I'm going to talk about quantum sensors
or sometimes we call them PAR sensors,
photosynthetically active radiation
sensors. They look the same as pyranometres, but instead
of being designed to measure all of the
shortwave wavelengths from the Sun or an
electric light they only measure the
photosynthetically active wavelengths
that is typically defined as 400 to 700
nanometers. The same wavelengths that the
human eye sees are the same wavelengths
that drive photosynthesis in plants.
It's an often-used measurement in plant
research because it tells us how much
radiation is available or how many
photons are available to make
photosynthesis happen. There is
multiple radiation sources available
used to grow plants. I'm showing some
here sun, lots of different electric
lights. Nowadays LEDs (light
emitting diodes) are becoming more and
more common. This is actually a
photograph taken from Bruce's lab at
Utah State University and they've got
all these little growth chambers each
one with a different colored LED or a
different combination of colored LEDs in
them to see what kind of effects
different color treatments have on plant
growth and plant morphology. We keep
pressing the wrong button. LEDs
are becoming more and more common all
the time especially in research
applications. LEDs are often narrowband
meaning they only output a narrow range
of wavelengths, whereas, radiation sources
like the Sun or some of these electric
lights are broadband; they output a lot
of radiation across the
photosynthetically active range. The main point that I want to make here
is that accurate photosynthetically active radiation or PAR
measurements are required for a
host of applications especially if we're
interested in photosynthesis. Before
we start talking about the sensors
here's just a few definitions you
probably have heard these terms before
and seen them to find but it always
helps to review a little bit. I
already mentioned that
photosynthetically active radiation is
the radiation that drives photosynthesis.
Photon flux density is just a number of
photons that flow through a unit area
per unit time. We're just counting
photons basically, and so either count
the micro moles of photons per meter
squared per second, that's an
instantaneous unit that we use, or we
count the moles of photons per
meter squared per day on a daily
timescale. That's how we characterize the
flux of photons. The photosynthetic
photon flux density (PPFD) is just the
photon flux density integrated over our
photosynthetically active range, 400
to 700 nanometer wavelength range.
When we have a quantum sensor or a PAR sensor, one of these guys, this is the
number that it's measuring right here.
They put out this unit so it
measures the micro moles of photons per
meter squared per second. Basically
the incident photons on the detector, and
then when we integrate that number over
a whole day we get the moles of photons
per meter squared per day. Yield
photon flux density is kind of like the
photosynthetic photon flux density but
instead of just integrating over the 400
to 700 nanometer range, to get yield
photon flux we actually weight the photon
flux by the plant response to photons and
then we sum it up. This one will
become a lot more clear on my next slide
because will show you a graph that
illustrates the difference between this
one and this one. A quantum sensor
is this thing; it's just the instrument
that we use to measure PPFD. The
reason that we often call them quantum
sensors is because one photon is a
single quantum of radiation, but quantum
sensor can be kind of a confusing term I
think. Nowadays you often hear them
called PAR sensors because they are
measuring photosynthetically active
radiation. PAR sensor and quantum
sensor mean the exact same thing.
Here's the the graph that I mentioned
when I was describing the difference
between photosynthetic photon flux and
yield photon flux. Wavelength on the
x-axis and then just relative quantum
yield or the response to photons on the
y-axis . The green line is actually
measured data from a number of different
places, I listed some of the publications
here. Keith McCree at Texas A&M
University was the first to do this back
in the early 70's and then others
followed up and found similar similar
data. What he did is he took a leaf and
he measured the the photosynthetic
response or basically how much carbon
did that leaf uptake at each wavelength
of light. He used a light with
specific filters over the top of it to
give him really narrow wavelength ranges
and he just measured the carbon uptake
of single leaves as a function of
wavelength. He found that the
photosynthetic response actually looked
like the green line. The plant leaves
respond a little bit more to blue
radiations than they do green radiation
and they're most sensitive or most
responses to the red radiation. The data that McCree and others
measured for single leaves we have to
remember that it was it was in the
laboratory, it was plants grown under
controlled conditions in many cases I
think they also measured field plants, it
was a low light environment, and again it
was single leaves.A more useful and
universal definition of
photosynthetically active radiation is
rather than trying to weight measurements
of PAR by this green line we just assume
that all the photons between 400 and 700
nanometers are equally efficient at
making photosynthesis happen in plant
leaves. The universally accepted
definition of PAR is what we call this
defined plant photosynthetic response,
where we just get equal weight to all
the photons within the 400 to 700 range
and we don't give any weight to photons
outside the 400 to 700 nanometer range.
We use that as our definition of
photosynthetically active radiation. PAR sensors, these kind of devices,
should have a spectral response or a
sensitivity but matches this black line
as close as they can.
There are really a couple of options for
for measuring PAR. One is to use a
quantum sensor like this and it's a
broadband device meaning it gives you a
single signal, one number. An analog
device means it would output voltage
that represents the sum of all of
the radiation that's incident on this
detector and again it should be the sum
of all of the radiation between 400 and
700 nanometers. That number (that
voltage) that it outputs is weighted by the
spectral sensitivity of the sensor
itself. You have a filter and
the purpose of the filter in front of
the photon detector is to try to give us
the best match that we can to 400 to 700
nanometers and then we have a single
detector that gives us an output that
should be proportional to PAR. The
spectral response of the sensor is
dependent on the transmission of the
filter and the sensitivity of the
underlying detector. Alternatively rather
than using a quantum sensor for PAR
measurement, we could use a spectroradiometer.
These are hyper spectraldevices, meaning we're getting signals at
multiple wavelengths. You can actually 
get spectroradiometers that give you a
measurement at every single nanometer
within the 400 to 700 nanometer range
that would be quite fine resolution.
Oftentimes, they give you a
measurement at every 5 nanometers for
example. The point is you're actually
getting a measurement at multiple
positions across the wavelength range of
interest. A quantum sensor only
gives you PAR. A spectroradiometer gives
you both PAR and the spectrum. Not
only do you get photosynthetically
active radiation but you get the shape
of the spectrum. In some applications,
that might be useful.
Here the detector array
determines the spectral response.
A spectroradiometer is in practice it's
a relatively simple device. You have a
prism or maybe a diffraction grattng,
where the light enters the unit and that
prism or diffraction grating is actually
separating the light into the individual
wavelengths. Then somewhere behind
that that prism you have a whole array
of detectors that measures the intensity
of the individual wavelengths, so we get
multiple signals. It looks like a
relatively simple device on paper but in
practice they can be quite complicated.
The only thing that really
determines the spectral response or the
sensitivity of a spectroradiometer is
how well is it calibrated. If we can
accurately calibrate each of the
wavelengths, then we have an accurate
quantum sensor independent of the light
source that we're trying to measure.
There's lots of different options for
for quantum sensors. Here I'm showing you
some of the more common commercially
available quantum sensors, and how well
their spectral sensitivity or their
response to photons matches our
definition of photosynthetic radiation.
The black line is what we're trying
to match because that's our definition
of the photosynthetic photons. Then
the colored lines, the blue and red lines,
are the actual sensitivities of
some of the different sensors
that are available on the market.
Kipp and Zonen has a sensor, Sky
Instruments has a sensor, Licor has a
couple the model LI-190 has now been replaced by a newer model from Licor called the LI-190R.
At Apogee we build a couple
different models the SQ-110 is the one
that we've sold for years and it's still
available, and the SQ-500 is a new model
that is a better match to the
photosynthetic photon flux weighting
factors. Having this information, we
can actually now make estimates of what
kind of errors we can expect when we use
these sensors for different light
sources, meaning Bruce talked a little
bit about how the solar spectrum can
change for clear versus cloudy
conditions so if we calibrate these
sensors in clear conditions for example
and we know that none of
them have a perfect match to our
definition of photosynthesis we can
expect some some errors when we try to
use them in cloudy conditions. For
example, if they were calibrated in the
laboratory under an electric light what
happens when we try to use them for
measurements under a different electric
light or what happens when we take them
out of the laboratory and try to use
them for measurements in sunlight. All we
have to know, we need three pieces of
information basically to
estimate spectral errors. That is we
need to know the spectral response of
the sensor we have that information here,
we need to know the spectral output of
the radiation source that was used to
calibrate the sensor, and then we need to
know the spectral output of the
radiation source that we're trying to
measure. To better illustrate that,
let's just take an example. You can
see here's the spectral response of the
Apogee model SQ-110. This is the
traditional Apogee quantum sensor that's
been available for for several years. You can see that it's not a great match
to our our definition of photosynthetic
radiation that we measure some of the UV,
it's not as sensitive to the blue as it
needs to be, and it misses some of this
red radiation out here when we get close
to 700. This mismatch between the
actual sensor response to photons and our
definition of photosynthetic radiation
is going to cause spectral errors. Kind of an extreme example: let's say we
are measuring an LED that was a blue and
red mix where the red peak was like 660
or 670 nanometers you can see there's a
lot of output from this led that's
beyond the sensitivity range of the
sensor that means the sensor is not
going to count those photons because
it's not sensitive to them and it will
cause spectral error. There's a
equation we can use to actually
calculate spectral errors and as I
mentioned previously the only bits of
information that we need this s is the
spectral sensitivity of the sensor in
this example would be the blue line, the
I calibration is the spectral output of
the light source we use to calibrate the
sensor it could be the Sun it could be
an electric light, and then this I
measurement is the spectral output of
the lamp that we're trying to measure it might be this led for example.
What I'm about to show you is error
calculations using this equation for all
six of these quantum sensors that I have
spectral information for right here.
The table shows data where we use sunlight on a clear day as
the calibration source and so all of
them have zero error under sunlight and
you can see what kind of errors they
have for a whole bunch of other light
sources. We've got sun spectrum on a
cloudy day, a Sun spectrum that's
reflected from a grass canopy. You can
actually in addition to measuring
incoming radiation you know some
applications you might want to invert
the quantum sensor like this and measure
reflected radiation from the plant
canopy. If you have a measurement of
incoming radiation and a measurement of
reflected radiation, it tells you how
much of the photosynthetic radiation is
being absorbed by the plants at the
surface. Bruce just mentioned in
the previous talk that if this were a
silicon-cell pyrometer you can't invert
it like that and make measurements of
the surface because there's large
spectral errors that are associated with
doing that and he showed a table of some
of those errors. Using a quantum sensor,
however, one of these PAR sensors that's
designed to measure photosynthetic
radiation instead of the solar radiation
you can take it and invert it like this
and these numbers here reflected from
glass gives you an idea of what kind of
error you can expect when you invert a
quantum sensor. Oftentimes, I'm going
to talk about this particular
application in a little bit more detail
in a minute, we want to put quantum
sensors underneath plant canopies and
measure how much of the photosynthetic
radiation is transmitted through the
plant canopy. Here's the kind of
errors you can expect if you measure
underneath plant canopies. This is data
for a wheat canopy. I think I failed to mention that all of
the numbers that you see here are
percentages so not actual radiation units,
so not micro moles per meter squared per
second but percentages. Then we have a
whole list of of electric lights that
people might use in a greenhouse or a
growth chamber environment. A couple things that you can see by
looking at the data in this table is
that all of them work quite well for
most of the radiation sources on there
except for the Apogee model SQ-110.
Some of these LEDs you can see the
errors get pretty large that's because
of the data I showed you on the previous
table it doesn't have an excellent match
to the photosynthetic definition or the
definition of photosynthetic radiation.
You want to be careful using
quantum sensors for certain radiation
sources. You want to make sure that they
are going to capture the photons that
you're trying to capture. You'll notice
of the other sensors some of
the worst errors also tend to be LEDs.
Here's a 3.8 % and
3.6 % that's an LED. A
3.0 % over there that's an LED.
LEDs are tricky to measure so for those
of you that are working in greenhouses
and measuring LEDs sometimes the
best option for that might be a spectroradiiometer.
As I mentioned when I
was talking about the differences
between quantum sensors and
spectroradiometers if you're
spectroradiometer is calibrated
accurately because it's measuring all
the wavelengths individually then
spectral error doesn't exist. We don't
have to worry about spectral error with
the spectroradiometer.
The main difference, so you might be
asking well if this is the case why
would I ever use a quantum sensor to
measure PAR wouldn't I just use a
spectroradiometer every time? The
difference is the price tag. Quantum
sensors tend to be in the low hundreds
to maybe the mid hundreds in terms of cost.
Spectroradiometers tend to be an order
of magnitude higher. They're in the
thousands, the low thousands to maybe even the mid to higher thousands.
That is the real difference. Quantum sensors are often the measurement of choice because
they give you accuracy that's good
enough for the application, and they're
much lower cost. This is just a
brief comparison of everything we've
covered up to this point quantum sensors
are subject to spectral error, spectroradiometers
are not if they're calibrated accurately. Quantum sensors
give you a broadband measurement meaning one number or one signal that's
proportional to PAR so you can only get
PAR from a quantum sensor. A spectroradiometer
not only do you get PAR, you
get the spectrum if the spectrum is
required. Then I just talked about
the differences in price. A couple
more review points there's lots of
commercial quantum sensors available and
I showed a least six of them and their
spectral errors and many of them have
minimal errors less than 5 %
when measuring lots of different light
sources. Then remember PAR is almost
universally reported as photosynthetic
photon flux density and units of micro
moles per meter squared per second, but
it can be converted to other units if
the spectrum is known.
The reason that I put this point up
here is often we get people that contact
us that they need measurements in units
of watts per meter squared or even units
of lumens per meter squared (lux) on
occasion. These units aren't often
used in environmental applications and
plant science but on occasion that they
are you can convert PPFD
measurements from a quantum sensor to
other units if you know the spectrum.
It's possible to do that. Bruce showed a
table like this for Apogee pyranometers
here's similar information for Apogee
quantum sensors just some specifications
to give you an idea of the performance.
The directional response or the
cosine response for a quantum sensor is
very, very similar to the cosine response
from pyranometer. The temperature response
is also similar. The stability is a
little bit better for the quantum
sensors that we tested and the
calibration uncertainty is listed there.
Let's skip over that summary slide just
because we've pretty much discussed
everything already. I mentioned i
wanted to talk about a specific
application, intercepted radiation. I want
to show you something really quick
before we go through the slides I
thought it was interesting. This is a
little brochure from Western Sydney
University that I picked up yesterday
here in the Hawkesbury Institute. There's a part I'll blow it up on the
document camera so you can see it.
Here we go.
This is actually a picture and a short caption from the brochure
that I found here at the institute
yesterday. It says this
photosynthetically active radiation
sensor measures light intensity below
the canopy. These are combined with
sensors above the canopy to estimate how
much shading is occurring, which
correlates to the amount of leaf area.
One application of these power
measurements is above canopy and below
canopy measurements to try to estimate
how much radiation is being absorbed by
the plant canopy, and how much is being
transmitted to the soil surface. From
that information, not only does it tell
us how much of the photosynthetic
radiation are the plants absorbing but
it also tells us something about the
leaf area, in between the sensor
that is above the canopy and the sensor
that's below the canopy. This
application is actually taking place
right here at this Institute. I'll
talk a little bit about it for just a
second. Let's switch back over
to the slides. I actually have a picture here of
what's called
a line quantum sensor and making a measurement
of the radiation underneath the corn
canopy. Here I'm showing a crop canopy
but you could do this for you a
pasture or an orchard or a forest,
whatever, you are interested in. A
line quantum sensor is just like this
quantum sensor except for there are
multiple sensors along the line. This
specific one has ten sensors. Each one
of these circles that you see here is a
sensor. Those ten sensors are spread along a bar that's approximately 70
centimeters long. The reason that you
want multiple sensors when you make
under canopy measurements is that, the
and you can see it in this picture, the
light environment underneath the plant
canopy is not uniform. You can see
these two sensors up here are in a light
Fleck where they're actually getting a
lot of sunlight all the rest of the
sensors are in the shade. When you
make measurements of radiation
underneath the canopy with the line
quantum sensor, remember that the
radiation is often non-uniform. It works
really well to measure with the line
quantum sensor because then you get an
average along the length
of the line and radiation under the
canopy should be measured in
multiple locations because it's
no-uniform. It often works best to make a
measurement in one location, move it and
make a measurement in another location,
move it make a measurement in another
location, and average the numbers that
way you get a pretty representative idea
of what the mean radiation
environment is like underneath the
canopy and also how variable it is.
Just to give you an idea the
equations are pretty simple where the
intercepted radiation, you just measure
the photosynthetic radiation above the
canopy and below the canopy. The
transmittance is the below canopy
measurement divided by the above canopy
measurement, and then the fraction
intercepted is 1 minus the transmittance. You have to have a quantum sensor
above the plant and then a line quantum
sensor below the plant. Pretty
straightforward and then you just plug
the numbers in to get intercepted
radiation, transmittance, and the fraction
of intercepted radiation. For a
relatively sparse canopy like this one
here, you know the leaf area index might
be roughly one meter squared of leaf
area per meter squared of ground
area. In the middle of the day, on a sunny
day, the transmittance would be about
50 % where roughly half of the
sunlight that you're measuring above the
canopy would actually get transmitted
below the canopy. If we were to move to
something like this, where we have a much
higher leaf area index a something on
the order of 5 meters squared per meter
squared in the middle of the day on a
sunny day the transmittance would be
about an order of magnitude lower, closer
to 5 % of the radiation above
the canopy is going to get transmitted
to the soil below the canopy. Once
we have this information, once we have
the transmittance we can actually get
some sense of the leaf area index if
that's what we're interested in.
This shows you a four different plost of
transmittance as a function of leaf area
index and this would be for a solar
zenith angle of 20 degrees on a
relatively clear day.
The reason that we have four different
lines and they are spaced like that is not
only does the leaf area influence
the transmittance but the orientation of
the leaves can also have a big impact on
the transmittance. The green line
would represent a canopy where most of
the leaves are positioned horizontally,
where as the red line would represent a
canopy where most of the leaves are positioned vertically.
Turns out the most plant canopies, I'm not as familiar with forests, but crops tend to
be somewhere near these two black lines
where you have a fairly even
distribution of both horizontal and
vertical leave. There's what the data
looks like now at a solar zenith angle of
60 degrees. I will toggle back once just so you can see.
It also shows use that not only does leaf
area in the position or the orientation
of the leaves have a big influence on
transmittance but so does the position
of the Sun. If we know something
about the the leaf angle distribution or
if we can make measurements when all of
the lines are you know converged or
close to one another, we can get a good
idea of the the leaf area index of our
canopy of interest. I've actually
been working on a research project in an
alfalfa canopy over the summer and we
wanted to to get some idea of the leaf
area index as a function of the height
of the the plant canopy, and so we made a
couple measurements in the recent past
at different heights. One was here when
the alfalfa was 17 centimeters tall. We
got a transmission of about
20 % that would correspond to
a leaf area of about 1.7 to 1.8 meters
squared per meter squared. Then we made
measurements later in the growing
when the alfalfa was much taller. It was
about 64 centimeters tall and we got a
transmittance that was an order of
magnitude lower that would correspond to
a leaf area of about 4.7 or 4.8 meters
square meter squared. It turns out that
these estimates of leaf area 1.8 for a
17 centimeter alfalfa and 4.7 for 64
centimeter alfalfa are very close to
actual values if we were to sample the
leaves, destructively sample the leaves
and measure the leaf area we would get
very similar numbers. You can if you're careful with the measurements use
transmittance data from from quantum
sensors measuring above and below the
canopy to get some idea of leaf area and
canopy architectures. Real quick, I
mentioned reflected radiation minute ago
so it won't spend too much time on this
slide. You can also measure reflected
radiation using quantum sensors where
you orient one up and one down and then
the the one measuring the reflectance if
you take the ratio of those two numbers
you get the reflectance and one minus
that number gives you the amount of or
the fraction of the photosynthetic
radiation that was absorbed by the
plant canopy. It gives you an idea of how
much radiation is being absorbed by the
plant canopy and used for photosynthesis.
Just a couple points here. The sensor
should be level for both the upward and
downward measurements. The upward measurement it's obvious why that
one has to be level Bruce talked about
this in his presentation. If it's tilted
a little bit one way or the other, you
can actually get pretty significant
errors on a sunny day. If you're tilted a
little bit towards the Sun it's actually
going to read quite a bit higher than it
should if it was level.
If it happens to be not level and tilted
a little bit away from the Sun, you'll
get a measurement that's a lot lower
than it should be. The reason it's
important to make sure the downward one
is close to level as well so that in a
place like Logan Utah where Bruce and I
come from it wouldn't matter that much
because the entire valley where we live
is surrounded by tall mountain, but in
places where it's flat when the Sun
first comes up over the horizon you want
to make sure thatthe direct
sunlight doesn't hit the diffuser and if
it's tilted a little bit one way or the
other in the in the early morning or
late evening hours you might actually
get some direct sunlight hitting the
sensor that's not reflected from the
surface> You just want to be careful that
you that you level both sensors.
I believe that's all I had for the quantum
sensors, any questions about
photosynthetically active radiation,
measurements above canopy or below
canopy?
 
I think that depends on I think that really depends on you and what you want to quantify for
application but and I think in my
opinion that, and I'm not a tree expert you
would want to know probably still a
horizontal measurement because you know
the bottom of the the tree canopy is
roughly pair parallel to the ground
correct? A right angle, okay there you go
but I think that you know it depends on
which orientation you're interested in
quantifying the radiation. I don't know
that there's a right or wrong answer to
that question. You could do either. Any
others? Yeah, Bruce.
Bruce: Let me mention this
people have long predicted
photosynthetic radiation from shortwave if you measure one you can precict the other. Mark and Ijust wrote a
book chapter for American Society of
Agronomy on shortwave radiation, photosynthetic, and UV.
The chapter talks about the challenges of predicting one from
the other. It is in more detail than it is here, but that's a forthcoming book
chapter about ratios of radiation.
Mark: This is actually a really good point.
Historically weather stations have
never had quantum sensors on them.
Quantum sensors have really been only
been in existence since I guess the
early 1970's but they've only really been
in widespread use for maybe the past few
years. It's not common that you would
find a weather station that has a
pyrometer on it to give you solar
radiation and the quantum sensor on give
you photosynthetically active radiation,
but there could be you know applications
where you need to takeweather data from a weather station and
one of the pieces of information that
you need is the photosynthetically
active radiation. Bruce is correct
you can actually
estimate photosynthetically active
radiation from pyranometer
measurements of solar radiation. If
you're interested in doing that, again
grab my card and send me an email. I can
provide you with some information. The
model that allows you to calculate
photosynthetic photon flux from solar
radiation measurements is pretty simple
but there are some coefficients in there
that can introduce some error and the
coefficients are also dependent on the
sky conditions. If you're interested
in that or want more detail, I would
gladly talk to you, just see me
afterwards or grab my card.
Any other comments or questions? Cool
just a couple of slides on UV that I can metntion
briefly. I can do those now I think
lunch was supposed to be 12:30 so we are right on schedule.
I'll just chat about UV real quick. I don't
have a lot to say. I actually failed
to mention one point, sorry about that.
If you're anxiously awaiting to get to
UV wait we'll get there in one
second I forgot this last point on my slide. You can actually
measure reflected radiation using a
single sensor if you want to. What I
said before is you have one mounted like
this that is level and then a second one
mounted like this that was level. That's
how you would do it if you wanted to
like continuously measure it over the
course of a growing season. If you
want to do this instantaneously, you can
actually take your sensor and measure it
in this position and then flip it upside
down and measure it in this position.
Just be careful when you do that that
during the time interval that it's here
to here that the radiation doesn't
change, otherwise you'll get an incorrect
number because the incoming number
changed. So on a clear sunny day, no
problem. Measure here, flip it over and
level it and measure here but on a day
when it's intermittently cloudy you have
to be really careful doing something
like that. I probably wouldn't recommend
it because the clouds can actually
change the incoming radiation very
quickly. You can put a sensor and maybe
we can even do this at lunch if you're
interested because I think it kind of is
intermittently cloudy today we can go
plug this USB sensor into a computer and
you can watch how dynamic the radiation
is on a cloudy day. It changes rapidly
and sometimes by a large amounts. If 
it's cloudy, especially intermittently
cloudy you don't want to measure here
and then flip it over and measure here.
You want to be confident the radiation,
the background or the incoming radiation
stays the same if you're going
using the same sensor and then invert it
because the measurements are separated
by time.
