Investing in new lights LED or HID?

[secondtry]

Hey Vg,

what i dont understand is, what environs AREN"T power limited?
even if you can afford the electricity, you should be limiting your power consumption for environmental reasons anyway.

Sure I agree, and I use a digital ballast for the reason, but I am not willing to sacrifice growth, yield and quality to save on my electric bill.

VG wrote:

like i said before, for me, if i replace my 250 hps in my cab with a 205 watt LED unit and get better results over a few runs, that will prove it to me way more than any calculations or light meters could

But that is not a valid comparisons of LED vs HID; it's only valid for you using your growing methods. We want to be able to say either LED are or are not as good at growing cannabis as are HID and for that we need to use (at least) PPFD.

If you have two of the same aged clones growing with 100% the same environment and growing method except that one is getting 600 PPFD under LED and 800 PPFD under HID than the HID plant will do better.

No matter how you grow if you plants are not getting 1,300-1,500 PPFD then they will not preform as well as plants which are getting that much irradiance. 1,300-1,500 PPFD offers peak Pn and higher growth and yield.

Has what I have written given you any pause about LEDs at all? (I sure hope it has...)

VG wrote:

because i have run the cab for a year or so with hps and cmh and basically pretty much maximised the yield i can get. if i run the same strains with the same soil in the same environment and get better results with LED then i dont see how anyone can argue with that (although im sure they will )

Because yield is dependent upon so many factors you can't control without lots of money and equipment using yield as a comparison method is invalid (unless you can control and quantify the variables effecting yield).

We know that high PPFD offers higher Pn, growth and yield (up to 1,500 PPFD), thus it follows we want as high PPFD as we can get...

VG wrote:

no-one knows 100% how plants use light and what wavelengths they utilise better than other afaik - so any light meter testing will not tell the full story imo. useful info for sure but not the answer.

Yes we do, well pretty close to 100%, that is what I have been writing about this whole time (sigh, I think much of what I wrote went over people head). The Photosynthesis Action Spectra (PAS) is what you seem to be referring to. However, it should be noted when McCree made the PAS he used single wavelength lights when measuring their effects upon Pn, the problem with that is wavelengths often work in synergy, for example, green light drives Pn greater than red light under bright while light (ie. HID) but not under LED. That is why I suggest we use PPFD when comparing LED vs HID, because irradiance is more important than spectral quality (SPD) when considering PAS.

Also, there is a phenomenon termed "midday depression of PS II QE" (Photosystem II Quantum Efficiency) which means around midday outside plants use photons within PAR less efficiently to drive Pn, thus PPFD again is the more important factor because AFAIK light saturation levels do not change throughout the day.

P.S. i will be starting the 205 watt LED in my 'quarters' cab in 2-3 weeks cant wait!

GL.

 

[secondtry]

Hey Mad L,

are you going to call my power company and tell them to start charging me in PPFD?

That is silly, unless they are shining a huge HID on your house!

kilowatt hours is the input cost we are trying to keep down! Otherwise why bother looking at LED?

I have no idea why people look at LEDs besides that they like the wow factor. I don't think most people use LED to reduce their electric bill. Regardless, most people I know grow to reach the highest growth, yield and quality,not the lowest electric bill... (those two are mutually exclusive IMO)

Mad L wrote:

I'm with you often 2ndtry, but you got me real confused here. What do i care about PPFD if in the end I will be financially and environmentally screwed by implementing a technology?

Why would you be financially and environmentally screwed? It's not like the difference between LED and HID is megawatts. You should care about PPFD because we know what levels of PPFD are best for cannabis Pn, growth and yield.

Mad L wrote:

I like the PPFD thing, really. But in the end, the calculus is "what's it going to cost me to make a gram of weed". Maybe look at potency on the side and other qualities as well. But don't leave out an estimation of cost/benefit. That's the whole point bro!

I see your point but I disagree about LED cost/benefit outweigh that of HID. I strive for the best and most cannabis I can get per grow, not for he lowest electric bill.

So let's get you a hypothesis, and let's test it. In vivo. And i say measure at the outlet, because the driver technology can't be separated any more than you can discount the digital ballast.

Measuring at the outlet says nothing about the amount of energy the lamp is emitting which plants can use for photosynthesis; that is why using PPFD is better than watt or kW. PPFD is a direct measurement and watt or kW is very indirect measurement.


All the best

 

[mad librettist]

VG wrote:

no-one knows 100% how plants use light and what wavelengths they utilise better than other afaik - so any light meter testing will not tell the full story imo. useful info for sure but not the answer.

Yes we do, well pretty close to 100%, that is what I have been writing about this whole time (sigh, I think much of what I wrote went over people head).

sigh. 2ndtry, this is standard scientist hubris. One thing we should have learned by now is that when science heads say "we know 100%" anything, they are on their way to being wrong.

you gotta give a little 2nd, people want to see efficiency. And while you are focused on controlled trials, it's not the only helpful way to experiment. One can look at trends, much like the collective brain at IC does all the time. I believe most of the choices made by this brain turn out to be wrong. But that's trial and error, which if you are studying theory, is what Popper believed is the only science.

So please don't discount yield, "sense of grow", or anything subjective you reflexively reject. It's all useful data. Give your limbic system a bit of exercise before it turns on you.

science and intuition/feeling are not mutually exclusive...

-2ndtry

Measuring at the outlet says nothing about the amount of energy the lamp is emitting which plants can use for photosynthesis; that is why using PPFD is better than watt or kW. PPFD is a direct measurement and watt or kW is very indirect measurement.

yeah but it's the one I get charged for! and the one that equals coal fumes being belched onto some west virginia family that did nothing wrong. My electricity bill makes me feel wretched! Like a total asshole.

I have no idea why people look at LEDs besides that they like the wow factor.

well maybe you need to be around for making a list of subjective metric categories. People have all kinds of situations (notice the root "situ" in that word) and need to be able to think for themselves by being presented with dispassionate info. I think you'll find it actually feels better and helps more people than constant advocacy.

 

[secondtry]

VG wrote:

2ndtry wrote:

Mad L wrote:

no-one knows 100% how plants use light and what wavelengths they utilise better than other afaik - so any light meter testing will not tell the full story imo. useful info for sure but not the answer.

Yes we do, well pretty close to 100%, that is what I have been writing about this whole time (sigh, I think much of what I wrote went over people head).

sigh. 2ndtry, this is standard scientist hubris. One thing we should have learned by now is that when science heads say "we know 100%" anything, they are on their way to being wrong.

It's a good thing I didn't claim science knows 100% then isn't it?

Mad.L wrote:

you gotta give a little 2nd, people want to see efficiency.

Efficiency IS the ability of a lamp (either LED or HID) to emit a goal of PPFD with as low wattage as possible (hence I use a digital ballast). Measuring lamp efficiency by yield is not valid unless one can control and quantify the myriad of environmental variables effecting yield.

And while you are focused on controlled trials, it's not the only helpful way to experiment.

True, but it's the result we should use as a method to tell people if lamp X is good at growing cannabis, or if lamp X is better than lamp Z at growing cannabis.

.
So please don't discount yield, "sense of grow", or anything subjective you reflexively reject. It's all useful data. Give your limbic system a bit of exercise before it turns on you.

Sure it's useful, but not as a method to judge lamps. Yield is really only useful to the grower, it's not really applicable to other growers as a means of comparing LED vs HID because yield is a phenotype trait which means it is effected very much by growing environment and growing style.

2ndtry wrote:

MadL wrote:

Measuring at the outlet says nothing about the amount of energy the lamp is emitting which plants can use for photosynthesis; that is why using PPFD is better than watt or kW. PPFD is a direct measurement and watt or kW is very indirect measurement.

yeah but it's the one I get charged for! and the one that equals coal fumes being belched onto some west virginia family that did nothing wrong. My electricity bill makes me feel wretched! Like a total asshole.

So use an HID with digital ballast and grow MORE cannabis per grow (due to increased PPFD), thus you can do fewer total grows over a year and save on coal that way, and save on your electrical bill too, and you would be reducing the chance of getting busted due to the decrese in grow time...

 

[mad librettist]

So use an HID with digital ballast and grow MORE cannabis per grow

1)self-imposed power limit
2)life and wife imposed space limit

see where I am going? I'm trying to make a rational choice for my situation. It's good to know thanks to your work what I am sacrificing (crucial actually, and perhaps you are not thanked enough in all the yelling).

But i need to be equally honest about what I gain! And we need to talk about it as relevant.

I'm resilient. Given the parameters, I will come up with a good grow. But try not to make me feel inferior for considering LED! (choice still not made, have about 5 months to make it.)

correction/clarification as you are right about the misquote:

One thing we should have learned by now is that when science heads say "we know pretty close to 100% of" anything, they are on their way to being wrong.

 

[VerdantGreen]

Hey Vg,...


But that is not a valid comparisons of LED vs HID; it's only valid for you using your growing methods. We want to be able to say either LED are or are not as good at growing cannabis as are HID and for that we need to use (at least) PPFD.

If you have two of the same aged clones growing with 100% the same environment and growing method except that one is getting 600 PPFD under LED and 800 PPFD under HID than the HID plant will do better.

No matter how you grow if you plants are not getting 1,300-1,500 PPFD then they will not preform as well as plants which are getting that much irradiance. 1,300-1,500 PPFD offers peak Pn and higher growth and yield.

hi again st

on one hand you are saying that you cant use yield as a measure because there are too many variables, and i agree with you up to a point - but IF the same wattage of LED consistently gives better yields than HID then i willl be happy to draw conclusions from that.

and then on the other hand you claim that this measurement of PPFD will give us the answer as to which light a cannabis plant will grow better under - and im afraid i think this is even more flawed in its reasoning - because it is just one factor in many that the lights will differ by - surely you have to consider that the huge amounts of mostly unwanted heat thrown out by HID lamps will have an effect on the way a plant will grow?, or how the spectrum of each will also be a factor?

Has what I have written given you any pause about LEDs at all? (I sure hope it has...)

well im lucky enough to have been given the light in return for documenting my grows under it - i have heat problems in my loft during the summer months so i am very pleased to test out an alternative to HID. having actually grown a plant under one of the tiny 63watt units i am confident that it will work well - whether it works better than my 250 remains to be seen .

Because yield is dependent upon so many factors you can't control without lots of money and equipment using yield as a comparison method is invalid (unless you can control and quantify the variables effecting yield).

invalid is a very absolute term. whilst i agree it is not perfect, given reasonable efforts to keep as many other variables constant as possible, i feel it will give some useful data - and whether you like it or not, yield and quality and documented grows - even subjectively reported - is what will have the biggest influence on most growers decisions when it comes to which light they will buy.

Yes we do, well pretty close to 100%, that is what I have been writing about this whole time (sigh, I think much of what I wrote went over people head). The Photosynthesis Action Spectra (PAS) is what you seem to be referring to. However, it should be noted when McCree made the PAS he used single wavelength lights when measuring their effects upon Pn, the problem with that is wavelengths often work in synergy, for example, green light drives Pn greater than red light under bright while light (ie. HID) but not under LED. That is why I suggest we use PPFD when comparing LED vs HID, because irradiance is more important than spectral quality (SPD) when considering PAS.

Also, there is a phenomenon termed "midday depression of PS II QE" (Photosystem II Quantum Efficiency) which means around midday outside plants use photons within PAR less efficiently to drive Pn, thus PPFD again is the more important factor because AFAIK light saturation levels do not change throughout the day.


GL.

see my examples above of why PPFD data is not the only factor when choosing a light.

V.

 

[supermanlives]

LED GROWS ARE LIKE FLORO GROWS BUT COST WAY MORE TO SETUP.i could also probably grow with halogen,blacklight or incandesent lights, but why? hell i could probably grow with my coleman lantern if i had enough fuel. led work but are a joke compared to hid.end of story

 

[secondtry]

+1!!

 

[secondtry]

Hey VG,

hi again st

on one hand you are saying that you cant use yield as a measure because there are too many variables, and i agree with you up to a point - but IF the same wattage of LED consistently gives better yields than HID then i willl be happy to draw conclusions from that.

Cool. But your results won't be applicable to other growers, that is why we should use PPFD to compare LED vs HID (or HID vs HID, or LED vs LED). PPFD is not subjective, while your results would be. People want to know before they buy a lamp if it's good, and using PPFD is IMO the best qualifier of "good".

VG wrote:

and then on the other hand you claim that this measurement of PPFD will give us the answer as to which light a cannabis plant will grow better under - and im afraid i think this is even more flawed in its reasoning - because it is just one factor in many that the lights will differ by

I don't follow you. If using PPFD to compare two lamps the one emitting greater PPFD (up to 1,500) is the better lamp, assuming each lamp is hung at reasonable distances from canopy.

VG wrote:

- surely you have to consider that the huge amounts of mostly unwanted heat thrown out by HID lamps will have an effect on the way a plant will grow?, or how the spectrum of each will also be a factor?

Heat from HID is good if used correctly. I keep my room at 30'C which is ideal for cannabis under Co2 enrichment and high irradiance, so I use the heat form the lamp as it gets blown around the room.

I wrote about spectrum a lot already; it is less important of a factor then PPFD. And the spectrum emitted by LEDs often inferior to that of 'plant' HID lamps like Horlitlux Blue. Please see my big post on page two for more info. I do consider SPD (spectral quality of lamps) when buying a lamp, I compare the SPD to the PAS. That is why I chose the Hortilux Blue 1000w over other lamps, the SPD of the Blue is great (and it offers a lot of irradiance in green range) and being a 1,000w it should reach at least 1,000 PPFD. However, PPFD is a more important factor when choosing a lamp than spectral quality.

well im lucky enough to have been given the light in return for documenting my grows under it - i have heat problems in my loft during the summer months so i am very pleased to test out an alternative to HID. having actually grown a plant under one of the tiny 63watt units i am confident that it will work well - whether it works better than my 250 remains to be seen .

Cool, good luck, sounds fun.

VG wrote:

invalid is a very absolute term. whilst i agree it is not perfect, given reasonable efforts to keep as many other variables constant as possible, i feel it will give some useful data - and whether you like it or not, yield and quality and documented grows - even subjectively reported - is what will have the biggest influence on most growers decisions when it comes to which light they will buy.

I agree it's not invalid for you, but it is for other growers to use the data you compile, unless they have the same clone and environment as you do. A problem I see tho is you still need to quantify the light emitted by the lamp which is used by the plants, using the power of the lamp/ballast in watts is inaccurate and error prone (ex. irradiance decline of lamp, etc). Ideal PPFD has been found for cannabis and it's the same in each of the many different experiments I have read about, all using different varieties of cannabis (thus the results are applicable to cannabis as a whole).


All the best

 

[secondtry]

P.S.

I agree people will believe the guy with pics even if the science is not correct and what the person claims is not accurate. For example, look at how you don't agree with me, even though what I have written is correct and accurate (save typo's I am unaware of); if people believed what they read more than what they see this thread would not have progressed past the first few pages. I think a problem is most people don't understand the science, and don't want to learn it, so they default to believing in that which they can understand: eg. pics without proof. (this is a common trait in humans it seems, how many people got laughed or yelled at when they agreed the Earth is not flat even though all the maps people had access to show the Earth as flat?)

 

[VerdantGreen]

hi secondtry, re heat from HID it is the high radiant heat from the bulb that i have read is stressful to plants, and this would obviously not be factored into your PPFD measurement. also you say PPFD is most important but you also concede that spectrum is a factor - which again isnt factored in your PPFD

P.S.

I agree people will believe the guy with pics even if the science is not correct and what the person claims is not accurate. For example, look at how you don't agree with me, even though what I have written is correct and accurate (save typo's I am unaware of); if people believed what they read more than what they see this thread would not have progressed past the first few pages. I think a problem is most people don't understand the science, and don't want to learn it, so they default to believing in that which they can understand: eg. pics without proof. (this is a common trait in humans it seems, how many people got laughed or yelled at when they agreed the Earth is not flat even though all the maps people had access to show the Earth as flat?)

by a similar token, its a good idea that the bumble bees dont listen to the scientists - as they would then be convinced that they couldnt fly

also if you are trying to convince people of your beliefs, it doesnt really matter what you think is the most convincing method - its what they think is most convincing that counts rightly or wrongly.

i have many books about gardening and i have spent a long time reading them, years in fact - and i learned a lot, but i have learned a lot more still from the time i have spent actually gardening.

cheers

V.

 

[mad librettist]

by a similar token, its a good idea that the bumble bees dont listen to the scientists - as they would then be convinced that they couldnt fly

actually physics never claimed bumble bees can't fly. It's an old anecdote that mutated.

2ndtry,

When they teach you in school that correlations don't indicate causality, do they also remind you that correlations are still incredibly useful, and that you use them every day to make crucial decisions.

I really, really can't suggest more strongly that a genius like you become more well-rounded in the arts. And also maybe cognitive sciences.

Even imprecise data can be useful if you have enough to look for patterns or trends. In fact, it's how many lab experiments begin.

 

[secondtry]

Hey VG,

hi secondtry, re heat from HID it is the high radiant heat from the bulb that i have read is stressful to plants, and this would obviously not be factored into your PPFD measurement.

Heat is a separate issue to be controlled by the grower, it's not an issue when comparing the lamp's ability to grow cannabis. For example, I use an air-cooled hood and light mover, so radiant heat is not a concern of mine, but that has nothing to do with the innate ability of a lamp to grow cannabis. That would be like saying an HID lamp makes a plant use water faster (which it does) and thus the water usage should be factored into PPFD; but just like controlling heat watering is the responsibly of the grower and not a factor in choosing a lamp by it's ability to grow cannabis.

VG wrote:

also you say PPFD is most important but you also concede that spectrum is a factor - which again isnt factored in your PPFD

Yes that's true. PPFD is the deciding factor and SPD is like a tie breaker.

VG wrote:

by a similar token, its a good idea that the bumble bees dont listen to the scientists - as they would then be convinced that they couldnt fly

That is not really accurate: science doesn't say bumble bees can't fly, science claims it can't explain why they can fly (yet); I on the other hand can explain the issues of plant-light quantum physics.

VG wrote:

also if you are trying to convince people of your beliefs, it doesnt really matter what you think is the most convincing method - its what they think is most convincing that counts rightly or wrongly.

I think this is where we get stuck, what I have written are not my "beliefs", it is proven scientific theory, or well evidenced scientific theory, they are light-years apart from personal beliefs. What most everyone else offers is beliefs due to lack of correct data sets and that is what I am trying to fix.

I also agree it's what people think is the most convincing is that which they believe, and that's a shame because most people can't be bothered to learn it, or read about it if there are no pics...

All the best

 

[renz]

I don't even know what a PPFD is...

I don't care about lumens because it doesn't apply to narrow spectrums at the edge of visible range...

absolute power from the wall is eventually a good parameter to measure by vs HID, but since 95% efficient LED drivers are available it doesn't matter as much right now as power through the LEDs...

That has nothing to do with pages of math. You run a setup, you measure the voltage across the LEDs and the current through them and that is your power level to measure vs HID.

I don't care so much about theoretical capabilities of LEDs because you need to run them, at temp, for VERY EXTENDED periods of time to do any sort of reasonable output and reliability testing, and pages of math wont tell you how the LEDs perform in a given packaging solution, nor does it account for the MUCH HIGHER versatility in placement you get with LED lighting. Hell you could hang the LEDs on the plant itself shining on individual leafs and buds if you wanted, then the inverse square law that says higher output HID is always going to be better doesn't matter at all.

There needs to be extensively documented, repeatable side-by-side testing. Grams per watt and tons of images. Controlled testing is hard, because of all the variables, but that's science. Put your work or be a philosopher.

 

[secondtry]

Renz,

If you don't know what PPFD is then I failed miserably in my attempts to help.

PPFD is a quantiation of photons plants use (those within PAR range; 400-700 nm) in a space of a square meter per second. We should use PPFD to measure the irradiance of a lamp at the canopy. Comparing LED vs HID with PPFD is the way to go, using power as watts or kW is not accurate at all; plants don't use watts, they use PPFD thus comparing lamps by what plants use is the best option. That is in terms of what's best for the plant, not the growers wallet (in terms of an electricity bill).

I think maybe you should read the last two pages again, at least. What you mention has already been discussed ad nauseum (in my opinion anyway) and honestly I have put so much time, effort and thought into my posts that it kind of bothers me you didn't read them before you made judgments, especially considering you don't even now what PPFD is (by your own admission) and yet you are still offering to sell LED panels.

The only math I wrote about was for finding daily light integral and UV-b irradiance. I wrote no math for the comparison of LED to HID; we need PPFD for that.

PPFD is basically a 1 for 1 method to compare LED and HID. That is, 1 PPFD from a LED is the same as 1 PPFD from an HID (SPD not considered at this point, please see this post for info about SPD, i.e. spectral quality). Distance from lamp to top canopy effects PPFD and that is why I always write "lamps hung at reasonable distances", to me that means ~> 8" from canopy.

I do plan to make SPD's of many LED panels and HID lamps with a spectroradiometer but that will take me a while, they aren't cheap. One of the best ways to compare lamps by quality (SPD) and irradiance (PPFD) is to weight PPFD by wavelength with the relative ability of each photon within each wavelength to drive photosynthesis. At that point we would be finding a single number which represents both the irradiance and ability of lamp to drive photosynthesis by spectral quality (ie. SPD). For cannabis we want to ideally reach an irradiance of 1,300-1,500 PPFD, but if PPFD is lower than 1,300 the higher the better.

For example, say you compare a LED panel with an HID, you would do so by finding the PPFD when the lamps are hung from reasonable distances; the lamp with higher PPFD is the lamp which is better suited to grow cannabis (assuming both SPDs from LED and HID are of the quality commonly used today). There is a so-called "burn in" period for HID lamps, I am not sure if LEDs also suffer them the drop in irradiance after about 100 hours of use. Because of the burn in period I plan to test lamps when brand new, and after they have been running for a total of 100 hours (12 hours a day for 9 days). The two data are good to see how the irradiances changes over time, and a good example why buying new HID lamps every grow or two is a good idea.

 

[Avenger]

Why only go half way with getting proper data, with the quantum sensor?

Why not get a spectroradiometer and end the guessing game once and for all?

 

[secondtry]

Hey nice to see you again,

Why only go half way with getting proper data, with the quantum sensor?

Why not get a spectroradiometer and end the guessing game once and for all?

I plan to, but I can afford the quantum sensor/data reader sooner than the spectroradiometer. And the quantum sensor/data reader is useful for testing lamp irradiance when a spectroradiometer is overkill, like when hanging a lamp, or finding PPFDi, etc. I am going to get the OceanOptics RAD spectroradiometer. Once I have that data I will use it to weight PPFD with PAS (Photosynthesis Action Spectra; by quantum efficiency of each wavelength).

• Li-Core LI-190 quantum sensor (~$360 link) and LI-250A light meter (~$650 link) = $1,000

• OceanOptics RAD spectroradiometer (link) = $4,700

All the best

 

[renz]

Plants don't use watts, people do.

You compare HID to LED by using available information to set up controlled experiments and use the HID and the LEDs in the best way they are capable. Which means the plants will probably be surrounded by LEDs at point blank range, and the HID will be offset several inches from the top of the canopy. LED packaging both on the emitter level and at that lamp chassis level will have a lot to do with how much light is getting to the plant. you cant park HID 2" from a plant from every direction with passive cooling setup. HID is gonna spend a lot of energy lighting up the room instead of the plant, so light output comparison output is pretty trivial if you're just using up way more of it.

The obvious solution is 1000W of LEDs vs 1000W of HID, but whatever no one has the balls or money or something.

Also I'm making LED arrays regardless if they work for plants or not, because power LEDs are win for all sorts of lighting. Most people are into custom made stuff at near cost. The whole point of the thread is if the guy should try LEDs, one of the main points being made is cost, and I'm telling people they can do it themselves for half the retail cost.

It's not like they really need me to drill a bunch of holes in an aluminum extrusion and buy some high output high high temp LEDs for them. But hell I'm setup for it -- it would be rude of me not to offer, don't you think?

And these are bar arrays, LEDs on extrusion heatsinks, not panels. I kinda think panels is a really dumb format that limits application.

Also good for the plant is good for the wallet so it all goes back to good for the wallet. If PPFD is just a lot of words for how much light output in a given area at a given distance within a given EM frequency band, 400-700nm, who cares. A lot of that is wasted energy anyway, green is right in the middle of that, unless the amplitude of the light is equalized to account for that, what are you talking about?

Seriously, who isn't into 40W of LEDs on a 24" bar even if it doesn't work for plants!? HAVE YOU NO SOUL?

 

[secondtry]

@ Avenger:


EDIT:

I forgot some researchers have already created an updated method to find quantum yield of a light source, I will email them tomorrow. I have the paper and I need to re-read it:


"Green Light Drives Leaf Photosynthesis More Efficiently than Red Light in Strong White Light: Revisiting the Enigmatic Question of Why Leaves are Green"
http://pcp.oxfordjournals.org/cgi/content/full/50/4/684

The literature and our present examinations indicate that the intra-leaf light absorption profile is in most cases steeper than the photosynthetic capacity profile. In strong white light, therefore, the quantum yield of photosynthesis would be lower in the upper chloroplasts, located near the illuminated surface, than that in the lower chloroplasts. Because green light can penetrate further into the leaf than red or blue light, in strong white light, any additional green light absorbed by the lower chloroplasts would increase leaf photosynthesis to a greater extent than would additional red or blue light. Based on the assessment of effects of the additional monochromatic light on leaf photosynthesis, we developed the differential quantum yield method that quantifies efficiency of any monochromatic light in white light. Application of this method to sunflower leaves clearly showed that, in moderate to strong white light, green light drove photosynthesis more effectively than red light. The green leaf should have a considerable volume of chloroplasts to accommodate the inefficient carboxylation enzyme, Rubisco, and deliver appropriate light to all the chloroplasts. By using chlorophylls that absorb green light weakly, modifying mesophyll structure and adjusting the Rubisco/chlorophyll ratio, the leaf appears to satisfy two somewhat conflicting requirements: to increase the absorptance of photosynthetically active radiation, and to drive photosynthesis efficiently in all the chloroplasts. We also discuss some serious problems that are caused by neglecting these intra-leaf profiles when estimating whole leaf electron transport rates and assessing photoinhibition by fluorescence techniques.

-----------------------------



Here are is a paper where Keith McCree proposes the use Quantum Flux Density (QFD), not PPFD. QFD is basically PPFD weighted with PAS (like I will do once I get a spectroradiometer):


"The measurement of photosynthetically active radiation"
http://www.linkinghub.elsevier.com/retrieve/pii/0038092X73900108

In crop ecology, the two most popular definitions of photosynthetically active radiation are the irradiance (radiant power flux density) in the waveband 400 to 700 nm, and the quantum flux density in the same waveband. Instruments calibrated in either of these two units are available. Calculations show that the quantum flux measurement is less subject to the systematic error caused by the spectral response not matching the action spectrum for photosynthesis in an “average crop plant” (11) than is the irradiance measurement. The range of errors is ±6 and ±16 per cent, respectively, for the 9 natural and artificial light sources examined. The imperfections of the instruments themselves are not included.

 

[renz]

PPFD is basically a 1 for 1 method to compare LED and HID. That is, 1 PPFD from a LED is the same as 1 PPFD from an HID (SPD not considered at this point, please see this post for info about SPD, i.e. spectral quality). Distance from lamp to top canopy effects PPFD and that is why I always write "lamps hung at reasonable distances", to me that means ~> 8" from canopy.

There is a so-called "burn in" period for HID lamps, I am not sure if LEDs also suffer them the drop in irradiance after about 100 hours of use.

The two data are good to see how the irradiances changes over time, and a good example why buying new HID lamps every grow or two is a good idea.

Hahaha, 8"... comparing LEDs and HID at 8" is pretty silly when you can run the LEDs at under 2" from multiple sources.

If you don't consider spectrum for considering LED lighting, you obviously missed the whole entire point. To not consider it now is to go ahead and prove the obvious, that they make less absolute power in the visible spectrum. Have you ever seen LED vs HID output plots vs frequency?

Also, if they're not driven insanely and run at high temps, and are from a reputable manufacturers, LEDs will last for years driven 24/7. So even if they do burn-in, it wouldn't matter because they will still have very high output for a very long time. Just get a bump at the beginning. Also V_f is temperature dependent, so LED power draw is temp dependent, and LED optical output is power and temp dependent, so you better take all this data too when you run your experiments.