Measurement and Measurement Error of
Light Used for Photosynthesis & Plant Growth
Richard Garcia
April 20, 2010
TRANSCRIPT
SLIDE 1 [00:01]
Speaker:
Thanks Ashlee, good afternoon from LI-COR Biosciences here in Lincoln, Nebraska. Thanks for joining us.
Probably the most important process on our planet, is Photosynthesis and the so called light reactions of
Photosynthesis, light energy from the sun, is captured by plant foliage and used to reduce ADP
SLIDE 2 [00:21]:
and NADP to ATP and NADPH this store of biochemical energy, can be used to fix carbon from
atmospheric CO 2 into sugar compounds in The Calvin Cycle or the Dark Reactions.
SLIDE 3 [00:40]:
Although the sun provides energy across a broad range of wavelengths, plants can only use energy in a
much narrower range of wavelengths. Today, we typically think of plants absorbing light in visible wave
bands between about 400 – 700 nm, represented here by the yellow bar. This wavelength range
represents about 50% of the total energy we receive from the Sun.
SLIDE 4 [1:09]:
Research and technical advances in the 1960’s and early 1970’s, allowed Scientists to refine the
definition of light used for plant growth. Keith McCree, published a series of classic articles which he
outlined the need for a definition of photosynthetically active radiation which would be accepted across
the scientific community.
SLIDE 5 [1:35]:
McCree studied the leaf spectral properties of 22 different plant species grown under a range of
conditions. In his study he specifically evaluated: A, in the top figure, the light Absorptance spectrum
determined by measuring the total incident light on the leaf, the reflected light from the leaf surface,
© 2012 LI-COR, Inc.
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Measurement and Measurement Error of
Light Used for Photosynthesis & Plant Growth
Richard Garcia
April 20, 2010
and the transmitted light through the leaf. And then, by difference, he calculated the light absorbed by
the leaf. This procedure requires a Spectroradiometer and an integrating sphere. B, the Action Spectra,
was determined through normalizing the photosynthetic rate, by the light energy incident on the leaf.
And finally C, the Spectra of Quantum Yield, was evaluated by normalizing the photosynthetic rate to
the quanta of light absorbed across the light Absorptance spectra.
SLIDE 6 [2:32]:
McCree showed that there is some variation in Spectra properties of leaves, due to growth history and
species. As you can see in this figure, which shows the variation in the absorption spectra of maze
leaves, with varying degrees of chlorophyll density. McCree’s conclusion was that, we cannot define a
single sensor with a perfect spectral response. He went on to reiterate the importance of establishing a
well defined measuring stick that the scientific community would accept.
SLIDE 7 [3:11]:
After evaluating all his data, McCree concluded that the best definition of photosynthetically ctive
radiation or PAR, is a flat response between 400 and 700 nm. In this figure, we have overlaid this
proposed definition over a quantum yield spectra of a typical plant. The scientific community has
accepted this definition, and it now represents what would be the ideal sensor response. Besides the
term PAR Photosynthetically active radiation, other terms that scientist use to describe light used for
photosynthesis and plant growth, include PPFD or Photosynthetic Photon Flux Density, and PFD, Photon
Flux Density.
SLIDE 8 [4:04]:
Now that we have a definition of what we want to measure, let’s talk about the measurement. Light is
typically measure using passive solid state electronic devices, such as photodiodes. There are hundreds
of these devises from various manufacturers, all with different spectral properties. None of these
devises match the ideal sensor response. But with judicious selection of sensor and the use of optical
filters, we can come close to the ideal sensor response.
SLIDE 9 [4:38]:
© 2012 LI-COR, Inc.
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Measurement and Measurement Error of
Light Used for Photosynthesis & Plant Growth
Richard Garcia
April 20, 2010
In this slide, we chosen to compare three commercially available sensors that have been used to
measure PAR. The response of the sensor in each case was compared to the ideal response, represented
by the rectangle in each figure. The sensors we have chosen to compare are, in figure A; the Gallium
Arsenide Phosphide sensor, which is an unfiltered, smaller, more economical sensor. B; a Kipp & Zonen
model PAR Lite, which is a filtered photodiode. And C; the LI-COR model LI-190 filtered photodiode. In
these figures, any area colored in blue represents overestimation by the sensor within the defined 400
to 700 nm range.
Any area colored with yellow, represents underestimation by the sensor within the defined 400 to 700
nm range. And finally, any area colored with red, represents overestimation by the sensor, due to
response outside the defined 400 to 700 nm range.
SLIDE 10 [5:52]:
The standard LI-COR calibration process for each sensor manufactured includes analysis of the specter
response of the sensor, to a tungsten halogen reference lamp by output comparison to a reference
sensor, with a known spectral response. The procedure by definition results in a total sensor error of
zero, where the overestimates and underestimates cancel each other out, as you can see in the table in
the bottom left hand corner of this figure.
For example in figure A, the Gallium Arsenide Phosphide sensor, has substantial errors, but the underestimation at the low ends and high ends of the PAR region, have been balanced out in the calibration
by the overestimates at the mid-range PAR response.
Once a sensor’s been calibrated with a specific light source, if we change to a different light source with
different spectral characteristics, the sensor error will change depending on how the sensitivity of the
sensor lines up with the spectral properties of the light source. In order to illustrate these types of
errors, we first need to look at typical spectra for light sensors.
Moderator:
Rick, may I interrupt you with a question? The question I have is, I see that some manufactures offer
calibration that are tailored for a specific light source, how does that work?
© 2012 LI-COR, Inc.
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Measurement and Measurement Error of
Light Used for Photosynthesis & Plant Growth
Richard Garcia
April 20, 2010
Speaker:
Well, in that situation, in the calibration process the manufacturer would use a reference lamp, with
spectral properties similar to the properties of the light source the sensor’s being tailored for. The
problem with this approach is, that the sensor is likely to have greater errors when it’s used under other
lighting conditions. Let’s move on.
SLIDE 11 [7:45]:
In this slide, we’ve plotted the spectral properties of seven different lighting situations. The three most
extreme examples in these plots, or plot A, where we have open sky conditions. Plot D, where red LED
with a peak output of 680 nm, and plot G, where we have daylight under a fully developed soybean
canopy. In the figure at the top of this slide, we’ve superimposed the light source spectra for daylight
under a soybean canopy,
SLIDE 12 [8:20]:
over the Kipp & Zonen sensor response, to illustrate how sensor errors change depending on the spectra
of the light being measured. In this particular case, where most of the incident light is between 650 and
750 nm this sensor substantially overestimates PAR, because of the sensors high sensitivity outside the
PAR waveband, there which is greater than 60% is computed as the integration or summation of the
products in the equation, that’s above the table.
SLIDE 13 [8::59]:
In another illustration, we have overlaid the light source spectra for a red LED with peak output at 680
nm over the sensor response spectra of the Gallium Arsenide sensor, although most of the output from
the LED falls within the defined PAR region, a significant portion of the LED’s output is outside the
Gallium Arsenide sensor’s range of detection and therefore the sensor underestimates the PAR from this
source by more than 84%.
SLIDE 14 [9:32]:
© 2012 LI-COR, Inc.
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Measurement and Measurement Error of
Light Used for Photosynthesis & Plant Growth
Richard Garcia
April 20, 2010
In this table, we want to examine the measurement errors of the LI-COR LI-190 sensor across the full
range of lighting conditions. While the response of the LI-COR sensor is not a perfect reflection of the
peak PAR definition, it does follow it very closely. And, it is the only sensor which gives reasonable
accuracy with errors less than 5%, regardless of the source of light.
SLIDE 15 [10:01]:
In this presentation, we set out to compare sensors to the ideal measuring stick response. We also have
a Technical Note, technical note #126 which you can download from our website. Thank you.
SLIDE 16 [10:17]:
Moderator:
Thanks Rick, I have just one question for you, and that is; I know that LI-COR uses Gallium Arsenide
Sensors for some of the LI-6400’s Portable Photosynthesis System Chambers, why do you use them if
there measurements are not as actuate?
Speaker:
That’s an excellent question Ashlee. In Photosynthesis measurements, our objective is to quantify the
light as close as possible to the leaf surface; the problem is that the packaging for our standard quantum
sensor, which has several filters in it, requires a much larger package. To get the quality of sensor we
want, and it’s not practical to get that sensor very close to the plane of the leaf in some measurement
conditions, so we’ve chosen to offer the Gallium Arsenide sensor, which is a much smaller sensor and
can be located right at the plane of the leaf, so that it’s seeing the same light that the leaf is seeing.
Now, in the LI-6400 System we also offer an option to include a standard quantum sensor, so the –
scientist can get both a very accurate measurement of the light that’s available—with the standard
quantum sensor the LI-190, but at the same time can measure light with the Gallium Arsenide Sensor,
right at the plane of the leaf, very closely with the very small sensor that we have in that package.
© 2012 LI-COR, Inc.
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