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Hi,
I'm hoping to get some clarification on how to compare the light output of various lamp types. I am familiar with the WPG rule, but all watts are not created equal, right? I assume a 30w T5HO bulb will put out more light than a 30w NO bulb. So what is the conversion ratio? 1w NO = ?w HO = ?w CFS ?? Put another way, if I need 60w NO to reach 2wpg for a 30gal tank, how many watts HO do I need to achieve the same light level? Based on a (kind of confusing) chart I found on this site, it looks like the ratio is 1w NO = 1.22w HO. Using this, I would conclude that, for my 30gal tank, I would need 49w HO to equal 60w NO, with both options giving me 2wpg. Is that right? How do CF lamps compare? Thanks very much!
- Marc
 

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It was a happy coincidence that T12 fluorescent lights could be characterized by a watts per gallon formula, but even that was only practical when a standard shape tank was being used, with the lights mounted right above the tank as close to the top of the tank as you could get. Today we have a variety of tank shapes, we mount lights in fixtures that sit right on the top of the tank, that sit on legs above the tank, that are hanging a foot above the tank, etc. So, that happy coincidence isn't valid now.

Light is not like fertilizer, where you put in a certain amount and it mixes with the water to give a uniform concentration throughout the tank. Instead, light just shines down onto whatever is below it, dropping in intensity approximately with the square of the distance from the light. The light intensity from a given fixture is the same at 20 inches (for example) when that light is over a 50 gallon or 200 gallon tank. You can't just fake it and make up a formula relating watts of light to gallons of water.

You need enough light bulbs to uniformly distribute the light over the substrate both in length and in front to back depth. Those bulbs need to be bright enough to give the intensity you want at the substrate level. A T5HO bulb, with the typical single bulb reflector, gives enough intensity for low light plants at about 30 inches from the bulb, or for high light plants at about 20 inches. Other types of lighting would do the same, but at different distances. The wattage of a tubular fluorescent bulb is a result of the bulb length, not the brightness, which is nearly the same for all fluorescent bulbs of one type (T5NO, T5HO, etc.).

For "power saver" screw-in fluorescent bulbs, there is a good thread in the stickies section here that gives good data for them.

I don't know of anything similar for MH/HQI lights or for T8 fluorescent lights.
 
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