PAR readings off on LEDs? - The Planted Tank Forum

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post #1 of 19 (permalink) Old 04-19-2011, 02:53 AM Thread Starter
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PAR readings off on LEDs?

I took my club's par meter to my LFS to test out some of the LED lights they have there.

I tested an illuminarium fresh box 120watt, which is supposed to be a 400watt metal halide equivalent.

Tested the par. The thing matched a crappy 2 bulb t5ho fixture. at best. No way close to a metal halide.

Anyways, got to talking with the owner, he said that with LEDs it's hard to measure the light output using par because they put out a spectrum of light a PAR meter can't measure. Which is why companies use LUX.

Any opinions on this?
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post #2 of 19 (permalink) Old 04-19-2011, 02:57 AM
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Yeah... and my wealth is so high that dollars can't measure it.

PAR stands for photosynthetically active radiation. So, if what he said is true, that doesn't matter from the perspective of planted/reef tanks.
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post #3 of 19 (permalink) Old 04-19-2011, 02:59 AM Thread Starter
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He has the reef version of this over his corals, they're growing, very quickly.

This thread isn't for calling the owner an idiot, so please don't come here if that's what you want to say.
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post #4 of 19 (permalink) Old 04-19-2011, 03:16 AM
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Who said that?

I just did some PAR readings over the weekend of an LED fixture (forget which one) on a reef tank and they were much higher than I'd expected (150-500mms) where the corals were. Not all fixtures are created equal.
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post #5 of 19 (permalink) Old 04-19-2011, 03:32 AM
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A PAR meter uses a filter and circuits to limit the wave lengths of light it will respond to to the range of 400 to 700 nanometers, approximately. Like all light meters the sensitivity of the meter isn't absolutely flat over that whole range, but it is flat enough for the readings to be useful. It makes no difference where the light comes from, if it is in that band it is measured by the PAR meter. So, I can't think of any theoretical reason why a PAR meter would fail to read LED light correctly. If a LED puts out light that is outside of the PAR meter band, it isn't photosynthetically active radiation, so it shouldn't be measured as PAR.

Having said that, I have been suspecting that LED give more effective light than T5HO bulbs, in that less PAR may be needed for the same growth rate. That could be the case if the LED does not produce spikes of light at a few wavelengths, with much lower intensity between those wavelengths, the way a fluorescent bulb does. If that is the case, then it is the light from T5HO lights that is being measured incorrectly by the PAR meter, not the light from LED lights. But, our expectations for what a given PAR means are formed based on T5HO light. (I'm just brain storming, thinking as I write.)

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post #6 of 19 (permalink) Old 04-19-2011, 03:56 AM
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Having just spent an evening combing the interweb for as much info as I could find on LED lighting for planted aquaria (answer: little conclusive), the general trends I've seen come from hydroponics groups who have developed wonderfully hideous purple lights for maximizing plant growth; a mixture of 2:1 red to blue LED's with a hint of orange mixed in gives the densest, fastest growth. This however is in direct contrast to full-spectrum LED's which are designed more and more for higher CRI and lux, the spectral profile of which is almost antithetical to that needed for plants, aquatic or otherwise.

The major developments for LED's for aquariums has been around marine, which is why we're seeing such a boom in actinic/blue setups, and less for planting. If you're game to use a 420-480 nm-heavy rig on your tank (which I personally like on my tank), there are tons of options. For the lower K type lights we're still a bit SOL for now.

I would be interested in setting up a plant tank with the ugly purple hydroponics LED's, but there's no way the wife will let me set up another tank right now... especially one that will "ugly up the place".
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post #7 of 19 (permalink) Old 04-19-2011, 04:27 AM
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If I set up ten absolutely identical tanks, with everything exactly the same, except that 5 of the have 50 mms of PAR with the spectrum on the left, and 5 have 50 mms of PAR with the spectrum on the right, will there be a significant difference in plant growth rates in the two sets of tanks? I'm suspecting that there would be, and the faster growth would be with the one on the right. This, because too much of the PAR from the one on the left is from the narrow peaks at 3 wavelengths vs. the more uniform distribution of the one on the right.

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post #8 of 19 (permalink) Old 04-19-2011, 04:44 AM
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If I set up ten absolutely identical tanks, with everything exactly the same, except that 5 of the have 50 mms of PAR with the spectrum on the left, and 5 have 50 mms of PAR with the spectrum on the right, will there be a significant difference in plant growth rates in the two sets of tanks? I'm suspecting that there would be, and the faster growth would be with the one on the right. This, because too much of the PAR from the one on the left is from the narrow peaks at 3 wavelengths vs. the more uniform distribution of the one on the right.
Doesn't this kind of make judging lighting systems based on PAR meter readings somewhat useless at best? You could have a source peaking hard in regions outside of a plants PUR that still gave huge PAR reads, but wouldn't do much for the plant, right?
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post #9 of 19 (permalink) Old 04-19-2011, 04:49 AM
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Doesn't this kind of make judging lighting systems based on PAR meter readings somewhat useless at best? You could have a source peaking hard in regions outside of a plants PUR that still gave huge PAR reads, but wouldn't do much for the plant, right?
It makes a headache for me Fortunately, neither spectrum is exactly smooth, so I think PAR will continue to be by far the best measure, but I am intrigued by this. I think you posted before about having a light with a single peak in the spectum, with say 90% of the PAR coming from that peak, but the peak being green. Would 50 mms of PAR from that theoretical light be as good as 50 mms of PAR from a typical light? Obviously the answer is no. (I think.)

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post #10 of 19 (permalink) Old 04-19-2011, 03:48 PM
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Doesn't this kind of make judging lighting systems based on PAR meter readings somewhat useless at best? You could have a source peaking hard in regions outside of a plants PUR that still gave huge PAR reads, but wouldn't do much for the plant, right?
Well, yes.

This also will be different for different plant SPECIES and each species will adapt differently to these spectra over time, than another species of plant.

So do you or anyone else happen to have a 3000-20,000$ radio spectrometer?

If not and if you do not like taking every 1-5nm readings over the 400-700nm range and trying to relate this to growth for say, 50-200 species of plant............

It's a lot more labor and cost and time.
PAR is a trade off, asking for each and every band is asking a bit much for most anyone to chew off.

Yes, it's be nice to know............but measuring that and growth and making sure CO2 etc is also correct while taking the growth measures will be a lot to ask.

Seems much wiser to simply stick with PAR for our needs.
Light measurement is by and large not simple or easy, there are a great deal of trade offs, this is true with respect to Biologist that measures Plant Ecology and Physiology as well, most just use the PAR meter unless they have a more specific question about light.

We might get a nice aesthetic color for ourselves and be able to mimic if we happen on a bulb with like, say a Giesemann powerchrome 6000K, then look at the graph.

Say this bulb grows the widest range and with the nicest color, then you'd focus and narrow the effort/labor and cost down and have a wide range of species that folks have good successful growth with etc. You will have trade offs with one species preferring/having higher growth or some morphology that an aquarist might like, that preference of the aquarist plays a large role there as well. How do you account for that aspect? It's not quite as simple and there's going to be a fair amount of trade off here.

Some reasonable approaches and trade offs must be accepted.

Regards,
Tom Barr
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post #11 of 19 (permalink) Old 04-19-2011, 03:49 PM
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Hoppy, did you want to borrow the Light and Aquatic Photosynthesis book by Kirk I have?

I'll leave it by the porch.

Regards,
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post #12 of 19 (permalink) Old 04-19-2011, 05:54 PM
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Sounds good, thanks Tom and Hoppy, I appreciate the time and effort you both put in here on a daily basis, I've learned a lot from the both of you.
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post #13 of 19 (permalink) Old 04-19-2011, 08:11 PM
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Hoppy, did you want to borrow the Light and Aquatic Photosynthesis book by Kirk I have?

I'll leave it by the porch.
Under other circumstances I would, but right now I'm trying to get my condo in shape to list it for sale - very time consuming. Once I get settled in an apartment I may take you up on the offer.

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post #14 of 19 (permalink) Old 04-19-2011, 09:52 PM
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My understanding for LEDs is that they are more accurately measured using (in terms of Apogee sensors) the Sunlight calibrated sensor, but most of the hobbyists that own a PAR meter will have an "Electric" calibrated sensor, which is designed for halides and T5 type lights.

The only reason I can suspect this is exactly what Hoppy mentioned--- the more natural spectral curve of an LED is much closer to the curve of sunlight than a halide or T5 puts out.

Anyone who can explain this better, or correct me, please do.

Quote:
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So do you or anyone else happen to have a 3000-20,000$ radio spectrometer?


Why, YES!

Well, I don't own it. It belongs to UNC Wilmington. We're studying a pile of common LED brands here soon--- on April 30th.


We will be looking at the actual graph of the individual LEDs, and then the resultant spectral graph of various planted tank and reef combinations.

Here's a list of what we will study (along with a myriad of common/likely combinations of these)


Cree XPG in cool, neutral, and warm white.
Cree XML in cool white
Cree XRE in royal blue and cool white
Cree XPE in blue, royal blue, and red.
Bridgelux 402 cool white 5600K.
Satistronics 10w pure white and 455nm royal blue
Satistronics 20w 15,000K cold white
Satistronics 3w pure white and red
Rebel neutral white, royal blue, cyan.

This basically covers the gamut of what we're likely to use--- LEDs from the "big three" name brands and then the chinese import Satistronics.


We will also take PAR data from raw LEDs with no secondary optics.
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post #15 of 19 (permalink) Old 04-20-2011, 02:10 AM
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Redfishsc, that's the most awesome thing I've heard all night. Thanks in advance for your time, and I can't wait to see the results.
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