Metal Halide requirements

My tank is a 110gal tall (48X18X30). I have a single 400W MH light pretty much centered over the tank, and in an enclosure whose lens is approx. 4" from the water surface. It was mentioned to me by one of your mods (Jason) that I probably need two of them, because each one has a light spread of approx 2 sq. ft.
My question has to do with light spread, wattage requirements, and what it all means in relation to one another (including water depth, and light source distance from water surface). My light was purchased from a "non-aquarium" company, with a pretty expensive optics system, which I was sold on due to the fact that it would (supposedly) spread the light of the MH over a larger surface at a closer range. So, in effect, does the lens also reduce the light penetration? The tank is lit brightly from end to end with no evident "spotlight" effect, but I'm now questioning what the "optics system" is doing to the big picture. I apologize if my question isn't clear, but I'm far from a writer, and even farther from a great explainer! ;D

 

30 inches is a ton of water for a single bulb to penetrate through. But by all means I believe a 400W can do it. But your thought process IMO is correct. the bulb is only outputting a certain amount of light. That amount is constant regardless of the lens put in front of it. So one must assume that diffusion occurs in order to spread the same amount of light over a greater surface.
Now similar theories could be said about reflectors, but they would be untrue. The quality of a reflector can enhance the spread and not diffuse the light. But increasing the ability of reflection you can actually increase the usable light.
To make a long story short Reflector good, lens bad.

Lets see what other say

J

 

I'm totally with you Jason, except that the light output quality of this particular "optics" system supposedly has as much to do with the reflector design as it does with the lens (diffuser). At least that's what I was told when I purchased it. I'm not arguing your point, I just really want to understand this. The person I bought this from knows nothing about aquariums, but has umpteen years of experience in the lighting industry. Very interesting.

 

Metal halide bulbs/lighting fixtures contain two pieces of glass. One encompassing the glass tube while the other is the reflector which helps to lessen the amount of uv rays that are hazardous to corals in large amounts as they are to humans(sun burn) and skin cancer! The reflector also increases the amount of light redirected into your water for the benefit of your corals and inverts etc. by as much as 50%!
Metal halides are different from the tube lighting like t5's, power compacts and fluorescents. Halides are a point source bulb that emits a crisp, more intense light from a small bulb as oppossed to the tube which disperse the light over the entire length of the tube, thus not as intense but covers more of the tank than halides IMO!

 

At 30 inches you should be using at least 2. 400 watters the rule of thumb is 1 lamp per 24 inches of tank length But with 30 inches then 3 400 watt lamps would not be out of the question and also use them with properly designed parabolic reflectors to aim the lamps light to the correct focal point in the tank. Like Jason said lens bad ju/ju that would be the first thing I threw away I always have even with the old early lamps and fixtures

 

OK, so it's the lens that I should ditch. I'm convinced that I'll get one more fixture too (before any corals go in). With the bulb(s) being so close to the water surface, I'm a little concerned about splash. Would a piece of clear tempered glass hurt the light quality? I think I need something on there. The housing is a polished aluminum parabolic reflector, and seems to do a nice job.
Again, everyone, I really appreciate your input.

 

I would do 2 400w's mount the bulb around 11"s off the surface of the water , you shouldnt have any splash around there unless a fish is trying to exscape, lol. That doesnt mean mount the fixture 11"s above, the distance from the bottom of the bulb to the surface of the water. Most of what everybody is saying here is a general rule of thumb. Nothing is set in stone since tanks very so much in length/width and height.

 

Whenever you place anything, glass included, under your lights, you will reduce the amount of light reaching your water and corals below!
Their are other variable that also impede the amount of light reaching your bottom of the tank such as DOM(dissolved organic matter) like gelbstoff and turbidity factors such as plankton detritus air bubbles and the such! Good water flow, carbon and good filtration will suffice and keep these to a minimum!

 

lol if I wasnt a smart cracka and know your lingo, I wouldnt have a clue what you just said, haha

 

OK, sounds like a plan. It makes sense to raise the lights instead of putting more glass between them and the water. The only problem I have is that the hood is totally enclosed, so in order to raise the lights I think I'll just cut some holes in the top large enough for the reflectors, and mount the lights externally. To tell you the truth, that was kind of my original plan until I decided to mount the light under the hood. That will also reduce the heat from the light and allow me to actually get the water below 82 degrees.
Thanks a bunch folks, I'd better get busy!