches or strobes,
the images obtained with such a setup are inferior to those obtained with a dedicated blue light torch,
as the following two example photos and two example videos below demonstrate:

[White light]

White light torch with dichroic filter

[Blue light]

Dedicated blue light torch (notice red fluorescence)

White Light torch with a single dichroic excitation filter

Blue Light torch with a single dichroic excitation filter

(Note that you can play both videos at the same time for easier comparison by clicking on the two “play” icons in rapid succession)

Note the greyish-whitish background produced by the white light torch (left), as opposed to the blue background produced by the blue light torch (right).
Note also that the white light torch (left) is visibly weaker than the blue light torch (right), although both torches have about the same power and both use three high-power LEDs.

The following spectrographs of a few (white) dive lights without (left) and with (right) a dichroic filter further illustrate what was said above.  Note the loss of spectral content and hence the overall output luminosity of each torch.  These are not unique examples of poor torches, in fact all of these make wonderful white light dive torches from well respected manufacturers.  However, they make very poor fluorescence torches if one simply places a blue dichroic filter over the front and expects good results.

UK C8 Cannon HID:

[UK C8 Cannon HID]

UK C8 Cannon HID

[UK C8 Cannon HID with filter]

UK C8 Cannon HID with dichroic filter

As one can see, this is a rather “noisy” torch. It’s a great dive light but it is marginal as a filtered fluo dive light.

UK Xenon:

[UK Xenon]

UK Xenon

[UK Xenon with filter]

UK Xenon with dichroic filter

Like the UK HID, this is an exceptional white light dive torch but as a filtered fluo dive unit, it is completely unacceptable.

 

 

 ScubaPro Fuego LED Light:

[ScubaPro Fuego]

ScubaPro Fuego LED Light

[ScubaPro Fuego with filter]

ScubaPro Fuego LED Light with dichroic filter

The Fuego unit has less output power than the above units, but it has more spectral content in the blue range around 430-440 nm.

Tekna 6:

[Tekna 6]

Tekna 6

[Tekna 6 with filter]

Tekna 6 with dichroic filter

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