Why do I need a PAR/Quantum Meter?
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I’m Bruce Bugbee, president of Apogee
Instruments, and this is part of an ongoing series of principles of
environmental measurement. Today we’re going to talk about measuring light for
plant growth in just a short talk. In particular, in this talk we’re gonna focus
on measurements of light for humans, or for people, and measurement of light for
plants. Now meters to measure light for plants are more expensive than meters to
measure light for people. So, people often buy the lower-cost meters to measure light for people, and then use them to measure light for plants. This can result in
significant errors if you don’t understand the principles and we’re
gonna go through these. Here’s an example this is a meter to measure light for
people, foot-candle meter reads out in foot-candles, here’s an Apogee meter
looks similar to measure light for plants, and then of course you can buy
these with separate sensors, there’s a lot of variations, but we’re going to focus on
the key differences in these. There’s a lot of terms for this but let’s go
right to the next slide, blank slide. First, I’m going to draw a line and we’re gonna put 400
and 700 nanometers on this line. In between this region are all the colors
of light. We go from over here is UV, then we go to blue, green (GRN), red, and
then right after this is infrared, and we can’t see infrared
and plants can’t see it. So, both plants and people see blue, green, and red
light. Here’s the difference, if we draw a curve
of how well we can see the different colors of light it looks like this. It’s a bell-shaped curve, like this. Let’s see if we can get that coming down… okay we see green light more than ten times better than
either blue or red light. Now this is designed with people looking in a box at
dim light and really it’s not intuitive. The green light looks very
bright to us, and blue and red we have to turn up the intensity so we can see them.
So, it’s a big difference. Now, plants see all these colors equally. So here’s plants
and here’s people- humans right here. Now this is Lux, and foot-candles (Fc) these are all the same thing for people, very different shapes of curves as long as the colors
didn’t change you could measure one and predict the other from it, but the colors
do change when we go from different kinds of light sources. Light for plants is
also the same light for any photosynthetic organisms that includes
coral in an aquarium. We’re not interested in the colors of light that
humans see, we are interested in the colors of light that plants see. So as a
dramatic demonstration of this we went to my laboratory at Utah State
University where we’re doing research on multiple colors of light for plant
growth, and we’re going to compare quantum meters that measure light for plants
with foot-candle meters that measure light for people and see the extent to which
we can convert back and forth. So let’s take a trip to the lab with this next video and see what this looks like. So you can see all the chambers there. These are all chambers that we used for plants in space.
There’s a picture of an astronaut that’s holding the plants. These are these little
chambers that are on the space station where we’re using LEDs so we want to measure
light for plants. Alright now let’s take the first, and here’s a quantum sensor an Apogee
quantum sensor, this is a research grade foot-candle sensor, this is $1,000 foot-candle sensor to measure light so we got that with perfect accuracy, an excellent
accuracy. Then the quantum meter here. Let’s take a look at what we get in the
first chamber with cool white fluorescents, every chamber’s designed to have 200 for quantum flux photosynthetic
radiation and then this is what the foot-candles is, now that first chamber was about 1,200 and this was about 200, so this is about six times as
much as this. At the end of this, we’ll come back to these numbers and compare these ratios. Now let’s go to the next chamber remember there was a six to one ratio there. Now we close the door about 200. Oof, this is almost an eight times ratio
in this chamber so this has equal light for plants, but if you measured it for
this you would be way off. You’d think this should grow better
plans and it does not because the ratio changes. Let’s keep going with chambers.
Now we’ll go from a neutral white LEDs. You can see the color of the warm
white LEDs. Close the door. Keep going just a bit we get there,
200, now look at this 1300 here. So yet a different ratio
even though the light for the plants is the same. Now, lets
take one more chamber. There was 200 and 1300. Now we’re going to take this sensor out of this chamber. You could see the ratio of the two there, and now we’re going to go to an LED chamber. This is red and blue LEDs,
very commonly used for plants. Now look at the ratio, 200, whoa! The other ones were 1,200 to 1,600 this is only six hundred and forty-nine. A foot-candlemeter dramatically under measures LED lights when they’re blue and red LEDs. That shouldn’t be surprising because humans don’t see red and blue light very
efficiently, what we see is green light. So particularly, if there’s any LEDs foot-candle meters have huge errors, so this is an example of these measurements very
quickly made under different types of lights. Now let’s go back to our slide
and summarize this data. Now we’ve watched this video many times to get
these numbers. Here’s a summary of the data first cool
white. The PPF was 200, the foot candles was 1278, and the ratio is 6.4. Now this would be ok if it’s always a 6.4 ratio we could just
measure this and divide by 6.4, and predict plant growth. Now lets look at what happens when we do neutral white, PPF 200, boom, this jumps way up 7.9x. This difference right here is 23%. So using a foot-candle meter just with cool white and neutral white, 23% error. That’s pretty significant. Let’s keep going. Warm white, that’s in between 12, 15, 13 all with the exact same light for
plants, different ratios here. So, if 20% accuracy was good enough
you could get away with the foot-candle meter, but let’s look at LEDs. Remember
how low that number was 620, 3.1x this is off by like 100%
compared to these other meters, even though this would grow excellent plants.
So for any kind of precision work for plants you really need to buy a photosynthetically active radiation meter they’re called quantum meters, they’re
calibrated and designed to measure light for plants. Think of the investment that
people make in growing indoor plants they spend hundreds of dollars on the
lights, and without a meter to measure the lights you can’t tell if your lights
are aging, you can’t tell why the plants are growing in one place better than another. You really need a quantum sensor or a PPF meter. Thanks for listening, we’ll talk to you again soon, bye.

13 thoughts on “Why do I need a PAR/Quantum Meter?

  1. If plants see all the light between ~400nm to ~700nm the same, why do I see all these grow lights advertising their lights covering special ranges for growth?

    Example: Light Spectrum: 430~440nm, 450~475nm 620~630nm, 650~660nm, IR, and white

    Is this just marketing or do grow lights need specific spectrums of light in conjunction with a good PAR output to work well for growing plants?

    If you have already covered this in a video please shoot me the link so I can watch. Your videos are quite educational.

  2. I wish more companies would put up technical info like this. It helps sell a product much better than marketing people screeching at you. Gonna order a SQ-520 USB come end of month. Thanks!

  3. what is the unit for PPF in the last video??? you said the FC to human like PPFD to plant, so I got confussed. for this measurment, it should be PPFD not ppf? right????????///

  4. thanks
    I tried the quantum sensor, here is the link:

    https://igrowlightkit.com/blogs/news/sun-system-par-meter-w-remote-sensor-unbox

  5. Hi,
    The sq-520 model… does this sensor come with a meter or is the sensor only available with a USB port? Is it Mac compatible?

  6. Can you please send me a number to multiply with lumens over your app to get par if i send you my light spectrum by email?

  7. This info saved me hours of messing around trying to figure out why my plants were burning under my LED grow lights. Found out the cheap 3-in-1 moisture/light/ph meters are reading FOOT CANDLES, not Lux as a PAR meter does. After purchasing and using a PAR meter I have tuned my indoor plants beyond what I thought possible. Felt like I was starting from scratch. I've since found out what reflective surfaces add to the growth response and where to place them. My PAR meter has found dead spots in my grow space and allowed me to spread out the brighter spots. Finally my plants respond to all my little micro tuning. Lastly I have discovered exactly how much LED lighting I need AT the plant and where on the plant. Its completely changed which type of light I use now. Hint, I don't use the heavily marketed large area LED assemblies. When you get and start using one of these meters and see the actual plant response you will know what I'm talking about.

  8. Hello, thanks for the video. You started very well, but I waited a little more. After all, if we have from the manufacturer, a graph of the spectral distribution and brightness in lumens, then understanding the principles of the breakdown you voiced, we can recalculate the lumens to ppf. It seems to me that this shouldn’t be anything extra complicated or secret, maybe you know a software that can do it automatically?

  9. Plants see all thesse collors equaly? ERROR, plant does not see green light by night , it does not disturb the night cycle, plants does not see all the lights equaly ^.^`

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