PAR meter

[Ray]

Maybe I missed something, but in the graph Naoki posted above, the "action" spectrum appears to be nothing more than the sum of the two chlorophyll absorption spectra - not a bad estimate, I suppose, but it does seem to ignore any other absorption.

[DavidCampen]

Originally Posted by naoki

Thanks David. So it was an error about the PAR meter. So you are saying that PAR meters are actually measuring actual photon counts, right?

Yes the response of the photocell is a function of the number of photons per unit time that are striking the cell and is independent of the energy of the photons. What may confuse people are graphs like this:
Spectral Response | PVEducation
but the sloped line there is because photocell response is being plotted at constant photon flux power as the wavelength varies and for a given power the number of photons per unit time increases as the wavelength increases (because the energy of a photon decreases as the wavelength increases). If the vertical axis had been plotted as amperes per photon instead of amperes per watt then the line would have been horizontal.

Yes, they did use the absorption spectrum (instead of the action spectrum) for the rough approximation. The soybean action spectrum in the paper you linked is pretty different from what I've seen in the typical undergrad textbooks. I guess different species can have different action spectra, but the difference seems to be rather big.

Yes, I think the action spectrum (and the absorption spectrum) are highly dependent on how it is measured.
The action spectrum you noted gives a 3:1 difference in quantum efficiency for red vs. green light. It would seem that the ratio is not greater than this for most all measurements.

In your old thread, you compared Fluorescent and HPS with deep red LED. Do you happen to know the difference in efficiency between blue vs deep red? I guess efficiency in terms of number of photons per electricity watt or the value corrected for the different energy per photon for corresponding wavelength is what we care, right? I vaguely remember deep red is less efficient than blue or red (the common 630nm), but you probably know this better.

Cree has claimed a quantum efficiency of about 50% for some of their blue LEDs but I have not seen any production blue LEDs with this great of a quantum efficiency. For red LEDs at normal operating conditions the quantum efficiency is about 25% to 30%; blue LEDs may be a bit better but I am not aware of any that approach 50% quantum efficiency. If Cree is actually producing production quantities of 50% efficient blue LEDs then they must be using them to drive the phosphor in white LEDs. I don't remember that there is much difference in quantum efficiency between 630 nm LEDs and 660 nm LEDs but I will have to check on that.

At equal quantum efficiencies then a red LED at 660 nm would produce 40% more photons than a blue LED at 460 nm and for photosynthesis it is the number of photons that matters not their energy (assuming they have at least a certain minimum energy, for photosynthesis this is about equal to the energy of a 700 nm photon).

[naoki]

Ray, I was thinking that the other pigments which absorb other color (carotenoid etc) are at a relatively low frequency in the antenna complex of the photosystems, or energy transfer from these pigments to the reaction center of photosystems is not efficient. That's why the shape of action spectra seem to be dominated by chlorophylls. But after seeing the article David linked. I looked at a couple of old primary literature (instead of undergrad textbook). The one which David pointed out seems to be more common than the figure I linked. Well, at least in crop plants according to this paper:
The action spectrum, absorptance and quantum yield of photosynthesis in crop plants

Basically, green is more useful than the figure I linked, so the action spectrum is flatter. Also the red peak seems to be generally higher than the blue peak. So the figure of absorption spectrum you have in your web page is closer match than the figure I linked.

Then I found a couple other info:
Action spectra for photosynthesis in higher plants
I couldn't get the full text, but this abstract talks about the negative correlation of action (carbon fixation) between green and blue. So plants with low action at the green light has high action at blue (creates double peak). So this suggest that there are quite a bit of variation in the shape of the action spectra. This is an opposite conclusion from the crop paper where he didn't see much variation in action spectra (except pale wheat leaves, which shows a significant green valley).

Then this paper talks about the usefulness of green light in high light plants (such as crop plants)
Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green
It seems to suggest that penetration of green light into the lower layers of chlorophyll become important.

There are lots of things I don't know about PS, but now I start to wonder whether high light (or thick leaved) plants has the flatter action spectra, and low (thin leaved) plants has more pronounced blue+red peaks.

Thank you for additional info, David. yes, the photocell stuff is very confusing to me. One minor point; you said "for photosynthesis, it is the number of photons that matters not their energy". But quantum yield curves generally show that for a given number of photons absorbed, the rate of carbon fixation differs depending on the frequency. In the crop paper, quantum yield is max around 620nm and the blue peak (around 440nm) is about 70% of the red peak.

[DavidCampen]

Originally Posted by naoki

One minor point; you said "for photosynthesis, it is the number of photons that matters not their energy". But quantum yield curves generally show that for a given number of photons absorbed, the rate of carbon fixation differs depending on the frequency. In the crop paper, quantum yield is max around 620nm and the blue peak (around 440nm) is about 70% of the red peak.

Yes, I should have said "at 100% quantum efficiency for all frequencies then it is the number of photons that matters not their energy". Quantum efficiency can vary according to wavelength but it is not a fundamental effect of the photon energy but just a factor of what pigments the plant uses.

Silicon solar cells also, while theoretically the current produced when the cell is illuminated is proportional to the number of incident photons without regard to the energy of the photons, in actual practice the photovoltaic cells have quantum efficiencies that vary according to the wavelength of the light. Licor and all the other quantum PAR sensor manufacturers employ filters to produce an equal quantum response across the wavelengths of interest.
Terrestrial Light Sensors | LI-COR Environmental

Why all the manufacturers other than Licor employ filters that cutoff before 700 nm is a mystery to me.

[Ray]

OK, let me throw this out:

Thinking about sun-loving versus shade-loving plants. Red tends to bend around edges more than does blue. Should we expect lower-light plants - those for whom the light has had to traverse a number of obstacles to reach them - to react better to higher levels of red wavelengths, while high-light plants do the opposite?

[naoki]

Ray, that sounds interesting, but I have no idea. Inada's paper (which I don't have at this moment) could talk about this kind of thing. In addition to diffraction, absorption by the leaves of the upper canopy is more dominant factor, isn't it?

David, LI-190 looks like a good one, and you are right, others have fairly different response. But $500 for just the sensor is a bit steep (well it's better than $1000 for the Licor meter). Mine shows one of the worst response curve, so it's not so useful for 680nm LED. http://wpc.306e.edgecastcdn.net/8030...-MAN-S-LIA.pdf

I'm a bit disappointed, but it's fun to learn new things.

The Li-Cor site has lots of good info. The page you linked has a link to this: http://www.licor.com/env/pdf/light/TechNote126.pdf

[Ray]

Originally Posted by naoki

In addition to diffraction, absorption by the leaves of the upper canopy is more dominant factor, isn't it?

Certainly, but my thought was that not only is the light intensity different, but the spectrum might be shifted a bit, as well.

Anyone got a portable spectrophotometer they're not using?

[naoki]

That's what I meant (spectra will be shifted by what is absorbed by canopy in addition to the physical property of different wave lengths). If the canopy absorbs red+Blue, then the shady plants are left with greenish light. With regard to red shifted vs blue shifted, I wasn't sure the direction. You should play with the DIY spectrophotometer which uses a piece of broken DVD + webcam!

I googled, and check the 1st figure. I'm not sure this is from real data or a conceptual figure. But this seems to suggest there are more red than blue under canopy.
BASIC ENVIRONMENTAL PHOTOBIOLOGY

I don't understand why more red than blue, but this may be a part of the reasons why the bottom of leaves from some shady plants are red (instead of blue). It's their strategy to reflect back the red light into the middle of the leaves where photosynthesis occurs. Begonia is a good example. Paph concolor is polymorphic, and Lance Bark book mentions that the green form without red bottom (f. chlorophyllum) seems to occur in more sunny location.

[Ray]

Originally Posted by naoki

I googled, and check the 1st figure. I'm not sure this is from real data or a conceptual figure. But this seems to suggest there are more red than blue under canopy.
BASIC ENVIRONMENTAL PHOTOBIOLOGY

I don't understand why more red than blue, but this may be a part of the reasons why the bottom of leaves from some shady plants are red (instead of blue). It's their strategy to reflect back the red light into the middle of the leaves where photosynthesis occurs. Begonia is a good example. Paph concolor is polymorphic, and Lance Bark book mentions that the green form without red bottom (f. chlorophyllum) seems to occur in more sunny location.

Thanks, Naoki. That seems to confirm my supposition.

More red than blue would seem to be simple physics. Think back to white light going through a prism - the shorter wavelengths pass through with a very slight deflection, while the longer wavelengths of the red end of the spectrum bend far more severely.

White light passing through the canopy would have little bending of the blue end of the spectrum, while the red end would diffract more easily around the edges and continue their journey.

[naoki]

So red might be even more useful for orchids than for crop plants.

This is in vitro, but they seems to suggest seedlings grew best with red:blue=4:1 than 100%red.
EFFECTS OF LIGHT EMITTING DIODES ON MICROPROPAGATION OF PHALAENOPSIS ORCHIDS

With a related note, this is interesting:
LED GROW LIGHTS | Cree LED Grow Lights | American Made LED Grow Lights | Best LED Grow Lights
White+Red LED seems to be getting more PAR than B+R (per watt). Even if white contains fair amount of green, the 5W white diodes may be more efficient for plants?