Measuring LED source with LUX meter

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Apical Bud

Well-Known Member
I know I should buy a par meter if I want to measure led intensity, and I know if I want to get fancy about it I should add a fudge factor to my reading that accounts for the differences in electronvolts per photon at different frequencies. But please, I just want to know what range of lux is good for led grows in veg and in flower. I don't want to buy a par meter, I just want the answer to the following scenario, because I can't find it online:
There is a magical grow tent where everything is optimal, and in it there is an optimally powered LED light with the standard purple color consisting of mostly red and some blue LEDs. The light intensity at the canopy is PERFECT for the plants. You have two devices, one a lux meter and the other a PAR meter. You use both to measure the canopy. What are the readings?
If you can answer this question you will forever be immune to spidermites.
Thank you.
 
RandomHero8913

RandomHero8913

Well-Known Member
Apical Bud said:
I know I should buy a par meter if I want to measure led intensity, and I know if I want to get fancy about it I should add a fudge factor to my reading that accounts for the differences in electronvolts per photon at different frequencies. But please, I just want to know what range of lux is good for led grows in veg and in flower. I don't want to buy a par meter, I just want the answer to the following scenario, because I can't find it online:
There is a magical grow tent where everything is optimal, and in it there is an optimally powered LED light with the standard purple color consisting of mostly red and some blue LEDs. The light intensity at the canopy is PERFECT for the plants. You have two devices, one a lux meter and the other a PAR meter. You use both to measure the canopy. What are the readings?
If you can answer this question you will forever be immune to spidermites.
Thank you.
Click to expand...
Credit to @Randomblame

10-15k (~150-220µMol/s) Early Vegging
25-30k (~350-350µMol/s) Late Vegging
50-60k (~750-900µMol/s) Flowering

As for PAR numbers, you will be at the mercy of whatever PAR meter is being used and how accurately it can measure usable wavelengths (i.e. what nanometer red/s are you using? 630nm, 660nm, 730nm?)

If you look at these you'll be able to see what I'm getting at. This is just an example of two of the main companies that offer products to consumers. In the case of using monochromatic LEDs as in the Purple/Blurple lights you can see how what tool you measure with can impact your results.

 
nfhiggs

nfhiggs

Well-Known Member
Apical Bud said:
I know I should buy a par meter if I want to measure led intensity, and I know if I want to get fancy about it I should add a fudge factor to my reading that accounts for the differences in electronvolts per photon at different frequencies. But please, I just want to know what range of lux is good for led grows in veg and in flower. I don't want to buy a par meter, I just want the answer to the following scenario, because I can't find it online:
There is a magical grow tent where everything is optimal, and in it there is an optimally powered LED light with the standard purple color consisting of mostly red and some blue LEDs. The light intensity at the canopy is PERFECT for the plants. You have two devices, one a lux meter and the other a PAR meter. You use both to measure the canopy. What are the readings?
If you can answer this question you will forever be immune to spidermites.
Thank you.
Click to expand...
You can't measure Lux values for blue/red mono colored LEDs. It just does not work. Lux is a value for white light..
 
wietefras

wietefras

Well-Known Member
You can potentially use lux meters for purple lights as well, but you need to know the correction factor.

For 3000K to 3500K 80 CRI light it's around 70. So if you measure 70k lux then that equals about 1000umol/s/m2 PPFD.

You need to know the spectral power distribution (SPD) of the light source. This is usualy in the datasheet. Or find the lux-par conversion factor in one of the threads on this subject here or get it from Apogee's site.
 
A

Apical Bud

Well-Known Member
If I put my light meter under a purple led it will give me a lux reading. It won't be the true lux under the definition of lux, but it will give me a reading. What reading should I shoot for for veg, and what for flower?
All other considerations are irrelevent.
, thanks,
AB
 
nfhiggs

nfhiggs

Well-Known Member
Apical Bud said:
If I put my light meter under a purple led it will give me a lux reading. It won't be the true lux under the definition of lux, but it will give me a reading. What reading should I shoot for for veg, and what for flower?
All other considerations are irrelevent. , thanks,
AB
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Honestly, this is not a question anyone can possibly answer with any reasonable accuracy, without seeing a spectral graph of your lights. Here's the thing - A lux meter is strongly biased toward green light. So with just a red/blue fixture, its going to give relatively low readings than it would for a white light of the same PAR value. How low of a reading would just be a wild guess without an actual spectral graph.
 
wietefras

wietefras

Well-Known Member
Apical Bud said:
If I put my light meter under a purple led it will give me a lux reading. It won't be the true lux under the definition of lux, but it will give me a reading. What reading should I shoot for for veg, and what for flower?
Click to expand...
Do you have an SPD for that light? Then we could calculate a conversion factor.

If it's just 450Nm and 660Nm light then it will probably be somewhere between 6.5 and 7. So then you would need to divide the lumen by say 7 to get an estimated PAR value. This will be extremely imprecise though. It's so far away from the regular lumen curve and also a cheap lux meter will probably not actually follow the lumen curve very exactly.

Most burple fixtures will have some other wavelengths and often also whites mixed in too. So then the factor would be different.
 
A

Apical Bud

Well-Known Member
wietefras said:
Do you have an SPD for that light? Then we could calculate a conversion factor.

If it's just 450Nm and 660Nm light then it will probably be somewhere between 6.5 and 7. So then you would need to divide the lumen by say 7 to get an estimated PAR value. This will be extremely imprecise though. It's so far away from the regular lumen curve and also a cheap lux meter will probably not actually follow the lumen curve very exactly.

Most burple fixtures will have some other wavelengths and often also whites mixed in too. So then the factor would be different.
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Hmmmm. Thank you for your input, this is giving me a raging clue.
Yes, let's assume the light is just 440 and 660, with 75% 660 and 25% 440. At the canopy the reading is 60000 lux evenly spread across the entire canopy (this is where I'm at now with my tent). I can take that lux over 6.75 to yield almost 9000 par units. That seems to be about 9 times the flowering recommendation, so I believe I am doing something wrong. Does that 9000 figure account for distance from the light source?

What I hope to do, once I get a good conversation factor from hid lux to blurple lux, is get a rough equation for watts over distance squared
To me that would be reliable enough, as each blue photon energy isn't that much greater than red.
I'm sure this sounds stupid but I believe I can do it. I was hoping someone might have experience with a purple light and be able to tell me that their lux was X and their grow went well. Other wise I will just experiment with different grows and hopefully find a general ballpark estimate.
 
wietefras

wietefras

Well-Known Member
Apical Bud said:
At the canopy the reading is 60000 lux evenly spread across the entire canopy (this is where I'm at now with my tent).
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That's sounds extremely high for just blue and red yes. On the other hand, if I add 10% of "white" the the a lumen to par conversion rate ends up around 25. It all changes really quickly when you add some green.

When you say "watts", that would need to be PAR watts or radiant watts or something. Otherwise efficiency would change things a lot.

The difference between blue and red in number of photons produced is also quite high. You get less than 60% of blue 400Nm photons at the same radiant power where you'd get 100% 700Nm photons.
 
A

Apical Bud

Well-Known Member
I won't have access to the meter until next weekend. Let's assume the reading is accurate. Is the lux/7.5 figure still agreed to be a good rule of thumb? (Based on the cri).
If so I'm all set, thanks all!
 
heckler73

heckler73

Well-Known Member
Apical Bud said:
I won't have access to the meter until next weekend. Let's assume the reading is accurate. Is the lux/7.5 figure still agreed to be a good rule of thumb? (Based on the cri).
If so I'm all set, thanks all!
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When you know what you're measuring with, let me know, too. Then I can perhaps do a little calculation for it...just for fun, if nothing else.
 
A

Apical Bud

Well-Known Member
Hmmmm so I used the photometer on my phone, which uses the camera and an app called physics toolbox, and got a reading of 30000 lux instead of 60000. Does that sound more reasonable? I noticed that my buddy's meter, once it reached about 30000, didn't follow the inverse square law when halving distance to the light, so maybe it's busted? My phone, however, did follow the law.
I will get you that info for sure, sir. Thank you.
 
wietefras

wietefras

Well-Known Member
@Apical Bud Inverse square should not apply when you are measuring a full fixture. Or at least not when you start at some distance from the light. Preferably the distance where you would normally have your plants.

A smartphone lux meter usually suffers from light at shallower angles not registering fully. A lux meter will have something called cosine correction (that white disc) to make the measurement less sensitive to incident angles. That also means that a smartphone will indeed follow inverse square sooner since it only measures the light source directly above it while it fails to register the overlapping light sources more to the sides.

It probably also indicates that either the smartphone or your lux meter (or potentially both) don't measure the lux curve very accurately, but are digitally corrected to sort of give reasonable readings in sunlight.
 
heckler73

heckler73

Well-Known Member
wietefras said:
It probably also indicates that either the smartphone or your lux meter (or potentially both) don't measure the lux curve very accurately, but are digitally corrected to sort of give reasonable readings in sunlight.
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I think the bigger issue here is the spectra of lights.
However, I think this is an enlightening graphic worth considering. What it shows is different phone/app combinations measuring reference sources (the solid lines). They generally are off by anywhere from 20% to "fuggedaboudit", so no, they are not the best to use if accuracy is your desire.



https://www.dial.de/en/blog/article/luxmeter-app-versus-measuring-deviceare-smartphones-suitable-for-measuring-illuminance/
 
wietefras

wietefras

Well-Known Member
heckler73 said:
I think the bigger issue here is the spectra of lights.
Click to expand...

Yeah that also. These things are only supposed to work to dim the screen somewhat when there is less ambient light. Why would they bother making sure they react even remotely linearly and/or correctly to spectrum.
 
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