Science

Photo © 2012 S. Beyer

FireDiveGear.com prides itself on offering some of the finest quality products at the most reasonable prices. All our hardware is calibrated against calibrated intrumentation, machined at close tolerance and use the highest quality LEDs and filter materials available.

We offer custom mask filters and made to order camera filters.

A great deal of research has gone into the development of these products and we are certain you will enjoy the results of your Fluo diving experience.

In the following sections we will explain the science behind underwater fluorescence in general as well as the science behind our products in particular.

The photo at the top right shows a fluorescent brain coral.

Discussion: We must start our discussion with a look at the electromagnetic spectrum (EM), see figure to the left. All radiative energy has an associated frequency (f) and wavelength (λ). One is a function of the other: f=C/λ and λ=C/f, where C is the speed of light (3x108 meters per second, or m/s). Wavelength is the distance in meters from one peak (or any other reference point on a wave) to the next peak (or next reference point), see the figure above. Frequency is simply the number of times that wavelength occurs in one second. The visible portion of the EM spectrum is a very narrow band of wavelengths from about 400 to 700 nanometers (nm or 1x10-9 meters). The area we are most concerned with is what is called near ultra violet (UV) or "actinic" light. This is light whose intensity is centered around a peak at approximately 420 nm. The term "actinic" is somewhat of a throwback to the early days of photography. Early films were not sensitive to red light and hence the term "safe lights" refers to dim red lights which were used in darkrooms during processing. Films of that era were sensitive to blue or "actinic" light, however. The term "actinic" is also used in many scientific papers on the subject of coral fluorescence. Referring again to the figure to the left, you see that this is in the deep blue, nearly violet portion of the spectrum. As all divers know, blue is the last color of the spectrum to be absorbed by water, hence near-actinic light is the light most sea creatures are exposed to, and that is also the light which is available for their photosynthesis.

What we as divers need to have in order for the effect of fluorescence to occur and in order to be able to observe it are two things:

1. A light source that emits light near or slightly above the actinic band (~430-450 nm). This can be a blue LED, or a dichroic filter which blocks everything from a white light source except blue, or preferably a combination of both a blue LED and the filter. This "assembly" we shall refer to as the excitation source or exciter. This exciter is used to shine on the subject target organisms.

2. A second filter is needed to block the blue light from coming back into your eyes. This is called a barrier filter and is made of a yellow acrylic material which blocks the intense blue light reflected from the target, and which at the same time passes the emission light (fluorescence) in the green to red range. This filter is placed over your dive mask (and/or camera lens).

Referring again to the figure of the electromagnetic spectrum above, note that as one moves to the bottom of the scale, the energy levels increase. Therefore UV light has more energy than red light. This will play an important role in our discussion below.

Please also refer to the above figure whenever you see references to wavelength and frequency throughout this discussion.

A "Full Spectrum Light" is a light source which emits all the wavelengths of the visible spectrum in the same proportions as natural sunlight. A lamp, LED, or bulb labeled "full spectrum" means that it emits light over the entire visible spectrum with a spectral output similar to that of the sun. This is what a standard white light underwater dive light (or torch) produces.

Fluorescence:

Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore lower energy, than the absorbed radiation.

Fluorescence occurs when an orbital electron of a molecule, atom or nanostructure relaxes back to its ground state, thereby emitting a photon of light, after being excited to a higher quantum state by some type of energy.

In our case, we are "hitting" an organism with higher energy light (relatively) in the near-actinic range, and lower energy light (relatively) in the green, yellow and red portion of the spectrum is being emitted. The actual color emitted is determined by how many quantum states the electron has "decayed" or relaxed back to.

The figure here to the right represents this schematically.

When energy (UV or violet/blue light in our example) strikes an atom, it knocks an electron up to a higher energy state. When the electron decays back to its normal state (usually instantly, after a few nanoseconds), it emits a photon of light (in the more visible, lower energy part of the spectrum in our example).

It is not well understood why it is that some corals and other sea creatures fluoresce. What is known is that some marine organisms (such as corals, sponges, anemones, jellyfish, clams, nudibranchs, shrimp, crabs, worms, fish) produce proteins which react to light causing this effect. The curious reader is encouraged to do a web search as there are a number of detailed peer reviewed scientific papers on this topic and about the scientific and medical implications of this phenomenon (see e.g. Green Fluorescent Protein (GFP) for an introduction).

Note that fluorescence is different from phosphorescence (after excitation, light is emitted over a longer period of time, as can be seen e.g. in cathode ray tubes, i.e., in pre-digital age television sets) and from luminescence (some marine organisms actively produce their own light using certain enzymes or symbiotic bacteria).

Some fluorescent corals have been discovered because they were bright red despite the fact that at the depth that they were found, red light is completely absent, because red light is the first to be filtered out by water (which is also the reason why water appears blue from above, and why underwater images have such a blue tint, unless corrected). See also the article Red at depth: Colors disappear underwater, but not always which shows many examples (photos) of such corals.

When divers first dove with torches under water, in the 1940's, they discovered that many organisms were actually red. It was a biological mystery why organisms would spend energy to produce a pigment which would appear black below a certain depth anyway. It was speculated that this was used for hiding, but this hypothesis was not very satisfying.

New results from scientific research show that many fish, even deep sea fish, can actually see red light. One wonders why, since there is no red light at these depths.

It has been found recently that underwater organisms actually use fluorescence to transform the only light available to them, namely ultraviolet and blue light, into visible light of longer wavelengths, such as red (of all colors!), among others, for a number of purposes:

Besides from apparently protecting themselves from the harmful effects of ultraviolet radiation, as a kind of sunscreen, corals seem to do this in order to feed their symbiotic algae, which live inside their tissues. This allows the corals to dwell at greater depths, where corals without this capability are unable to thrive.

More recent discoveries seem to suggest that fish also use fluorescence in order not to be easily discernible from the background of fluorescing corals, which otherwise would make them easy prey, and in order to communicate between each other (within the same species), at least over short distances, see Red fluorescence in reef fish: A novel signalling mechanism?.

Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative energy as a function of its wavelength or frequency.

Spectroscopic data is often represented by a spectrum, a plot of the response of interest (amplitude) as a function of wavelength or frequency. This is often referred to as spectral power distribution (SPD).

At FireDiveGear.com, we use spectroscopy to analyze and calibrate our torches, excitation filters and barrier filters for peak performance.

The figure here to the right is a representation of what happens when you shine a white light through a prism. The light is broken up into its component colors because each color has its own energy level and hence has its own path it takes through the prism.

We use an ASEQ Spectrometer in all of our analysis work. Basically it is a box with an input port for light to enter. The light is then sent through a prism ("diffraction grating").

The pattern similar to that shown in the figure adjacent hereto is then deposited onto a photo sensor where each color generates a corresponding electrical signal. These signals are then sent to a computer where the data is processed and displayed as an image like that in the left figure below.

The higher the signal for each particular color, the higher the amplitude displayed on the screen for that particular wavelength. This is what is meant by spectral power distribution. Using this technique, we can custom design filters and calibrate filter and LED combinations to exactly the performance parameters we desire.

The right figure below shows the test setup to perform a filter analysis.

Light output from a UK 100 HID dive light

Setup to perform a filter analysis

A dichroic filter, or interference filter, is a very accurate color filter used to selectively pass light of a small range of colors while reflecting all other colors.

Dichroic filters use the principle of thin-film interference, and produce colors in the same way as oil films on water. When light strikes an oil film at an angle, some of the light is reflected from the top surface of the oil, and some is reflected from the bottom surface where it is in contact with the water. Because the light reflecting from the bottom travels a slightly longer path, some light wavelengths are reinforced by this delay, while others tend to be canceled, producing the colors seen.

In a dichroic filter, instead of using an oil film to produce the interference, alternating layers of optical coatings with different refractive indexes are built up upon a glass substrate, usually by vacuum deposition. The interfaces between the layers of different refractive indexes produce phased reflections, selectively reinforcing certain wavelengths of light and interfering with other wavelengths.

By controlling the thickness and number of the layers, the frequency (wavelength) of the passband (the band of frequencies or wavelengths that will pass through the filter) of the filter can be tuned and made as wide or narrow as desired. Because unwanted wavelengths are reflected rather than absorbed, dichroic filters do not absorb this unwanted energy during operation and so do not become nearly as hot as the equivalent conventional filters (which attempt to absorb all energy except for that in their passband).

ScubaPro Fuego LED Light

ScubaPro Fuego LED Light with a dichroic filter

This figure shows the spectral output from a white light torch (in this case a ScubaPro Fuego LED Light). Referring back to the figure near the top of this page showing the EM spectrum, one can see that this particular underwater white light torch has most of its content in the 500 to 650 nm range, with a significant spike in content centered on roughly 430 nm. This is a fairly common characteristic output from a white LED source.

This figure shows the same torch with the light passing through a dichroic filter. Again referring back to the figure near the top of this page showing the EM spectrum, note that nearly all the (low energy) light above about 475 nm is gone (reflected away, not absorbed!) with the exception of a small hump in the infrared region. This hump is so small in amplitude that it can be ignored for this analysis, though. The peak at 430 nm (higher energy light) is still there however giving this torch a violet/blue output.

You may be wondering: "When I already have a torch with blue light, why do I still need a blue excitation filter in addition to that?"

See the special page about excitation filters for an answer to that question; that page contains several pictures which will demonstrate the difference between blue light with and without excitation filter.

As noted above, the above torch/filter combination emits a very deep blue colored light.

However, this (strong) light overwhelms the (weak) fluorescent light coming back to your eyes (or camera lens) off of the target subject, which is the reason why a blue barrier filter is required. It simply blocks all light below about 480 nm.

In the two figures shown below you can easily see the effect of using a barrier filter. Any blue spectral content has been completely eliminated:

White LED torch

White LED torch with a barrier filter

This barrier or blocking filter is placed over your dive mask and/or the lens of your camera and is custom-cut by a laser cutter to your specifications. See the products page for more detailed instructions on how to submit your template information.

See the special page about barrier filters for a demonstration of the importance of choosing the right filter material.

See the special page about spectrographs for a number of spectrographs of various common white light dive torches, in order to give you an idea of what you may already have at your disposal in case you opt to simply use a white torch and a filter.

As mentioned earlier in this text, the best combination is a torch with high performance blue LEDs and a dichroic filter. However, some have found the results of just using a dichroic filter over a white light source "gets the job done" to their satisfaction.

This is why we offer custom filters and mounts for many white light torches. The images of the spectrographs page are provided to help you decide whether what you have will perform for you.

See the page technology for an explanation of the technology and particular choices of our products.