Light: Part 1


In this series of articles, I would like to introduce the theoretical and practical aspects of light in the aquarium. How much do you need? Of what kind? From what source?

But first, a more fundamental question.

What is light?

Actually, we don’t know. It’s “sort of” an electromagnetic wave, i.e., an oscillating change in an electromagnetic field.

That brings up the question: “What is an electromagnetic field?”

Actually, we don’t know that either.


Well, so are a lot of university physics majors, who aren’t let in on science’s “dirty little secret” – that we don’t really know what light, electrons, protons, people, and mongooses actually are – until their first quantum mechanics course, usually in their fourth year of studies. We have very good theories that tell us how these things (or at least the first three) behave, but there is no understanding of what they are.

Light has associated with it a property called “wavelength”. Since light really isn’t a wave, the wavelength of light isn’t what you picture when you see, for example, a water wave, but it is both useful and traditional to think of light as a wave, and so we call that property (whatever it actually is) “wavelength”.

The shorter the wavelength of light the more energy it carries. But a real light beam is made up of light with a range of different wavelengths. The intensity of the light at a given wavelength might be different than that at another wavelength. How the intensity changes with wavelength is the light’s spectrum.

Our eyes react to light that has a wavelength between 0.4 m m and 0.7 m m. Furthermore, we perceive light with different wavelengths within that range as different colors: the longest wavelengths are red, then as wavelengths get shorter you progress through all of the colors of the rainbow to violet.

Green plants also react to light with about the same spectral range, but in their case they use a light sensitive pigment to absorb the light, usually a type of chlorophyll. Chlorophyll absorbs light most strongly at the red and blue ends of the spectrum, and is less efficient at absorbing light that is yellow or green. Because the green light that is not absorbed is reflected, green plants look green.

So why do both our eyes and green plants react to (more or less) the same wavelengths of light? Its because that’s the light we have the most of.

Our major source of light is the Sun. The Sun is a ball of hot gas, and its surface has a temperature of about 5500K. Such a hot object radiates away energy in the form of light, with the most intense light being in the wavelength range 0.2 m m and 0.8 m m. But the light with wavelengths shorter than 0.3 m m gets filtered out by the Earth’s atmosphere, and wavelengths longer than 0.7 m m carries comparatively little energy, so the useful radiation that reaches the ground is mostly between 0.3 m m and 0.7 m m: almost exactly what our eyes can see.

The spectrum of light produced by hot object depends on that object’s temperature. For example, the Sun, at a temperature of 5500K, produces a spectrum that your eyes see as “white” light. That is, when your eyes see that particular mix of red-through-violet light, your brain combines the colors so that you perceive the color white.


But an incandescent light bulb has a tungsten filament with a temperature of only about 2600K (depending on the wattage), which is considerably cooler than the Sun. So an incandescent light bulb does not produce nearly as much high-energy short-wavelength blue and violet light as the Sun does. Incandescent lights therefore look yellowish to human eyes. We say that incandescent lights have a lower color temperature than sunlight, because it is produced by a radiating object with a lower temperature, and thus has a different (yellower) color.

Halogen lights differ from simple incandescent lights in that they have their tungsten wire encased in a high-pressure gas with traces of a halogen (usually iodine). The purpose of this halogen is to return evaporated tungsten to the filament. After tungsten evaporates from the filament, it condenses on the inner surface of the bulb. The halogen gas in the bulb reacts with this thin, warm tungsten deposit to produce tungsten halides, which evaporate fairly easily. When the tungsten halide gas reaches the filament, the intense heat of the filament causes the halide to break down, releasing tungsten back to the filament. This increases the lifetime of the tungsten filament and allows it to operate at a much higher temperature. Thus light with a higher color temperature can be created by halogen lights than by ordinary incandescent lights. The color temperature of halogen lights is about 3500K. They also are about 20% more efficient than normal incandescent lights and last about three times longer.

There are advantages and disadvantages to using incandescent lighting in aquaria; and in a sense the same advantages and disadvantages exist for halogen lighting, except that both the advantages and disadvantages are more pronounced in halogen lighting. The biggest advantage, and this is a very big advantage, is that they are both very inexpensive. The incandescent/halogen fixtures can be purchased and easily hand wired for a pittance when compared with other forms of lighting. The bulbs themselves are also relatively inexpensive. Normal incandescent bulbs can be purchased for about a dollar per globe if you shop around and buy in larger numbers. Halogen lights cost about $8 but are still economical when compared to incandescent lights given their greater longevity and intensity. Another major advantage is that the lights are very compact, and many of them can be placed over an aquarium. Very high light intensities can therefore be achieved.

The primary disadvantage of halogen or incandescent lights is their inefficiency. Most of their monstrous power consumption goes into creating waste heat, and this can cause serious problems in the aquarium. They also produce a yellowish light that does not show fish colors to best advantage. Freshwater plants, however, do well under these lights (provided the intensity is adequate) as their spectrum matches well the red peak of chlorophyll’s absorption spectrum.


Fluorescent lights do not produce light by simply heating an object until it glows, as do incandescent and halogen lights, but instead they are a little more complicated. They produce light by subjecting a gas to a very high-voltage electric field, which excites the electrons in the gas’ atoms, so that they are in high-energy state. But the electrons will also (spontaneously) jump back down to a lower energy state, and do so by releasing energy in the form of light. But the jump between energy levels is very discrete, as each energy level has a very specific energy. The light released therefore has a specific energy too…and thus a very specific wavelength.

In neon lights, for example, the light released has a specific wavelength in the red region, which is why neon lights are red. Other gasses produce other colors: for example blue “neon” lights are actually full of argon. The mercury/argon gas in fluorescent lights, however, glows in the ultraviolet, which human eyes can not see. What happens next is that the ultraviolet light is absorbed by the phosphor paste on the inside of the tube. This phosphor produces a secondary glow in visible light in response to the energy of the ultraviolet light.

Fluorescent lights require more voltage than the 110 volts of your AC outlet, so they incorporate within them a transformer that boots the voltage to a high enough level. This transformer is called, for reasons I can not fathom, a ballast. In typical fluorescent light fixtures this is a black metal brick about 20cm long. These iron ballasts a “simple” transformers that use wire-wrapped iron core to step up the voltage. They themselves consume up to 16 watts of power (converting it into heat) so fluorescent lights aren’t quite as efficient as they could be: a fixture with two 40-watt fluorescent tubes actually consumes 96 watts of power rather than 80. But there are also electronic transformers that do the job much more efficiently and run much cooler. They also have the added advantage of eliminating the 60-Hertz flicker of fluorescent lights, as they operate at about 20000Hz (far faster than anything your eye could ever perceive). There is also a 10% to 15% increase in light output caused by their shorter cycle time. They also greatly extend tube life. And with electronic ballasts in place fluorescent lights become dimmable. But electronic ballasts cost big bucks…I found prices that ranged for $75 to $270 for electronic ballasts that can handle two four-foot, 40-watt tubes. Neither does anyone seem to stock these ballasts locally, so they must be mail ordered.

Fluorescent tube manufactures have invested a lot of time and money developing mixes of phosphors that produce different kinds of light spectra: some can mimic the spectrum of sunlight very well.

We can therefore say that a fluorescent light has a color temperature too, even if that color is only reproduced artificially (there is nothing inside the tube that is actually heated to that temperature).

Your typical “cool-white” fluorescent tube has a color temperature of around 4100K, while a warm-white tube has a color temperature of around 3400K. This means that the light they produce has a spectrum that is most similar to a glowing hot objects of temperature 4100K and 3400K, respectively.

Note that “warm-white” tubes have a lower color temperature than “cool-white” tubes. That is because we call yellow, orange, and red “warm” colors because of their association with fire and sunsets, while blue is a cool color because of its association with the sky and the sea. The yellower light of the warm-white tube is therefore thought of as “warmer” than the bluer light of a cool-white tube. But a hotter object produces light with more blue than a cooler object, and so a bluer light has a higher color temperature.

Unfortunately, warm white and cool white tubes do not recreate the spectrum of sunlight very well. They produce far too much green light, which your eye is most sensitive to, but little red or blue light, which plants are most adept at using. So although they look bright, they produce comparatively little light that is usable by plants. If you use these lights, you will need more of them to produce the same plant growth than tubes that reproduce a solar spectrum more accurately.

A daylight fluorescent tube has a more advanced (and thus expensive) “rare-earth” phosphor mix than either the warm or cool white tubes. These phosphors mimic the 5500K spectrum of sunlight quite well. Daylight fluorescent tubes typically cost about $8 at Revy and other home-building supply stores.

Daylight tubes are also available that have very advanced “triphosphor” compound that produces more light per watt of consumed electricity than the dual-phosphor coatings in regular fluorescent tubes. And they also have a very good color-rendering index (CRI). The CRI is a measure of how natural colors appear under the light. A perfect score of 100 means that a special multi-color card looks the same under the light as it does under “high noon” sunlight. Triphosphor daylight fluorescent lights typically have a CRI of better than 90.

Unfortunately, triphosphor tubes are only locally available from stock at pet stores, where they sell those made by Coral Life and a few others. The cost for such tubes is prohibitive, at about $45 for the four-foot size. However, a tube that is essentially identical to the wallet-busting pet shop lights is the Philips Ultralume 50. I don’t imagine you’ll find anyone who has this tube in stock, but you can order it from industrial lighting stores, such as Litemor, in lots of 10 for $8.50 per tube, or in lots of 30 for $7.50 per tube. These lights are about the same price as the dual-phosphor daylight tubes sold at Revy, and you get a much better tube, but the need for a bulk special order requires a much larger single outlay of cash.

Those pinkish aquarium and plant “grow and show” tubes have phosphors that produce a spectrum that closely mimics the absorption spectrum of chlorophyll. They therefore produce a lot of “photosythetically available radiation” (PAR) per watt of consumed electricity. Unlike a daylight tube, which has a spectrum with a broad peak of intensity centered in the yellow-green, a plant tube’s spectrum has two peaks, one centered in the blue and the other in the red. There is a dip in intensity in the yellow and green, since chlorophyll does not absorb green very well. This enhanced blue and red helps bring out the color in your fishes, but this color enhancement is unnatural. No colors look particularly natural under these lights, as their odd spectrum gives them a near-zero CRI. And, because the dual-peak spectrum is nothing like that produced by a glowing hot body, these lights really can’t be given a meaningful color temperature.

Also available are actinic tubes, which have spectra that are very enhanced at the blue end. These tubes are used in saltwater aquaria because the mimic the light seen under the sea. Because water absorbs red and yellow light much better than blue light, the light more than a few meters down in the ocean (if the water is clear) is quite blue. Photosynthetic organisms from coral reefs are used to such light, and do better with an enhanced blue spectrum. But few freshwater aquarium residents, with the possible exception of the open-water East African lake species, live under such blue-enhanced light, so actinic tubes are of little use in freshwater tanks. The color temperature of these lights is very high, as much as 30000K, but their CRI is abysmally low.

In the past, a normal fluorescent tube consumed 10 watts of electricity per foot of length. So the four-foot length consumed 40 watts. But the standard of 10 watts per foot was thrown out the window in 1996, at least in the case of cool-white tubes, which must now be 34 watts in the four-foot size. Thankfully, these regulations do not apply to “high quality” tubes such as daylight tubes (with a minimum CRI of 69), or special purpose tubes, such as plant tubes.

The standard four-foot tube you will soon see will also be smaller in diameter than the current lot, 1 inch rather than the current 1½ inches. Most fluorescent tubes are nowadays of the T12 variety, where the number 12 refers to the tube diameter in eighths of an inch. But the industry is moving toward T8 varieties, which can fortunately use the same iron ballasts and hook to the same connection points as are in the current standard fixtures, and they also produce the same amount of light for the same wattage.

But I’m told by a local commercial light supplier that the industry standard four-foot fluorescent light fixture will soon have a 32-watt T8 powered by an electronic ballast. And these new-generation electronic ballasts are unable to operate the current-generation 40-watt T12 tube (Aarrrgggh!).

Light output from an electronic-ballast 32-watt T8 fixture is only slightly lower than that of a current 40-watt T12: the 20% decrease in wattage is offset somewhat by the 10% to 15% increase in light output provided by the electronic ballast. But a simple switch in a current fluorescent fixture from a 40-watt T12 tube to a 32-watt T8 tube results in a full 20% loss of light output if the fixture’s iron ballast is not upgraded to the new electronic variety.

The technology exists in the new triphosphor fluorescent lights to create a fluorescent tube that is not only more efficient but one that produces a much more natural light. Hopefully, the move to reduced tube wattage will be accompanied with an increase in the availability of triphosphor tubes to restore our lost illumination.


Compact fluorescent lights are intended as an energy-wise replacement for incandescent lights. They consist of an electronic ballast unit which screws directly into a normal incandescent light socket, and the tube itself. The tube is looped into a U shape so that both sets of connector pins can connect to the same ballast connector socket.

Since fluorescent lights are about four times more energy efficient and last 10 times longer than an incandescent bulb, the relatively high initial cost of these lights (about $16, with replacement tubes being about $6), are offset by their low operating cost.

As of yet, compact fluorescent lights are only readily available in the “warm-white” and “cool-white” colors. Nevertheless, they are great for an inexpensive and energy efficient refit of all your old incandescent aquarium hoods.

In addition to the compact fluorescent lights that are intended to replace household incandescent lights, there have also been introduced high-output compact fluorescent lights for business lighting applications. Although these lights are slightly less efficient than long fluorescent tubes, they are perfect for aquaria, providing intense lighting with excellent spectral characteristics.

These systems are however very expensive. Mail order sources have been selling these units for reef aquarium use, but they are not price competitive with metal halide systems (discussed below). Mail order price for a 192-watt unit carries a mail order price of about $1000, or double what local hydroponics stores ask for a 400-watt metal halide system.

One aquarium manufacturer has recently introduced the OSRAM power compact fluorescent light in a “regular line” aquarium hood. These lights are however still expensive (about $190 mail order for the 2 x 55-watt, 4-foot size) and they have yet to arrive in local shops.

These fixtures are not readily available for the do-it-yourselfer to wire together, as they are a specialty building item that has yet to show up at Revy. They can however be ordered from industrial lighting supply stores: the most commonly available example is the Philips PL-L 40-watt compact fluorescent light, but you may be hard pressed to find it listed in anything but the typical warm-white or cool white colors (it is available in daylight as well). The cool-white tube costs about $18, and the parts for the fixture and ballast cost about $85. But for the extra cost over regular fluorescent lights, these lights offer no advantage besides a smaller size, and thus the ability to stuff more of them over the aquarium for more intense lighting.


High output and super high output fluorescent light look like the normal long-tube variety but they have a wattage double or triple that of a normal tube of the same length. But they are less efficient than normal tubes, so they do not produce as much light per watt. Nor do they last nearly as long as normal tubes. And they require very expensive electronic ballasts rather than the mass-produced and available-at-every-hardware-store iron variety. And I have never seen HO or SHO lights for sale locally, and the industrial lighting stores I have approached do not even have them listed for order, so you are looking at a mail order to one of the pet supply firms in order to get them. Figure on $340 for the ballast and connectors to run four 4-foot tubes (110 W), plus $44 per tube. All this seems like far too much cash and hassle to be worth it, so I would forget about high output fluorescent lights.


These lamps contain a mixture of gases including mercury, argon, and metal halide gases such as sodium iodide, strontium iodide, and a few others (the exact mixture being a manufacturer’s secret). These gases fluoresce when subject to a strong electric field. In this respect they work like neon lights, but instead of producing a single pure color, they produce a broad spectrum. They can also be made to produce a sun-like spectrum with a near perfect CRI and a color temperature of around 5500K. Such “daylight” metal halide lights do however come at a premium price, and most general-purpose metal halide lights have a color temperature only about 4300K.

The current generation of metal halide lights are extremely expensive, starting at about $300 per operating fixture, plus $40 for the cheapest possible bulb (4300K), or $120 for a daylight bulb (5500K). And that is for a unit with just an inefficient iron ballast: double the price for one equipped with an electronic ballast.

An iron-ballasted metal halide light requires a good 10 minutes to “warm up” before it reaches maximum light output. And if it is turned off while fully warmed up, it must be allowed to cool completely before the power is restored, or else it will not start again. So a brief power outage while you are at work will leave your aquarium dark all day.

The smallest size of metal halide bulb that is locally available is 400 watts. This is actually a little more power than you are likely to want for a planted freshwater aquarium unless your tank is enormous, since a single 400-watt bulb, although not outsized for a 400-liter or larger tank, won’t light the corners of such a tank very well…you really want two 200-watt bulbs for a 400-liter tank. But you can’t find these locally, and so you are back to making a special order. And even if you did order them, the cost would be the same per unit as the 400-watt unit: about $300. So you are looking at $600 for just the fixtures needed to light your 400-liter tank. Ouch. Well, its still better than the $2000 a commercial metal halide aquarium hood would set you back I suppose.

I am also not impressed with the reliability of the metal halide I purchased, as it shorted out and fried itself twice in three years. And its iron ballast ran very hot…so hot that you would burn yourself by touching its metal case. I was never convinced that I hadn’t purchased a fire waiting to happen.

But the advantage of metal halide lights is great: they produce a concentrated light source from a small bulb, unlike the diffuse light of a much larger fluorescent tube, so very much higher light intensities can be achieved. They are about as efficient as fluorescent tubes and they last almost as long as fluorescent tubes.

Metal halides are still the only way to go when it comes to reef aquarium lighting, as they are the only readily-available lights that produce a daylight spectrum and are compact enough that you can fit enough of them over an aquarium to give you the required light intensity.

Traditionally, metal halide lights over reef aquaria are supplemented by blue actinic fluorescent tubes to reproduce the blue-enhanced color of sub-marine light. This will soon be unnecessary as high color temperature metal halide bulbs (color temperature of 20000K) are already getting onto the market.


In the future, we may be able to choose between one of several high intensity, high efficiency lighting sources that are already seeing development. Foremost of these is the sulfur lamp, which uses radio frequencies to stimulate a sulfur-argon gas to radiate visible light. These lights have proven to be about 20% more efficient than fluorescent lighting, while generating practically no heat or ultraviolet. Bulb life is an incredible five years. The color temperature is a good 5200K, but the CRI is a relatively low 79. These units are as yet only available in a gigantic 5900-watt size (for about $2500) which may be fine for the midnight pharmaceutical herb growers, but aquarists would be hard pressed to make use of this much power. Smaller units are however under development, and if they prove to be price competitive with existing lights and as efficient as the 5900-watt unit, they will set the standard for all aquarium lighting. ?