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Discuss water requirements, "soil" (growing media) and suitable planting containers

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By davinstewart
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Posts:  101
Joined:  Sat Jul 13, 2013 2:29 pm
#389273
What's the best way to measure the light your plants receive? Are you giving them enough? Too much? How much light is that flourescent bulb giving off and how much can your plants use?

Sure, you could eyeball it and guess but wouldn't it be nice if there was a much more precise way to do it that's free and available to everyone RIGHT NOW!?

Enter the Daily Light Integral (DLI). It's a measurement of the total amount of photosynthetically active light that strikes your plants on a daily basis.

Don't know your DLI? Then ...
By davinstewart
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Posts:  101
Joined:  Sat Jul 13, 2013 2:29 pm
#389275
Huh, so you can't edit your own posts eh? Alrighty then. Some small corrections...
Hope that helps and that this is useful for you!
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By elaineo
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Posts:  444
Joined:  Tue Jul 24, 2012 4:07 am
#389277
Lux is not a useful measurement for plants, since it measures visible light whereas plants primarily use red/blue spectrum. Eg, a purple grow light will give a very low lux reading using the app above even though it may provide greater PPFD than a white light. To get a useful reading, you want to measure PPFD directly. I don't know of any free apps that do that.
https://growlightmeter.com/ppfd-or-lux- ... ifference/
By davinstewart
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Posts:  101
Joined:  Sat Jul 13, 2013 2:29 pm
#389278
So you're saying this approach is no good?
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By elaineo
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Posts:  444
Joined:  Tue Jul 24, 2012 4:07 am
#389280
davinstewart wrote:So you're saying this approach is no good?
It won't give you an accurate number to compare to the chart. It might be okay for fluorescent lights, which have a more similar spectrum to natural daylight. Actually, your last link includes a link to a post on this forum, which explains how to calculate PPFD from LED power ratings :) Here: how-much-light-do-you-really-need-t31035.html
By Z_Y
Posts:  147
Joined:  Wed Jun 24, 2020 10:19 pm
#389281
I have my doubts with using a phone too, phones have different sensors and there is no way an app is calibrated for them all. It's like one of those noise dB meter apps, you get two different phones and you'll get two different results.

What it might be useful for is if you're using the same phone for a comparison. For example you go out into full sun to get a reading, then you go inside with the same phone and check your grow lights. However, I also have my doubts at how good a phone is at reading sun brightness. I'd imagine the sensor would saturate really fast and give you an overly optimistic reading when you go check your grow lights.
By davinstewart
Location: 
Posts:  101
Joined:  Sat Jul 13, 2013 2:29 pm
#389282
Thank you for your responses.

I agree the sunlight saturation issue with phone sensors is probably an issue. Not sure how to address that other than scaling the reading down somewhat. For sunlight, you should be able to estimate the intensity based on weather readings for your lattitude, though.

The breakdown on how much light do you really need is excellent and I agree with everything on that post. The difference here is this approach is a point measurement, not a theoretical calculation and so addresses a different need.

I've found this approach to be very useful and helps me dial in my plants light requirements much faster than waiting weeks or months for the plants to show signs of need.

One of my challenges is that I lack the equipment to make an accurate PAR measurement and compare the results to this method. I'd be very curious to see someone do that and see just how close this approach gets. Is anyone up for that challenge? :D

Thanks.
By davinstewart
Location: 
Posts:  101
Joined:  Sat Jul 13, 2013 2:29 pm
#389300
One additional note, the Lux app states that for my phone the maximum reading it can take is around 32,000 lux so definitely an issue with sunlight. Other than that, though, should be a decent approach.
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