What Are My Light Choices?

There are two types of lighting that are with maintaining healthy coral: fluorescent and halide.

Halide bulbs produce a lot of light in a small package. They are a type of HID, or high intensity discharge, lighting. There is a good, simple description of how HID/halide lights function at Halcyon.net. Here is a quick summary. Electricity is passed through metal vapor that is under high pressure. Now a little chemistry. Electrons orbit the nucleus of an atom in shells of energy. They are not specific orbits as much as they are clouds surrounding the nucleus of the atom. Electrons can be forced out of their orbits when supplied with energy. After electrons are knocked out of their orbits, however, they fall back to the original orbit. As they fall back, the excess energy is released in the form of light, heat, and other types of radiation. The metals used in the "metal vapor" and the energy supplied to the bulb determines the light given off.

In my post Lighting is Complicated, I talked about PAR (photosynthetically available radiation) and PPFD (photosynthetic photon flux density) ratings. Halide bulbs can be given PPFD ratings. As Sanjay Joshi describes in his article, Facts of Light-Part 2 , a PPFD rating does not consider the spectral distribution of the light produced. This is the downside to halide lighting. Without testing your bulb, or finding the results of someone who has tested your bulb, you don't know the spectral distribution of that bulb. If you don't know what wavelengths of light the bulb is emitting, you don't know what wavelengths of light are entering your aquarium. In other words, some bulbs may be more beneficial to your aquarium than others.

Fluorescent bulbs work differently than HID/halide bulbs. Blurtit.com has a relatively simple definition. To summarize, a gas that can produce a smaller wavelength radiation than visible light, like ultra violet radiation, is excited by electricity. The UV radiation in turn produces excess electrons that excite another gas/chemical/etc that will produce light when the extra energy is applied. This is called fluorescence. The color and intensity of the light depend on what the molecules/atoms/substances are fluorescing. Fluorescent bulbs produce light, heat, and other types of radiation; but distribute it over the length of the fluorescent tube. The result is that fluorescent bulbs do not feel as hot as halide bulbs do. Fluorescent bulbs come in different sizes, wattages, and colors. The color of a fluorescent light is more specified than a halide light because atoms (etc) fluoresce certain colors.

High output fluorescent bulbs don't work any differently than normal fluorescent bulbs. They just use more energy. More energy in turn yields more light. This article, How Fluorescent Lighting Works, seems to explain it pretty well.

The downside to fluorescent bulbs is that you are not buying one bulb. One fluorescent bulb can not duplicate what one halide bulb can produce. An array of fluorescent bulbs can, however. They usually come in 4,5,6,7... etc. bulb arrays to encompass better color variations in a reef. Even though multiple bulbs are used, less energy is used in a fluorescent array than would be used to run a comparable halide bulb. The difference is in PAR ratings. PAR ratings from a fluorescent system are not as high as what you would get from a halide system. What makes it a viable alternative is that the PAR ratings produced (under the right conditions) are more even through out the tank. Less light overall, but the light present is more usable.

How beneficial light is to an aquarium depends how well the light is reflected in to the tank. Here is an example of experiment done to test PAR values of different Halide lamps 8 inches below the bulb. If you follow the link, and then look at the graphs at the bottom of that page, you will see that not all halide bulbs are the same. In fact, some are just useless.

Here is a website that does the same thing (sort of) with T5 fluorescent lamps. Small note, T5 refers to the size of the fluorescent bulb. It is a high output bulb, and the only affordable alternative to halide lighting on the market right now.

The jury is till out as to which lighting system is better overall, but here is one of the only tests published that compares T5 to halide lighting. In Coral Coloration and Incident Light a T5 system was tested against 3 halide lights. One of the conclusions drawn is that the T5 system had an almost equal effect on SPS corals, the most light intensive corals, as a 10K halide bulb. The other conclusion is that a 20K halide bulb has the most dramatic effect on coral coloration.

In my next post I will talk about which light I am choosing.