Lux and Purple LEDs

Hi there everyone. I'm hoping someone can help me out here.

I know the conventional wisdom is that you can't use a lux meter to measure purple LEDs. But surely there is a conversion fact one can use to approximate good distance from purple LEDs. I'm asking anyone who has a lux meter, a PAR meter, and a purple LED light to measure that LED with both meters and post the results. I don't care if the accuracy is perfect.
See, I have an LED panel that I want to use for seedlings and to veg. It draws about 37 watts from the wall ( I measured). My phone has an app called physics toolbox that among other things has a photometer on it and will spit out lux over time graphs, which is great for mapping my canopy. However, this app (and an old hand held meter my buddy has) give me a reading of about 8000 lux at the seedling level. However, 8000 lux drawn from the best parts of the spectrum is different than 8000 lux of HID.

So I'd like to find a good conversion factor for purple LEDs. In other words, if I use a lux meter with these lights and it gives me a reading of 2 lux (just 2!), yet the plants grow well, that would tell me to use a conversion factor of around 1000. I could calculate that conversion factor if someone could take two readings of purple LED light, one with each meter. from the same spot in their room.

Can anyone help? Thanks for reading!

What is the led you have at 37 wall watts?
What size is dimensions of the fixture ?

id be extremely surprised if an app on your phone gave you an accurate lux reading without a sensor.

on my iphone those apps are absolute dogshit

Moflow said:

What is the led you have at 37 wall watts?
What size is dimensions of the fixture ?

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It is Chinese Communist Party LED Company; about 12 by 12 of light aperture. Let's say 75% of the leds are red and 25% blue. Let's use that assumption as the spectral distribution of wattage.

But none of this really matters if someone can test two meters on a similar purple light, right?

For "white" light a lux meter works just fine, but purple light is very far away from the green which is measured by the lumen curve. Plus it will depend a lot on the actual sensitivity curve of the lux meter. They don't really accurately measure the lumen curve, but some approximation of that curve. So you are compounding errors on what is already a very error prone conversion.

Especially the light meters in phones are very inaccurate. They are simply meant to get some semblence of a light intensity indication to adjust the screen brightness.

So even if you get a conversion of a PAR meter on that purple light, you will have a high margin on error on your own side still.

On the other hand, why do you care? The only thing really important is that you distribute the light as best as possible. That you can do with a lux meter. After or during the grow you will be able to tell if the light is too much or too little by judging the plant development.

So with my phone Lux meter I am getting about 20,000 lux at the canopy with blue and red LEDs. Using a black star flowering LED advertised as 500W that's actually about 240. My plants are acting like I'm giving them too much light. So, does anyone know if 20,000 LED lux translates higher or lower than a white light?

Apical Bud said:

So with my phone Lux meter I am getting about 20,000 lux at the canopy with blue and red LEDs. Using a black star flowering LED advertised as 500W that's actually about 240. My plants are acting like I'm giving them too much light. So, does anyone know if 20,000 LED lux translates higher or lower than a white light?

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your phone doesn't have a diffused lens to capture light from all angles. its just measuring from straight above. if your plants say theyre getting too much light, they are.

do not trust those phone apps for anything.

try turning it at your tent wall and see what it says then.

pulpoinspace said:

your phone doesn't have a diffused lens to capture light from all angles. its just measuring from straight above. if your plants say theyre getting too much light, they are.

do not trust those phone apps for anything.

try turning it at your tent wall and see what it says then.

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Right. That's a good point.
I think it's red herring to suggest that I need accurate readings, though.

Let's say I'm not using lux. Let's make up a unit, so people don't get caught up on the accuracy. The issue is, if most people have these poorly performing light meters (maybe on their phone or maybe they got one on amazon for 30 bucks) these meters have trouble with blurple lights because they are tuned to the wrong part of the spectrum or they only take readings from head on, or whatever, but they will still give similar (albeit incorrect) readings. I have used the same app on two different phones and I have used my buddy's amazon meter that works well for non-LED lights. They all give me ballpark the same readings. So I'm assuming that these meters will be just as inaccurate for other people if they try to measure blurple lights.

So the original question is still, can someone or has someone used a non-quantum meter on a blurple light (LED made up of red and blue LEDs) when that light was a proper distance from healthily growing plants?
I'm hoping for an answer like, "hey man, right now my plants are doing well and I'm using a flowering LED, or a veg LED, and when I measure at the canopy it says X lux." Or something like that.

See, we're in a position where I'm asking if 20,000 lux off a red-blue led could be too much for my plants and we're quibbling over accuracy of the meter that I know isn't accurate. I appreciate the support, though, and thank you for your help

For this kindnof lights most people use a par meter. 20000 lux if accuarate is probably a bit too much if there is only red and blue dies and no white, but its very hard to say. It depends on optics and a few other stuff. As per the watts of that unit it could maybe flower one small plant but thats pushing it.

Edit didnt realize it was actually reading 2 lux.