Lighting How much is "too much" light?

[Yozhik]

I edited the post, look at this added link https://www.icmag.com/ic/showthread.php?t=127379

WE HAVE A WINNER!!

temperature and light optima for photosynthesis was concluded to be at 25–30 °C and ∼1500 μmol m−2s−1 respectively.

,,, nice work @Corgy

 

[thetreeman]

You can do too much light with LED and it will negatively impact your grow. Ive seen several plants bleached white at the tips cuz the led killed the chlorophyll. Also your plants will grow very short and stocky, especially with too much blue end of the spectrum. From my experience about 30 watts / sq foot works very well for bloom using led but you can go up to 45. 15-20 watt / sq foot is really all that is needed in veg or your plants wont get as much height as they could before kicking into flower. For some that might not be a bad thing, but for me it is. Also with LED, color spectrum can be controlled and is important, wrong spectrum will mess things up from the beginning. Ive found that good results can be obtained using about 20 percent blue, close to 450nm is optimal, at least 50% red, 640 to 670 is good here. Im currently growing some plants using custom led strip lights using this pattern of 3w leds, 660red, 6500k white and 2700k white repeating. This provides roughly this amount of light.
Blue (400 – 499nm)
13%
Green (500 – 599nm)
29%
Red (600 – 699nm)
56%
Far Red (700 – 750nm)
3%
-

 

[Hal 8999]

You can not make too much light. It may be too hot from being close. Just keep in mind the more light, the more everything else it needs. Nutrients, water, and air are the everything else. I have only had bleaching occur at two inches or less distance from a 400 and a 600 panel. The sun is a ball of ongoing nuclear fusion, we are not anywhere near its power.

 

[Magic]

I'm not sure but I would think you'd suffer from too much heat before too much light. I'm sure others will correct me if I'm wrong.

Right on the money, you'll get too much heat before too much light.
If there is a way to keep heat down then I couldn't really say how much light can a plant take before efficiency suffers. I'd Google it you get a lot more info from universities that use LEDs for growing

 

[Magic]

So, it would seem there IS "too much light" ... but we don't know how much that actually is?

i.e. how many actual watts per sq.ft would be considered "too much light", and morph into photoinhibition?

Photosystem II is damaged by light irrespective of light intensity.[16] The quantum yield of the damaging reaction in typical leaves of higher plants exposed to visible light, as well as in isolated thylakoid membrane preparations, is in the range of 10−8 to 10−7 and independent of the intensity of light.[10][19] This means that one PSII complex is damaged for every 10-100 million photons that are intercepted. Therefore, photoinhibition occurs at all light intensities and the rate constant of photoinhibition is directly proportional to light intensity. Some measurements suggest that dim light causes damage more efficiently than strong light.[11]

 

[81camaro]

Good link Corgy. Haven't looked at the light question much since I bought my light. I went with the LEC315 (for UV), and I believe in the tech and I've had good results with it. I did come away with a feeling at that time that ANY artificially produced light needs to maintain a minimum distance from the canopy to allow for proper blending of the light spectrum produced. I was considering the purchase of an LED rig and I believe the bleaching seen in older LED grows supports this, particularly where the spectrums produced by early LED tech centered mainly on either the red and blue grow peaks and ignored other parts of the spectrum. The addition of white spectrum LEDs in these rigs in later generations and their improvements in growth patterns also, to me, would bear out this line of thought. At this point, LED manufacturers use chips that produce light across the entire spectrum, furthering the consideration of proper height to allow blending as well as the use of diffusers (GN HSI Hologram!). Just a thought.
To get back on point there are grows documented here using 1000s of watt and they are quite successful. Removal of the heat produced appears to be the limiting factor, agreeing with posts above. In seedling I think its unnecessary and a waste of electricity. I start at 90 watts of blue LED at this point. When the leaves look big enough and a few nodes stack up supporting more photosynthesis, the big light goes on, but high at 36 inches, and lowering through veg. In flower, I get as low as I can, and with my setup consider 20-24 inches the sweet spot, based on light charts and personal observation of my grows. That being said, I just finished a Thai that was 12 inches from the bulb the last 4 weeks, which would of absolutely scorched (and has, LOL) anything else. So strain choice and its ability to absorb light is also a large consideration. In addition, taking advantage of overlapping fields of low level light is a proven strategy to boost light field intensities, Think CFL and FWIW LEDs (100 pieces x 3w each or whatever). I am also aware of a post but unfortunately cannot remember the link that details a plant's reactions to ratios of red and blue light and how these ratios drive growth.