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The Basics Of Plant Lighting

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The same questions get ask repeatedly and in a number of different ways. In this post I’m going to cover all of the BASIC information about plant lighting, how much of what type of light you need, why some systems work better than others, wavelengths, lumens, PAR, the Emerson Effect, PS1 and PS2, and a few other topics that may be of interest. I am going to break this into a few post so people can scan to what information they want to see.
I will do my best to answer any questions you may have as well as update the post to clarify points that may not have been worded clearly. Most importantly, I hope this helps a few people out.

What is light?
Light is an electromagnetic wave. This wave has a wavelength that can be measured in meters, nanometers to be exact. Mathematically speaking that is 0.000000001. This is abbreviated by using nm. White light is a mixture of all wavelengths of light. The hue or color that a light gives off is due to the amount of light from a curtain wavelength. Below 400nm is called ultra-violet or UV. Above 700nm is infrared or IR lighting. The wavelength of each color is as follows: (note that as colors get closer to each other the hues blend together)
UV C: 100-280nm
UV B: 280-315nm
UV A: 315-400nm
Violet: 380-450nm
Blue: 450-495nm
Green: 495-570nm
Yellow: 570-590nm
Orange: 590-620nm
Red: 620-750nm
IR A: 700-1400nm (near infrared)
IR B: 1400-3000nm (mid infrared)
IR C: 3000nm-1mm (far infrared)
The human eye sees light from ~400nm to ~700nm. There are a large number of measurements used in lighting and optics. I am going to cover the common ones in this thread. If you would like to see more click here.

This is directly from Wikipedia, as they explain it very well.
“ The lumen (symbol: lm) is the SI derived unit of luminous flux, a measure of the total "amount" of visible light emitted by a source. Luminous flux differs from power (radiant flux) in that luminous flux measurements reflect the varying sensitivity of the human eye to different wavelengths of light, while radiant flux measurements indicate the total power of all light emitted, independent of the eye's ability to perceive it. A lux is one lumen per square meter. “
Lumens are skewed toward what the human eye sees better. Therefore, lights that contain a high amount of greens and yellows have a higher value given to them than those that contain less or no green and yellow. This is an okay measurement to use when comparing two bulbs/panels of the exact same lighting type or wavelengths, but very poor when comparing light sources of different types. For example: When comparing two HPS bulbs (with roughly the same spectral output) the one with the higher lumens will be a brighter bulb. However, if you are attempting to compare two CFL bulbs one that is cool white and one that is warm white, the warm white will have roughly 5 more lumens per watt.

Photosynthetically active radiation
Photosynthetically active radiation or PAR is a measurement of all light between 400 and 700nm. This is wavelengths of light that control MOST photosynthesis. Unlike lumens, PAR does not skew the results of the photon count toward any given color and is expressed in µmol photons/ m2/second. It simply records the number of photons of the wavelength(s) being measured. You may also see the term PPFD or photosynthetic photon flux density expressed as W/m2. This is the measurement that is most valuable to us as plant growers.

Radiant Flux
“Radiant flux or radiant power is the measure of the total power of electromagnetic radiation (including infrared, ultraviolet, and visible light). The power may be the total emitted from a source, or the total landing on a particular surface.” Again, Wikipedia has a great definition. This is mostly used in technical publications and development, but is the most accurate measurement that can be given. It would be great to use this measurement for growing but you are not likely to see it on any packaging.

Chlorophyll and light absorption.
Chlorophyll is the main part of photosynthesis. This is why leaves are green when illuminated by white light. Chlorophyll is a green pigment which is found in all plants. However, there is more than one type of chlorophyll. There are at least six known types of chlorophyll and possibly more (the last chlorophyll to be discovered was in 2010). We all know that chlorophyll is what makes plants grow and they absorb the sun/light to make that happen. Many people are not aware of all of the other photoreceptors (light catchers) in plants. There are between 200 and 300 photoreceptors in every plant. Some botanists believe that there are still hundreds if not thousands to be discovered. These antenna or accessory pigments play a much larger role than most people think. The reactions that occur within a plant are very complex. Light hits the leaf, excites a photoreceptor pigment that then performs a chemical reaction that is passed on to the next. As PS1 loses and electron it gains one from PS2, this is called the Hill reaction. This process occurs in chlorophyll A, move through lutein, zeaxanthin, beta-carotene, lycopene, and then to chlorophyll A. This is just for the production of sugars that the plant uses to grow. When we talk about cannabis or other higher plants there are a number of other reactions that occur for the building of cannabinoids and other compounds used within the plant. Below are some of the wavelengths for plant photoreceptors. (Note that most photoreceptors have multiple peak absorption points.)
Chlorophyll A: 430, 662
Chlorophyll B: 453, 642
Alpha Carotene: 442, 444, 478
Beta Carotene: 425, 480
Zeaxanthin: 423, 451, 483
Lutein: 420, 447, 477

If you compare that list with the above list of wavelengths, you will see that none of these photoreceptors are in the green or yellow region. This is another example of how the lumen measurement does not equate to the growth of plants.

Emerson Effect
This leads me to the Emerson Effect or Emerson Enhancement Effect. This has been used in almost every plant photoreceptor study since he published his first paper on it in 1957. When illuminating plants with 700nm wavelength red light, he noticed that photosynthesis was slowed greatly. The same occurred when illuminated with 650nm red light only. However, the combination of the two generated a symbiotic response and increased the growth rate to double that of the single sources combined (a fourfold over each light alone). His later work indicated that there was not one but two photosynthesis traps. We now call these PS1 and PS2, where each system transfers energy and electrons to each other increasing productivity.

Types of Lighting
HID lighting is high intensity discharge. This refers to Metal Halide (MH) and High Pressure Sodium (HPS) most often. HPS is the second most efficient light source and has been used in the growing lighting and greenhouse industries for years. They work very well, put out a lot of light, and even more heat. I’ll go over coverage areas later, but HPS and MH lights can cover a lot more area from a single point source than any other lighting type. That is something to keep in mind if you are running a commercial grow operation or have a very large flowering room.

CFL and Tube Fluorescents
Compact and Tube Fluorescent lights are used extensively for growing. They are cheap to purchase, widely available, produce little heat, and have a wide range of color temperatures. When using CFLs, you can place them very close to your plants (within 2 to 3 inches) due to their low heat output. This helps the light penetrate deep into the plant and increase photosynthesis. Be careful about using a large portion of CFLs in a small confined space. While they put off a small amount of heat, placing large amounts in a confined space without proper ventilation can cause the heat to rise rapidly. Fluorescent lighting is not as efficient as HID or LED lighting, so as your grow area increases, the energy inefficiency causes a greater problem.

Light Emitting Diode (LED)
LED lighting is the newest form of lighting and has gotten a really bad rap for the most part. I’ll be honest; I’m an LED person and have been testing them for years. This probably makes me a little bias when it comes to this subject. LEDs have come a long way over the past few years. Everything we thought we knew about lighting and what produced more light and what grew plants better is out the window when speaking of LEDs. First, the measurement of lumens that we have all went by for decades only applies to a very small portion of LEDs, those that produce white light. Because lumen is a scale based on brightness to the human eye, color LEDs have a very low lumen count. Radiant Flux is the measurement used by designers when attempting to make panels and get a certain percentage of each type of light. The photoreceptors in plants use different wavelengths to perform different functions. This is the main reason that LEDs got a bad rap over the past 5 years or so. When panels first started coming out they only had 2 colors 630nm red and 480nm blue. While this will grow plants, it is missing a lot of the key elements that the plant needs to perform to its full potential. As companies (and individuals) started testing these LEDs to grow, they noticed this. Soon you seen 4, 5, 7, 11 and even 15 band LED grow panels. The results started to improve but the bottoms of the plants where not producing very well if at all. Then secondary optical lenses came to improve penetration. Most companies have learned that hitting wavelengths for accessory pigments and adding these lenses finally made LEDs with it for growing. I want to point out that it was the MMJ/Cannabis growers that performed the large amounts of these test and drastically improved the industry. There are some great people on this forum and many others that have contributed to the advancement of LEDs for growing and the production industry noticed. They started producing better diodes in the color sector as well because of these people and they deserve credit for their work. LEDs are the most energy efficient form of lighting, produce the least amount of excess heat, and have the highest lm/w rating of any lighting type. The area where LEDs fall short is in coverage area and penetration.

Lighting efficiency
The following is just for reference on energy efficiency in lighting types and is not to be construed as suggesting a lighting type for your grow room. A 100% efficient light would produce 680 lumens per watt. The following is the current efficiency of available lighting.
Incandescent: 1.9-2.6% (10-17lm/w)
Halogen: ~3.5% (35-60 lm/w)
CFL and Tube Fluorescent: 9-11% (CFL 50-70 Tube 30-100 lm/w)
HPS: 27-32% (100-150 lm/w)
MH: 25-30% (75-100 lm/w)
LED: 17-46% (1mw -167+ lm/w)


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Which Lighting system to use?
Each grow room and grower is different. I’ll be the first person to tell you that I cannot give information that is one size fits all. The wants, needs, and requirements for your grow room will be different from mine or someone else’s. The first thing we need to think about is what kind of light we are going to use. I’ll cover the basics here, if you have more advanced questions, post them in a separate thread and I’m sure you will get plenty of help. There are a few things we need to cover before we get in to lighting systems. The first is color temperature. Kelvin (K) is the standard rating system for point source white lighting. The sun has a color corrected temperature of ~6500K on a normal overcast day and ~ 22,000k on a completely clear day. This number varies greatly depending on a number of things like: cloud cover, latitude, humidity, and other atmospheric conditions. The more blue the hue of the light the Higher the color temperature will be. Likewise, the more red hue it has the lower the color temperature. This rating type works well for most single point light sources, however, it does not work for single color LED lights.
Now let’s get down to the information that everyone really wants. What type of lighting should I use? How much should I use? How far away should my lights be?
The type of lighting you choose will be based on the area you need to cover, the amount of ventilation you have available, and the amount of money you want to spend. We have already covered the basics of each lighting type. Let’s take it a little farther this time.

Cost is a major factor when setting up a new grow room or expanding a current set-up. CFLs and tube fluorescents are cheap to buy and cheap to replace bulbs (which needs done every 4-6 months). HPS and MH systems can cost quite a bit more to purchase starting at around $200. The bulbs can be pricy as well costing up to $60 or more each. LEDs are by far the most expensive. Panels can cost as much as $3 to $4 per watt (and more for some companies). However, the “bulbs” last for as much as 5 years without any noticeable degradation. The “driver” or power supply (called ballast for HID lighting) will be the first thing to fail with an LED system. If you are considering LEDs, make sure the company has a good service/repair policy.
Remember that heat and ventilation will play a role in this selection as well. If you do not have any venting available for your room, HPS/MH lighting is NOT for you. Likewise, if you have heat problems in your grow area you will want to avoid HPS/MH lighting as well.

How much light should I use?
The following is a MINIMUM light requirement for productive growth in the vegetative stage:
CFL: 42.25 watts per square foot
HPS/MH: 32.5 watts per square foot
LED: 22.75 watts per square foot

The following is a MINIMUM light requirement for productive growth in the flowering stage:
CFL: 65 watts per square foot
HPS/MH: 50 watts per square foot
LED: 35 watts per square foot

I know someone is going to say it so I will do it for them… “But I grow 3 pounds of bud off of x# of watts.” Yes, I’m sure you do. However, by following these basic guidelines, you have a greater chance of success and will have a higher product. You can get better results with more lighting. Like I said at the very beginning of this thread, everyone has a different grow room and results will vary on a number of factors. This is simply general information to help new people start out. I encourage everyone to tweak their grow room and get it set up for optimal production for their area.

How far away should my lights be?
There are a number of factors for this question. Do you have heat issues? What kind of light is it? Does it have secondary optics? How much room do you have?
For HPS/MH lighting, a good rule of thumb is: hold the back of your hand out over your plant. Raise it until you start feeling the heat from the bulb. The distance that your hand is from the bulb is 1 inch too close for your plant to be.
For CFLs and Tube fluorescents, it is almost impossible to bleach or burn your plants. Remember that the closer they are the less area they are going to cover. Other than that 2 to 3 inches is more than enough distance.
LEDs are where it starts getting tricky. It largely depends on the light. Does it use 1 watt diodes or 3 watt or even 5 watt diodes? The lower power the diode the closer the light can be. Does it have secondary optics (lenses) over the LEDs? If so, you should start at around 24 inches and slowly move it down until you see the leaves starting to bleach. Move the light up 2 inches and that is the closest you want to keep the light at any given time. If it does not have secondary optics, start at around 18 inches and perform the above method. There are simply too many variables and differences in LED systems, panels, manufacturers, and diodes to give exact numbers.


Member of the Month: July 2012, July 2014 - Nug of the Month: July 2012


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Hey hose.
I just picked up this this LED panel and need a lil advice on it. its a 120 watt panel but no name.

I m trying to determine how i want to set this up. My plants are already sitting under a homemade CFL set up.


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Are you reading 120 watts from a kill-a-watt or is that what the seller told you it was? I counted 112 diodes, which means that it is probably made up of 112 1 watt diodes and is being driven at ~300mA. Without a little more information it is really hard to tell. I'm guessing that if you hook a kill-a-watt up to it that it will only pull about 90 watts and 10 or so of that is from inefficiency of the drivers. It may work okay for the plants you show for veg growth, but again that depends on the spectral output of the device as well.

As far as where that light falls into my post. That appears to be a first or second generation panel with 1 watt LEDs and no secondary optics. The problem with buying cheap LEDs is that you will never truly know what the spectrum is and what the quality of the components are.


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Hello Hosebomber, yet another great posting. Thanx!

But why didnt you mention that the Emerson effect har a use of 730-740nm? You posted a comment here on the subjekt Custom 400 piece, 3w, led panel, help me choose led ratio, or is it old knowledge now?

I was looking on the internet for any 720nm to 740nm starleds to build a own growlight, but i couldn't find any.
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Ah and i forgot this:
Where can i find star LEDs that hit the 640nm??

If i want to enhance the Chlorophyll B with 453nm and 642nm.

Well i have some problem location a several LEDs too have a good light (to my knowledge). When i want to enhance
Alpha Carotene: 442, 444, 478
Beta Carotene: 425, 480
Zeaxanthin: 423, 451, 483

I cant find the 420nm either. I have search many big company like Luxeon, Cree and so on.

Great thread Hosebomber!!


New Member
Are you reading 120 watts from a kill-a-watt or is that what the seller told you it was? I counted 112 diodes, which means that it is probably made up of 112 1 watt diodes and is being driven at ~300mA. Without a little more information it is really hard to tell. I'm guessing that if you hook a kill-a-watt up to it that it will only pull about 90 watts and 10 or so of that is from inefficiency of the drivers. It may work okay for the plants you show for veg growth, but again that depends on the spectral output of the device as well.

As far as where that light falls into my post. That appears to be a first or second generation panel with 1 watt LEDs and no secondary optics. The problem with buying cheap LEDs is that you will never truly know what the spectrum is and what the quality of the components are.

thats what the back of the panel says. theres nothing more than the label on the whole panel.
Power 120W
Input/ current 1.1A
Input voltage/ Freq 110/60hz


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The Emerson Effect is often misused and misunderstood. His orginal test was 660nm and 730nm. Then he used 650 and 700, then 680 and 740. The test has been duplicated a number of times using everything from 630-680nm for the low end and 700-770nm for the high end. The key is having a light source at the high end that will trigger Pfr receptors that turn Pfr back to Pr. 730nm has proven to be the most efficient at doing this from my research.

As far as what Cammie's company does... well I don't trust anything he/she says.

The high end companies do not make many colored LEDs anymore. Bridgelux (which most panel manufacturers/resellers claim to use) only makes blue and white LEDs now. Many of those companies have sold their patients and production rights to smaller Chinese firms to produce and they focus on R&D and white lights. Phillips has moved into fixture production very heavily as of late. You can special order phosphor blends from some manufacturers (Chinese). As for finding those LED on stars, it is not very likely. You can purchase the starboards at a number of places (ebay, steves's, rapidLED, etc.) and then order the raw diode from mouser, digikey, avnet, or similar. A good 630nm LED with have a + or -15-20nm peak. Meaning that 80% of it's power distribution will be between 610 and 650. Likewise, remember that those are only the peak absorption values and not absolute values. They will still absorb other wavelengths of light, just not as efficiently.


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dubbrollin, I would personally just put a kill-a-watt on it to get the true power draw. The wider spectra of light couldn't hurt in your current CFL setup. Just make sure that you don't put it too close to the babies and let them get adjusted to the light.


New Member
AS I mentioned in the above post, there are too many different brands of LEDs and lenses on each to give exact info for every case. For your one watt LED version start at 18 inches or so and move them closer until the plants start to fill out rather than stretch. If the spectra of the light is correct it should be pretty noticeable when that happens.


New Member

Thanks for taking the time to post this information.

I'm was going to add a 600 watt HPS bulb and hood then starting reading about MH being a better light source during veg.
And I also read about the hybrid bulb that has both the MH and HPS light source.

Do you think using a MH only bulb during veg, then switching to HPS for flowering, is better then the MH and HPS mix to run for the whole grow?

Sorry, I'm ripped, I don't think I can explain it coherently.



New Member
It depends on the mixture of lighting and what ratios. The benefit of running MH for veg is that red light causes elongation of inter nod length. When we are growing inside (in most cases) the shorter we can keep the plant the more room we have for lighting and waste air removal. The chemicals used in the plant during flowering are a complex collection of photo receptors and internal chemicals that perform better under red wavelengths. A small amount of MH light during flowering will help the plant continue to make sugars and starches from the lower wavelength photo receptors. Nature is a balance of these light wavelengths all the time, slightly skewing the hue as seasons change. Some seem to think mimicking these actions of the sun will produce as the sun does.

Hope that answered your question.


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i have to admit, i'm more than a little intrigued by induction lighting, yet i can't find a whole lot of first-hand knowledge on the subject other than the claims of the manufacturers (particularly igrow). are they just too expensive ? too new to the scene ? ineffective ? just wondering why they are rarely included in the conversation when it comes to grow lighting. this seems like a very knowledgeable community, so i would love to hear any current thoughts on the subject from real growers(experts imho).


New Member
Induction lighting falls under the category of tube florescent lighting in almost every form. They are more energy efficient at around 20-25%, but they are still not as efficient as HPS or LED lighting. Spectrum wise, they are nearly identical to cfl and tubes. There is a induction company adding what they call "pontoons" of 660 and 730nm red LEDs to increase their performance in the flowering stage. I do not believe the test results are complete yet but I may be mistaken on that. iGrow says that they have a different blend of phosphors used in their light but from what I have seen they have very little red in their light at all and are simply making bold claims (sometimes known as exaggerations of the truth) that their light is better than anything out there. I personally do not own their product and would not say that they are lying.... but the grow test performed by others suggest that their product is not all that they claim.


New Member
i did find a blog called "grow with induction" that are performing side by side grow tests with 400w igrow vs 1000w hps and they are claiming 90% yield with less energy being consumed, less heat generated, less need for exhausting heat, and less water/nutrient uptake. now, i don't know if this guy is for real or just a shill for igrow, but it has piqued my curiosity. i am probably going to use a T5 6 bulb & supplemental cfls for my first grow for start-up cost reasons, but i have to be honest, these guys did their job and really put a hook in me. i just HAVE to know more so i can make the most informed decision that i possibly can.


New Member
That is 100% some employee of iGrow (or a paid blogger). They are adding random amounts, multiplying by 2 to get to a 6 plant coverage (which the 1000 watt HPS can do), and many other sketchy math applications to make it look like the iGrow performed well.
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