Ten myths of growing under lights

shrubs_n_bulbs(z8/9 UK)March 10, 2005

I have been browsing some of the large number of internet pages devoted to growing plants under artificial light, some commercial, others purely informational. The amount of incorrect information is astounding. A few things crop up time after time, probably copied from page to page in true urban myth fashion.

Here is my top ten myths on this topic. Feel free to add more. Or, if you're that way inclined, defend one of my myths or just trash my explanation :)

1. HID lamps give off more heat.

All lamps of the same wattage give off more or less the same amount of heat. A 400W metal halide or HPS lamp gives off the same heat as ten 40W fluorescent tubes or four 100W incandescents. The HID lamps will be hotter because all the heat is being given off in such a small space, but the total heat is the same. A fluorescent tube will gently warm the whole shelf while a HID bulb will fry a few square inches of plant and hardly touch the rest.

2a. HID lamps have better "penetration"

2b. Light from fluorescent lamps drops off too quickly with distance.

Two ways of saying the same thing really, but both wrong. The same wattage of fluorescent light used at the same distance as a HID lamp provides equal "penetration". This myth arises because people tend to use fluorescent lights very close to plants (to compensate for having less power) and HID lights quite far away. The difference in light intensity between the top of a plant 2" from a light and the bottom maybe 12" from the light is very large, while the difference between 20" and 30" from a light is relatively small. No difference if you have the same amount of light, the fluorescents may actually perform better because the light is distributed more evenly.

3. GRO-LUX type lamps are less effective because they are less bright.

GRO-LUX fluorescents appear dim to our eyes because they emit light that our eyes are not sensitive to. However the total light power emitted is basically the same as an average fluorescent tube. The light emitted at wavelengths good for plants is as much as any fluorescent and more than many.

4. Plants only use red and blue light, green light is useless.

Plants use light at all wavelenghts from near ultraviolet to near infrared to convert water and carbon dioxide into sugars. They use red light near 650nm most efficiently, then blue light near 620nm, then light in between. Some plants are much less efficient at using green light, some use it almost as well as red and blue. All plants have some capacity to adapt to best utilise the available light.

5. Blue light is for growth, red light is for flowering.

Plants will grow under monochromatic red light. Plants grow faster with a little blue light (with the same total light power). Plants grow slower under monochromatic blue light because the same power produces fewer photons. Approximately 25% of the power in blue light appears to be best, but probably varies for different circumstances and plants.

6. LEDs are ("ultra-bright", "extremely low power", "used by NASA", "monochromatic", or pick your own catchphrase) therefore they are the best grow lamps.

LEDs look ultra-bright because they are small and emit all the light in one direction. This is great if you want to grow a single blade of grass and not much help for most anything else. LEDs are extremely low power because they are small and emit very little total light. This means you need a lot of them to grow anything useful and then they use a lot of power. NASA has experimented with LEDs because they are small, light, and nearly indestructable. None of these features are particularly valuable to the average plant grower. Many LEDs emit a very narrow range of light wavelengths, which means they can be calibrated to emit only the light most useful for plants. That's exactly what a GRO-LUX lamp does and the fluorescent GRO-LUX does it about five times more efficiently.

7. T5 fluorescents are the newest and best.

The new generation of T5 fluorescent tubes is extremely efficient and compact, both desirable features for a grow light system. They are as efficient as some metal halide bulbs. Unfortunately they are mostly being sold as premium items with a premium price tag and often with expensive custom fixtures. For the same money, a good HID setup would be better and for less money you can get the same light from standard fluorescents.

8. A windowsill isn't bright enough, you have to use artificial lights.

Direct sunlight through a window is far brighter than any grow lights most of us will ever see, use it when you can. The limitations are that there is very little direct sunlight in the middle of winter at temperate latitudes and there is only so much space directly in those south-facing windows. Indirect light in a south-facing window is poor, indirect light in any other direction is completely inadequate, and light more than a foot or so from a window is virtually useless to a high light plant.

9. Two fluorescent bulbs aren't enough, you need at least four.

The number of fluorescent tubes you need depends completely on how much light you want. Some plants need more light than others. Lighting a larger area requires more light. It is difficult to use more than two fluorescent tubes to light a strip the width of a seed tray, so two tubes are sufficient for a single row of trays. At the other extreme, four 40W fluorescent tubes are completely inadequate for lighting twenty mature tomato plants.

10. Cool white bulbs emit blue light and warm white bulbs emit red light, so mix them if you want both red and blue.

Warm white bulbs emit most of their light in the red part of the spectrum with only a small amount of green and blue light. Standard cool white (4100K) bulbs emit more blue light, but still more red than blue. "Daylight" 6500K bulbs emit roughly equal amounts blue and red light power, which still means about twice as many red photons as blue photons. So any of these tubes emit a lot of red light and some blue light, the difference is in the details.

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biology_brad(z4 MN)

I would take exception with statement #4.
Chlorophyll A absorbs red light most effectively at 680 and 700 nm , not 650 nm.
There is no blue light at 620 nm, you need to get smaller, say 500nm.
this is textbook stuff, hard to see how a "myth-dispeller" could be so far off.

    Bookmark   March 10, 2005 at 8:41PM
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shrubs_n_bulbs(z8/9 UK)

Sorry, that was a typo. I meant "blue light near 420nm", not 620nm :)

    Bookmark   March 11, 2005 at 6:10AM
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Interesting. I like #5 because a few weeks ago i started growing lettuce and radishes under HID 400w HPS, and was worried about the red-spectrum thing (that i should be using blue, but am using mainly red).

    Bookmark   March 12, 2005 at 12:07PM
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jwmeyer(Z8 OR)

I know I'm a broken record but, I'm having outstanding results w/ a ceramic metal halide 400w. by philips. To my knowledge, It provides the best spectrum of any HID on the market right now. I don't worry about blue for growth or red for flowering...This is a white light that is as close to the sun's spectrum as your going to get in an HID light source.For you HID lovers, I've supplied a link. Look for yourself. This is truly a light that is good for all stages of plant growth! If your interested in getting one, go to bulbs.com You'll pay 62.00 including shipping for one bulb. It operates off a HPS ballast. It also states that it is intended to be used in a vertical position. Since they have been available in 400w, about 1 1/2-2 years, many people have used them in a horizontal configuration w/ no ill effect. I talked to a philips tech myself, at most you may lose 5-10% life on the bulb burning it in a horizontal position. Well worth the lose of bulb life considering it's advantages, IMO.


Here is a link that might be useful: ceramic metal halide.

    Bookmark   March 13, 2005 at 12:23AM
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nygardener(z6 New York)

A nice list, but a few comments:

2. The same wattage of fluorescent light used at the same distance as a HID lamp provides equal "penetration".

HID lamps are more efficient than most or all fluorescent lights. So even allowing for the greater width of fluorescent bulbs, you'll get more light and therefore more penetration from an HID lamp of the same wattage.

9. It is difficult to use more than two fluorescent tubes to light a strip the width of a seed tray, so two tubes are sufficient for a single row of trays.

Yes, if the trays are arranged lengthwise (end-to-end). If they're side-to-side, then four bulbs will fit over them and will generally produce better results.

    Bookmark   March 18, 2005 at 9:56PM
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drvinnie(z8 AL)

Humm, I have a comment on #1. "All lamps of the same wattage give off more or less the same amount of heat." For the same amount of power input the three types of lamps discussed have wildly different amounts of visible light output. The sum of the energy of visible light, UV light and IR light are the same for the same wattage, just distributed differently. HID's have the least heat (infrared) output of the three types since most of the energy goes to visible light. Incandescents have most of their spectrum in the Near Infrared, thus give off much more heat per watt than the other two types. A long fluorescent tube gives off more heat per watt than an HID lamp but it typically is of lower wattage and that heat is spread over a much larger surface area, thus feeling cooler. A 15 W CFL is not cool to the touch.


    Bookmark   March 20, 2005 at 2:54PM
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1000w of any kind of light does not throw off the same amount of lumens. The efficiency of the bulb is a ratio of amount of power used in the generation of useable light versus the amount of heat generated. I guess you could call heat a form of light but we're growing plants in ambient temperatures so the wavelength of "light" given off as heat is waisted energy.
compare PAR ratings for a 400 watt Halogen flood lamp and a 400w HPS. Better yet place each bulb (and ballast on the hps) in a 4'x4' box and place a thermometer in each box and tell us which reaches 120f first. I'll put my money on the 400w halogen. (Caution that this could start a fire)
If 400w of sodium or Halide is the same as 400w of tubes then why do commercial growers spend thousands of dollers on expensive HID ballasts and bulbs?
As far as penetration; I almost agree but; 120,000 Lumens WILL not penetrate the same as 68,000 Lumens.
I would agree that the same amount of lumens WILL penetrate the same no matter what the source is.
Most of the info. YOU gave us IS a myth IMO.

    Bookmark   March 20, 2005 at 7:12PM
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shrubs_n_bulbs(z8/9 UK)

Its good to see that plenty of people still believe the myths and are even prepared to defend them :)

#2 for nygardener. You're still confusing the total amount of light with the penetration. You said you'll get more light and therefore more penetration from an HID lamp of the same wattage. Firstly, is the 10%-20% more light really such a big deal, because that isn't what people talk about when they repeat this myth? More importantly, more light doesn't mean more penetration, they are two completely different things. Penetration is the relative amount of light reaching the bottom of a plant compared to that reaching the top of the canopy. If half of a high intensity light hitting the top of the canopy reaches the bottom of the plant, that is the same penetration as half of a low intensity light reaching the bottom of the plant. Same penetration but a different amount of light. The important factor is the inverse square law, light intensity drops very rapidly close to the light source and slowly when you are more distant. Moving a foot further away from a fluorescent tube makes a big difference, moving a foot further away from the sun is not even measurable. Penetration from sunlight is superb, only the plant shading itself makes any difference. Penetration from any bulb, fluorescent or HID, hung one inch from the top of the canopy is very poor.

#9 for nygardener. Are you agreeing with the myth, or just quibbling over how to arrange seed trays? Either way, whatever arragment you make that will work with four tubes, I can show you an arrangement that won't work, just by adding more trays. And I can show you plenty of setups where two tubes is just fine. The myth is that four tubes is in some way the right number, and it simply isn't.

#1 for Vince. Do you really think that a 400W HID lamp gives off significantly less heat than ten 40W fluorescents, or two 200W compact fluorescents? Alantown has proposed an experiment, and I really wish he would go and perform it. He says Better yet place each bulb (and ballast on the hps) in a 4'x4' box and place a thermometer in each box and tell us which reaches 120ºf first. 400W is 400W, the box will heat up identically in each case. The only way it would be different is if some of that energy could escape. A light bulb is sending out some energy as visible light, but only about 20% of the total energy in the case of a HID bulb and about 16% for a fluorescent. Even if you allow 100% of the visible light to escape without being absorbed by anything, the difference between 400W of metal halide and 400W of fluorescents is about 16W, just about irrelevant. So, I repeat, the heating effect of all bulb types is basically the same.

Alantown asks why do commercial growers spend thousands of dollers on expensive HID ballasts and bulbs? You know as well as I do why - the best metal halide bulbs are around 10% more efficient than the best fluorescents and HPS another 10% more efficient, plus they last longer and are more compact. That 20% adds up to serious money when you have thousands of square feet to light, but simply isn't the make-or-break factor for modest home growers. Commercially, it is also very tax-efficient to make capital expenditures in order to reduce ongoing expenses.

And another Alantown quote: I would agree that the same amount of lumens WILL penetrate the same no matter what the source is. This shows exactly why people have so much trouble with this concept. Lumens is not the important thing, 10,000 lumens at one foot has a totally different penetration to 10,000 lumens at one inch. Light from a point source falls off inversely with the square of the distance. That means that the few inches from the top of a plant to the bottom makes a huge difference for a close light source and a small difference for a distant light source. How bright they are doesn't matter. 120,000 Lumens WILL not penetrate the same as 68,000 Lumens This totally meaningless statement compounds the problem. You need to be talking in terms of light intensity, measured in lux or foot-candles, which means you need to know the distance from the light source. 120,000 lumens at the same distance as 68,000 lumens will decrease in light intensity from top to bottom of the plant in exactly the same way, and will be nearly twice as intense at both the top and the bottom of the plant. The improved penetration becomes apparent when you place the 120,000 lumen source further away. For example, 120,000 lumens at three feet from the top of a two-foot plant produces a lower light intensity at the top of the plant and a higher light intensity at the bottom of the plant than the 68,000 lumen source place two feet from the top of the plant.

Want to trample on this myth even more? A fluorescent tube is an extended light source and light intensity falls off more slowly from an extended source than from a point source like a HID bulb. This isn't usually very important because of the effects of reflectors, but it can be relevant in large-scale setups. For example, if the ceiling of a grow room was covered in fluorescent tubes, the light would have much better penetration than a single HID bulb putting out the same lumens. The HID bulb would be able to create a high light intensity if placed close to plants but the light would drop off more quickly with distance. Of course, the very use of the word "penetration" is a problem since it isn't even a term that is properly defined. However, it has come into popular use with certain macho growers, perhaps because they like the macho connotations of the word :)

    Bookmark   March 20, 2005 at 8:34PM
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John_Dal(z6 IL)

#8 ....light more than a foot or so from a window is virtually useless to a high light plant.

Is this true? I've recently added a sunroom and am anxious to see how my plants grow in there. Depending upon the time of day, plants will usually be more than a foot from the source of the sunlight through the window. But since it IS a south-facing sunroom, plants will get sunlight almost all day.

Also, my sunroom windows are treated. I wonder how much of the useful light spectrum will be blocked by the windows? Anyone have any information about this?


    Bookmark   March 21, 2005 at 2:18PM
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shrubs_n_bulbs(z8/9 UK)

John, light intensity does drop off incredibly fast when you move away from a window. Larger windows are better and some form of light meter is really the only way to measure it since your own eyes adapt too well to the drop in light. If a spot is receiving direct sunlight even several feet from a window then that is OK, but indirect light will be much dimmer than right at the window.

Your sunroom is presumably mostly glass? You probably wouldn't have to worry about distance from the glass. The light intensity will drop when you move to areas that are not surrounded by glass. I can post how to measure light intensity using a camera if you want to measure around your room and don't have a light meter.

Normall glazing glass passes virtually all visible light with less than 20% loss, but it stops UV very effectively. Some modern glazing is coated to reflect infra-red, either to keep heat inside a room, or to minimise solar heating of a room. If the coating isn't visible then visible light shouldn't be affect. Some coatings are visible, either silver/gold, or a grey tint, to further reduce solar heating. These coatings do reduce visible light substantially.

    Bookmark   March 21, 2005 at 3:27PM
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Since when is MH only 10% more efficient than the best flouro bulb?

Flouro's are ok for rack growing or even plants as tall as a couple foot tall but; have you ever tried to cover a 6' tall tomato plant with a bank of tubes? (Good luck) Guess I'll stick with my Miniature sun for a mans size harvest and keep the cool tubes for the violets.

    Bookmark   March 22, 2005 at 10:52AM
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shrubs_n_bulbs(z8/9 UK)

Thanks for the constructive input Alantown ;) Nobody is suggesting you buy a dozen fluorescent fittings to provide largescale lighting (well actually they are, just not me, see this link), but you should at least know why you are making your decisions, not just getting caught up in the "metal halide is the brightest" hype.

1,000W Metal Halide grow lamp, typical output 110,000 lumens. Actual input power including ballast 1,100W. Efficiency 100 lumens/watt. Those sneaky vendors tend to avoid mentioning the ballast! You might be able to find a bulb with 115,000 lumens or even slightly more, but be careful that they aren't just using a higher power ballast to overdrive it!

T5 or T8 HO fluorescent efficiency, including ballast, up to 104 lumens/watt, but only for fairly small tubes. More typically 90-95 lumens/watt in the higher powers. That's within 10% of the best metal halid lamps. Also remember that metal halide efficiency drops once you get below about 400W, although this is starting to improve as electronic ballasts are used.

HPS, up to 130 lumens/watt, including ballast load.

    Bookmark   March 22, 2005 at 11:43AM
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Yepper my 1000w sodium is one hell of alot brighter than my halide and I can cover much more area. I have to run it about 18" away from any growth or; it will bleach the leaves.

    Bookmark   March 23, 2005 at 9:06AM
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lenrapp(New Jersey, USA)

I am new to this forum and read your postings with great interest. I have begun to sell HO Fluorex Fluorescent Grow and Aquarium lights on eBay. Have been doing quite well by converting a HO full spectrum flood light that I purchase from Lights of America to be used for indoor use. The lights are all electronic and the bulbs draw 65 Watts while the output is equivelent to many 500 Watt lights. The lumens rating of the lights are 4550 or 70 lumens per watt consumption.

I have had great personal success growing my plants with these lights and my aquariums are doing great. They the lights get to around 90 degrees hot vs. halides that can get to around 350 degrees so my aquariums stay cool and I can keep the lights directly on top of my glass tops with no ill effects to water temperture. This means no "Chiller" units required and great diffused lighting in the tanks. I have found the main difference that I can see is that a Halide is a point light source like a spot light and the HO Flourescents I use are a softer, diffused light. Frankly the spread on my lights are wider then the halides at equal distances.

I have also began to offer color correction filters that are used in the motion picture and photographic industry to adjust the 6500 Kelvin lights to 10,000-12,000K or lower down to 5,000K with great success.

I am not looking to sell any nor do I want this post to be considered a commercial message, but I am still learning and recently had a question about the PAR ratings of these lights. I also notice on this forum differences in opinion on the importance of those ratings and how many grow light manuf. use them for marketing purposes and that the measurements they offer have little meaning.

I have found it easy to sell my lights at $65 each to marine aquarium users because of the cost, low costs of replacement bulbs ($15) that last a full 10,000 hours, low heat output, color temp adjustments through my filters, low operating costs ($25 per year, 10 hours per day every day) vs. almost $200 for a 500W Halide, etc.

Am I on the right track or am I missing something here. I want to understand more about the quality of these lights so I can talk to my customers with full knowledge of the lighting and the benefits or drawbacks of each.

Any help would be appreciated. I am not listing my web site or eBay auctions because again I don't want anyone to think I am trying to sell lights on this forum and hope this posting is OK.

Thanks in advance for you input and knowledge.

    Bookmark   June 19, 2005 at 1:20AM
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So much for the price. I bought mine as a LofA Worklight over a year ago at WalMart for only $24.95. I have seen the Worklight available elsewhere, but not at WalMart anymore. The 65w bulbs are available at both Lowes(for $9.99)and Home Depot(for $12.99).

As for information, LoA really hypes their output. A 150w halogen bulb emits 2400 lumens(16 lm/w). A 300w halogen bulb emits 5200 lumens(17.3 lm/w). A 4550 lumen halogen bulb equivalent would consume approximately 265w, NOWHERE close to 500w!

Want incandescent? A 200w incandescent bulb emits 2850 lumens(19.3 lm/w). A 300w incandescent bulb emits 6820 lumens(20.9 lm/w). A 4550 lumen incandescent bulb equivalent would consume approximately 229w, also NOWHERE close to 500w!

Most of the hobbyists in this group would be much more inclined to be helping the other hobbyists in this group to find an inexpensive source for your light, not to help you sell it.

Not to be mean, but that is the way we like it. You can do a search here for previous discussions on Flourex, or other keywords. They have been discussed.


    Bookmark   June 19, 2005 at 2:34AM
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shrubs_n_bulbs(z8/9 UK)

I guess in one respect you are on the right track because you are making sales and presumably making some money. A product is worth what the other guy will pay, right?

Personally, I think the compact fluorescent type of bulb fills a useful niche for plant lighting, but they are in no way a replacement for a metal halide because they are less efficient when you need 40,000 or so lumens. They are similarly not a useful replacement for regular fluorescents if you wish to light an area around four feet long and a foot or more wide. The only real big-win is over regular incandescents and halogens which no right-minded person would consider for plant or aquarium use. The niche for these bulbs is when up to about 10,000 lumens is needed over a fairly small area, then I can recommend this type of bulb.

The claim that this lamp puts out less heat than a halide lamp is also misleading. The only reason there is less heat is because there is less light. A metal halide bulb is a very small hot bulb, the fluorex is larger and cooler, while a regular old T8 is even larger and even cooler, but the total heat output for the same light is more or less the same. The smallest metal halide bulbs designed for aquariums put out about three times as much light as the fluorex and so put out about 3 times as much heat. If you don't need that much light then maybe the compact fluorescent is the right bulb.

Filters to change the spectrum are frankly an utter waste of time to any plant or aquarium hobbyist. They waste a big chunk of the precious light which is just not acceptable. Bulbs are available which produce the light in the right spectrum, or use a combination of bulbs.

    Bookmark   June 19, 2005 at 7:27PM
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Jumpin_Timmy(z5 NE)

Shrubs - the March 22 post correctly noted the substantial wattage consumed by HID ballasts, but we also need to address the HUGE lumen depreciation of metal halide (and other HID) lamps.

The "typical" 110,000 lumen lamp only produces 71,000 lumens after 4,800 hours of operation... dropping the efficiency to 65 lumens per watt! (See Philips Lamp Specification & Application Guide 2004, for example, MH1000/U lamp). In other words, the metal halide lamp continues to draw full wattage but produces only 65% of "new" output after just 9 months of operation at 18 hours per day! Output continues to decline during the balance of the relatively short 12,000-hour lamp life.

"Super T8" fluorescent lighting can provide 96-99 initial lumens per watt efficiency and end-of-lamp-life efficiency of 88-91 lumens/w. Average life is a whopping 30,000 hours (12 hrs. per start), and 29,580 lumens can be achieved from a 2x4 foot fixture. (Example: TCP 8-lamp Power Bay fixture with Ushio Ultra 8 F32T8/850 lamps. 3,698 lumens per lamp w. 1.2125 Ballast Factor).

A 232-watt 6-lamp Super T8 fixture can more than replace the light output of a 465-watt metal halide fixture on a "one for one" basis. (Fixture efficiency - ability to direct the light where we want it - is also superior with Super T8 linear fluorescents). The energy savings are so substantial that a number of power companies in the US are offering major fixture rebates in support of this transition in business settings.

    Bookmark   July 1, 2005 at 5:13PM
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I know this is an old post but I was hoping you could describe the science behind myth #1 - "All lamps of the same wattage give off more or less the same amount of heat."

I recently came across an experiment that tested the heat output from a single 100 watt bulb against four 25 watt bulbs. The conclusion was that the single bulb produced much less heat.

Here is a link to the details:
Do Four 25 Watt Light Bulbs - One 100 Watt light Bulb?

How does this experiment differ from what you described in Myth #1?

Thanks for any insight!



    Bookmark   August 24, 2007 at 11:55AM
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I only read part of it.

Didn't even bother to read it all.

Because it seems to have failed to recognize that light, when absorpted by objects will turn into heat.

The only time light is not converted into heat is when it's energy is used to cause a chemical change or it's converted into some form of kinetic energy and stored away (solar cell charging a battery)

Otherwise, 100 watts will generate 100 watts of heat.


    Bookmark   August 24, 2007 at 1:09PM
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shrubs_n_bulbs(z8/9 UK)

First, myth #1 does not include incandescent bulbs. They are radically less efficient than any plant lighting and so they emit a lot less light than the same power of fluorescent or HID lighting. Put another way, an incandescent light system producing the same light as a fluorescent light system produces a lot more heat.

I wouldn't read too much into a high school (grade school?) science project. The methodology is so full of holes you could use it as a sieve. I'll leave you the fun of tracking down why each and every part of the experiment is horribly flawed ;)

The science is simple: 100W in, 100W out. In the case of all the light sources we are interested in, (ie. NOT incandescent), approximately 30% of the input energy is converted into light. The rest is converted directly to heat either warming up the ballast, warming up the bulb/tube, or being radiated directly as infra-red radiation. So within a few percent, the amount of heat produced by a fluorescent, metal halide, or HPS lamp is simply proportional to the actual power consumed. Note that the nominal power is not always the same as the actual power consumed, so a 1000W metal halide typically consumes over 1,100W of power (including the ballast), while a 32W T8 typically uses about 28W including the ballast. There are variations in the proportion radiated directly (HID lamps tend to radiate more infra-red which makes them "feel" hotter) and the temperature reached by the bulb (which is where the confusion tends to arise), but add it all up and you have to come to the same total. Its called conservation of energy and it is a fundamental rule of the universe.

    Bookmark   August 24, 2007 at 4:32PM
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dcarch and Shrubs,

Thank you both for replying and setting me straight on this topic.

On to further reading of the "Conservation of Energy Principle"...



    Bookmark   August 27, 2007 at 8:45PM
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shrubs_n_bulbs(z8/9 UK)

I was just browsing through all ten myths again. Apart from the unfortunate typo in #4, most still seem fairly sound. But its interesting to see how far LEDs (myth #6) have come in the last two years. Although 90% of the marketing hype about LED grow lights is still extremely misleading and often just plain wrong, LEDs have gone from roughly five times less efficient than a fluorescent tube to almost as efficient in straight light output terms. There are LEDs in existence considerably more efficient than any fluorescent of HID lamp, but I'm not aware of any commercially available that exceed the efficiency of a good fluorescent. Other advantages do make them worth looking at if you are the experimental type and want to try something different in a small space.

    Bookmark   August 28, 2007 at 12:35PM
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Would LED's work for growing orchids on a wall shelf? I have no south facing rooms and would like a small light so I could grow something on a shelf. Just wondered if LED's were getting that efficient.

    Bookmark   September 5, 2007 at 2:01PM
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Nicely done shrubs_n_bulbs. My first fluoro set up was a bank
of 12 twin tube strip fixtures mounted side by side. The bank
is mounted on the ceiling, and my garden consists of a reinforced piece of 1/2 inch plywood, hanging by chains and hooks from the ceiling. The first time I turned on the bank
and stood underneath it, it seemed as if I had just turned on the sun. I was just as if I was standing outside on a sunny
day. A single fluoro is more like a line source, and a large bank of fluoros is like a plate source.

    Bookmark   September 9, 2007 at 1:09AM
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shrubs_n_bulbs(z8/9 UK)

just as if I was standing outside on a sunny day

You won't get much of a tan though :)

    Bookmark   September 9, 2007 at 4:18PM
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You won't get much of a tan though :)

No, for that I would need to mount some U.V. lamps! Not so good for growing. I am planning on overdriving my fixtures 2x
and I have lined the reflector with mylar using 3M double sided tape. Is it a myth that overdriving is inefficient? It seemed the article by zink shows 54 watts through a t8 delivers 1.7 times the light. This efficiency seems just about right. I am planning on using a hefty fan to keep the lamps cool, and even use a toggle switch to be able to turn overdrive on and off.

    Bookmark   September 9, 2007 at 4:30PM
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shrubs_n_bulbs(z8/9 UK)

Is it a myth that overdriving is inefficient?

Fluorescent lights operate most efficiently at a particular power level, and then progressively less efficiently at higher or lower power levels. Tubes are typically manufactured for a nominal power level near the peak of the efficiency curve. High Output (HO) and Very High Output (VHO) are designed to operate at a nominal power level above the most efficient power, these are overdriven lamps by another name. That's worth remembering when you are drooling over a 54W T5HO lamp, a standard 28W T5 is considerably more efficient!

The most efficient power level for a 48" T8 tube is around 35W-40W, and it operates pretty close to maximum efficiency anywhere from about 25W-50W. Note that this type of tube is generally run at 28W on power factor 0.88 ballast, as a "power saving" feature although it is in fact less efficient than running at a higher power. The power saving comes from being a direct replacement in the same fitting for an old 40W tube or a somewhat less old 36W, 34W, or 32W tube.

So a T8 tube running at 54W is really not far from the peak efficiency levels and may actually be more efficient than running at the "out of the box" 28W power level. Certainly less efficient than running at the designed level around 35W-40W though. Some people have made a lot of running with this fact and tried to make out on the basis of a single case that overdriving does not reduce the efficiency of a fluorescent tube. There is also a much referred-to web article by someone who claimed to demonstrate that overdriving was in some cases more efficient and generally not much different from running a standard tube, although the experiments were actually flawed and really only proved that it is incredibly wasteful to run a single tube off a four-tube ballast :)

Overdriving a 40W T12 significantly reduces the efficiency, these tubes are not generally underdriven by design and doubling the applied current takes them way beyond the most efficient part of the power curve. The same applies doubly so when you 3x or 4x overdrive, or if you were to overdrive the already overdriven T5HO tubes. You may be able to track down in the specifications of some manufacturers actual light output curves against the applied current, or lumens/W curves against current or power. You will find that peak efficiency comes at around 400mA, or a little less for thinner tubes. Off the top of my head, I think 400mA is the nominal current for T12 tubes while T8 tubes (ballasts really) in the USA run nominally at 265mA for 32W power and obviously at an even lower current (about 240mA) on a 0.88 ballast. You see that a 2x overdriven T8 could be running at only 480mA while an overdriven T12 would be more like 800mA. One is near peak efficiency, one is way off.

Hope that wasn't too confusing :)

    Bookmark   September 10, 2007 at 1:03PM
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Thanks, I think that really helps to shed a lot of light on this matter! It really makes sense to maximize the light per lamp, without getting into the inefficient range, because the alternative, of just packing in more lamps, leads to inefficiencies of the lamps blocking each others light output, and blocking the reflected light.

    Bookmark   September 10, 2007 at 11:03PM
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khyberkitsune(Redlands, CA)

With the current gen of LED the company I work for is developing, we can expect to outperform HID bulbs in terms of heat and efficiency across any board you point us to. Current tests on lettuces and tropical plants such as hibiscus show very promising results. (We've been at this for 15 years, I joined two years ago, and no, I don't work for SolarOasis.)

We're already past the lumens/watt of HID bulbs. Space is an entirely different issue, on the other hand. Unless we, as of this post, can double the efficiency of LEDs, we're looking at about a 50% increase of needed overhead area to match HID lighting with it's ballast and reflector included, so right now the real issue is space. Our next project will be a flexible LED mesh that can be used to create customizable light domes for smaller plants, like violets or such.

    Bookmark   September 16, 2007 at 3:40AM
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shrubs_n_bulbs(z8/9 UK)

You mention "space". LEDs will always likely require spreading over a greater surface area than a little HID bulb, but they should also be able to be used in quite a thin sheet, possibly less than an inch thick. Provided that such sheets can produce light in excess of 2,000 lumens or so per square foot of LEDs then there shouldn't be a problem. The small size of HID bulbs is generally more of a problem than a convenience in plant lighting, requiring mounting at a distance from the plants with large reflectors.

As always, availability and cost remains an issue. LEDs more efficient than HID lighting have existed for years, but so far as I know are still not available to schmucks like us, and certainly not at any sensible cost. We'd love to known where and when such high performance products are available.

    Bookmark   September 16, 2007 at 10:11AM
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jessicavanderhoff(7 Md)

All lamps of the same wattage do not give off about the same amount of heat. Those compact fluorescent bulbs were developed because they convert a larger percentage of the electricity they use into light, and a smaller percentage into heat. You can feel a clear difference if you touch the bulbs. I thought we were dispelling misinformation!


    Bookmark   April 29, 2008 at 2:45PM
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Jessica, the cfl gives off less heat than an incandescent of the same lumen because it only uses one seventh of the wattage to generate the equivalent brightness, therefore the heat is only one seventh. Remember back in physics, the first law of thermodynamics: Energy is neither created nor destroyed. The light does not just disappear. A tiny part is converted to plant growth, and the rest ends up as heat. It has to. All of the points given in the diary are 100% accurate. A cfl is just more efficient. If you had 100w of cfl it would give the exact same heat as 100w of incandescent, 100w of HID, 100w of LED etc. The difference is the light efficiency, NOT the amount of heat. You would need 700w of incandescent to equal the light of 100w of cfl. I use 22 tubes of straight 48" fluorescent tubes for my grow. At 32 watts per tube, this means 704 watts total are used. My lamps feel barely warm, much cooler than a single 100 watt incandescent lamp even, much cooler than a 400 watt HPS lamp, but the total heat, for my grow is always going to be 704w. This wattage keeps my plants warm, and keeps the room in the cellar where I have my grow nice and warm. If I turned the lights off, and plugged in a 704w electric heater, the room would have the EXACT same temperature with the heater, as with the fluorescent lamps, even though you can feel the heat being given off by a heater, but can't feel anything being given off by my lamps. Cheers, Paul.

    Bookmark   April 30, 2008 at 3:07AM
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Shrubs opines:

"Plants use light at all wavelenghts from near ultraviolet
to near infrared to convert water and carbon dioxide into
sugars. They use red light near 650nm most efficiently,
then blue light near 620nm, then light in between. Some
plants are much less efficient at using green light, some:
use it almost as well as red and blue. All plants have
some capacity to adapt to best utilise the available

(Lermer) The most efficient red light is about 670 nm.
The most efficient blue is about 420 nm. See: McCree.

(Shrubs) Myth # "5. Blue light is for growth, red light is
for flowering."

"Plants will grow under monochromatic red light. Plants
grow faster with a little blue light (with the same total
light power). Plants grow slower under monochromatic blue
light because the same power produces fewer photons.
Approximately 25% of the power in blue light appears to be
best, but probably varies for different circumstances and

(Lermer) Plants grown under monochromatic red will tend to
have excessive stem elongation. Blue has many specialized
functions: production of growth hormones, rooting, and
reduction of stem elongation. These functions can also be
performed by other spectra at high intensity, but less
efficiently. Red produces flowering hormones and is the
most efficient monochromatic light for plant growth.

    Bookmark   April 30, 2008 at 4:07AM
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Shrubs said:

"The small size of HID bulbs is generally more of a
problem than a convenience in plant lighting, requiring
mounting at a distance from the plants with large

(Lermer) Horizontal HID reflectors tend to be not large.
Longer bulbs (like 4' fluorescent) require larger
reflectors per watt compared to HID, and hence block more
natural sunlight and are less convenient to use.

    Bookmark   April 30, 2008 at 2:03PM
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Object 16 said:

"It really makes sense to maximize the light per lamp,
without getting into the inefficient range, because the
alternative, of just packing in more lamps, leads to
inefficiencies of the lamps blocking each others light
output, and blocking the reflected light."

(Lermer) Lamp efficiency tends to drop off in the lower
wattages (except that 1500+ MH is less efficient than
1000w). Also depends on what you mean by "efficiency";
full-spectrum lower wattage lights would tend to be more
efficient for plant growth than non-full-spectrum higher
wattage lights.

Multiple lights do not block each others' light output or
reflected light (not the same thing). On open sided
horizontal reflectors, bulbs may shine unto each other,
but the amount is miniscule. Most horizontal reflectors
are not open on the side. There are both advantages and
disadvantages to open-sided reflectors. Generally, if the
reflectors were at the same height, they would not block
light from each other.

    Bookmark   April 30, 2008 at 2:20PM
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I'm talking about packing 3 1/2 " t12 striplights together. There is a total of 3 " of transverse distance of fluorescent tube per 3 1/2" of total width of fixture, so the lamps get it the way of most of the reflected light. Instead, I now use a 6 1/2 " fixture with 2 t8's, so 2 inches transverse out of 6.5" total width of fixture, so this is only a third of the obstruction compared to the t12 3.5" striplight. Using a light meter, I get similar illumination with far less wattage, which proves my hypothesis to be correct.
You keep repeating the myth that full spectrum is more efficient for growth, but it is not. Please stop repeating this over and over, because the scientific proof is that the fluorescent tube with maximum lumens is always the best for growth, compared to a full spectrum lamp of lower lumens per watt. This thread is supposed to dispel myths, not propagate them!!!

    Bookmark   April 30, 2008 at 6:09PM
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(Lermer) In the "Ceramic Metal Halide Life-Lights" thread,
Object 16 told us about his bank of t-8 fluorescents
totalling 1000w. He measured foot candle readings:

6" = 3000 fc
12" = 3000 fc
18" = 2900 fc

With only two t-8s, I got these readings:
6" = 500 fc
12" = 270 fc
18" = 170 fc

With a 1000w HPS, horizontal reflector with 98% reflective
material, I get these readings:

6" = offscale of meter
12" = offscale of meter
18" = offscale of meter
24" = 5000 fc
30" = 4500 fc
36" = 3300 fc
48" = 2000 fc

Obviously, the HPS has greater penetrating power. I'm
getting 3300 fc at 36". Compare to fluorescents, 2900 fc
at 18". At twice the distance, I get 400 fc more.

When moving the 1000w HPS by a light-mover, the high
intensity is shared over a larger area. Stationary, the
1000w HPS with this horizontal reflector, covers about a
8' x 10' area with 500 fc at the edges. When tracked, this
covers about 14' x 10' area; far more than Object 16's
4'x 6' area using the same watts. Growing plants that are
6"-48" tall, HPS blows away fluorescents; particularly
over a larger horizontal area. Which explains why most
growers do not take fluorescents seriously, except for
cuttings and the first six days or so of veg.

The reason the bank of large number of fluorescents has
less % drop-off, is that there are so many points of
light. A line is an infinite series of points. If you
covered all of a fluorescent tube, except for one point,
the inverse square law for point source of light would
apply. As the distance increases from the point, the cubic
area filled by light increases exponentially. Hence the
rapid drop-off. With many points of light placed in close
proximity, the light footprints overlap. With
fluorescents, each tube has many points of light, and with
several tubes, there are even more points of light. As the
meter goes further from the lights, it is picking up light
from many sources.

Rather than using 1000w hps, I recommend instead multiple
400w Ceramic Metal Halide. This gives these advantages:

1. Using multiple lower-wattage lights results in multiple
points/angles of light. Having more angles of light
results in fewer shadows on leaf surface.

2. Multiple points of light creates several light
footprints, and less drop-off with distance.

3. CMH has far better spectrum than HPS.

I heard something yesterday. In the UK, buyers of HID
lights must obtain a license from the police. Buyers of
fluorescent lights aren't required to do so. If true, this
creates a motivation on the part of retailers to push
fluorescent lights as opposed to HID. Quo bono?

BTW, Shrubs, tell us again your theory that glass barriers
on vented hoods result in only about 1% light loss.

    Bookmark   May 1, 2008 at 9:57PM
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"Object 16"--did you actually read the article you posted?
The article does not agree that "...plants have very
EQUALLY WELL.." In fact, it showed two charts that refute
what you claim.

Here's another, one of many scientific references on the
"The part of the spectrum that humans can see, called
visible light (380 to 770 nanometer wavelengths) is
measured in lumens."

(Lermer) The extremes are UV and infra-red. These portions
of the spectrum have specialized functions, but by
themselves would result in very stunted plant growth.
If "Lumens" were the only factor to consider, the LPS (Low
Pressure Sodium) at 180 lumens per watt would be the best.

"A balance of light across the PAR range is considered to
be preferable, however there is increasing research being
conducted in the area of light spectrum modification for
improved plant growth.
The rate of photosynthesis peaks in most plants around 450
nm (blue light) and 650 nm (red light).
A clear film will transmit the most amount of light. Blue
and green coloured plastics will transmit a lot of the
light in the blue to blue-green wavelengths, but cut out
much of the light in the red wavelengths. From the diagram
above looking at PAR, it can be seen that red light is the
most efficient waveband for plant growth. A blue plastic
is likely to produce a slower growing, shorter, tougher

(Lermer) Generally plants prefer a full spectrum light,
with special emphasis on the blue (at about 420nm) and red
(about 670nm). Blue helps rooting, shortens stem
elongation, and induces growth hormones. Red is the single
most efficient spectrum for over-all plant growth, and
induces bloom hormones. That is why LED promoters favor a
combination of red and blue LEDs. However, other portions
of the spectrum have other functions, and there is an
issue of spikey vs. continuous spectrum.

For example, conventional Metal Halide, if used as the
sole source of light in the bloom phase, would result in
less bud relative to leaf and take longer to finish. If
standard HPS were the sole source of light in the
vegetative phase, the tendency would be to stem elongation
and poor rooting. So if the choice were limited to HPS and
MH, I would prefer a mixture, or (with some strains) the
super-HPS like Hortilux that has higher intensity
(including blue). But our choices are not limited to those
two--and I prefer either Pulse-Start Metal Halide (PSMH)
or Ceramic Metal Halide (CMH).

That being said, high intensity can compensate for
deficiencies in light spectrum (at a cost to efficiency),
and different strains may respond differently (perhaps
they have been genetically adapted). Roses happen to
prefer HPS light. HPS works best in plants over 2' or so.
And so on.

Roughly speaking, you want a light source that is high in
lumens and also high in CRI.

There are other factors, such as those relating to
distribution of light. Generally you want multiple points
of light (resulting in more angles of light and fewer
shadows). This can be accomplished by a light mover and/or
many points of light.

4' fluorescents are a line source of light. A line is a
series of an infinite number of points. Cover up all but
one point of light on a linear fluorescent, and the
inverse square law will apply. Uncover the light, and add
many more fluorescent tubes, and you will have many more
points of light. But that is not the only factor, so
unlike Shrubs, I don't favor fluorescents except in
special applications. For various reasons, such as the
greater penetration of high-intensity lights.

What you want is a balance of features, between spectrum
and intensity, and distribution and penetration.

    Bookmark   May 8, 2008 at 10:15PM
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for sure different plants may have slightly different action spectrum for photosynthesis, but not the sharp bimodal peaks that lamp manufacturers want you to believe.

Action spectrum of photosynthesis of the green alga Chlorella pyrenoidosa (quantum yield of oxygen production as a function of wave length). Please notice the drastic drop in the red part of the spectrum (R. EMERSON and LEWIS, 1943).

    Bookmark   May 8, 2008 at 11:09PM
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I can't find Object16's earlier link to the complete
article. In the link you now post, the sharp drop-off is
really in the infra-red, not red. What labels we apply to
the various spectra have a subject element. This 1943
article does not conform to the verbal color labels that
most other plant growth curves use.

Most lamp manufacturers don't even talk about plant growth
curves. That is because about 97% of their bulbs are sold
for industrial/commercial purposes. Of those who do talk
about plant growth curves, some emphasize full spectrum,
some emphasize red and blue. Both are right; and to
repeat, intensity and distribution are also important.

    Bookmark   May 9, 2008 at 1:40AM
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mistascott(7A VA)

This should be the bible of the Growing Under Lights section. Only change is that T5HO are becoming more and more affordable as an alternative to metal halide these days.

    Bookmark   February 22, 2012 at 4:00AM
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" For the same money, a good HID setup would be better and for less money you can get the same light from standard fluorescents. "

I do not understand this at all? Let me break it down.

"For the same money, a good HID setup would be better "


"and for less money you can get the same light from standard fluorescents"

Did I read this wrong???????

HPS= the most highest lumens per watt. HID is more economical then T5's.


    Bookmark   February 24, 2012 at 11:33PM
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chilliwin(EU DK 7)

Still I keep looking any useful information for container gardening and I found this thread, I consider it is useful for me.

Thank you all the forum members who were participated on this discussion and making this forum more valuable. Due to lack of knowledge of gardening I cannot share anything else here. I do not have any grow light, I use table florescent lamps for my baby plants :( After I read this thread I started to use Phillips Energy Lights HF3308. I am expecting positive result.


    Bookmark   December 14, 2012 at 3:48PM
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I am running a 4' x 4' garden with T8. Total of 14 lamps. The lamps are overdriven x 2, the usual way (4 lamp ballast runs 2 lamps) I am using 5000K Litetronics T8 lamps, which have extra high lumen rating, and cost 3.50 per lamp. I measure light with a PAR meter. At 6" from the lamps, the PAR is 500 - which will easily grow any high light requirement plant. My Daylight-integral (total photons) for a 16 hour day is 30, and a DLI of 20 is actually sufficient to grow high quality tomato, so this is actually excessive light, by 50%. The total power for this garden is roughly 700W. Right now, I am growing peppers, tomato, and rosa rugosa Hansa. The stems on the plants are very thick and sturdy, the internodes are extremely close, and the roses are sending out additional canes from almost all of the bud-eyes. This is using standard shoplights, reballasted with used easy to get cheap ballasts from ebay. The complete set-up costs around $200, which is completely cheaper than any other available - and totally superior to HID in terms of uniform light coverage, and being able to grow the plant to within an inch of the lights. totally superior to LED due to cost and
I am anticipating that I can grow 2-3 foot plants with this garden. - paul mozarowski.

    Bookmark   December 15, 2012 at 10:37AM
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