Instructables
Picture of High-Brightness LED Grow-Light
Note: This project page has morphed into a design discussion.  (Constructive suggestions and collaboration are still welcome.  See the comments section at the end.)
**There are no recommended construction-plans here.**
Instead, I recommend shopping around for a good deal on standard White LED Light-Bulbs.  Please try using them as grow-lights, and let us all know how it goes.  My calculations suggest they should should be economical, energy efficient, and great for plants.

After many hours of research I've concluded that it doesn't make sense to build DIY grow-lights, and I can't honestly recommend it to anyone.  This is partly because it is technically difficult (and making it easier makes it more expensive), but more-importantly because commercial products are much less expensive and higher quality in the end.

You're better off buying an off-the-shelf product.  And surprisingly, I wouldn't even recommend buying a red & blue "LED grow light", because the best option seems to be conventional white LED lights!  Sure, grow-lights are approximately 25% more energy-efficient for growing plants, however, they are priced 2 to 8 times more per unit of power.  LED grow lights cost $4/Watt-electric typically, or nearly $2/W-e if made in China.  White LED bulbs cost $1/W retail, and sometimes $0.50/W if there is a green-subsidy instant-rebate. 
White LEDs emit more green-light than typical "grow-lights", which unfortunately isn't used very efficiently by plants; however, paying for the electricity to emit that inefficient green-light actually costs less than buying a grow-light that doesn't emit green-light!  Even if hypothetically you have sky-high highest electricity rates of $0.50/kWh, and zero-percent financing, when I try to amortize the cost of grow-lights, I figure that the LEDs burn-out before accruing enough saved electricity to make up for their high up-front cost.
 
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arnookie3 years ago
If you are thinking of building a diy grow light please take note of this.
Each and every led needs to be atleast 1watt each. Anything below that will be useless so don't use old leds out of toys or old boards, they simply will have too little Total lumens versus lumens per watt. You need atleast 10mm LEDs with 1watt per LED to supply enough light to the plant anything lower will not work. A good combination is a pannel made from 75% 1watt red high brightness leds, 20% 1watt blue high brightness leds and 5% 1watt amber high brightness leds. somewhere in the region of 660nm for red and 460nm for blue
There is also no effective difference in penetrative power for horticultural purposes between a 1W LED and a 3W LED. So anything over 1watt is just wasted. This means brightness has very little to do with the benefit you will get once you use 1wat leds. Don't confuse this with a pannel made from say 20 LEDs rated a 10watt as to one with 10 LEDs rated at 10watt. As the 20 watt pannel will use the useless 0.5watt leds verses the 10watt pannel that uses 10x10watt 1watt LEDs that are ideal. This has been tested and proven that 1watt single LEDs have great benefit to plants and anything less is just a waste of time and has no benefit at all to plants. The same applies with going brighter than 1watt has no benefit either.
Hope that may help some of you. Especially if you are growing indoors.
Also LEDs are more efficient than any other form of grow lighting available.
The commercially available LED growlights outperform all other growlamps from HID lamps to including high pressure sodium (HPS) and metal halide (MH) lamps.
So prepare to see other grow lamps become obsolete as LED growlight take over.
Just to clear a point up. If you make a 100watt pannel with 100x1watt leds you will benefit. But if you use 200 0.5watt leds to make a 100watt pannel it will not benefit the plant at all. The same applies if you use say for eg 10x10watt leds to make a 100watt pannel then you are only really getting 10watts overall because the extra 9watt per LED is just wasted as the plant will not benefit. SO using 100x1watt LEDs will benefit the plants by 100watts. 10x10watt LEDs would not because 9Watt of each led is just wasted giving the plant just 10watt of usefull light. Sorry but thats just the way plants absorb light.

are you talking about led outputs?

I recently built a LED grow lamp using LED strip lights (a 8mm wide flexible PCB with LEDs and resistors on one side and adhesive on the other) It has about 468 LED s each with an output of less than one watt. total power consumption is 40watts with about 5 watts being consumed by the 12V power supply. The light panel is 6 inches by 24 inches and is in fact a glass shelf mounted on the wall about 20 inches off the floor.

My small collection of orchids are growing just fine. In one pot I accidentally got two weeds started growing (one a Snapdragon and the other a vine hummingbirds like (both came from by carden) The Snapdragon has flowered and the vine is also growing well but has not yet flowered but is growing.

the issue isn't power. Instead it is light penetration If you have poor penetration you will only have leafs on the ends of all the stems and the interior of the plant will be all stems and no leafs. While having more power helps penetration, it is not the best solution to the. problem..

For the best light penetration you want light from multiple sources and reflective surfaces . The leds I have don't have any lenses so they emit at a wide angle. Light that doesn't travel straight down to the plant gets reflected off of reflective Mylar plastic sheeting around the growing area. any leaves near the top don't block light coming in from the sides. I have excellent light penetration with no shadows.

One advantage of low power LEDs is thermal management becomes easier. My light panel doesn't have a fan or heat sink. Most LEDs have a maximum temperature rating of about 130C. My light panel is running at 30C and produces no unwanted noise.

mr_man (author)  steven48728 months ago

Thanks Steven... I think light "penetration" (or light diffusion), and power are important. You say you're using about 35-40 watts electric for a 1 sq.ft. array. How much growing area is this illuminating? From my quick market research on it, I read that people recommend 25-50 watts electric (LEDs) per sq.ft. of growing area, for intensive growing comparable to full-sun.

When LED grow-light products cost around $4 / Watt electrical, this equates to a significant investment of $100-200/sq.ft of growing area.


The sun is not a diffuse source, however, since the earth turns, sunlight effectively is rather diffuse over the course of a day. Using reflectors, as you say, is a good way to make the light more diffuse, as well as being more efficient overall by directing more of the light to the plant that would otherwise be wasted.


Pre-fabricated arrays like you mention are a good option that I should have researched more, but my experience with LEDs happened to be with single devices, and I hadn't gotten around to researching arrays. Fabricating arrays at home likely doesn't make sense to do DIY, especially for most people, even though I've been stubborn on this point, and maybe this is *part* of the reason why this instructable has languished (to put it nicely). The availability of these arrays with high power output and lower cost seems relatively new, or at least it is rapidly advancing.

By the way, when you mention a max temperature of 130 deg.C, that is the internal die/substrate temperature. The LED's max case temperature is *a lot* lower than that! Like 60 deg.C. Anyway, for the greatest light output, and good efficiency (efficacy), and consistent light output (lumen maintenance) over the life of the LEDs, and long life, it's often better to run somewhat below the max temperature. At least reference the thermal operating characteristic chart/graph in the LED device datasheet.

You might think that you don't have a "heat sink", but technically you do...it's just that in this case the large 1 sq.ft. circuit board is the heat sink. It almost certainly has an (industry standard) aluminum core, assuming it has good light output (good efficacy or efficiency) and long life. If you don't need a fan or a heat sink, you might consider that to be good, and if it works, okay, but it doesn't necessarily mean that you've completely sidestepped the thermal issue, and of course it doesn't mean that physics no longer applies. I'm *not* saying that all LED luminares require more fans and heatsinks, but I am saying that generally LEDs benefit from improved cooling.

I agree that the use of "medium" power LEDs on a large array *could* be a fairly well optimized design, in the sense that it *might* have found a "sweet-spot" with good compromises (or "trade-offs") in cost, light-output, thermal management, and power-density (or diffusivity).


Fan noise depends enormously on how fast it's run (at what voltage). You won't hear a fan turning at a moderate rate, but it can still move a worthwhile amount of air.

steven4872 mr_man7 months ago

The array is 1sqft and with the curtain I have a 1sqft growing area. Witout the curtain it will cover about 9sqft but growth rate would be slower.

"By the way, when you mention a max temperature of 130 deg.C, that is the internal die/substrate temperature. The LED's max case temperature is *a lot* lower than that! Like 60 deg.C."

That is the maximum LED junction temperature. It will very a little from manufacture to manufacture and case styles but most I looked at averaged around 130C. My grow lamp is operating at only 40C, The maxim case temperature is not generally listed but it is going to be a lot higher than 60C. The case has to be structurally stable when it is soldered to to the PCB. lead free solder melts at about 200C and the soldering iron will be even higher. If the case gets soft while soldering the delicate wire attached to the actual led would break if tj\he metal solder tabs moved.

"You might think that you don't have a "heat sink", but technically you do...it's just that in this case the large 1 sq.ft. circuit board is the heat sink."

Replace the word board with flexable ribbon. This is a what I am taling about http://www.superbrightleds.com/moreinfo/top-emitti...

It is a flexable plastic ribon with copper foil conductors and adhesive. It is only 0.5mm thick (excluding leds and resistors). There is no aluminum core. Some of the ribbons I purchased use transparent plastic and I can see through the areas with no copper foil wiring. With 468 leds and 35watts power consumption, each led it only dissipating about 10mW of heat that is easily transferred to the surrounding air . Most resistors are rated at 1/4 watt max power dissipation and they don't need heat sinks..

The problem I had with LED ribbon was finding companies that specified the wavelength of light and finding the colors I wanted. It was fairly easy to find 440nm and 460nm blue but finding 640nm and 660nm red was difficult to imposible. I never found a source of 660nm red. Everyone carries 626nm and I only found one company selling 660nm. I also found the 440nm, 460nm, and 626nm ribbons at clearance sail prices but I had to pay full price for the 660nm, Overall I spent about $160 on the LED ribbons allone. Not the cheepest way but it did ellininate a lot or wiring and assembly work.

mr_man (author)  steven48727 months ago

Right, the junction temp.

Usually air-convection is the bottle-neck for heat-transfer. There's a very good reason why radiators and air conditioners have many finely spaced aluminum fins.

you could still sort-of attach a flex circuit to a heat sink. Flex can be a pain though. It's not very robust or reliable. Only use it if you need its flexibility.

660 nm LEDs are hard to find. I mentioned mouser.com had some. Even the surplus dealers are expensive...especially when you consider all the work to make the component into a viable end-product. That's why I suggested using off-the-shelf white LED bulbs.

http://www.allelectronics.com/make-a-store/item/LED-250/3W-RED-LED-WITH-STAR-HEAT-SINK/1.html


It's not easy to make phosphors work well. Here are some links. The simpler ones, and ones without rare-earth elements may be cheaper. Often they're not that efficient, nor long-lasting....

http://en.wikipedia.org/wiki/Phosphor#Standard_pho...

http://www.google.com/#q=phosphor+site:sigmaaldric...

That would appear to be the most flawed logic I have ever read.
mr_man (author)  antennas1 year ago
@antennas, I don't get what @arnookie is saying either. All I can guess is that he's referring to the problem of excess luminuous intensity at the top leaf and shading of lower foliage. As I said above, it would seem that a top leaf could easily shade a lower leaf. This is not optimal. If the illuminated leaf is saturated with all the light it can handle, then conceptually I imagine that additional light emanating from the same lamp would not benefit the plant. I don't know if this is a valid theory or not. I'm not a botanist or horticulturist or biologist. However if this theory is correct then it would be preferable to use indirect illumination, and "diffuse" the light, so that it is spread as evenly as possible over all the leaves. LEDs are highly directional sources, and are *not* diffuse sources in the way that florescent lamps are. Fortunately, this is not at all difficult to do and there are many ways to diffuse light. The light can be reflected off of a bright-colored or reflective surface, or shown through frosted-glass or any material that is translucent (passes light) but not transparent (don't pass an image).
Leafy plants aside 10W of light is 10W of light regardless if it comes from 100 0.1 W lights or 1 10w light. That's light not the power used...
saib30 arnookie2 years ago
Dear Arnookie,
Do you have a reference to a scientific experiment that proves what you are saying about 1W LEDs? Because for me, it does not sound correct, as some of the 1W LEDs are actually multi core LED with lets say 2 core of 0.5W!! check the following:
http://www.ledwv.com/en/images/LED%2080W.jpg

Second, what does matter for plants growth is the Luminous and wave length, and LED Luminous efficiency does deffer between manufacturers, then not all 1W LED will behave the same. Check the following:
http://en.wikipedia.org/wiki/Grow_light#Luminous_efficiency_of_various_light_sources


Regards,
Saib
mr_man (author) 8 months ago

I did some numerical integration in a spreadsheet, and I calculate that even a white LED is about 70% to 80% as good as a single monochromatic LED on the peak for photosynthesis. A "cool white" lamp will have more blue, which is good for vegetative growth, and "warm white" will have more red, which is better for flowering (according to all the weed growers on the 'net).

Even if the LED only gives you an extra 10 lumens per watt compared to a CFL (60 vs. 50) , the savings in electricity can still in some cases justify the cost of a replacement bulb. I picked up a 60 watt-replacement (13.5 Watts electric) , 850 lumens, for about $7 or $8 at a wholesale-club/ big-box store. (Yes it was in a neo-liberal state that automatically subsidized half of it. We also have much higher electric rates too @ 20 cents/kWh). Anyway, this lamp cost $0.50/Watt with discount, or about $1/watt normally. Sometimes the LED device/components alone can cost that much! so to get the whole package for that price is amazing.

steven4872 mr_man7 months ago

Keep in mind that Chlorophyll (the main food generating pigment in plants) has 4 absorption bands, 440nm and 460nm blue and in red 640 to 660nm. A 2700K 90 CRI (color rendering index) white LED lamp will cover all of the red spectrum. However no white led will cover much of the blue spectrum. The phosphor used in LEDs works best when energized by 460 nm to 470nm blue light. I know of no LED white lamp that cover both blue absorption bands of chlorophyll.

I have tried white LEDs par 20 lamps (8watts each) one 4000k (no CRI rating on the package)and the other 3000K 85CRI . I wasn't satisfied with the result (minimal growth and the lamps were running quite hot (75C).

I am beginning to think the optimal LED grow lamp design would be multiple blue wavelength LEDs and red phosphor. I have found companies that make LED phosphor and and from the specks I have seen one type of red phosphor would cover most of the red spectrum. However the phosphors appear to be quite expensive (based on only one site that listed prices for sample sizes. And you would still have to make some sort of paint to apply to the LED's

mr_man (author)  steven48727 months ago

It's not clear to me where the exact peaks are for which pigments and exactly what effect each of them have. I don't see a chlorophyll peak at 460nm. The info I have (in the graph above) is that there is only a carotenoid peak at 460nm. Also these 2 links.

http://en.wikipedia.org/wiki/File:Par_action_spect...

https://en.wikipedia.org/wiki/Chlorophyll#Spectrop...

There is a "chlorophyll-a" peak at ~440 and "chlorophyll-b" peak at ~490.

One's opinion on this may depend up whether more importance is given to the absorption spectrum for all pigments or the photosynthesis rate.

It's true that "chlorophyll-b" does have a very nice absorption peak at ~490nm, that is at least 3-4 times higher than at 660nm. However, the proof is in the pudding, and if you believe the published data, it says the photosynthesis rate isn't significantly different between the two. I can speculate that perhaps this is due to carotenoids absorbing some of that 490 light, and/or "chlorophyll-a" also being a bit more active at 660 than at 490, but that's a guess, I don't really know.

The individual pigments and non-phosphor LEDs have fairly narrow spectral widths. At first glance this looks like an opportunity, and it's what got me interested in this topic. However, now it seems to me that there are enough pigments with overlapping spectra that it gives the PAR curve a relatively wide spectrum. It appears that any light in the ranges of about 390-510 and 640-690 should be relatively efficient. It's tempting to want to line-up LED peak wavelengths with the PAR peak wavelengths. But where efficiency is concerned that isn't the true objective . The true objective is to make the *area* under the spectral curves overlap as much as possible, for the light-source and PAR. the efficiency is the ratio of the area that is overlapping. (In math, the wavelength integral of the product of the 2 spectra)

So because of this, now it seems that the narrow spectral width of non-phosphor LEDs aren't necessarily the breakthrough in spectral-efficiency that I thought they might be, especially when costs are taken into account. The breakthrough has more to do with their radiant-power efficiency.

800 lumens from a CFL consumes 14 watts, but an LED uses only 9.5 watts. The difference of 4.5 watts seems trivial, especially when the LED costs $8 and the CFL only $1. However, the electricity savings puts breakeven at about 1-2 years of continuous use (for electric costs of $0.20/kWh to $0.10/kWh respectively)

Anyway, if cost is a factor, then it makes sense to use the light that gives the most moles of photons @ PAR per unit currency.


I agree that a "warm white" lamp is relatively more efficient in the red, and a "cool white" phosphor is relatively more efficient in the blue, and neither is particularly efficient at both. To get both red and blue using only "white" lamps, I suppose you could use both "warm white" and "cool white", or compromise and use "neutral white". Or to change the ratio of blue/red, switch between one or the other as needed.

mr_man (author) 2 years ago
UPDATE: I think the supplier stopped carrying the LED I specified, although there are newer, cheaper, better LEDs coming out all the time. Available for instance from mouser.com

Also my original design called for ordering a custom circuit board. While that would give a higher quality result, now I'm thinking that might be unnecessarily complicated, or expensive, or not in the DIY spirit.
It may be possible to simply solder the LED to a couple of strips of copper sheet metal, (e.g. roofing flashing), or even a couple of solid copper coins, such as a pre-1981 U.S. penny, of which there are plenty still in circulation. Total cost, 2 cents! If the coin is not flat enough, then it can be sanded down a bit. The copper serves as both the electrodes and the heat spreader, which is then thermally coupled to the larger heat sink. If the LED is the type where the heat sink pad in the middle bottom of the LED is electrically connected to one of the electrodes, then you just have to make sure that the external connections match the internal ones so that the circuit isn't shorted out.
mr_man (author)  mr_man2 years ago
Regarding the electrical circuit design. The main difficulty is that the high-power LED ideally should be driven with constant-current, yet most power supplies are considered "constant voltage". However, it occurs to me that these power supplies are only constant voltage up to their maximum current rating. Any attempt to lower the load resistance and draw more current beyond this point will cause the voltage to drop so that no additional current flows, This is actually what a constant current source does. So to summarize, at low currents the power supply will act like a constant voltage source, but at high currents it will act like a constant current source. This is actually exactly what we want to drive a LED. We still have to take care to match up the power supply's rated voltage and current with the LED's rated voltage and current, but assuming that these parts are available and can be obtained, then if I'm correct, then we don't really have to worry about limiting the current with resistors or with the current limiting circuit that you see on some of the other LED instructables that has a current-sensing resistor and 2 transistors.
steven4872 mr_man8 months ago

All a LED cares about is the voltage across its terminal and that it is operative at the correct temperature. If you have the correct voltage the current will also be correct. The temperature however depends on how you dissipate the heat generated. Semiconductors, including LEDs, are susceptible to a failure mechanism called thermal run away.

If a led heats up is voltage requirement will change. that will lead to current increase and that will cause higher power dissipation. The higher power dissipation caused the temperature to increase will causing a even greater power dissipation. When this occurs the LED will quickly overheat and burn out..

You can mitigate the problem with a constant current source. Constant current sources limit the current so that the LED will always have the correct voltage across its terminals. However if the glue holding the LED to the heat sink failures the LED will heat up to its maximum rated temperature and fail. The down side to constant current sources is that all LEDs must have the same current rating and dimming by pulse width modulation is more difficult. If the leds have different current ratings some will be to bright and may fail while others will put out less light. the series parallel design in this project would work out just fine as long as the power is safely dissipated to prevent the LEDs from overheating.

Parallel configurations have the advantage that if a LED should fail for whatever reason, the others will continue to stay on.

So you can drive the LED safely with a constant voltage source and in fact most of the time that is what is used. What really matters is how the LED is mounted to its heat sink. The easiest way to mount a led is with thermal glue. But glues will over time fail and when that happens the LED could burn out. Securing a LED to a heat sink with screws is the pest way but sometimes the manufactures doesn't supply any mounting holes. Another option is to use lower wattage LEDs and have enough space between them so that the heat will be safely carried away by the wiring and dissipated.. Unfortunately that means more parts and more assembly time.

mr_man (author)  steven48728 months ago

I know... I'm an electrical engineer.
For LEDs is better to think in terms of current, not voltage. I discussed some of this in the article.

Power LEDs are surface-mount devices that are soldered to an aluminum metal-core pcb (MCPCB). Sometimes the thermal pad is electrically insulated, and sometimes it's not.

Where screws or adhesive are concerned, you're thinking of a pre-fab module where the LED device has already be soldered to a MCPCB.

mr_man (author) 12 months ago
I'll try to post a quick update. The LED market is growing by leaps and bounds. This is good since the value is ever-improving (more light at less cost), but presents the problem of high-turnover and non-stocked/obsolescence of products. The design has to be updated when new LEDs are selected every few months.

Anyway, if I was going to build a grow light today, budgeting about $25, I would buy 3 of these red LEDs: http://www.mouser.com/ProductDetail/LED-Engin/LZ1-10R200/?qs=sGAEpiMZZMu4Prknbu83y1kHJl487Qqnh7qFW6eM39A%3d
and 1 blue: http://www.mouser.com/Search/ProductDetail.aspx?qs=vmkU9SbkviSmfeFA%252b68R1g%3d%3d
These devices appear to be already soldered to an industry standard Aluminum core circuit board (MCPCB). This greatly improves handling, workability, robustness, electrical shock discharge protection, and thermal heat-sink cooling. Basically these just have to be glued to a heat sink (salvaged from an old computer), and soldered into a simple loop circuit with the power supply. The whole project should go much faster and yield a better result. I intend to update the instructable to reflect this.

I would wire all of these LED's in series with a current limiting resistor and drive it with a 12 volt, 1 amp power supply. I would try a resistance value of around 2 ohms, then decrease this if needed until there is 1 amp of current. The minimum resistor *power rating* in watts should be 2 watts, preferably 3-5 watts.

To save money and simplify the design, I'm thinking of omitting the 612 nm "orange" LED. I noticed that on this graph, carotenoids appear to be inactive at the 612 nm wavelength. http://en.wikipedia.org/wiki/File:Par_action_spectrum.gif
So I wonder if the patent I referenced is accurate, or supposing there is an empirical benefit to the plant, what is the mechanism? If additional carotenoid stimulation helps protect the plant, then why would 612 nm be better than say 500 nm? I suppose some experiments are in order to confirm or deny what that patent claims. Does any one have any good "scientific" literature references? I don't mean to lean so heavily on one patent, it just seemed at the time to be one of the better open-source references I could find on the 'net.
mr_man (author)  mr_man12 months ago
Maybe I answered my own question. I found this graph: http://www.blackdogled.com/photosynthetic-active-radiation-par.html
which shows that 612 nm is the peak for "phycocyanin". Interestingly, with these LED lamps it appears that the "phycocerythrin" isn't activated at all.
netzerite1 year ago
After reading the patent you mentioned above
@: http://www.google.com/patents/US6921182

I found the completed device for sale on the patent owner's website
@: http://www.led-grow-master.com/gardeningproducts.html

Compared to HID lighting prices and power consumption
as well as the devices 100,000 hour life expectancy;
$299 per light bar is really not bad.

However; that is still WAY out of my personal budget.
I for one would LOVE to see a complete DIY that is similar for my personal creation and use as they still have several years of patent protecting them from legal competition and there by lower pricing.

Keep up the good work!
mr_man (author)  netzerite1 year ago
Thanks for the info, and your enthusiasm and encouragement!
I agree with your sentiments. I think the $299 price tag may have to do with LEDs being a somewhat new technology and a low-volume product. The ideal wavelengths (or exact "colors") are not easy LEDs to source. I'm skeptical that the patent is keeping the price high, mainly because a red & blue LED grow light is not under patent protection as far as I know. I figure the patent probably only has design protection for certain wavelengths of light in combination, and light intensity, or lumens (or number of LEDs), particularly with orange light, or something like that. In other words I doubt there's any really strong patent protection on LED growlights, since it's a fairly obvious application. Anyway, to keep it simple and reduce cost, it may be prudent to skip the orange LEDs anyway.

Anywho, I agree on the open-source design concept.

I've been pretty busy lately, but I'll try to find time to work on it more, especially this winter! I think it could be an excellent way to have a healthy, productive indoor herb garden. Many herbs like a lot of sunlight, and this is hard to get indoors, especially in winter. Plants like basil taste best fresh, and are highly perishable, and rather expensive to buy at the store. So there's a lot to be said for a small, inexpensive lamp that doesn't produce a lot of heat, etc, and LEDs are clearly an excellent technology for this application.
thanks again.
mr_man (author) 2 years ago
I think we are in agreement that the efficiency (or efficacy) of LEDs varies widely. In other words, in general, when comparing various LEDs, the number of electrons going in is *not* proportional to the number of photons coming out.

Hence, knowing the electrical power is better than having no information at all, but knowing the luminous output is better still. However, even this can be misleading, especially for the blue LEDs where the human eye is not nearly as sensitive as to red LEDs. Better still is knowing the "radiant" data. Sometimes for blue LEDs the datasheet will spec the radiant output in WATTS. I think this is the best data, because it tells you exactly what the optical power is, *and* it is trivial to calculate the efficiency, simply by dividing by the electrical input power in WATTS.

I guess what arnookie might be trying to say is that a plant leaf can only make use of light up to a certain intensity. Once that threshold is passed, where all of the plant's photochemical processes are saturated (or "maxed out"), then any additional light has no further benefit to the plant. I am speculating on this point, which I shouldn't do, but it seems reasonable to assume that somewhere there will exist a point of diminishing returns.

Also, it would seem that one leaf could easily shade another leaf, and it seems reasonable to assume that there would be losses here, so it is probably preferable to "diffuse" the light, so that it is spread as evenly as possible over all the leaves. This is noteworthy, because LEDs are *not* diffuse sources in the way that florescent lamps are. Fortunately, the laws of optics allow a point-source to be diffused (just not the other way around, you can't focus a diffuse light to a point).

There are many ways to diffuse light. One can reflect the light off of a white wall for example, or shine the light through any material that is translucent but not transparent.

lightfans3 years ago
LED Grow Light is used for home garden, greenhouse, farm where need artificial lights for the plants.Idea for all phase of plant growth, and work well with indoor garden, hydroponics, horticultural and soil base. from http://wayet-lighting.com