Light Intensity Question

Setting up my new tent - 2x4 Gorilla with a 320 XL light by HLG which is a QB, 3000k, and the light has an adjustable dimmer knob on the power supply. At the very lowest setting, the light pulls 36 watts from the wall and the highest setting pulls 349 watts from the wall.

I have an Apogee PAR meter that plugs into my laptop USB, so I can set my uMol of light by dimming the power supply.

Is there a chart that is easily linked to show how many uMol of light the seedling needs to do best…then shows the lighting requirement as the plant grows? I can do the math and find the DLI for the number of hours I plan to keep the light on. I looked for a uMol chart but cant find what I’m looking for.

@dbrn32, I thought you may know?


You’re looking for a Daily Light Integral (DLI) Chart That will measure for PPFD, which I have included at the bottom of this post.
( quick disclaimer I mean no offense when I provide definitions and/or formulas below; I assume you’re a rather smart person with a sound understanding of how this all works, but I don’t want to ramble off a bunch of crap without explaining myself. Plus, it may help others reading this post who may not know. I certainly don’t mean to patronize you, so I hope it doesn’t come across as such. :slight_smile: )

DLI measures the actual number of photons (specifically, photons capable of being used in the photosynthetic process*) within a given area per 24 hours.
Photosynthetic Photon Flux Density measures the amount of photons packed into your light and is measured in micromoles per meter squared per second: umol/m2/s

The packaging for your light should have some sort of information regarding the PPFD of your light.

DLI calculation formula is:
umol m-2s-1
DLI = (PPFD) x (3600 x photoperiod) / 1.000.000 – moles/m2/day
(3600 is how many seconds in an hour; 1.000.000 = micromoles / mole)

HOWEVER - we’re getting monster specific here and isn’t exactly necessary. Your light should be able to generally dim the intensity. For your infant seedlings you should be at 15/30/30. As they become a wee bit more established, like after they’ve popped their cotyledons and have their first real leaves, you can increase to 30/60/60. You should have your light minimum 6" from your babies but if they’re getting too hot you can lift the light. Try not to go higher than 12" though because they could stretch and that sucks. Obv.

I hope this helps out a bit. You really know how to test a girl’s long forgotten physics this early in the day!! Dayum!! :grin::upside_down_face::dizzy_face:
Let me know if I can help another way or if I didn’t address your issues.
Be well!!
*photosynthetically capable photons are photons anywhere in the range of four to seven hundred nm


Thank you for the reply. No matter my particular skill, your detailed post helps a variety of members and not just me exclusively. With that said, your post will benefit many members and I thank you on behalf of the forum.

I generally start the seedlings off with a light height around 12-18 inches so the light wont cause any heat burns. If I see stretching I adjust, and if the plants shy away from the light, I’ll adjust the other way. I feel I have an advantage with an adjustable power supply, plus I can vary the light height for a large range of settings.

To clarify, what specifically is meant by 15/30/30? I’m not familiar with that number sequence, and I hope it isn’t imposing to ask. In many circles some folks say set at 300 uMol for seedlings and 600 uMol for larger plants. I’m not sure if this one size fits all is accurate, hence the reason for my post. And since I have a PAR sensor, figured I should put it to the best use possible.


Lovely post, thank you. I’m going to book mark it and steal it haha.


Oh it’s no problem at all to ask questions!! Questions are how we all learn!!

Those numbers represent your spectrum channels - red/blue/white and represents a percent. So if you’re instructed to run 15/30/30, you’re at Red 15%/Blue 30%/White 30%
This helps you adjust your light spectrum to properly cover the required light spectrum for a given stage of growth.
Some LEDs are full spectrum and they don’t offer the user the capability to make any adjustments so you’re going to have light covering the full light spectrum for your whole growth period. Other LEDs are specific to veg so that light will always be running with blue (7000 K), others are specific to flower, so that light is stuck in the flowering spectrum (3000 K). But with these awesome, more professional, commercial grade LEDs you can custom adjust your spectrum to fit you and your plants needs. There are some great articles and papers on exploiting the light spectrum to manipulate and increase your yield and growth. I’m happy to provide some if you’re interested.

Another really easy way to figure out the effectiveness and efficiency of your lights is to inquire about your light’s footprint. So if you have a light that supposedly guarantees 7’x7’ vegetative coverage and 5’x5’ flowering coverage at an average of 18-24" above your canopy, then you can avoid having to work out all these calculations. Then just factor in your Kelvin requirements and you’re golden!

The only thing you may need to calculate and continue to be mindful of is what power you have available, and how much you’re using. The last thing you want is to be throwing fuses all the time. You also want to take into account whether or not you’re maxing out your space’s available power. You don’t want to be running your energy supply ragged - you won’t get the maximum efficiency out of your equipment and you ultimately risk damaging your power supply as well as your expensive ass equipment!!


You are a great source of information, @the2409labs. Since I have a QB light(s) that is an all purpose spectrum, I never had the chance to learn about the various settings you mentioned. But, there is no such as too much knowledge - or at least in my opinion, lol.

I worked with the owner of Grow Lights who helped guide me** to the right tent and light combination since I bought them together from him at the same time. He was very informative and helped me select the right light for the size tent as we discussed the footprint/coverage as you mentioned.

He also helped me setup my original starter tent which was (is) a 2x2 with a 135W QB. I get some amazing yields in this tiny tent with over 7 ounces on my last Blue Dream grow from one plant. I’ll go with 2 plants in the 2x4, and I hope to get great yields with another QB light.

Thanks for sharing your knowledge with all of us.

**I also wanted to mention @dbrn32 as a huge source of information as he also helped guide me into these premium lighting choices.


Where would I find a device that measure the voltage the light is pulling at the source and how can I measure it at the light.


Kill-a-watt. You can get em at Harbor Freight too.


Thanks a bunch


I use that same light and love it!



If you use the PPFD to figure the DLI as well you will realize that higher value is crammed into a shorter interval of time. (i.e. 16/8 vs 12/12)

Just an FYI.


Thanks for the chart, @blackthumbbetty. Once I start my Maui grow in the new setup, I’ll be sure to tag you.

I remembered on my last grow to adjust my math to get in my DLI when I went from 18/6 down to 12/12 to flower. I did my blue dream grow at 32 mols per day which gave me a good yield w/o pushing too hard. Good reminder.

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That’s great information @blackthumbbetty, thanks for sharing.

I did order the meter as well, thanks for that @Myfriendis410

What is everyone using to measure their PPFD?

I use a Sun Systems PAR meter with remote (6 ft cable) sensor. It ran ~$300+ US.

I mounted the sensor on a 3 ft long paddle/stick so I can reach all parts of my growing areas without interfering with the readings with my body.

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The reviews suck on the 2 par meters on Amazon…

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I use an Apogee SQ-520 sensor that has a USB plug that works with my laptop. I downloaded the app from Apogee and it has additional features and benefits above and beyond what a dedicated meter offered.

They are spendy as I paid a tick over $300 on Amazon last year. But I use this sensor all the time and consider it one of my more important purchases.

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Personally; I’d buy another light for that price. I pulled the data sheet for my strips and use that as a reference. Good enough to grow big plants with high yields…and I don’t have a meter.

I use a lux meter on my phone (my phone has a very, very good lens set-up), then plug that into HLG’s converter to get ppfd.

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