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Interesting Article on Lighting

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Light is not always white. The color of light is called color temperature. It is based on a hypothetical model called Blackbody Radiation. I won’t bore you with the theorems, but give you a analogy you can work with. Picture this. You have this big block of black iron, maybe 3 feet across in all directions. You put it in a furnace, close the door, where you can watch it through a little window. There is a temperature dial on the wall, and you start turning it up until the furnace gets real hot. When the temperature of the furnace reaches about 1,500 F, the block of iron is so hot that it starts to glow a dull red color. You then crank it up a bit higher until the temperature gauge reads 4,000 F. Now that block of iron is really toasty. It’s so hot it changed from red to a bright glowing yellow color.
At this point, the yellow color you see is the same color temperature as a 40 watt incandescent light bulb. If we convert 4,000 degrees Fahrenheit to the Kelvin temperature scale we get 2,477 degrees Kelvin or 2,477K for short. A regular old incandescent light bulb has a color temperature of 2477K. It doesn’t matter if its a 40W bulb or a 300W bulb. The 300W bulb will be brighter, but both bulbs have the exact same the color temperature.
Just for fun, we continue to crank up the furnace even higher. When we reach 5,500 degrees Kelvin, the iron starts to glow a bright white color with not much yellow in it. This is the color temperature of the sun (5,550K) as it strikes the earth. If you continue to crank it up even higher, the iron will glow in a crisp white color. Now we’re at 10,000K. A little higher still and the iron begins to change color again. At around 15,000 degrees it is so hot it goes from white to a bright blue color.
Since we were kids, we were all taught that red is hot and blue is cold. In reality, red is hot and blue is much hotter.

This is how color temperature works. It is a measurement of color given off by an imaginary, theoretical object when it is heated to a certain temperature. But its not all theory. In nature, lava flowing out of a volcano has a temperature of around 1200 C, which is 1473K. Amazingly, the color temperature of molten lava is exactly the same as its real temperature.
We can’t always see correct color temperatures. Our eyes can play tricks on us. Humans have the ability to change the color temperature of what we see on the fly. And it’s completely autonomic so we don’t even know it’s happening. Our eyes and brain create our own “white balance”. In other words, if the lighting is different than the sun, we will see the light as white once we are exposed to it for a short period of time. A good example of this is the standard “cool white” fluorescent bulbs. They have a color temperature of around 3400K, which is a very yellowish, almost greenish light. But once you’re in an office building chock full of cool white fluorescents, it only takes about 10 seconds for your eyes to adjust to it and see it as a white light. If you take a picture under cool white bulbs using a conventional film camera (with no flash) the photograph will have a definite yellow cast to it.

Our eyes are most sensitive to light in the 3000K-4000K range. In this range we “see” light much brighter than it really is. This is why cool white bulbs were invented in the first place-to get the most bang for the buck. Or better put, to get the brightest light for the watts. While cool white bulbs and conventional fluorescent fixtures are cheap, abundant, and give humans bright light, they are not suitable for use in most marine aquariums.
The color temperature of the sun is often sited to be 5500K. So why don’t we all use 5500K bulbs on our marine aquariums? First off, when the sun’s rays pass through our atmosphere, certain bands or wavelengths get filtered and change the color temperature. The sun’s color temperature can appear to us in a range of 2,000K (sunrise/sunset) to 30,000K (cloudy skies over the ocean). So 5500K is used as the standard for “midday sun over the equator”. The color temperature of pure sunlight at the surface of the ocean is 5500K. BUT-as the sun light penetrates through water it is unevenly filtered and refracted, which changes its overall color temperature. The infrared color band is completely gone within the first 1 meter of water depth, as is most of the lower wavelengths (predominantly reds and yellows). Within 3 meters, the color temperature changes to almost double the sun’s original value, or roughly in the range of 8000K -10000K at a depth of 3 meters.


If you’re keeping corals or inverts that live in atolls or shallow tide pools, then 5500K or 6500K bulbs is really the optimum choice. Fish look good under these bulbs. Your tank looks very bright, though most people see this as a yellow-tinged light.

Aquarium bulbs are now available in a wide variety of color temperatures. In the last few years, 20,000K bulbs have become very popular. They simulate color temperatures at ocean depths beyond 15 meters. Most all of the tropical corals and inverts sold into the trade don’t live at these depths. But people like the blue color of these lights, despite the fact that they are not optimal for coral growth. Most commercial coral farming operations use 5500K and 6500K bulbs.

What about Actinics? Do they have a color temperature? Yes, it’s 7100K but the number is somewhat meaningless. This is because color temperature is the average of all the wavelengths (or colors) that come out of the bulb. For all practical purposes, Actinic bulbs only give off one narrow spectrum that lies between the bands of longwave UV and the lower visible blue spectrum. You’ve seen or read the ads that claim “True Actinic – 430 nm”. The “nm” stands for nanometers – it is another measure of light which defines its full-wave frequency, called the wavelength. In general, we don’t really care about this.

So why do we use Actinics in the first place? Actinic bulbs (or the correct name for this spectrum of bulb, “Actinic 03”) was not developed for the aquarium industry. They were invented for the health care industry to treat jaundice in infants. Jaundice is a disease that causes the skin and whites of the eyes to turn yellow, usually because of poor liver function.
When exposed to the Actinic 03 bulb, the light would increase vitamin synthesis on the skin, which battles the disease. As the story goes, somebody at the hospital took one of these bulbs home because he thought it would look cool over his marine aquarium. Not only did it look cool, but it had a marked increase in livestock growth. Whether this story is true or just another reef urban legend, the fact of the matter is that the Actinic 03 bulb has a wavelength that is pretty much dead center on the peak value required for marine photosynthesis.
Many of us use fluorescent lighting fixtures containing one or more 10K bulbs with an equal number of Actinic bulbs. Flip the fixture over and look at the bulbs. Those 10K bulbs look ten times brighter than the Actinics. However, they are brighter only to our eyes. Those of us who use light timers or controllers to sequence the lights on and off usually turn on the Actinics first, followed by the 10K bulbs. In the eyes (or photo-receptors) of your corals, this is backwards. The Actinics are far brighter to them than the 10K bulbs.

Actinics are very important because they supply a relatively large amount of light in the right wavelength. Think of it this way. If you wanted to reproduce the wavelength and brightness of an Actinic bulb using a regular bulb over your tank, you would probably need several thousand watts to get there.
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