PAR readings and different length fixtures - The Planted Tank Forum
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post #1 of 6 (permalink) Old 03-04-2013, 06:14 PM Thread Starter
Algae Grower
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PAR readings and different length fixtures

I think this question applies to PAR and linear light fixtures in general so I'm posting in this lighting forum, although I'm using the Finnex Ray II fixtures as an example. I'm hoping this question will be of general use to everyone.

I have a standard 55 gallon tank measuring 48" wide x 12" deep x 21" high. I'm thinking about buying the Finnex Ray II DS, and I'm trying to decide whether to buy 2 24" fixtures or 1 48" fixture.

For the 24" fixtures, the manufacturer states that at a depth of 18" with the fixture 3" above the surface of the water, the PAR reading is 37.

For the 48" fixture, the manufacturer states that at a depth of 18" with the fixture 3" above the surface of the water, the PAR reading is 66.

If I get 2 24" fixtures, what will be the PAR reading at a depth of 18" with the fixture 3" above the surface of the water? is it simply 37 x 2 = 74?

For maximum PAR, would you recommend 2 24" fixtures or 1 48" fixture?

thanks in advance everyone...this is a great resource!
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post #2 of 6 (permalink) Old 03-04-2013, 07:21 PM
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If you put the two 24" lights side-by-side and pointed them at the same spot, then you may well get ~74. If you put one on the left and one on the right, I'd expect something closer to 37 throughout the tank.

I have a real problem with some of these PAR statements. If you look at Marineland's PAR chart, it gives the same rating regardless of size. Many give different ratings by size in the LED Compendium but I often cannot find the source data to verify the information.
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post #3 of 6 (permalink) Old 03-04-2013, 07:21 PM
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I am not sure with LED fixtures. Do they have the same amount of bulbs per length? For example, does the 48 in fixture have 2x as many bulbs as the 24 in? If so, it shouldn't matter.

With florescent lights, it's a bit easier. A tube 2x as long with double the wattage would put out the same amount of light, more or less, than two tubes half the length. I imagine it would be the same with LED's, assuming you have the same total number of bulbs of the same wattage, in the same places.
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post #4 of 6 (permalink) Old 03-04-2013, 08:02 PM
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LED fixtures aren't like T5 fixtures where you can just add the PAR of two fixtures together UNLESS they are side by side (front to back) in the same exact position. Across a 4' tank, if you were to get 2 24" fixtures they would overlap in the middle and be ~37 PAR every where else. The 'overlap' would not necessarily be double the PAR, but it would be more. Hoppy states this in a few threads previously posted. Use the search tool I'm sure you could find more info about it. IMO from the info given I think one 48" fixture would give you more PAR and more evenly throughout the tank.

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post #5 of 6 (permalink) Old 03-04-2013, 10:23 PM
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LED lights are different primarily because they have a lens the concentrates the light into a cone, with the middle part of the cone having the highest intensity. That means if two LEDs are close together, their PAR will be about 2X that of one LED, but if they are far apart, the light from one won't extend far enough to overlap that of the other, so they won't add together. With a string of LEDs in a row, only a limited number of them will contribute light to a spot under them. But, if that lens is a wide angle one, a 120 degree angle cone, for example, then most of the LEDs in the row will contribute something to the light everywhere in the tank. For those, a short light will have fewer LEDs contributing to the PAR than for a long light. That's why the Finnex lights give more PAR for longer lights than for shorter lights.

With a fluorescent tube most of the tube contributes some light to every spot under it, only if the light is short enough that little of the light will reflect off the water surface. With the light sitting right on top of the tank, only the section of the bulb that is close to the spot will contribute much, if any PAR to the spot. If you plot the PAR vs distance for a fluorescent tube light the light drops off a little faster than if it were directly proportional to the distance from the light, until that distance is about equal to half the length of the bulb, or a little more than that. At longer distances, the light begins to drop in intensity almost equal to the square of the distance from the bulb. All of that means that 2 foot T5HO bulbs give about the same intensity as 4 foot bulbs for typical aquarium shapes used with those bulbs.

LED lights with 60-75 degree or less lenses seem to drop intensity directly proportional to the distance from the bulbs.

All of this is approximations only.

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post #6 of 6 (permalink) Old 03-04-2013, 10:50 PM Thread Starter
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Aquavibrant's site says that the 24" fixture has 192 LEDs while the 48" fixture has 384 LEDs. So I would expect that if I were to put one 24" fixture on the left and the other on the right, that it would be similar to one 48" fixture at least in terms of PAR.

If I'm understanding all of you correctly, then talontsiawd agrees with that statement, Hoppy and Ashnic05 kinda agree, while Atari does not. (thank you for your opinions regardless!)

Anyone else?
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