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I've used one of those in my 2x2'. I found it really helpful to measure the light intensity at the canopy and use that to inform light distance / dimming. With a white LED light like that HLG QB, a cheap lux meter (I use this one) will be accurate enough. A PAR meter would be better, but they cost significantly more.
The light intensities that make my plants happy have been fairly consistent across grows, though they depend on the light schedule -- which makes sense, because 24/0 is already giving them 20% more light per day than 20/4. I'd suggest starting with less light, watching how your plants react, making gradual adjustments, and keeping notes. My numbers might be different than yours, due to other environmental factors like ambient levels of CO2, but in 24/0, I tend to use something like 6-8k lux for seedlings, gradually increase to 14-18k in veg, then 20-25k in flower. (Each would be a little higher for 20/4 or 18/6.) In practice, that means I start with the light about 30" away, bring it down an inch or two if the seedlings are really stretching, but otherwise let them grow towards the lights. Once they're close enough that they exceed those thresholds and risk light stress (roughly 18") I raise the lights, checking the light intensity daily during the flowering stretch.
I'm not sure because I honestly don't trust any phone app readings. Get something like this:
You want to run you plants pretty hard and if you are actually at 30klx then I'd be running those lighting levels higher. Don't go below 40klx.
You might want to consider picking up a LUX meter, like this one, and use it to give you an idea of how much light you are giving your plants. I didn't think they were useful for that purpose until I found a conversion table on HLG's site for their lights. I have an HLG light, so I've got an accurate conversion number, but you can still get a fairly good idea of how many ppfd your light is putting out.
It may also lead to light stress. You may be better served by getting a lux meter first and making sure that your current lights are positioned effectively. I use this $20 one.
I found that when I added more light than my 135W QB, I just stressed my plants, unless I raised it sufficiently far away or dimmed it.
Phone apps are inherently incapable of measuring lux accurately, because the phones don't have a dome for cosine correction -- making tiny change to the position of the phone can cause the reading to jump around. Since lux meters are pretty cheap (I use this $22 one) it's worth picking up one if you're using QB or COB lights.
Okay. My plants' upper fan leaves look a lot like that when my LED lights are too close. I'm trying to not jump to conclusions and see it everywhere just because it took me a while to figure out and fix in my grows, but if it's all at the top / closest to the light, light stress would be the top thing to rule out.
"80%" and "24 inches" doesn't objectively mean that much, especially since plants 24" away but directly under the light will still get quite a bit more light than 24" down but in the corners. I found it really helpful to get a cheap lux meter and actually measure. While it's not as accurate as a PAR meter, it's close enough to be useful (and much cheaper); the thresholds I've started to see light stress have been very consistent across grows (depending on light cycle). As long as I stay under them, there's no damage, and then if flowering stretch brings one branch an inch closer to the light, I see symptoms within a day or two. I have damaged fan leaves that look almost exactly like that on the very top branches of a really stretchy Hubbabubbasmelloscope right now, but the rest looks fine.
A lux meter app on your phone won't be accurate. The phone doesn't have the translucent white plastic dome for cosine correction, so instead of getting an area reading it's looking through a pinhole, and readings will vary widely due to small adjustments to the phone position/angle. The sensor hardware is fundamentally not set up right for that use.
Also, if it is light stress -- in my experience, that damage doesn't heal, you'll just see them dry out and curl up and fall off later in the grow. They probably can't photosynthesize very well due to the damage, so it may be beneficial to remove a few a day (gradually, to avoid extra stress), prioritizing the ones that are shading healthier leaves and flowering sites below.
I don't have that light, but I've used a similar one (their 135W kit) in a 2x2. I had best results keeping the light around 18-20" away from the top canopy in flower, but rather than measuring vertical distance, it's probably more meaningful to measure light intensity across the canopy. With that light, you can use an inexpensive lux meter (I use this one) to get fairly accurate readings. (A PAR meter would be more accurate, but they are considerably more expensive.)
The specific lux targets will depend on your environment and your light schedule, but if you adjust your light and plants to keep the lux reading relatively even across the canopy, then gradually increase intensity and see how the plants react, you should be able to settle on light position/dimming that grows effectively without stressing the plants.
That sensor on your phone is great for photography, but they're generally pretty terrible for getting an accurate reading from a grow light. This light meter was recommended to me in here and has made a huge difference in my grows
This is what I recommend for most people:
I have -- I've switched back and forth between 20/4 and 24/0 for a couple years now. The main difference I've noticed is that I get light stress more easily with 24/0 unless I dim my lights slightly / position them slightly further away. Based on light intensity measurements* and conversions, I consistently see light stress appear at a DLI somewhere very close to 40, it just takes a little less light intensity to reach that with 24 hours rather than 20. My hypothesis has also had predictive value -- in cases where individual branches stretched up towards my lights and exceeded the threshold consistent with that light schedule, I saw light stress symptoms appear within a day or two, right when I would have expected it.
While I don't have the resources to do a formal experiment, with enough plants to make any strong conclusions with statistically significance, all my experience thus far is consistent with the DLI mattering more than which particular light schedule you use. (Within reason -- you might not get great results with 24x as much light for 1 hour.)
I usually recommend using whatever light schedule helps to manage your environment best -- if your tent runs cold or humid, use 24/0, if it's too hot use 18/6 or 20/4, etc., because that's more likely to have a significant benefit.
* Using a lux meter, which is not as accurate as a PAR meter, but accurate enough for use in a homegrow. Also consistent with its own measurements over time.
This one has been recommended by others and it’s worked well for me.
Dr.meter LX1010B Digital Illuminance/Light Meter, 0-100,000 Lux Luxmeter https://www.amazon.com/dp/B004K0A7I6/ref=cm_sw_r_cp_api_glt_fabc_9WDP3Q8155H23XCGWVVW?_encoding=UTF8&psc=1
PAR meters are more expensive ($300+), but lux meters are around $20-40. I use this one.
Lux meters will only work with white LEDs, though. If you have "blurple" LEDs, you need a PAR meter, because lux meters aren't designed to measure that weird spectrum.
I linked the one I use below literally you can move your plant a few inches and get a extra 10k lux or more.
Dr.meter LX1010B Digital Illuminance/Light Meter, 0-100,000 Lux Luxmeter https://www.amazon.com/dp/B004K0A7I6/ref=cm_sw_r_cp_api_glt_fabc_V276Q2KAZ7EAC8APV99W?_encoding=UTF8&psc=1
I see! Good luck with your future grows man! It is a rewarding hobby but also a patient mans hobby LOL. I’m using a Mars Hydro TS1000 and as far as how close the light needs to be I use a luxmeter Dr.meter LX1010B Digital Illuminance/Light Meter, 0-100,000 Lux Luxmeter https://www.amazon.com/dp/B004K0A7I6/ref=cm_sw_r_cp_api_glt_fabc_XYRYWS9P05FTJ5MPZQWE?_encoding=UTF8&psc=1
With that style of light, you can use an inexpensive lux meter (I use this one) to measure light intensity, since just the vertical distance between the canopy and the light doesn't account for things like light intensity decreasing away from the light's center.
I'm reluctant to post specific numbers and say, "you should aim for these targets" because there are way too many variables, but I tend to notice light stress symptoms when I go past particular numbers (which vary based on the light schedule) -- if you use the meter to even out coverage between the plants, then make gradual changes and see how the plants react, you can probably settle in on light positioning that will work well for you.
Lux meter apps are horribly inaccurate, because your phone doesn't have a translucent dome over the sensor to compensate for the directionality of light. It's probably using a camera's ambient light sensor to measure light, then converting that to lux as a unit; even if it were accurate, which is unlikely, it's would still be very unstable -- like trying to measure the pH of RO, which has very little buffering.
A while ago I tried several lux meter apps on my Android phone (a Pixel) and none of them agreed with each other or my actual lux meter. If you depend on lux measurements for light positioning, I'd recommend buying a real meter for $20.
Also, I highly recommend grabbing a light meter for ~$20 if you don't already have one. Without a doubt.. it's the most useful tool I have in my housplant-toolkit.
I do use this lux meter to help set my light height., but honestly the spectrometer and what not is over my head (and out of my price range).
I understand proper testing would be needed to say definitely what is the best or ideal lighting for each situation. but I was hoping you might have a general recommendation based on your testing and exp. My google fu has failed me at finding side by side grows that mix color temp vs 2700k/3000k only during flower.
I am in the process of tying to read your whole lighting guide, but I haven't been able to find the advanced section on color temperature mentioned here at the bottom of the color temp basics
> So, this covers the basics. In the advanced version we'll discuss how not all color temperature is the same and how a 2700K LED can give different results than 2700K CFL with spectrometer pics to illustrate why.
This applies to me completely b/c outside of my tent that has COBs, my buckets will run LED bulbs and I got a 40W equivalent 6500k for my aerocloner bucket (hoping to prevent stretch with the higher temp).
Lux is light intensity over area. If you had an idealized light source that emitted equally in all directions then the math would be fairly straightforward by taking the total lumen output and dividing by the area of a sphere with a radius of the distance you are removed from the light source.
But in reality the light doesn't emit equally in all directions and there are so many other variables that it's easier just to measure it at the place you're interested in. I have this lux meter, and it works fairly well: https://www.amazon.com/dp/B004K0A7I6/
The lumen output of the Hue lights does change depending on the color temperature. I don't think Philips publishes the output curve, they just give the output at the brightest temp (4000K as you mention). It will be dimmer at lower and higher color temperatures.
I have a Dr. Meter LX1010B.
Ideally you'd get a PAR meter, but they're expensive ($250-400ish?). I use a $25 lux meter, then use this lux -> PPFD converter using the High CRI 3000K and Low CRI 3500K values as an estimated range (I use 3500K autoCOBs, which are probably high CRI), then plug that into this PPFD -> DLI calculator to get an approximate DLI. The lux->PPFD conversion is approximate, but the PPFD->DLI conversion is exact (it's accounting for hours per day and doing unit conversions).
Whether a lux meter is a viable option depends on your lights -- they won't work well with blurple LEDs because of their atypical light spectrum -- but with full spectrum LEDs they're probably good enough for homegrowing use. Accuracy will vary across individual meters, for all I know my meter always reads 5% low, but I've got consistent results from it for a couple years. It's great for finding poorly lit spots in the canopy I could never spot visually, I start to see light stress if I go past consistent light thresholds (depending on light schedule), etc.
Using a smartphone's light sensor as a lux meter won't work very well, because the hardware isn't set up properly for that use case; the white dome over the sensor is important, without it slight changes to the phone's position will cause readings to jump all over. Some mobile app suggests putting a piece of some specific weight of paper over the phone sensor as a diffuser (not even a dome shape!), but I've worked on configuring light sensor hardware for commercial electronics devices and the whole idea seems ridiculous.
A quick take-away is that DLI has a linear relationship to time, so if you look at light levels people recommend for flowering photoperiods in 12/12, you can flower autos with roughly half as much intensity in twice as much time (24/0).
This is a good question but the answer is complicated.
I use a lux meter to measure my light intensity, and that informs the position. (While a PAR meter would be more accurate, it would also cost about 25x as much...) For my LEDs, the lux meter is accurate enough to be useful, and it's great for finding uneven spots in the canopy that are hard to see with the naked eye. Just saying "X inches away" or "Y true watts" sweeps a lot of important variables under the rug -- vertical distance is only useful straight down, with most lights intensity decreases off-center, and LED lights vary in efficiency; for two lights with the same wattage drawn from the wall, one may have more efficient hardware, and the other may also divert some of those watts to power a cooling fan.
Anyway. I start with my lights about 30" away with seedlings, and if they stretch excessively I bring the lights an inch or two closer. Otherwise, I position my lights to even out light intensity throughout my tent (2x2') and let the seedlings grow towards the lights through veg. As the plants stretch upward, I make sure the lights are always at least 18-20" away (particularly as they stretch in early flower), trying to avoid going past a certain lux threshold beyond which I consistently seen symptoms of light stress appear. This may involve moving the lights around or bending upper branches to even out intensity at the top canopy, especially when the plants are different heights.
I'm reluctant to share my lux threshold numbers without heavy qualification, because my meter might be 5-10% off, my environment's ambient CO2 might be unusually low (ambient CO2 dramatically impacts how much light cannabis can handle), it may depend on my specific model of lights, and it definitely depends on my light schedule (it's about 20% higher in 20/4 than 24/0). None of this stops GrowWeedEasy, of course; they suggest lux targets that are more than twice mine, probably because they don't take light schedule into account, and in my experience can seriously burn autos. Their numbers seem more reasonable for photoperiods flowering in 12/12.
Rather than start with specific lux numbers as a target, I started with my lights further away, made gradual changes, and watched how the plants react. Every time I've run into light stress, there's been a similar progression of symptoms that started on parts of the canopy getting the most light intensity. The measurements have been pretty consistent for each light schedule (I switch back and forth seasonally between 20/4 and 24/0), so I treat those as an upper limit and adjust accordingly.
Also, if you do enough searching you'll find I've essentially typed up this answer at least a dozen times so far. I should probably bookmark one of the more thorough versions of it and just link to that from now on.
It probably varies from one setup to another due to ambient CO2 levels and who knows what else, but I see symptoms of light stress in flower when I go past about 20-25k lux in 24/0 or 25-30k lux in 20/4*. It's been very consistent for several grows for me. They're fine in that range, but when plants stretch too close to my lights and exceed those, I see the serrations on the leaf sides standing up (right at the limit) followed by damage to the most directly lit fan leaves. If it matters, I use two autoCOBs, in 3500K.
I'm not aiming for "optimal", but that's what seems to keep my plants happy and lead to a low-drama, low-maintenance grow.
* Both seem to correspond to a DLI around 40, as do lux targets I often see for flowering photoperiods in 12/12 (without supplementing CO2). I switch back and forth between 20/4 and 24/0 to help manage the temperature inside my tent. There's also some fuzziness in the DLI measurements, because I'm using a lux meter rather than a PAR meter and then doing approximate conversions.
Dr.meter LX1010B Digital Illuminance/Light Meter, 0-100,000 Lux Luxmeter https://www.amazon.com/dp/B004K0A7I6/ref=cm_sw_r_cp_api_glt_fabc_VTGW67TCNMX55EN1J4QE?_encoding=UTF8&psc=1
You need to be able to measure light intensity. A PAR meter is ideal, though they're pretty expensive. With full-spectrum LED lights, it's possible to get reasonably accurate readings with a ~$25 lux meter (I use this one), and then use some conversions (1, 2) to convert lux to approximate ppfd and then ppfd to DLI, which you could think of as ppfd (a measure of photosynthetically useful light intensity) per day.
It won't work with a blue+red LED light, because lux meters aren't tuned for that weird spectrum. Phone apps can't measure accurately either, because they lack a plastic dome for cosine correction -- they're effectively measuring straight-line light intensity through a pinhole, but what you want is more like putting your hand out and feeling sunlight coming from all angles and over an area, since that's what's actually happening with leaves. If you use a lux meter app, you'll notice that slight movements with the phone will cause the reading to jump around a lot.
I'd recommend picking up a cheap lux meter from amazon. I own this one https://www.amazon.ca/Dr-Meter-Digital-LX1010B-Display-Luxmeter/dp/B004K0A7I6/ref=sr_1_5?keywords=lux+meter&qid=1579567193&sr=8-5 and it helped immensely even though I still ended up burning them a bit. I was running about 45k lux 20hrs on and ended up toasting a few of the top leaves. For reference I've got a 260w QB and was running at full power about 24" from the top colas.
Yeah, sorry, I tried a couple phone apps too. They didn't agree with each other or with the meter I bought later. I got this one for $20 (doesn't look available now), maybe try this $14 one?
Lux meters are about $15-20 or so. Your eyes have a logarithmic response to lighting intensity, meters will have a more appropriate linear response.
Measuring true total light output actually requires an integrating sphere. It's not as easy as one may think. Fortunately these LED bulbs have already been measured but for the amount of light on the plant leaves you can just use one of the inexpensive lux meters.
The color temperature of the white light source makes very little difference in a certain common range with lux meters. The color rendering index (CRI) can make a big difference but most of these consumer lights are roughly the same and you should not worry about it.
You phone is highly likely not cosine correct so it does not really make a good light meter.