# Double the light = Double the PAR values?



## Rush3737 (Jan 15, 2013)

Good question, I'm interested to know the answer myself.

Edit: My guess from perusing around would be somewhere in between. I'd think the extra bulb would marginally increase the overall PAR, but not double it.


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## Whiskey (Feb 15, 2005)

In my testing I'd answer very close to doubling it, yes - but that depends to how close to ontop of one another that you make the bulbs, and if any sacrifices have to be made in the reflector department to get the two bulbs up there.

If on the other hand you have a very wide tank,.. say a 4 foot cube, and you have one bulb on the very front, then put one bulb on the very back, you would end up with slightly higher PAR right under the front bulb, but mostly you would be improving spread for more even coverage.

So it really depends on the situation.
Whiskey


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## jmrmotorhead (Sep 27, 2011)

Hoppy once explained to me that especially with vertical CFl's in the work lamp reflector, the par is not added or doubled. The par levels fall down quickly as you venture out from directly under the bulb, and the overlap is low level light. He said adding more would only ensure that the coverage would be more even. Again, this is with the work lamp reflectors, where one bulb is in each reflector. If you had a white good, with just a bunch of CFl's inside, increasing the # of bulbs would lost likely increase the total amount of light reflected into the tank, this increasing the par at substrate. 

Make any sense?

Matt


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## hitmanx (Jun 24, 2012)

in my proposed tank, i'm dealing with a narrow width and length and a deep water column: 13.5 x 13.5 x 20"H... i'm going for a low light setup, so light fall-off is a concern at the substrate level.

In order to modify the existing hood I could add up to three 13w CFL bulbs in a space approx 1foot long... obviously the bulbs cannot occupy the same space so it makes sense that it wouldn't truly be doubled at any given area of the substrate, but surely it must be close. 

Unless PAR has no relation to lux or lumens, it seems probable that just like adding another light bulb makes an area seem brighter to our eyes, the usable energy from the lights would significantly be raised as well. 

According to Hoppys chart, a single 14w horizontal CFL would give about 18 PAR at 15" and about 10 PAR at 18"... this of course if very low light - not enough i gather for even crypts without them being long and leggy.

So a 2nd and maybe a 3rd light would get me in the realm of lowlight if in fact they could come close to doubling or tripling the PAR. Of course the amount of light in the upper reaches of the tank would also double or triple and that would mean 50 PAR for each CFL bulb at the 10" mark below the lights.

What's my tank going to like if the upper half is high light and the bottom half is low light? Algae on the top and not the bottom? :eek5:


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## Greystoke (Jul 24, 2010)

Hoppy’s method is based on the “spot light” theory. A method that requires you to buy a Quantum meter and measure the PPF (PAR/m²) underneath the spot light at a certain height. It then allows you to use the “square root law” to estimate the PPF value at different heights based on your measured value.

There is – of course - nothing wrong with that method, although – in my opinion – it makes life a bit difficult, and certainly almost impossible if you wish to install TWO spot lights.

In South Africa we work with the *average light intensity* which equals the strength of the light source (in Watts, Lumen or PAR) divided by the area of the water surface in m², and all you need to ensure is that the light is as evenly distributed across the top area as you can manage, and preferably without any losses.

This average light intensity must be used as the base-level of your plant.
Of course, you may have areas of higher intensity or even spots, but that’s up to you to even out. Maybe you can even use it to some plant’s advantage.

Now, if you double the light source, then you obviously also double the average. It certainly makes life a lot easier (and we haven’t even used a Quantum meter).


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