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An Aquatic View of the Complexities of Light

By Ian Ashdown. Presented June 7th, 1998 on #reefs IRC.

This talk is about aquarium lighting, with an emphasis on the word "light." It is *not* about who makes the best products or how various species of marine life respond to different luminous environments. An important caveat: I owned a 100-gallon freshwater aquarium some 35 years ago when most people thought of aquaria as two goldfish in a glass bowl. I am *not* familiar with today's vastly more advanced aquarium technology.

As a lighting research engineer, I am however very familiar with my favorite topic: light. It is an enormous field of discourse, ranging from the physiology of human vision to photobiology and quantum mechanics. I cannot hope to cover the entire field in relation to aquarium lighting in a short talk. Instead, I will present four brief topics and then attempt to answer your questions.

Measuring Light

Light consists of photons, with each photon having a specific amount of energy. We see these photons as having different colors; photons with increasing amounts of energy appear to us as pure red, orange, yellow, green and blue light. Beyond the visible spectrum is infrared light (low energy) and ultraviolet light (high energy). Most light sources emit photons with a wide range of energies. What we perceive as "white" light is really a mixture of photons ranging from infrared to ultraviolet. The proportion of photons with different energies determines the color we perceive the light source to be.

Our eyes do not see all photons equally. They are most sensitive to yellow-green light, and are less sensitive to red and blue light. Of course, we do not see infrared or ultraviolet light at all. Here's a problem: when we measure light with a light meter

(typically illuminance in units of footcandles or candela per square meter), we are measuring according to the color sensitivity of the human eye. Unfortunately, it is highly unlikely that marine life will have the same sensitivity to different colors of light. Water absorbs light of different colors, and the rate of absorption depends on the presence of suspended silt, plant and animal material, and living organisms such as plankton. For example, the transmittance of water per meter for selected locations is:

  1. Morrison Springs, Florida
  2. Gulf of Mexico
  3. Long Island Sound
  4. Thames River (Connecticut)
Loc 1 Loc 2 Loc 3 Loc 4
Ultraviolet (400 nm) 92 % 56 % 38 % 0 %
Blue (450 nm) 95 % 83 % 40 % 1%
Blue-green (500 nm) 97 % 95 % 45 % 4 %
Green (550 nm) 95 % 94 % 48 % 5 %
Orange (600 nm) 83 % 80 % 45 % 6 %
Red (650 nm) 74 % 73 % 40 % 12 %
Infrared (700 nm) 58 % 55 % 33 % 13 %

Remember, this is per meter of water. If you are trying to simulate an environment at a depth of 4 meters, for example, and the transmittance is 74 % per meter for a given color, the amount of light reaching this depth from the surface will be 0.74 * 0.74 * 0.74 * 0.74 = 30 percent. For a transmittance of 97 % per meter, it is 86 percent. The spectral distribution of light, even in the same geographical location, clearly varies with depth.

My point is that marine species that occur at a given depth will mostly likely have evolved their visual systems to take maximum advantage of the available light. If you are measuring light with the intent of simulating a species' natural enviornment as closely as possible, you should consider the spectral distribution of the light. Measuring light according to our visual system's color sensitivity may not be appropriate. To do this properly is admittedly a problem. You need to know the spectral transmittance of the water for the species' natural environment (a good excuse to visit the Great Barrier Reef), which requires a fairly expensive spectroradiometer (and a waterproof one at that). You also need to know the spectral distribution of your light sources, which may not be readily available from the lamp manufacturer.

These, however, are resolvable problems, especially in these days of worldwide dissemination of data on the Web. The question is whether the problem itself is a true concern or a theoretical exercise. (As an aside, lamps for horticultural purposes are designed to produce most of their light in the orange and red portions of the visible spectrum. Various plant pigments involved in photosynthesis and plant growth are quite sensitive to the spectral distribution of light in their environment. The same is likely true of symbiotic marine algae in corals.)

Another issue regards underwater light measurements, particularly in environments with light-colored sandy bottoms. Skiers and high- altitude mountain climbers know all too well that the ultraviolet light reflected from snow-covered slopes can cause sunburn almost as quickly as direct sunlight. (Imagine being sunburned on the inside of your nostrils -- it happens.) Relating this to keeping deep-water fish in an aquarium brings up the same issue: the light reflected from the bottom of the aquarium is not measured by a light meter facing the water surface.

Seeing Colors

We may think we can distinguish millions of colors, but we really can't. Instead, the retinae of our eyes have color-sensitive "photoreceptors" that can only distinguish three overlapping ranges of colors - red, green and blue. What we see as a particular color are really electrical signals to the brain from these photoreceptors. The color we perceive is due to the relative strengths of these signals.

This leads to an interesting physiological effect and a possible problem for aquarium lighting. Sunlight consists of photons with a continuous range of energies from infrared through visible to ultraviolet light. We perceive the color of sunlight as "white". However, because the human eye is sensitive to only three ranges of colors, we may also perceive a mixture of three specific colors - red, green, and blue - as being the same color of white.

Lamp manufacturers take advantage of this effect by producing fluorescent and high-intensity discharge (HID) lamps that emit light only in narrow bands of the spectrum. We may see the emitted light as being white, but it is not in any way equivalent to sunlight. Is this important? It depends on how your aquatic friends perceive and respond to light. Some species of shrimp have been shown to perceive ten different ranges of colors (compared to our three). The evolutionary reasons for this capability are unclear, but it is likely that it offers the shrimp benefits in identifying food and fellow membera of their species. If you attempt to recreate their environment using fluorescent or HID lamps, they may be unable to see some colors. (Imagine living your life in an environment that has only blue light.)

As an aside on this issue, lamp manufacturers often rate their lamps with a Color Rendering Index (CRI). This index is a measure of how well the lamp will render colors of various materials in comparison to daylight (which is a mixture of direct sunlight and the clear blue sky). A low-pressure sodium vapor streetlight, for example, renders most colors very poorly and so has a low CRI. A lamp with a high CRI will allow us to perceive most colors such as they would appear in daylight. Again however, what we see in terms of colors is not necessarily what your aquatic friends will see.

Sunburnt Starfish?

Simulating the environment of a shallow water tropical reef can be challenging. The average illuminance of our homes and offices is on the order of 100 to 500 candela per square meter (10 to 50 foot- candles). Direct sunlight, on the other hand, is on the order of 10,000 candela per square meter (1,000 foot-candles). That is a lotof light.

One obvious solution is to use high-intensity discharge (HID) lamps. As long as you don't inadvertently cook your aquatic friends with an excess of infrared energy, you should be fine. Right? Not necessarily. HID and fluorescent lamps produce most of their light in the ultraviolet region of the spectrum. The lamps bulbs are typically coated with rare earth phosphors that absorb ultraviolet light and re-emit it as visible light. The problem here is that the process is not 100-percent efficient.

Many HID and fluorescent lamps emit a proportional amount of ultraviolet light well in excess of that found in natural daylight.

The water will absorb some of this excess ultraviolet light, but clear water still transmits up to 90 percent of UV light per meter. In humans, excessive exposure to ultraviolet light produces sunburn, skin cancer, cataracts and and morphologic alterations of the skin, including wrinkling, altered pigmentation, and thickening. It is likely that similar effects occur in aquatic animals and plants. (Amphibians appear to be particularly sensitive to excess amounts of ultraviolet light, especially in the egg and larvalstages.)

The real problem is that most lamp manufacturers do not publish any information on the relative proportion of ultraviolet light emitted by their lamps. As one example, the amount of UV produced by two otherwise identical compact fluorescent lamps was determined to differ by a factor of four. Lighting manufacturers often use UV-inhibiting acrylics in their fluorescent lamp diffusers, but this may not be a viable solution when the lamps are producing 10 to 20 the amount of light normally found in architectural light fixtures.

To address this problem requires an ultraviolet irradiance meter. Unfortunately, the energies for ultraviolet photons varies considerably, with wavelengths ranging from 257 to 453 nm. The ultraviolet spectrum is divided into several regions according to wavelength, and the biological effects of photons in these different regions varies. The problem is not a simple one by any means.

Getting Technical With Microeinsteins

Poor Albert Einstein! Horticulturalists, looking for some means of quantifying the effect of light from various sources (daylight, xenon lamps, and metal halide lamps designed for greenhouses), named a unit of light measurement after him - the einstein. Unfortunately, this is a *lot* of light, so the units of measurement for real world applications is the microeinstein. Marine biologists use the same unit of measurement when referring to the response of symbiotic marine algae in coral.

The key concept is the absorption spectra of chlorophyll, that wonderfully complex molecule that makes our world turn. (No chlorophyll, no oxygen, no animals, no humans). It has a significant absorption range between 550 and 720 nm, with peak absorption at 700 nm. Various pigments alter the actual absorption characteristics of different plants, so horticulturalists required some standardized means of measuring the photosynthetic efficacy of polychromatic light. Some bright biologist decided that if you plot the photon energy between 400 nm and 700 nm (the limits of the visible spectrum, from deep blued to deep red), it grossly approximates the spectral absorption characteristics of chlorophyll. This we have photon (quantum) flux, in which the number of photons (rather than their energy) per second per unit area is significant.

Plant biologists use what are called quantum meters to measure plant irradiance, which is nothing more than a light with a color filter whose spectral transmittance ensures that its response is directly proportional to the number of photons rather than their energy. Needing a unit of measurement, they turned to chemistry for the mole (6.23 x 10+23 atoms or molecules in a mole). This gave them the microeinstein, or a micromole (6.23 x 10+17) photons per second per square meter), of irradiance as a measure of photosynthetically active radiation.

If you know the wavelength of monochromatic light, you know its energy from Planck's law (see any college physics text), and so you can calculate the number of photons per second per unit area for a given irradiance. Integrate this over the region of 400 nm to 700 nm (the visible spectrum) for a given light source and you have the plant irradiance value in microeinsteins. It's simple once you understand the underlying concept.

As a rule of thumb: one watt/sec/m2 of sunlight or light from horticultural lamps such as xenon or metal halide arc lamps is equivalent to 4.5 to 5.0 microeinsteins. Similarly, one lux of visible light is approximately equivalent to 0.01 to 0.02 microeinsteins. Your mileage may vary for aquarium lighting, but you get the idea.

I doubt whether I have answered many questions with these discussions, but this was not my intent. Rather, I wanted to offer a few topics for discussion that may not appear in the usual literature. If you want to discuss lamps and ballasts or light measurements, then by all means post your questions. However If you have understood that there may be issues with respect to aquarium lighting beyond that presented by the lamp manufacturers (whose primary interest is in producing illumination for the human visual system), then I will have accomplished my goals with this talk.

- Ian Ashdown

OK, a microeinstein. It was in the talk, but it flew by too fast for most people. The basis of the "microeinstein" is a measurement of light irradiance. That is, it is a meaure of how many photons illuminate an area (say one square meter) per second. In lighting, we measure this quantity as "candela per square meter", or "footcandles". It's really just a measure of how much light is illuminating a surface. The problem is that we see light best when it is yellow-green in color. We see red and blue light (at the ends of the visible spectrum) very poorly - our eyes are not as sensitive to these colors. Plants (presumably including marine algae) "see" light differently. Photosynthesis is based on the ability of chlorophyll to absorb energy from from light, and it does this most efficiently in the red region of the spectrum. The microeinstein takes this into account by preferentially measuring orange and red light.

(The discussion has a more coherent explanation) -

Given that you say a microeinstien is designed for terrestral measurment what can we use for underwater measuremtn so we can converse with each other and compare apples to apples?

That's the problem - as Bretton Wade has informed me, there does not appear to be an appropriate unit of measurement.
Certainly the Illuminating Engineering Society of North America (which defines units of radiometric and photometric measurements) hasn't considered the issue. - IA

Are dimmable MH balasts of any real benefit?

It depends on why you might want to dim the lamps. If you are trying to provide a day-night cycle to maintain the biological rhythms of your inhabitants, then yes. However, this assumes that you have a programmable dimmer

Ian - you didn't use the term PAR - Photosynthetically Available Radiation - How do you feel about that unit of measurement?

PAR is an equivalent for the microeinstein, or micromoles per second per square meter.

What colors of spectrum seem to filter out quicker and is it a constant rate?

The yellow region of the spectrum goes first. The loss is geometric - if you have 74% transmittance per meter of water, you have 74% afer one meter, 55% after two meters, 40 % after three meters, and so on IA

Is there a chart we can refer to that shows the 'brightness' needed for aquaria?

Ah, "brightness." Lighting engineering cringe when we hear that term - it can mean too many things! What we see when we look at a surface used to be called "photometric brightness", but is now called "luminance". Simply put, it is the average amount of light emitted or reflected from a surface in the direction of the viewer. Of course, this is not what you want. What you are asking for is "irradiance", which is the average amount of light illuminating each sqaure meter (or square foot) of a surface. It is measured in watts per square meter. The problem is that you are not illuminating a surface; you are illuminating a *volume* of water. The water absorbs the light according to the depth, so that the light level at the bottom of an exceptional large aquarium may be significantly less than at the surface - as little as 50 percent for 12 feet, for example. For most of you (I am not an aquarium enthusiast), the aquarium depth is more like one to two feet. This means that the light loss is (assuming clear water) usually less than 10 percent. In other words, all you really need to worry about is: (a) how much light per unit area is illuminating your aquarium; and (b) what is the spectral distribution of the lamps used for the lighting system. Part (b) is the hard part, of course.

What about in turbulent water? Is there any way to approximate the effect on the refractive index?

I'm not sure why you are interested in the refractive index of light, but turbidity does not affect it. It is no different than trying to see through smoke or fog -

I guess, as far as the amount that actually travels downward, versus being redirected... or does it balance out?

for light travelling downwards, it has to enter the water perpendicular to the surface. The refractive index of water has no effect at this angle.

Will it ever be possible to ~fine tune~ lamps to favour zooanthelae growth and not undesirable algae?

If by "fine tune" you mean adjust their spectral distribution, then the best way is to use color filters.

Ian, is there any direct relationship between K ratings, nm and visually perceptible colors?

K ratings - ask me another time by e-mail ( - I have to investigate the topic

Ian, could typical UV leakage from aquaria bulbs ever be an eye hazard?

yes, for two reasons. First, looking directly at the lamps or their reflections for extended periods of time may cause eye damage. Second, high-powered HID lamps can generate small amount of ozone, which is a hazardous gas.

Is an electric MH ballast really worth $100 more than a tar ballast? Is there a significant difference in perfomance?

I assume you meant "electronic" ballast. These generally consume less power (ie, emit less heat), and should last longer. They also produce *much* less visible flicker. We can't usually see the flicker, but I wonder what effect it has on animals with faster visual system responses. in general, yes. Better lifetime, less power loss, less flicker, cooler operation. The newer electronic ballasts (at least for fluorescent lamps) incorporate "soft start" features that gradually increase the lamp current. This reduces the physical deterioration of the lamp electrodes and so reduces premature lamp end blackening However, I am not sure about HID lamp ballasts. These are newer entries to the market, and I have yet to design no, electronic ballasts generally do not affect the spectral distribution of lamps under normal operating conditions. again, a very general question with no single answer. See the manufacturers' data for more information

Does manufacturer of the bulb matter as much as your ballast type when predicting how long the bulb will live? Which dominates?

Most of the major lamp manufacturers have optimized their production to achieve maximum lifetimes for the cost of the lamp.

They could make lamps that last forever (one incandescent lamp in a baseball stadium has been burning continuously since the 1930s), but you wouldn't want to pay for it, and the light output would be less than what you would expect for the power used

Thanks for the Great chat Ian!

Created by liquid
Last modified 2006-11-26 18:09