See my recent 10 or so posts in this forum, in the de bulb thread. It's an issue in multiple ways. High ppf requires a high leaf temp for optimal photosynthesis. Cannabis requires similar temps for optimal metabolism. The ambient temp needs to be higher with led to achieve the same leaf temps. Blasphemy according to the Bibled thumpers here but a well recognized issue in tons of research and side by side performed by universities and experts.
Quite a few more honest led light designers agree, some even add IR.
"When growing under optimized-spectrum LED plant lights, the lack of excess infrared and other unusable light causing the leaves to heat up means that ambient air temperature needs to be warmer than for the same plant grown under any light (natural or artificial) which is not optimized for plant growth."
"When growing with plant-optimized LED lights, it is important to realize that ambient air temperatures need to be kept higher compared to other lighting to achieve the same metabolic rate. "
Using white leds only partly makes up for this issue. Especially those low in red, the most efficient for photosynthesis lol... and high in non-blue/red. The crappier the spectrum of the white cob, thus the less usable light in the optimal wavelengths for photosynthesis, the better they will be for warming up your plants... Ironically, led fans have taken the heat thing to a simplistic black and white level. They are essentially willing to sacrifice a major advantage of led (light quality control) by spending par watts on warming up the plants. And missing out on the full potential of led, blinded by par watt efficiency numbers and hps-envy.
The claim led runs cooler and is therefor better is typical led marketing from led companies (and thus led shills and fans). Horticulturists and unbiased pros often have a different view... The IR increases the leaf temp more than the ambient temp. Ambient temp needs to be higher under led than under hps in reasonable circumstances to be able to match hps. This negative is turned around into a positive, which led fans take a step further by pretending hps creates too much heat.
"LIGHT & INFRA-RED
Both sunlight and HID light sources produce copious amounts of Infra-Red (IR) radiation. IR is effectively heat, but it only exhibits that heat when absorbed by something. This means that IR can pass innocuously through the air having little effect on its temperature because air is transparent and absorbs little IR. But when it strikes the leaf of a plant, it is absorbed and it heats the leaf. There is some evidence to suggest that IR closest to the visible range can enhance the red band absorption of chlorophyll, but IR’s effect on the plants leaf temperature and metabolism are more pronounced. As an example, if your room temperature is 78f using HIDs, the IR striking the leaves will increase the effective leaf temperature by 5-7f. This means that while you THINK your room is operating at 78f degrees, the plant is actually seeing 83-85f.
[...]
It’s a common misconception that LED grow lights cannot compete with HID purely in terms of yield.
Actually LEDs simply require an increase in ambient temperature for comparable results with lower energy consumption and heat generation. LED-based light sources differ from sunlight and HID not only in their duo-chromatic, photo-synthetically tailored spectral output, but also in the fact that they produce virtually NO IR. So a plant growing in a room with HIDs at 78f will actually exhibit the metabolism of a plant at 83-85f, while the same plant in a room with LEDs at 78f will only have a 78f metabolism rate.
Tip #1 -So when you flower with LEDs, you must raise the room temperature 5-7 degrees f higher than you would run with HID, with all other conditions equal.
"
"Disadvantage of LED.
LEDs do not emit that much radiant heat. Conventional lighting systems, especially HPS, can produce heat to warm up and dry leaf surfaces faster. Radiant heat from the lighting can be an advantage in certain circumstances, such as in keeping leaf surfaces dry in order to reduce the occurrence of powdery mildew. This is one of the reasons that some greenhouses are using mixed LEDs and HPS in their production environments".
-Dr. Youbin Zheng, University of Guelph
University of Wageningen tested it too, on a huge scale, and it's a simple fact that LED requires higher ambient temps than hps.
https://www.blackdogled.com/bloglst/ This led company presents a fairly honest view on the matter.
What it comes down to is that the lack of radiation heat from led can be both an advantage and a disadvantage. Just heating to the point where ambient temp is equal is not enough.
