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High photosynthetic photon flux density (PPFD) and LED dimming (pulse width modulation)

Simon Cole

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With almost limitless-intensity LEDs becoming commonplace in the market. We must answer a fundamental question over the role of pulse width modulation (PWM) in mitigating photoinhibition in plants. In theory, LEDs shine at the same intensity, but this intensity is split into longer "off" and shorter "on" periods. PPFD has always been used to work out continual flux density from stable radiation sources of light like the sun or CFLs, whereas all LEDs follow a continuous pattern that is split between two modes, off and on. The human eye cannot detect these changes, but does it affect plant photoinhibition?
 

Simon Cole

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What is even more interesting is that some members have used dimming to reduce leaf melt! ... check out Ray's observations in this recent thread:
My reference is I have a 25cm cube with low light - I’m running a 15w ONF flat nano on it at 25% power with no added carbon. When I turned it up to 50% (roughly what you have) I had major leaf melt on my Staurogyne and Crypts. With CO2 however, that would have been fine.

Any observations more would be very welcome :)
 

zozo

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Not in my experience... :)

dscf7481-jpg.jpg


And this grew all under a DIY DC12 volt LED fixture dimmed with PWM, as you can see the left-hand side in the picture the TC420 PWM led timer/controller hanging to the wall.

dscf7314-kopie-jpg.jpg


The LED's used were the SMD 8520 dual-chip LED strip at 60 lumens per chip, 4 strips 50cm x 36 chips each, total 144 chips x 60 = theoretically 8640 lumen at 100%. Not counting the 1 strip it contained with Red and Bleu LEDs that came without lumen specs.

It's not something specifically for DIY, the Chihoros LED units that come off factory with a dimmer or the Chihiros Bluetooth controller also uses PWM... :thumbup: Thus PWM controlling already is used as long since LEDs are used in the hobby.

I can not fully explain PWM away and i think the explanation as an On/Off pulse is a bit too straight forward and simplified theory. It is also used to control the speed of electrical motors. And even a simple voltmeter is kinda fooled by it. I hooked a simple digital voltmeter to the PWM controller and it shows +/- 6 volts if it is turned down to 50% output. But in theory, it should stay 12 volts at different on/off frequencies.

That was here. :) The DC motor controller is a PWM unit with the voltmeter hooked to it to control the DC 12 volt pump.
35l4sbq-jpg.jpg
 

jaypeecee

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Hi @Simon Cole

The following would also be my response on this subject:
I don't think the plants would 'perceive' such fast pulses.

But, if you wanted to pursue this further, perhaps Dr Bruce Bugbee at Apogee Instruments has something to say on this? If the name isn't familiar to you, Dr Bugbee is one of the foremost names in plant LED lighting. He is engaged in cutting-edge research for the likes of NASA. Check out his YouTube videos to be enlightened (pun intended!).

Or, having just seen the above post, you could talk to @zozo!

JPC
 

jaypeecee

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I can not fully explain PWM away and i think the explanation as an On/Off pulse is a bit too straight forward and simplified theory. It is also used to control the speed of electrical motors. And even a simple voltmeter is kinda fooled by it. I hooked a simple digital voltmeter to the PWM controller and it shows +/- 6 volts if it is turned down to 50% output. But in theory, it should stay 12 volts at different on/off frequencies.

Hi @zozo

PWM has been around for donkey's years. I seem to recall that it was used 50 years ago when I dabbled with radio-controlled boats. And I wouldn't expect any multimeter - analogue or digital - to provide sensible measurements from PWM. Multimeters are limited to DC or 50/60 Hz sinusoidal mains waveforms. An oscilloscope would be required to do any sensible measurements.

JPC
 

zozo

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Hi @zozo

PWM has been around for donkey's years. I seem to recall that it was used 50 years ago when I dabbled with radio-controlled boats. And I wouldn't expect any multimeter - analogue or digital - to provide sensible measurements from PWM. Multimeters are limited to DC or 50/60 Hz sinusoidal mains waveforms. An oscilloscope would be required to do any sensible measurements.

JPC

Yes, i know it is already in use for decades as DC motor speed control. :) Especially in the private radio-controlled modelling hobbies. Anything using Servo Motors... :)

I guess what kinda is misconceived or incomprehensible for us hobbyists is the On/Off concept and how this reduces the output that it seems a lower voltage but it isn't. Similarly as how a computer binary code actually works simplified with Zeroes and Ones. 0 = Off and 1 = On. Same theory, the processors switch On and Off at an amazingly high frequency to do the tasks it does and show it on your screen.:eek: Sounds so simple, but explain away?. o_O
 

Simon Cole

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I just found an interesting article showing a positive "photo-inhibition recovery" correlation in wheat:
PWM (rSP) helped the plants to photosynthesise after the light intensity got too much.

"We confirmed that the rapid change in Y(I) at the beginning of rSP illumination was similar between two wheat cultivars and between LL and HL growth conditions, and the decrease in Y(I) was rapidly recoverable in both LL- and HL-grown wheat cultivars when pulse illumination intervals were prolonged (Supplementary Figure S2)."

Here is the figure from the article below:
PWM on wheat.png


Other growth studies have shown negative correlations (e.g. lettuce), but these methods were all different.
An important consideration is that we are not measuring growth. Instead we are looking at photo-inhibition.
I have found it very hard to find any articles on PWM vs photo-inhibition.
Perhaps it is just the way that I am looking at the data.

So yes, please continue to post your observations! :)
 
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jaypeecee

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Sounds so simple, but explain away?

Hi @zozo

Maybe one day. It's not easy to explain things like this without drawing a few graphs and throwing in some maths, as well. Or, I could simply refer you to a book by one of my University lecturers of years gone by. That's my cop-out! The book is 'Signal Processing, Modulation and Noise' by Dr J A Betts. ;)

JPC
 

dw1305

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Hi all,
PPFD has always been used to work out continual flux density from stable radiation sources of light like the sun or CFLs
I don't know, but my guess would be that it doesn't make any difference to the plant whether the light is pulsed or continuous.

The reason would be that the photosynthetic pigments are intercepting individual photons, so that even a continuous wave light source isn't necessarily continuous at chloroplast level. It is back to the deep green Bolbitis or Anubias, with their low light compensation points, they have a lot of chlorophyll to intercept all of the relatively rare photons.

There is also the effect of clouds to take into account. These are the solar panels on our roof, so facing SE and not shaded in any way, if this was a situation with trees as well, the differences between sun and shade would be even more pronounced, with the <"plant experiencing "flecks" of high PPFD">.

Showery day.png


cheers Darrel
 

jaypeecee

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Cara Wade says in the interview that she welcomes anybody willing to help and participate in experimenting to contact here via Facebook. It might be 😍 for you both... She might be able to explain a lot in detail or provide you with usefull data...

Hi @zozo

Your link took me to something that says "Cara Wade, a lighting expert from Build My LED, tells you exactly what you should be looking for when thinking about lighting your planted aquarium". The company, BuildMyLED (BML) no longer exists. I have a BML LED fixture as does none other than Dennis Wong. To the best of my knowledge, BML became Fluence (Osram). I will pursue this with interest even if it means looking on FB!

JPC
 

zozo

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Hi all, I don't know, but my guess would be that it doesn't make any difference to the plant whether the light is pulsed or continuous.

The reason would be that the photosynthetic pigments are intercepting individual photons, so that even a continuous wave light source isn't necessarily continuous at chloroplast level. It is back to the deep green Bolbitis or Anubias, with their low light compensation points, they have a lot of chlorophyll to intercept all of the relatively rare photons.

There is also the effect of clouds to take into account. These are the solar panels on our roof, so facing SE and not shaded in any way, if this was a situation with trees as well, the differences between sun and shade would be even more pronounced, with the <"plant experiencing "flecks" of high PPFD">.

View attachment 150154

cheers Darrel

Good point!... :) never really crossed my mind that it's something natural too... The ferns and or mosses, whatever low compensation point plants that grow on the forest floor below the canopy actually receive a kind of pulsating light intensity due to air movement through the dense canopy. (Now I'm shaded/now I'm not) So we could assume the plants are evolved not to care and to cope with this.
 

zozo

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The company, BuildMyLED (BML) no longer exists.

Indeed they stopped manufacturing aquarium lighting and since solely put their energy (so to speak) in the horticulture industry lighting. No idea if that's still the case today, it's a rather old article from a few years back. But Cara seems to be a fanatic Aquarium enthusiast and Hobbyist. So she probably is still as active in this field as before but just not professionally anymore.
 

Ray

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The reason would be that the photosynthetic pigments are intercepting individual photons, so that even a continuous wave light source isn't necessarily continuous at chloroplast level. It is back to the deep green Bolbitis or Anubias, with their low light compensation points, they have a lot of chlorophyll to intercept all of the relatively rare photons.

I think so too. I am pretty sure that from a plant perspective it is simply a matter of how many photons hit your chloroplasts (driving your chemical reactions) in a given time. Whether the photons arrive in pulses or a steady flow won’t make any difference at the chloroplast level.

BTW @Simon Cole there is a good CEG thread on melting here https://www.ukaps.org/forum/threads/what-causes-leaves-to-melt-and-what-to-do-now.20421/
 
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zozo

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It's the power driving the light that has high-frequency puls and in this case, it can be modulated. If i remember correctly all power supplied in whatever form has a pulsating frequency but it isn't or can't be modulated as such.

So i guess all artificial light pulsates but to fast for our eyes to see it...

What else does frequency mean?
 
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Oldguy

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From the internet: ' Most people cannot notice the flicker in fluorescent lights that have a flicker rate of 120 cycles per second (or 120 Hz). Flicker with LED lights may be more noticeable due to the fact that LED lights flicker between less than 10% and 100%, where as fluorescent lights dim to about 35% and back to 100%)'

So it appears that LED's flicker whether dimmed or not if powered from AC electricity. Car LED's (12 volt DC should be flicker free.)

Incandescent filament lighting should also flicker if powered by AC but filament reaction time would be slow compared with 50Hz AC. Have heard it claimed that DC powered filaments last longer than AC powered ones due to constant power supply. No personal experience however.
 

rebel

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I just found an interesting article showing a positive "photo-inhibition recovery" correlation in wheat:
PWM (rSP) helped the plants to photosynthesise after the light intensity got too much.

Can someone translate this study to English? ;)

Do the pulses increase or decrease growth?

Thanks.
 

Simon Cole

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@rebel ...so with a shorter "off" gap between pulses (on-off-on-off etc.) the lights were "on" for longer. This caused photo-inhibition in wheat plants, which stopped growing. When the "off" gap was extended so that the lights were off for longer, this enabled the wheat to recover and start growing again. This showed that for wheat, using pulse width modulation (PWM) was effective at achieving the right amount of light, and without PWM the crop would have failed. This suggests that dimming switches that regulate these on and off periods, can be used effectively to manage high intensity lighting that could otherwise cause plants to stop growing. Aquarium plants could respond in very much the same way, and having an AC current with a dimmer could be advantageous when optimising light intensity... I don't know whether that helps. Most plants just stop growing and show no sign of bleaching when the intensity is too high.
 

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