# Measuring LED PAR with a Lux Meter?



## kevmo911 (Sep 24, 2010)

I know that measuring PAR with a lux meter has always been a quick and dirty kinda thing, and it might give you a general idea, but no more than that. But I read this article the other day:
Aquarium Equipment: PAR Meters and LEDs - How Accurate are the Measurements? A Comparison of Three Meters and Lux to PAR Conversion Factors for LEDs ? Advanced Aquarist | Aquarist Magazine and Blog
...while looking into PAR meters. Down toward the bottom the author gives conversion factors for a couple colors of LEDs. And I've been thinking about it for a bit.

So, regardless of whether the author's factors are accurate, I'm wondering if the fact that LEDs tend to have a narrower color band than other lamp types makes it easier to get a more accurate reading with a lux meter? For example, measure the reds, blues, and whites separately, convert them separately, and then add. Something like that. Thoughts?


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## Hoppy (Dec 24, 2005)

I doubt that we planted tank users of PAR/LUX meters can gain much by getting more accuracy in our measurements of light intensity. First, the intensity varies a lot from high in the tank to the substrate level, and a significant amount from the ends of the tank to the middle, etc. Second, plants in the tank shade lower plants enough to have a significant effect on the intensity. Third, it takes a lot of care to use a light meter in the tank well enough to eliminate errors caused by how we use it.

If we are doing some kind of research where knowing the light intensity within 5% or so on the various parts of a plant, then good accuracy is necessary. But, if all we are doing is selecting a light fixture/bulb for our tank, or adjusting the height of a light from the top of the tank, or adjusting a dimmer on a LED light, then great accuracy isn't useful. Just getting a reasonably accurate intensity number at the center of the tank, at the substrate, will be enough for our needs. I think "reasonably accurate" means +/- 10-15% at best.


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## kevmo911 (Sep 24, 2010)

Hoppy said:


> I doubt that we planted tank users of PAR/LUX meters can gain much by getting more accuracy in our measurements of light intensity. First, the intensity varies a lot from high in the tank to the substrate level, and a significant amount from the ends of the tank to the middle, etc. Second, plants in the tank shade lower plants enough to have a significant effect on the intensity. Third, it takes a lot of care to use a light meter in the tank well enough to eliminate errors caused by how we use it.
> 
> If we are doing some kind of research where knowing the light intensity within 5% or so on the various parts of a plant, then good accuracy is necessary. But, if all we are doing is selecting a light fixture/bulb for our tank, or adjusting the height of a light from the top of the tank, or adjusting a dimmer on a LED light, then great accuracy isn't useful. Just getting a reasonably accurate intensity number at the center of the tank, at the substrate, will be enough for our needs. I think "reasonably accurate" means +/- 10-15% at best.


I understand and agree with all of that. But I'm trying to satisfy my curiosity. In theory, it seems to me that a cheap lux meter may be able to be more accurate when used carefully with LEDs than with other lamp types due to narrower spectrum spreads (by measuring colors separately). Would you agree? And is it worth $25 or $30 and my time to test?


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## jeffkrol (Jun 5, 2013)

kevmo911 said:


> I understand and agree with all of that. But I'm trying to satisfy my curiosity. In theory, it seems to me that a cheap lux meter may be able to be more accurate when used carefully with LEDs than with other lamp types due to narrower spectrum spreads (by measuring colors separately). Would you agree? And is it worth $25 or $30 and my time to test?


Well how much is your time worth?
Sorry I don't want to discourage DIY but some thing to consider:
$200 and a computer and full waterproof PAR meter..

Very few LUX meters have waterproof sensors so you must factor that into r/d and costs..
BioTek Marine SQ420 PAR Sensor with 15' USB Cable

now to be honest that is an Apogee sensor and has it's own warts..










A waterproof LUX meter..
Milwaukee Instruments MW700 Lux Light Meter


holy Grail:









LUX:
http://www.deterco.com/products/Spectronics/reative_spectral_response.gif


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## kevmo911 (Sep 24, 2010)

jeffkrol said:


> Well how much is your time worth?
> Sorry I don't want to discourage DIY but some thing to consider:
> $200 and a computer and full waterproof PAR meter..
> 
> ...


Okay, I appreciate the feedback, but I'm not being clear.

I am *not* trying to find a way to accurately measure PAR.
I am *not* trying to find a way to accurately measure lux.

I am attempting to slate my curiosity. I'm wondering if, for whatever purpose anybody or nobody might use the information, it is possible to *more* accurately measure lux, with any given meter, from LEDs than from other types of lamps, for the reasoning I already stated.

So, because I'm not very knowledgeable about lighting, I'm asking for advice from people who know more than I do.

Does my reasoning make sense? If there's already a fairly solid answer to my question, then it's not worth my time to test. So is there?

Thanks!


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## jeffkrol (Jun 5, 2013)

Ok so you need/want to improve on already fairly established "guesstimates"?

Conversion - PPF to Lux

The "work" to improve the estimate because of LEd's will probably not exceed the natural error of using the LUX meter

you can do some improvements w/ single spectrum LED's but when you get to whites you need to know the natural proportion of each "color group" and then correct from there.
In other words one correction factor per White K value and separate ones for each color, and in proportion to the fixture percentages..









in other words a "one number" correction factor is probably just as good as attempting to create multiple correction factors..

using a "my" real world example.. having a fixture of 4=RB 3W at 1000ma 4=660nm red @700ma, 4=cyan at 1000ma (70%), 12 =6500k white at 1000ma and 7= 3500k @ 1000ma and correcting for each light and output..
I'd rather just guess how accurate one number is......


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## kevmo911 (Sep 24, 2010)

Well, there are correction factors for each color, and knowing them, and taking current differences into account, you should be able to get a vague idea with just math and no actual testing, assuming you know what the total lux value is. And you're saying that it's all guesstimates in any case, since any meter, but especially a lux meter, is going to have a margin of error. So doing the math might seem pointless.

But since the LED bands are likely narrower and spike in certain ranges, would that possibly reduce the margin of error of a lux meter to a point where it might be worth doing the math?

If a person were to make measurements of an LED strip in air, one color at a time, current doesn't play a role. So if I had 30 6500K whites, 10 660nm reds, and 10 460 blues, and measured them as color groups (3 readings), converted, and then added, wouldn't that reduce the inaccuracy of a lux meter? (In my head, this involves taping over all but one color group at a time). I mean, isn't most of the inaccuracy due to the fact that we can't measure and convert each color section of a particular spectrum of, say, a T5HO? But we have the ability to do that with an LED strip because the color segments are physically compartmentalized, yes?

And so I get back to my original question. Does the ability to measure LED strips in individual color groups mean that using a lux meter can give a more accurate reading for LEDs (if we do the work) than for other lamps, such as MHs, T5HOs, or CFLs?


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## Hoppy (Dec 24, 2005)

I think the problem you will have is knowing the correct PAR or LUX for a 550 nm LED, for example. You won't be able to get an accurate reading with a Apogee sensor/PAR meter, nor with any low price LUX meter. To calibrate something you need a standard measurement that you know is more accurate than the accuracy you want with the meter you are calibrating. So, how do you determine that that 550 nm LED produces 2125 Lux at 10 inches, so you can compare that to the reading you get with the meter you are calibrating? I don't know any way to accurately calculate the true Lux from that LED, so I have to use a very good meter to measure it.


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## jeffkrol (Jun 5, 2013)

kevmo911 said:


> Well, there are correction factors for each color, and knowing them, and taking current differences into account, you should be able to get a vague idea with just math and no actual testing, assuming you know what the total lux value is. And you're saying that it's all guesstimates in any case, since any meter, but especially a lux meter, is going to have a margin of error. So doing the math might seem pointless.
> 
> But since the LED bands are likely narrower and spike in certain ranges, would that possibly reduce the margin of error of a lux meter to a point where it might be worth doing the math?


you may want to look at this:
Light Intensity Measurements for Light Emitting Diodes (LEDs); Correcting for errors

By interpolating from the Apogee vs LUX sensor response you could certainly derive similar correction factors.
note though a sensor w/ a more "correct" response than a LUX meter is useless for some colored LED's. You can't guess anything w/ a zero reading..
also note each "white" has a different correction factor. simply put in a mixed LED enviroment even a meter designed for PAR can fail ..



> IMPORTANT NOTE: LEDs that output a large proportion of radiation above approximately 660 nm will read very low and should not be measured with an Apogee quantum sensor/meter.


My main point is you are probably better off "starting from scratch" w/ a better photodiode and doing corrections from there than to use one w/ falloff (LUX sensor) in the most important photosynthetic bands..

you can use the Lux "box"... 

THAT is my opinion, no more no less..








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And like what Hoppy eluded to, you can estimate all the corrections you want but without testing it against a standard (Li-Cor) it is all a guess.

for fun:
http://www.lexingtonwx.com/techdata/Wp53SolarSensor.pdf
http://www.pmasi.org/frm/showthread.php?851-Cheap(er)-PAR-Meter
http://www.jstor.org/stable/2389327?seq=1#page_scan_tab_contents

2010:
http://www.plantedtank.net/forums/20-diy/123293-dirt-cheap-par-meter-kind.html

Really interesting:
http://blog.photosynq.org/what-you-can-measure-with-your-multispeq/


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