In a planted tank, lighting is very important. The general rule is 2-3 watts per gallon when using CO2 injection - which we will be. That would be about 500-600watts normally. But the tank is deeper than usual - about 30" - and we also will have the lighting fixtures hanging much higher than normal since we want people to see the plants behind the tank. Some light will also bleed off onto those plants as well. I tried to find some means to calculate the increase in light required to have the lights raised high above the tank. In photography you learn about the inverse-square law, the idea that if you double the distance between you and the subject, they receive half as much light. The problem is, it's half the light we had to begin with, it's relative. Watts don't translate to lumens because distance and other factors play a role. So I asked on forums online and tried to find information via Google but ultimately found that I had two options, a pair of 250watt lamps or a pair of 400watt ones. The 250's were what I would have needed normally, so I got the 400's. Below is the diagram I made to support my question online.
A couple years ago when building out the 150g aquarium, we talked to a great guy at one of local stores who answered all our questions. One of the comments he made was that metal halide lighting had a great lighting quality, that the light rays are distinctly visible in the water. So for this tank, that's what I wanted to go with. This was further supported by recommendations in articles online that you use metal halide for tanks 30" or deeper so the light reaches the bottom with sufficient intensity.
It turns out that pre-made metal halide lights of this size are in the range $1,200 which seemed rather extreme. I looked around online at various do-it-yourself sites and learned that there is quite a bit to know about metal halide systems. Apparently the bulbs are either single or dual end. The dual end ones require a special "ignitor" to get them started, so you can't use just any ballast with them. They also don't have shielding of the ultraviolet light they produce so they require a glass filter over them to perform that task. Single end bulbs use a socket similar to standard screw in light bulbs called a mogul socket. These bulbs are typically dual-envelope which means the bulb that has the gas in it and produces the light is enclosed inside a larger glass tube that provides protection and UV filtering. Some of these bulbs have orientation limitations, meaning they must be mounted vertical or horizontal or their useful life will be shortened. At $60-100 for a bulb, it's important to know these things. The traditional ballasts use wire coils to get the voltage up to what is required. These are very heavy and run very hot. Also, unless they include a special capacitor, they can cause electrical interference. New electric ballasts use solid state electronics to perform the same tasks. They can support any type of bulb and run cooler, but they are more expensive.
We found a great deal on a retro-fit kit from DIYReef.com which included a spider (shaped) reflector and socket, and a 400watt IceCap ballast (which seems to be one of the better electronic ballasts) for only $199. Add in a bulb for $60 and we were able to get a set of two for around $550. The reflector is shown below. I knew it was large, but wasn't prepared for the socket to be the size of my hand!