In-Depth Look at PAR/Quantum Meters

I’m Bruce Bugbee, president of Apogee
Instruments. This is a series of seminars on principles of environmental
measurement. I’m not sure what number we’re on but today we’re going to talk
about photosynthetically active radiation. To me every single component
to this is critical to understanding plant growth. This is one that I’ve been
particularly involved with for a long time in my career, and I want to give
credit to Mark Blonquist who is the chief scientist at Apogee. He and I
have worked on this together for more than 10 years refining this measurement
and understanding its effects on plant growth. So, let’s get started with this,
first thing we want to look at is what I consider to be the eighth cardinal
parameters for plant growth. So, what are these eight parameters? So, here we have a plant, and we always want
to show the anatomically correct roots of this plant. Well let’s take the below
ground parameters, first, of all we have nutrients. That’s one. Second, we have water. Third, we have oxygen. Three parameters below ground
that are critical to excellent plant growth. How about the ones above ground. Well we have temperature and I’m going to abbreviated that temp. We have humidity. So, 1 2 3 4 5 not necessarily an order of importance. Carbon dioxide; critical
parameter. 7- wind. We got eight, we’re missing one. The elephant in the room is
radiation, and I’ll put that in a giant box up here. We can measure all of these parameters better
than the plants can sense them but in the case of radiation the plant is so
exquisitely sensitive to that that our best sensors are only equal to what
the plant sees. So, I should put some arrows here for radiation coming in and
specifically today we’re going to talk about photosynthetic radiation, a subset
of this, but I put this slide in here to really emphasize how critical it is to
do radiation. All of these sensors, we can tell
temperature by it’s cold its warm. We can feel the soil for water. we
can tell if its high humidity. We think we can tell temperature and we can tell
radiation, but our eyes are a terrible light meter. Our irides contract when we
go out in bright sunlight and we don’t realize how bright it is. Then come back
inside they expand. We have a very poor ability to tell the
amount of light and the result of that is, I’m going to draw another little
figure here.Here’s our plant outside, now we get that same plant that looks like this,
tiny leaves. People say oh your plant must be low on potassium, it is low on light. And,
anytime you have indoor plants you really need to understand radiation. So, I
can’t emphasize more the importance of really understanding that. So, let’s
take a look at this, multiple sources of lights to measure plants. Right, we have the Sun. Here these are different types of electric lights: high pressure sodium,
ceramic metal halide, compact fluorescent all with different
colors of light, and there’s a whole body of studies about what’s the right color
of light for plants, and I’m not going to go into that. But, our research says there’s
not as much difference as you might think among colors of light. But, we’re missing one key color on here and that is LEDs, light emitting diodes. This is a picture of
some plant growth chambers with different colors of the LEDs. There’s a tremendous
amount of research going on with the colors of LEDs and people are using them
for plant growth. So now look, a sensor has to integrate all these different
types of light, and accurately predict radiation for plant growth and that’s
the focus of this seminar. How do we do that, what are the options for doing that, and making accurate measurements of this so we can predict the rate of plant
growth? Alright, so a couple of definitions.
First, and these confusing a lot of people P-A-R is photosynthetically active
radiation and that just means that radiation that drives photosynthesis. UV
light, that’s radiation but plants filter that out, it doesn’t result
in photosynthesis. Near-infrared light doesn’t have high enough energy for photosynthesis this is the
radiation for photosynthesis, and we’ll come back to this again, it’s 400 to 700
nanometers. This is close to the visible range as well what are eyes see
that’s the radiation that drives photosynthesis. Alright second definition,
photosynthetic photon flux or photosynthetic photon flux density. Now
these two things refer to the same thing in the next
slide I’ll explain the use of these two terms. This is the whole amount
of radiation again between four and seven hundred nanometers. So, really all
of these terms refer to the same thing. They’re all this radiation that drives
photosynthesis, these terms here are technically a little more exact and PPF
and PPFD. So we get then to what’s a quantum, and this gets back to
Einstein and quantum physics. A quantum is a mole of photons and this mole is
Avogadro’s number of photons so it’s these particles coming in and we know
from the Stark Einstein Law that one photon excites one
electron and that’s the first step in photosynthesis so we’re really
interested in measuring, with these, moles of photons and that’s what the sensors
measure, and we’ll talk about that in the next slide. But quantum sensor then, is
a sensor to measure all this stuff that’s when we say quantum sensor we
could say a PAR sensor, we could say a PPF sensor, or PPFD, they’re all the same thing,
but that’s where this word comes from quantum sensor. Or, if the sensor is
connected to a meter then it’s a quantum meter. Now I’ll show you some examples of that. In fact I’ll show you an example right
now. Here is a quantum meter, sensor plus the meter, and reads out all by itself
you can buy just the sensor and hook it up to a data logger and then it’s a
quantum sensor. That’s all it means between the two of those
definitions. All right now, I want to talk about this
issue of flux, and I’ve given myself a nice white board to do this. Flux, if we
are to talk to our physicist colleagues flux has two definitions and you would think by
this time in the evolution of science read out this term all worked out. One
definition is per time and in the case of photosynthesis this would be moles
M-O-L-E-S per second, that’s one definition of flux. Moles of photons per
second, but equally valid is per area and time. Together. And this means
moles per meter squared per second. Even in physics these two things are used
interchangeably. If we use this definition per time we would have PPF
and then where’s the area in here, we better add D for density to this
definition to make sure we’ve got the area. If we use this definition and we said PPFD well wait a minute we’ve got per area
and per area again, so this D could be redundant. We wouldn’t need it. If you
look through the literature on this half of the people say PPF and the other half
say PPFD, the terms are used interchangeably. They mean the exact same thing, but
it’s been confusing to people just as it’s confusing to physicist.
Is it flux only per unit time or does it mean per unit area and time? There’s the
difference. For the purposes of this we don’t need to worry about it PPF and PPFD
are equal. All right, now, let’s go to what we’re looking at. Here’s
photosynthetically active, photosynthetic photon flux, photosynthetic photon flux
density it means radiation between 400 and 700. Sharp cut offs. Now, remember that Stark Einstein Law that you’re remembering for the test, equal weighting of all photons 1 photon
excites 1 electron. Every photon coming in up here gets equal weighting
on this chart. These are blue in here, this is ultraviolet, this is blue, this is
green in the middle, and then red, and then right after 700 its
infrared, IR. Thermal radiation doesn’t cause photosynthesis its just this here
with equal weighting of protons. T his is what we’re after a sensor that can get
this sharp cut offs on the end so we can predict photosynthesis and plant growth
by measuring this. Right let’s take a look at how we do this. Here is our
quantum sensor we just talked about very simple. One detector, right
here, one filter and one output if we ran it over here to a meter. This is a quantum
sensor, but this has to be designed with filters
to get all of these colors exactly right and that’s a tricky business, very hard
to do. If we make the jump to lightspeed now we have a spectroradiometer and a
prism it breaks up the light and we have lots of arrays down here and this
goes off to a meter. This is more accurate it’s also more than ten times
the cost. So, we’re going to look at these how accurate are these compared to a
spectroradiometer and comparing back and forth. This is two options to do this
quantum sensor, spectroradiometer. Okay. Here is what they look like these are
quantum sensors one from Licor, one from apogee, and the apogee
spectroradiometer, little bigger a lot more stuff going on in here.
If we did x-ray vision we’d have a prism and here. The light comes in
it breaks the light up into multiple detectors down here, and these have a
single filter. These are the most widely sold in North America
Licor and Apogee. Licor came out with a new sensor, they have a better head. It
sheds water better now, than this one which tended to trap water.
It has a nicer cable, it has a cable like this. But, I’ll talk about the new
sensor in the old sensor they’re both equally accurate. It’s just how it
sheds water and how rugged it is for outside. So there’s the
comparison, about ten times the cost for this. So if you need very accurate
measurements, this is the instrument of choice. But, let’s talk about how close
we can get with these. Alright here’s our box again equal weighting of photons. Kipp & Zonen is widely sold in Europe, and here’s the Licor. Look at how
closely they match the curve now this is really good sharp cut offs because there is multiple
colors of light. It’s real important to have a sharp cut off here. These are their two model numbers. There’s the new Licor looks a lot like the old Licor and it’s equally accurate to this. Now let’s look at the original Apogee
sensor, that Apogee is short for some time here’s the curve. It comes up a little
slow in the blue and look how it cuts off here, at about six hundred and sixty so
what misses this radiation. That’s not serious because it’s calibrated to light
sources, unless you get a light source with a lot of radiation right here. Now
let’s take a look at an example of that. LEDs may come into the picture, this is a
red LED and its called deep red cause it’s very close to this cut
off I’m going to put deep red here but this is common in some LEDs.
Of course this is a blue LED, but look at the problem it cuts off before
it can capture that deep red LED. So, this is an excellent sensor for all types of
light sources except when there’s a deep red LED in the light source. So what can
we do about that? Let’s take a look this is the original Apogee that blue line
that we just looked at. Apogee developed a full spectrum quantum sensor that’s this gold line right here, that cuts off here. So you can see the gold the green and the blue line are all very similar comes up
better here. So if you need to measure red LEDs or LED spectra this is better than
the original, but the difference in cost for this full spectrum is
about two times as expensive, so all of these are quite a bit more expensive
than this. But, if you measure LEDs you need them. Let’s take a look at this here is the
original black sensor. Many people have this. Here’s the new one it’s
slightly taller slightly bigger head. Because it’s a higher end sensor it’s a
gold color instead of a black color, that’s what the two sensors look like.
Original and full spectrum. Now let’s go back to that graph and here’s the
difference again, right here. Now let’s take a close look at the accuracy of
all of these sensors and to do that we need to use this fancy integrated method
originally developed in 1966 where it takes all the colors of light and it
multiplies this by the accuracy of the spectral accuracies of these. This is
something that Mark Blonquist has done in great detail for all these. This
is the equation to do it. I’m not going to go into that now except let’s look at the
results how close do we get. Kipp & Zonnen, Licor and this is also the 190 R, is
very close to this, and then the original Apogee and the new
Apogee. Sunlight and cloudy, these are all tiny errors, so you really
don’t need the full spectrum to be accurate if you’re just doing sun light
measurements. Any errors less than certainly 1% are very tiny, even 2% errors are fairly small for these. All right, now we take
cool white fluorescent lights. All these errors, less than 2%. This
is calibrated here. They are calibrated to be 0. Small
errors. Small errors. Now look at the original Apogee here’s LEDs. Red and blue LEDs. Remember it didn’t capture that red peak? Big errors. If you’re not using it under
there it’s fine, but if you need these you need for spectrum. We jump to
full spectrum and everything gets better. Metal halide a little better. High
pressure sodium, better. But these errors here in particular are small and you
take a close look at this, and positive video and study it. You can see that the
errors between Kipp & Zonnen, Licor, and the full spectrum Apogee are all really
small under all of these different types of light sources. This one has an asterisk
because it has separate calibrations for Sun and electric lights.
That’s explained on the Apogee website. All right let’s take a look at the next slide here. Spectroradiometer, remember the ten times, bigger, more expensive sensor? It nails every single source. 0 errors, because its measuring
every color individually. So if you really need excellent accuracy you need
to spectroradiometer to do this, but most people if they have two or three percent
accuracy that’s good enough for their applications. Okay, I’m going to conclude on this slide Apogee also makes a line quantum sensor and it looks like this.
When we want to measure radiation underneath the canopy it’s
really variable, there’s bright light and shadows. This
sensor averages the light over the whole length of this. If you
start to cover some of these it gives you a proportionately reduced
reading. This is something that is helpful for people for transmitted light.
Of course I have examples here hooked up to meters. I
guess I should put them in the same order as this, but the old and the new sensors.
Again the new one is full spectrum and more accurate under all the different
colors of light. I hope this overview of some terminology and some differences
among meters is helpful to understand and make more accurate measurements of
photosynthetically active radiation. We look forward to another session on other
topics in environmental biophysics thanks for listening.

25 thoughts on “In-Depth Look at PAR/Quantum Meters

  1. Why is UV ignored, isn't UV used in photosynthesis and necessary for certain proteins? Are photons in different wavelengths really equivalent? Why then is there so much hype with red and blue LEDs?

  2. Great video. I have been looking for this information for a few years. We use the original par meter in our saltwater aquarium store. Will we be able to buy the new sensor and install it on the handheld?

  3. thanks can you give me the best reading for a 600w viparspectre LED grow light as I am about to buy a par meter (NOVICE) as I am new to measuring light, thank you

  4. Thank you so much,
    tried the quantum sensor, here is the link:


    What do you think are these PAR readings pretty high then?

  6. Hey guys! I used your SQ 500 Quantum Light meter to create an experiment explaining DLI. I matched DLI values while using 3 different day length cycles on hydroponic lettuce. I'd love to get your feedback and see what Dr Bugbee thinks about my explanation! Thanks for all of your great lectures. xD

  7. i can no help but make this comment the science of the study of light is amazing but God created light to work with the earth nothing will ever compare 🙂

  8. Glad I found this. Great science. I would like to suggest repeating McCree, 1972, using LEDs. Make measurements using Apogee Quantum Sensors, really dial-in the radiation measurements. Then analyse several popular COBs; 3K, 4K, 6K, 10K, just for comparison. Carefully investigate colour temperature. Could be a thesis proposal?

  9. Thanks guys this video was very helpful in showing the differences in the full spectrum meters to non. It seems more people need to know about this. We have found a few light manufacturers that have seriously under rated their lights due to the 660nm deep red leds. You guys may know me a little better from my other channel Aqua Nut. We are getting great use out of our MQ 510. I wish we went with the SQ 520 now that I am using it so much out of the water testing grow lights. My head hurts from all of the calculations I need to make haha.

  10. Excellent video thank you for uploading. I was always. confused with how people use PPF and PPFD interchangeably! Seems like PPF should be constant for a given light source (similar to lumens but calibrated to plants and not human eyes) and PPDF should depend on distance from the meter, reflectors, lenses etc (similar to Lux which is lumens per area). Apogee, you guys are a leader in this industry, be the change we all want and make PPF and PPFD make sense!

  11. Can somone help me , im measuring light intensity with a Voltmeter and LED diode , and it can measure , problem is i get 200mV result , and i dont know how much PAR is that
    Is there a way to know at least close figure , or convert mV in to PAR

  12. Wht about age old experiments that show that plants have 2 photosensitive parts to its photosynthesis. Where the first part wants the spectrum of PAR but the secondary process wants IR light.
    Meaning IR light on its own has no effect on photosynthesis. However when first giving the plant its precious PAR then at the same time introduce the IR spec at the same time there is nearly 2x more photosynthesis.. So shouldnt this effect be factored into to our sensors?

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