HighScience - HPS vs LED: What temp do I keep my canopy explained

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Chad.Westport

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#ChadWestport #Lighting #LED #HPS #HighScience

Publication: Plos One
Peer Reviewed: Yes
Authors: Jacob A Nelson and Bruce Bugbee
Published online 2015, Oct. 8
Title: Analysis of Environmental Effects on Leaf Temperature under Sunlight, High Pressure Sodium and Light Emitting Diodes


Why was I interested?
I’ve known that an ideal temperature for your canopy under HPS (High Pressure Sodium) lighting has been universally accepted as roughly 72 degrees. There are many factors that need to work in tandem for a perfect grow, but that temperature has been ingrained in my databank. LED technology has finally become an effective alternative and many home cultivators are using it, but why do users insist that the room temperature should be around 85 degrees? That would spell serious trouble if it were my HPS room.

The answer I got from online forums, was that the difference in desired room temperatures is due to LST (Leaf Surface Temperature). LST is important because increased leaf temperatures cause increased transpiration. In short, the plant sucks up water and nutrients faster. A slowing in the transpiration process would mean reduced yields, so that question was worth investigating.


The Study;
The goal was to compare leaf to air temperature differences among four radiation sources: clear sky sunlight in the field, sunlight in a glass greenhouse and indoor plants under either HPS or LED lighting. The plants used in the study are tomato, pepper, basil and broccoli. While Cannabis was not used in this study, the authors specifically note that their findings “would likely be representative of flowers, fruits and thick dense plant parts that have low transpiration rates, including high value products such as tomatoes, strawberries and Cannabis flowers”.

In the study, they took into account important factors such as PPF (Photosynthetic Photon Flux), stomatal conductance, wind speed, vapor pressure deficit and leaf size. Overall, I am satisfied with the process and considerations of this study and feel confident using this information going forward, until there is new evidence to dispute it. So why am I being told I need to run my room hotter with LED lighting?


The Why;
Professional grade HPS and LED lighting can generate an identical amount of thermal energy per photosynthetic photon but LED fixtures “dissipate much of their heat away from the plane they illuminate, while HPS fixtures dissipate more heat toward the plane they illuminate”. The greatest difference in terms of incident radiation between HPS and LED lighting was in the NIR (Near-Infrared) and longwave bands.

NIR is quantified in this study as the 700-2500nm spectrum and is found in high numbers with HPS lighting but very low numbers in LED. NIR light is poorly absorbed by leaves, although it is a significant source of energy it has little impact on individual leaves. “The effect of NIR on leaf temperature is relatively small”.

Longwave radiation however, had the largest effect on leaf temperature. “HPS lamps operate at higher temperatures and thus generate more long wave radiation in the same direction as the photosynthetic radiation”. Conversely, LED lights are typically thermally bonded to heat sinks and the thermal energy is directed away from the canopy.


The Results;
It’s a mixed bag. If the plants are not water stressed, leaves under both the LED and the HPS lighting systems were typically within 3.6 degrees of air temperature. In their “worst case scenario” which includes water stressed plants, low wind and high PPF, leaf temperatures would increase 10.8 and 21.6 degrees under LED and HPS respectively, compared to the air temperature. “When other environmental conditions are constant in the model, air temperature between 59 and 95 degrees has minimal effect on leaf to air difference”.


Conlusion of the study;
“The presented model indicates that the use of LED technology reduces leaf temperature by about two degrees compared to HPS technology under typical indoor growing conditions. While this is a significant difference for some applications, the difference is smaller than the difference between indoor and outdoor leaves”.


What are my take away thoughts;
There is some truth to running your room hotter using LED compared to HPS systems, although under ideal conditions it is only a few degrees difference.

Don’t get that confused, the LED won’t make your room hotter, in fact, HPS lighting operates at higher temperatures. If you are in cooler climates and keeping your grow room at the right temperature is an issue, HPS might help with that. Conversely, if you live in hotter climates, LED lighting might help keep your grow cooler comparatively.

The factor that seemed to have the largest impact on the question of air vs leaf temperature, is the transpiration and water stress level of the plant. An interesting note was added in regards to supplemental CO2. The authors noted that “supplemental CO2 can decrease stomatal conductance 10-40% and increase leaf temperatures”.

If I run an LED with a room temperature of 86 degrees as suggested online, and I happen to have water stress in my plants, the leaf temperature (without supplemental CO2) can reach almost 97 degrees. That scares me. If running a room with LED lighting, I’ll keep the air temperature around the upper 70’s, lower 80’s. I guess the real question now would be, what is the most efficient leaf temperature for Cannabis and does it vary by strain?





Link to study - https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0138930
 
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Chad.Westport

Chad.Westport

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Hey growers :) I have a habit of reading boring technical papers, so I thought I would start to summarize some of them and offer them to you all. I'm open for debate, I'm open to learn new things and I'm always happy to offer my opinion.

I hope you all enjoy the High Science ;)
 
Aqua Man

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Many more factors could be accounted for. Such as the spectrum difference which has a fairly large impact on stomata. This directly can affect transpiration rates and in turn leaf temps.

I also believe slightly different nutrient usage under different spectrums as they influence the type of growth of the plant a great deal.

Good info but I feel like again we cannot class LED as LED and this is a bit to general to hold as concrete imo.
 
Aqua Man

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Comparing a 400w led to a 1000w HPS? And I never heard of that LED.
 
Chad.Westport

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How does spectrum difference effect the stomata?

How do nutrients effect the leaf surface temperature?

In the study, they took into account important factors such as PPF (Photosynthetic Photon Flux), stomatal conductance, wind speed, vapor pressure deficit and leaf size.
 
Aqua Man

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How does spectrum difference effect the stomata?

How do nutrients effect the leaf surface temperature?

In the study, they took into account important factors such as PPF (Photosynthetic Photon Flux), stomatal conductance, wind speed, vapor pressure deficit and leaf size.
Blue light has the most effect on stomata.
 
Aqua Man

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Here is a good reference for spectral affects on cannabis.

 
Aqua Man

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Also different spectrums reflect more or less light. Red/blue doesn't give enough info imo. This is important to know since they are measuring reflection of light
 
Chad.Westport

Chad.Westport

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Comparing a 400w led to a 1000w HPS? And I never heard of that LED.

From the study -
-Our objective was to use a well-established energy-balance model to compare the leaf-to-air temperature difference in four radiation scenarios across multiple environments.

-All measurements were scaled to PPF to account for differences in fixture height. As a canopy gets closer to the fixture all types of radiation increase.

Despite wattage, all things were equal in the test.
 
Chad.Westport

Chad.Westport

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Blue light has the most effect on stomata.
Awesome, learning, its fun :)

The amount of blue light in the spectrum would be a variable in using this in the equation for the study I reference. In that study, they are using a multi spectral light, it wasn't monochromatic, although it was a blurple. Which, considering this came out in 2015, makes sense.
 
Chad.Westport

Chad.Westport

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Here is a good reference for spectral affects on cannabis.

funny, cause I've already read it and have it highlighted ready to go in a stack next to me.
 
Aqua Man

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Awesome, learning, its fun :)

The amount of blue light in the spectrum would be a variable in using this in the equation for the study I reference. In that study, they are using a multi spectral light, it wasn't monochromatic, although it was a blurple. Which, considering this came out in 2015, makes sense.
And blurple give off IR... the good ones don't give off hardly any. How can they call a blurple... I forget what they called it but something like industry leading
 
Aqua Man

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Hardly any, it would take a model with that specific IR feature.
Blurples? They give off more than full spectrum led.

Also just can't compare them as LED its misleading and green is very important. Photosynthesis is improved i would say significantly.

 
Aqua Man

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I have to look around for the OR but blurples put put more IR. Can test just with your hand.
 
Chad.Westport

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Yeah, I've got stuff about green light as well. The technology has changed since the paper was published and as I said, I'm happy with that info until new studies are created to dispute it. Overall, my recap of the study is accurate, but as the dude says - new shit has come to light man :).

and thats a pretty clever double pun dammit! Thanks for sharing @Aqua Man

The study is still very useful in understanding conditions that effect your leaf surface temperature. Worth the read even in you don't agree with the conclusion of the authors.
 
Aqua Man

Aqua Man

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Yeah, I've got stuff about green light as well. The technology has changed since the paper was published and as I said, I'm happy with that info until new studies are created to dispute it. Overall, my recap of the study is accurate, but as the dude says - new shit has come to light man :).

and thats a pretty clever double pun dammit! Thanks for sharing @Aqua Man

The study is still very useful in understanding conditions that effect your leaf surface temperature. Worth the read even in you don't agree with the conclusion of the authors.
Absolute all information is beneficial.
 
Chad.Westport

Chad.Westport

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Blurples? They give off more than full spectrum led.

Also just can't compare them as LED its misleading and green is very important. Photosynthesis is improved i would say significantly.

Here is a good read, about green light.

" Because green light can penetrate further into the leaf than red or blue light, in strong white light, any additional green light absorbed by the lower chloroplasts would increase leaf photosynthesis to a greater extent than would additional red or blue light."
 
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