PAR meter

[naoki]

I came across this page:

Product Review: A Comparison of Two Quantum Meters - Li-Cor v. Apogee — Advanced Aquarist | Aquarist Magazine and Blog

I thought that it might be interesting to some of you. It compares relatively inexpensive Apogee meter with more expensive Li-Cor model.

If you want to save a little bit more, you can get the Apogee sensor, and use digital multimeter:

PAR meter vs PAR sensor from Apogee - Aquarium Plants

Here is the comparison of inexpensive Apogee vs more pricey ones.
Apogee SQ‐120 vs. Kipp & Zonen PQS 1 vs. LI‐COR LI‐190 vs. Skye SKP 215

[Nexogen]

Good to know. Thank you very much.

[DavidCampen]

The problem with the Apogee sensor is just as the article says:
"The Apogee underreports blue wavelengths more so than the Li-Cor. The cutoff in the red portion of the spectrum is at ~650nm for the Apogee; the Li-Cor sensor extends its range (albeit slightly) into the near-infrared (cutoff at 700nm, but with a slight response to ~720nm)."

Because I want to measure LED lighting that is putting a lot of energy into 660 - 670 nm deep red photons and much of the rest of the energy into blue photons the Apogee sensor is useless to me. If you want to measure sunlight, fluorescent or HPS then the Apogee sensor would work OK.

I looked at the spectral response of every PAR meter I could find and only the Li-Cor is suited for measuring light that has most of its energy in blue and deep red.

[naoki]

You are right, that is a problem. I think the common red LED is around 630-640nm, but deep red seems to be a better match for photosynthesis. I wonder why they can't make a decent PAR meter with reasonable price. Maybe not enough demand, I guess.

This seems to be interesting; Public Lab: Spectrometer
I wonder if it is usable.

[Ray]

I question the utility of light meters to growers, including PAR meters.

Yes, a PAR meter covers more of the spectrum used by plants than does a "regular" light meter, but as PAR is the "volume" of photos per area per time, might not a high intensity blue light give you the same PAR reading as a high-intensity red light source, thereby telling you nothing about the quality of the light?

Seems to me what we really need is a spectral plot by intensity.

[naoki]

Ray, are you talking about the different energy of blue vs red photone? I don't completely understand physics of light, but looking at the conversion of light energy to chemical energy via photosynthesis (i.e. action (not absorption) spectrum of photosynthesis), the height of blue and red peaks seems to be similar even though blue light has a higher energy than red.

And I agree, PAR is not ideal metrix for measuring the usefulness of light to PS because it includes green light, which can be absorbed, but not efficiently converted to chemical energy. PAR meter is better than the normal light meter since it's less likely to be dominated by green.

As you said, it's better to have the spectra of different light source, but the cost of the real spec meter is a bit prohibitive. So I was wondering if the DIY spectrometer could give anything usable.

Maybe I'm not talking about the same thing as what you were pointing out?

[naoki]

I found this posted in a forum for another type of plants commonly grown under artificial light.

http://www.inda-gro.com/pdf/MeasuringPlantLight.pdf

Now, I think what Ray is talking is related to top of the 2nd page. And I didn't know about the issue.

The bottom of this document has interesting tables. For typical plant photosynthesis, LED grow light (not the white LED) is 50% more efficient per watt than HPS or MH. T5HO is about 1/2 efficiency of LED.

If I'm understanding this info correctly, the last table
"Yield PAR Watts Based on DIN 5031-10 Sensitivity Curve" is the most interesting one. So here is the relevant numbers (extracted common ones, and ordered by efficiency). The unit is W/m^2/s per given consumed W in electricity (and the W is adjusted by absorption spectrum).

LED (650W): 0.296
Induction (EDFL,420W): 0.200
HPS (600W): 0.190
T5HO (6500K, 54W): 0.150
Plasma (300W): 0.148
MH (4000K, 1000W): 0.145
T5HO (6500K, 54W): 0.130

Well, it is a rough calculation, but it is still interesting. I believe this number doesn't consider the differences in the efficiency of reflector (and penetration). So larger surface area of florescent light (and LED panel with multiple diodes) could have further advantage in penetration (amount of light which lower leaves receive) over point source light like HPS, right?

[DavidCampen]

I will have to check the calculations. His action chart seems more like an absorption spectrum and here is a statement not correct:
"This issue is further complicated by the fact that
PAR meters actually measure light intensity (not actual photon counts), it must assume a spectral
distribution to actually assign a uMol/M2-S value."

[DavidCampen]

Here is an earlier thread I posted. According to data from Philips Lighting and not scaling by an action spectrum, deep red LEDs and HPS are about equally efficient in producing PAR and about 50% more efficient than linear fluorescent.
http://www.orchidboard.com/community...tml#post554104

Here is an action spectrum that shows green light to be about 2/3 as effective (per quantum) as red light for photosynthesis.
http://www.plantphysiol.org/content/46/1/1.full.pdf (page 4)

[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? I didn't know the internal mechanism of PAR meter, so I believed the article.

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. This is the one which I frequently see in several textbooks:

I quickly looked for the citation for this figure, but I didn't find it.

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.

The article says that it is difficult to come up the metrics of light quality, and used an approximate method. But I guess the industry could directly report the plant photosynthetic rates using some standard plants (e.g. Arabidopsis, rice etc) under a standardized condition. IRGA (infra-red gas analyzer) isn't that expensive (it might be more expensive (or there might be a cheaper option) now, but LiCor IRGA was $20k or so 10 years ago).