I've covered this topic before but it appears that some of you newbies weren't listening or so dense it didn't sink in the first time. So Listen up!!! 
One of the biggest investments you will be making in setting up a new tank is the lighting. There are all sorts of posts here on RC debating which lighting is best. Obviously, if the author of these posts sold his wife's kidney and a lung to buy the ultra-sun, super blue metal halides he is going to make a pretty good case why those lights outshine all the others.
You hear that xyz lighting is the
only type lighting that will assure success in your reef tank. Let's try to set the record straight here and look a little deeper into lighting.
First, for those of you who fell asleep when they talked about Thomas A. Edison in class, let's get the technical part out of the way. In current reef setup there are two main classes of lighting, fluorescent and metal halide.
Fluorescent has some subsets such as PC's, power compacts, but they are in fact fluorescents going under an assumed name. A fluorescent is just a hollow tube with a filament at each end. The tube is filled with an inert gas, argon or krypton, and there is a very small amount of metallic mercury in the tube. The glass wall of that tube is coated with a blend of chemicals known as phosphors. There are many types of phosphors and, since their chemistry is on a need to know basis, you don't need to know it. In operation a high voltage is momentarily applied to the filaments. The tiny bit of mercury inside the tube gives off some mercury vapor. This makes the inside of the tube conductive and when the high voltage is applied a current flows down the length of the tube between the filaments. After the lamp starts it warms and more and more mercury is vaporized allowing more current to flow. Shortly after starting the resistance in the tube falls like a rock and the lamp would burn brighter and brighter. Finally so much current would flow that the lamp would wink itself out like a supernova and fail.
Enter the ballast. Most of you have seen a ballast. It usually looks like a brick inside the fixture. A ballast serves two purposes. It provides that extra voltage kick to start the lamp and it also limits the amount of current the lamp can draw. This latter function is why the lamp does not burn out the first time it is put in use. Now ballasts come in two major flavors. Magnetic coil ballasts, which work like a current limiting transformer, and electronic ballasts, that are a type of silicon controlled rectifier. Both keep current levels from exceeding the limit for that bulb. The cheaper, magnetic ballast does so mainly by dissipating the excess current as heat. This is not very efficient as the ballast itself adds wattage above and beyond that consumed by the lamps. Electronic ballasts limit the current by solid-state electronics that don't lose energy as heat. Here the ballasts and lamps usually draw no more wattage than the lamp alone saving some money. They also produce a higher frequency current going to the lamp. This eliminates the flicker affect that is associated with lamps run on magnetic ballasts. Electronic run much cooler and they can run multiple combinations of lights where a magnetic needs to be matched to the lights in use.
The main drawback is cost. Electronic ballasts costs much more than a magnetic. Also, like the electronics in your computer they don't do well with voltage spikes. A failed filament shorting out can not only end the life of the lamp but take the electronic ballast out too. If your kid is building a thermo nuclear devise for the school science fair the accidental detonation, and resulting electro-magnetic pulse, will fry the circuits in an electronic.
I can see the management at Icecap yelling to R&D, That darn WaterKeeper is stirring things up again. Quick, get a nuke and see if our ballasts fail.
The lamp is now fired and the mercury vapor is giving off photons and electrons. Problem is that most of it is in the lower wavelengths that don't supply much visible light. That emitted radiation strikes the phosphors that coat the tube and they begin to glow much brighter than the mercury discharge. That glow is usable light and our fluorescent is now functioning in its normal manner. When fluorescents first came out they used single phosphors that emitted in the red-green regions of the spectrum. Now they use all sorts of blends that allow them to produce light over the entire spectrum.
So far I have not said much about metal halides. That is because MH lights work almost the same as a fluorescent. A high voltage is applied to start an arc across the filaments. This in turn causes the gas inside the lamp to become hot and glow. The gas would become hotter and brighter as the current increased. In fact, so hot that the lamp tube would melt if things continued without limit. Once again, a ballast steps in and limits the current that the lamp will draw and prevents burnout. The main difference between the two systems is that the fluorescents depend on the phosphors to provide secondary radiation and usable light while the MH is more like an incandescent using a hot gas cloud to provide the light.
Ballasts for a MH lamp will not run fluorescents and vice versa. For one the starting circuits differ. A MH may have three elements with one being an igniter filament or some with only two that use what is called pulse start. If you have a hood with both MH and say VHO you need separate ballasts for each system.
Ok, we see how they work but which is best? Well I'm not going to tell, so there!
Actually both types make fine lighting for a reef tank and there is a lot of misinformation about the differences between the two. This centers on that the fluorescent lights are a continuum source while MH is a point source.
A big myth is that MH lights are much hotter than fluorescents. That is really not so. A bank of 500 watts in fluorescents puts out around the same heat as 500 watts of metal halides. The fluorescent tubes spread that heat over the entire length of the tube. A MH light concentrates the heat in a small area. This aids in the belief that they create more heat. The light output is also overstated for a MH. Many say that they produce much more light than a fluorescent (s) of the same wattage. This is just the same as the heat. The fluorescent spreads the light out while the MH emits it in a small area. Sure it looks much brighter but if you add up the total illumination for the fluorescent there is not that much difference.
Well that's about all I can figure your feeble Newbie brains can absorb in one sitting. I'll continue to illuminate this topic in my next post. I'll hit upon which lights work best where and some of the variations in the two lighting types. Light them if you got them!