Far Red light penetrates the canopy more than you think

[Prawn Connery]

I took three different spectrographs tonight at three different canopy levels. Have a look at how deep Far Red penetrates compared to the other spectra. Green not so much, but more than red and blue.

Top canopy

PPFD (400 - 700 nm)		933.97​
PPFD FR (701 - 780 nm)		158.53​
PPFD R (600 - 700 nm)		509.43​
PPFD G (500 - 599 nm)		292.49​
PPFD B (400 - 499 nm)		131.96​
PPFD UV (380 - 399 nm)		10.148​

Mid canopy

PPFD (400 - 700 nm)		76.991​
PPFD FR (701 - 780 nm)		30.73​
PPFD R (600 - 700 nm)		41.204​
PPFD G (500 - 599 nm)		25.191​
PPFD B (400 - 499 nm)		10.586​
PPFD UV (380 - 399 nm)		0.7753​

Bottom canopy

PPFD (400 - 700 nm)		10.312​
PPFD FR (701 - 780 nm)		10.689​
PPFD R (600 - 700 nm)		5.8184​
PPFD G (500 - 599 nm)		3.4398​
PPFD B (400 - 499 nm)		1.0518​
PPFD UV (380 - 399 nm)		0.0253​

And here are the lights and tent that were measured.

 

[OneHitDone]

Farmer Tyler displayed this quite nicely visually a while back also

 

[Rocket Soul]

I took three different spectrographs tonight at three different canopy levels. Have a look at how deep Far Red penetrates compared to the other spectra. Green not so much, but more than red and blue.

Top canopy
View attachment 5355808

PPFD (400 - 700 nm)		933.97​
PPFD FR (701 - 780 nm)		158.53​
PPFD R (600 - 700 nm)		509.43​
PPFD G (500 - 599 nm)		292.49​
PPFD B (400 - 499 nm)		131.96​
PPFD UV (380 - 399 nm)		10.148​

Mid canopy

View attachment 5355809

PPFD (400 - 700 nm)		76.991​
PPFD FR (701 - 780 nm)		30.73​
PPFD R (600 - 700 nm)		41.204​
PPFD G (500 - 599 nm)		25.191​
PPFD B (400 - 499 nm)		10.586​
PPFD UV (380 - 399 nm)		0.7753​

Bottom canopy

View attachment 5355810

PPFD (400 - 700 nm)		10.312​
PPFD FR (701 - 780 nm)		10.689​
PPFD R (600 - 700 nm)		5.8184​
PPFD G (500 - 599 nm)		3.4398​
PPFD B (400 - 499 nm)		1.0518​
PPFD UV (380 - 399 nm)		0.0253​

And here are the lights and tent that were measured.

View attachment 5355815

Id like to drop a link to one of my favourite posts here on riu since youre onto the same thing: what light gets absorbed by the plants and what travels down thru the cannopy.

Spectral lightmeter Gigahertz-Optik MSC15

Malocan. If you could by chance do a simple test with only a few metering points Of any difference from a fully reflective area vs anything less. Fully reflective as in white or silver floor ceiling and walls vs the normal non reflective floor and ceiling. Facing the meter up of course. Just...

www.rollitup.org

He had similar but contrasting findings. The poster had a spectrometer and a 3500k led light and measured the remaining spectrum puting one, two and three leaves on top of the sensor. The results was quite eye opening but not surprising it youve read up on spectrums; most light absorbed at 680nm leaving lots of greens and far reds.
Both looking at your results and contrasting with Malocans results theres a few things i find interesting:
- in your mid cannopy reading it looks like the blue peak decreased more than the violet peak. But then down in bottom cannopy the violet is almost gone completely with plenty of blue peak left. Seems to defy logic a bit.
- contrast to malocan: he had much more green left than you. 660 was also very clearly missing in the "low cannopy" shots but he didnt have any red sup. Also i dont think he did the exact same thing as you; i think he took a shot of one leaf covering the sensor, then another leaf on top and then a third leaf. The way ive read the thread it seems like rather than sticking the sensor into cannopy he actually covered the sensor with leaves in layers so that you wouldnt have any light sneaking into the sensor from the sides.

It would be very interesting to repeat the test in this way if you have the urge to get science-y with us.

Another thing that may skew results in cannopy measurements would be how close you are to those reflective sides. As in if Mid and low cannopy reading were closer or further away from the reflective sides.

More things: i find it somewhat odd that you have such a symmetric decrease over the spectrum. Especially of the 660 peak. Malocans results 660 dropped off almost as fast as 680nm, here you can clearly see the peak going all the way down to "low cannopy".

His light source was a cob which may have some influence here.

The reason i find it so interesting is due to the concept of "penetration". Ive often scorned it cause i dont think everyone is talking about the same thing: are we talking about buds all the way down or actual light levels or spectrum remaining at the bottom of the cannopy. Some have also argued that penetration is higher with point like lightsources or that you have to have really intense light at the source. As in having a 500w cob would have more P than a 12x12" board of 500w.

Having the monos come thru so clear in your test compared to how much red is absorbed in malocans tests is interesting in this context.

Oh, prawn youve made me feel like "ye olden days" of riu. Please lets have a bit of fun with this and make this thread to a xmas present to all of us and lets get some more data. Im getting popcorn on for this one.

 

[Prawn Connery]

Haha!

OK, the blue vs UV spike might be explained by my hand-held spectrometer, as the Lighting Passport measures every 10nm, not every 1-2nm. If you have seen the Teknik goniometer reading of this light, the 405nm peak is actually higher to begin with than the 450nm peak. But my spectrometer shows it lower because it is not reading the actual 405nm peak, however it does read the actual 450nm peak.

The readings were taken in the middle of the tent at the top, the middle of the tent just below the main canopry, and then the middle of the tent near the pots at the bottom, so it was a pretty linear measurement.

What surprised me was I thought there would be more green. I will do another set of readings tomorrow and try to be a bit more careful about other light sources, as I was in a lit room with the tent door open.

 

[Rocket Soul]

I think spectrometers probably have a response curve, in that they wont respond equally to all nm. I actually had another case of something similar in icmag just previous today.
Im also puzzled about the green light, lets try to figure it out

 

[cobshopgrow]

to add some food, this is some through leaf tests i made a while back.

modular overkill led striplight build

thanks, yes i am also a bit surprised how low i need to drive em. i think youre right, it will just take a little more to get above 600 ppfd in the 3x3, i havent tested. would need to make a complete par/ppfd map, if i find time and sense for it and have no plants in the tent. i can not tell...

www.rollitup.org

 

[OneHitDone]

Maybe this is the place to also get some clarification on "plants are green because they reflect green light". That would suggest they reject the green portion of the spectrum and not use it for photosynthesis. Then there are purple and other pigmented plants. With the fuzzy explanation of "reflecting that color" - purple plants clearly utilize the full range of the spectrum just as green plants do, would they not?

 

[cdgmoney250]

More modern studies show the impact and importance of green light on photosynthesis, particularly at high light intensities and also in the lower canopy. Green light also penetrates deeper into the leaf tissue.



This study talks about comparing red, green, and blue light to background halogen white light at different intensities. It’s an interesting read.

Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green

Abstract. The literature and our present examinations indicate that the intra-leaf light absorption profile is in most cases steeper than the photosyntheti

academic.oup.com

This is another good one on roles of green light in plants.

Don’t ignore the green light: exploring diverse roles in plant processes

Highlight. Green light has essential roles in plant biology, and may have a role in the regulation of canopy water-use efficiency.

academic.oup.com

 

[Prawn Connery]

So this isn't very scientific, I just pick a spot in the middle of the canopy and keep lowering the lightmeter down through it. Still an anomally with the UV and I wonder if it's just the light angle with a UVA diode having a clear path through the leaves, but it corrects itself later on.

Top


Upper middle


Lower middle


Bottom

 

[Lou66]

That's a neat demonstration.

But I would read the results different. To me it says that almost all light is absorbed and very little reaches the bottom. At the 2nd reading the values dropped to 8 % for the PAR and 20 % for FR-PAR. At the third reading it is basically a rounding error.

 

[Prawn Connery]

That's a neat demonstration.

But I would read the results different. To me it says that almost all light is absorbed and very little reaches the bottom. At the 2nd reading the values dropped to 8 % for the PAR and 20 % for FR-PAR. At the third reading it is basically a rounding error.

Yes, but there is a definite trend. Pretty much every colour goes down in relation to far red, but the green – especially around 550nm – is not absorbed as much in relation to the red and blue. It looks like 660nm is absorbed the most followed by UV and blue.

It's a shame about the hand-held spectrometer only reading every 10nm because it doesn't really pick up the red spikes around 610-630nm to see how they are impacted. Here is the original spectrum. These panels are being run at about 2A each, so around 3 umol/j at the LED level (not system performance):

 

[Prawn Connery]

Maybe this is the place to also get some clarification on "plants are green because they reflect green light". That would suggest they reject the green portion of the spectrum and not use it for photosynthesis. Then there are purple and other pigmented plants. With the fuzzy explanation of "reflecting that color" - purple plants clearly utilize the full range of the spectrum just as green plants do, would they not?

More modern studies show the impact and importance of green light on photosynthesis, particularly at high light intensities and also in the lower canopy. Green light also penetrates deeper into the leaf tissue.

View attachment 5356030

This study talks about comparing red, green, and blue light to background halogen white light at different intensities. It’s an interesting read.

Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green

Abstract. The literature and our present examinations indicate that the intra-leaf light absorption profile is in most cases steeper than the photosyntheti

academic.oup.com

This is another good one on roles of green light in plants.

Don’t ignore the green light: exploring diverse roles in plant processes

Highlight. Green light has essential roles in plant biology, and may have a role in the regulation of canopy water-use efficiency.

academic.oup.com

Green light is not reflected off the epidermis or outer layer of the leaf. Rather, it is reflected by chloroplasts inside the leaf. As it bounces through the leaf, its path is elongated – it is not a direct path through the leaf as shown in the above image, but rather a series of zig-zagging lines inside the leaf that make up a longer pathway.

This explains why, when you look at an absorption graph, it appears green light is not abosorbed, but when you look at the action spectra, green light contributes quite a bit to photosynthesis. The reason is because individual green photons are not as readily absorbed by the chloroplasts, but they bounce off a lot more of them – they interact with far more chloroplasts than red or blue light – and thus have more opportunity to be absorbed. That's what makes green light so efficient in terms of photosynthesis.

 

[Prawn Connery]

It's hard to find an image to depict the green light pathway, but see how the green light is reflected from within the leaf? Just imagine that it keeps bouncing around inside the leaf until it either exits (visible reflection) or is absorbed.

Far red is different because it is long-wave radiation that more readily passes between chloroplasts with less diffusion.

 

[Prawn Connery]

Good point about green being more visible to the human eye.

Anthocyanins. I just happened to have this page open when you mentioned it. The solid and dashed lines are two different models.

Carotenoids reflect orange light – who would have thought it!

And you can see how anthocynanins reflect red light (which gives stems and petioles their distinctive red-pink hue when exposed to high light levels). Anthocyanins absorb more green light when there are high light levels, because most of the photooxidative damage will be from red and blue light.



And the reason not to get too hung up about specific wave-length absorption graphs. Notice how peak absorption changes depending on the solvent used to dissolve the chlorphyll? In reality, chlorophyl is bathed in H2O not ethanol or diethyl ether. Although ethylene is always present as a hormone and is the hormone that triggers flowering, so is more abundant at that stage.

 

[cdgmoney250]

Green light is not reflected off the epidermis or outer layer of the leaf. Rather, it is reflected by chloroplasts inside the leaf. As it bounces through the leaf, its path is elongated – it is not a direct path through the leaf as shown in the above image, but rather a series of zig-zagging lines inside the leaf that make up a longer pathway.

That is correct. Here is a study that was meant to dispel any ideas that chlorophylls or the epidermis reflect green light.

“plant leaves are green because green light is less efficiently absorbed by chlorophylls a and b than red or blue light, and therefore green light has a higher probability to become diffusely reflected from cell walls than red or blue light. Chlorophylls do not reflect light.”

https://www.tandfonline.com/doi/full/10.1080/00219266.2020.1858930?scroll=top&needAccess=true

 

[Rocket Soul]

That is correct. Here is a study that was meant to dispel any ideas that chlorophylls or the epidermis reflect green light.

“plant leaves are green because green light is less efficiently absorbed by chlorophylls a and b than red or blue light, and therefore green light has a higher probability to become diffusely reflected from cell walls than red or blue light. Chlorophylls do not reflect light.”

https://www.tandfonline.com/doi/full/10.1080/00219266.2020.1858930?scroll=top&needAccess=true

At one point i was looking at the results above and those of malocan thinking that maybe the the difference was down to phosphor part of the light being less available in lower cannopy: i thought i could see the peaks of the actual led diodes(violet, blue, 660,730) was what decreased less while the part of the phosphor that decreased more. But looking at numbers and malocans spectrum i decided it was bunk, the two tests arent really showing the same pattern. Malocans seems to follow a more "action pattern" response but here its just more far red the lower you go. It was the "diffusely reflected" in your quote that brought it back to me but i thought id share it even if i bunked it.