The Planted Tank Forum - View Single Post - PAR readings and different length fixtures
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post #5 of (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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