For reference: Footcandle measurements for a two-bulb T8 fixture

sonnypippoJanuary 12, 2005

Here are the approximate footcandle measurements for a two-bulb T8 fixture (48", 32W, 6500K):

0" 4000 fc

3" 1500 fc

6" 1000 fc

9" 700 fc

12" 500 fc

Hope this helps.

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john_z(Z 7b Ga.)

Hi! Thats quite an impressive colletion you have listed in your home page! I'm an orchid fever afflicted human too, and love it. However, to put it in a nutshell, foot-candle measurement (from photometers) measure the wavelengths that are irrelevant to plants; they only measure what the human eye and brain perceive as "bright". This was intended for the area of the electromagnetic spectrum from 400 to 500 nanometers to match the human eye/brain sensitivity curve. Plants reflect these wavelenghts off and cannot use them for photosynthesis. Under artificial light, "bright" to the human retina is "dark" to the plants who don't have these senses - you know, pupils that constrict and adjust, etc. Most tubes are designed to meet the visual needs that the plant kingdom doesn't have.

Plants use the wavelengths perceived to use as "violet, blue, and red" - which actually appear as dim to the human eye no matter how much wattage. A spectroradiameter is the appropriate tool to use for photosynthesis concerns - not a photometer. They measure how much "phytosynthetically active radiation" or PAR is emitted from a light source, and it measures "PPFD" or photosythetic photon flux density irrelevant to us humans.

I wish I could share more, but time won't permit it right now (personal life stuff). Perhaps those who are knowledgeable in PAR and PPFD can fill in the blanks. But forget foot-candles measurements.

I appreciate and respect your concern for your leafy friends, as I do with mine! Keep well!

PS: To match what the pigments of plants actually absorb and utilise for photosynthesis, I use GE Chroma 75 and Sylvania Gro Lux Aquarium (aka Gro lux GRO) fluorescent tubes that are t-12, in a 1:1 combo).


    Bookmark   January 12, 2005 at 8:44PM
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hi John_Z- thanks for taking the time to reply and for providing some excellent insight. i am familiar with PAR although not very well-versed in the subject matter. i understand and agree with your statements, but is there a readily accessible way for us laypeople to measure PAR? seems like the best we can do is measure the brightness of daylight-simulating bulbs (e.g. 6500K) and hope that correlates with the historically documented light recommendations of the plants (assuming those recommendations are based on natural light).

    Bookmark   January 12, 2005 at 8:59PM
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john_z(Z 7b Ga.)

Hey Sonnypippo! I don't know if you will get this message because I've been having staying connnected problems with the Internet. Also, the "preview message" page is blank, so I cannot forward the posting. Anyway, I've decided to use you e-mail box. Also, with the PAR acronym , I mistyped "photosynthetically active radiation" and wrote "phytosynthetically". Please don't call the botany police. Ha. They're viscious.

I want to suggest graphs for you concerning "human eye sensitivity curve", "absorption spectra of chlorophyll a and b & carotenoids", and "spectral power distrubution graphs of Sylvania fluorescent tubes". You can type these keywords into your browser to get a jump start, while I check to see if the old Web sites I printed off are still in existence. Graphs make it so much clearer. I'll get back with you regarding measuring PAR via e-mail or whatever.


    Bookmark   January 16, 2005 at 7:42PM
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Which meter did you use for those measurements?

    Bookmark   January 17, 2005 at 11:37AM
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i used a digital light meter which measures light on a 2" diameter disc. the output scale is lumens or footcandles. accuracy is +/- 5%

Overall range for footcandles is 0-5,000 with subranges of 0-200, 0-2,000, and 0-5,000. Overall range for lux is 0-50,000 with subranges of 0-2,000, 0-20,000, and 0-50,000. Note: Footcandle=one lumen per square foot; lux=one lumen per square meter.

    Bookmark   January 17, 2005 at 11:53AM
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OK, thanks, I have a similar meter but have never gotten a reading as high as 4000fc even with the probe pressed directly against the lamps. Maybe the lumen output of your lamps is higher than anything I've been able to find...

    Bookmark   January 17, 2005 at 7:40PM
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john_z(Z 7b Ga.)

Hi Sonny! I think I got my disconnection problems straightened out, but we'll see.

The wavelengths that plant pigments need are shown on a site easier to reach by keying in, "Photochem CAD Spectra by Category", rather than the long address. Scroll down until you get to the "Tetraphyrrole" group, then click "chlorophyll a diethyl ether, and also "chlorophyll b". Beta-carotene is another significant plant pigment for absorbing PAR. All of these pigments (plus others) need the wavelengths/colours outside the range measured by photometers.

If these are what plant pigments absorb, then the question that follow is, "what is the spectra that lamps emit to meet the need"? Spectral power distribution graphs reveal this if you don't have a spectroradiameter and graphing devices. Looking at them you will see that the Kelvin ratings are an eye thing, not a photosynthesis concern. How much green, yellow-green, and yellow wavelengths that light emits(reflected off leaf pigments) are what photometers measure. It's measurement are limited to that area of the spectrum.

Examples of SPD graphs of lamps are found by keying in "spectral power distribution graphs of Sylvania fluorescent lamps". Regardless of whether you are using GE or whatever, this is good orientation to spectral output of lamps/tubes. Many photobiologists will disagree of what Sylvania wrote-up on their first few pages, as do I.

If the graphs, nanometers, or whatever are confusing - I'll be happy to explain. I was mind-boggled at first too years ago, being concerned with "bright" nonsense. I think that understanding absorption spectra and lamp SPDs are important before you consider buying any other light measuring equipment.

I'll be going to a tropical plant conference in FT. Lauderdale, Fla. this week. One of the workshops is on light measurement, so I'll let you know what I have learned. If they talk in "foot-candle" measurement, I will surely return to my hotel room, get drunk and cry in my beer.

Cheers! John_Z

    Bookmark   January 17, 2005 at 10:17PM
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Way to go John Z!

Now take a look at what wavelengths are emitted by the phosphors. First, though, figure out which lines on the SPD are actually the mercury emission lines. They are listed at the text beginning of the Sylvania document. They are rather insignificant in effect, but are always plotted wider than they actually are.(254, 313, 365, 405, 436, 546, 578nm)

Now, what is left are the peaks that belong to the phosphors. In the tricolor phosphors, there is wide blue peak, a narrow green peak and a narrow red-orange one, along with a few other lesser ones. THESE ARE THE SAME THREE PHOSPHOR PEAKS IN THE SAME LOCATION IN ALL TRICOLOR PHOSPHOR LAMPS. (450,543,611nm)

The BLUE peak(450nm) is a great match to the plants absorption spectrum(430,453nm). What people DON'T KNOW is that the RED peak(611nm) completely misses the RED band(642,662nm) where the chlorophyll absorbs at.

So whenever you opt for a warmer TRICOLOR PHOSPHOR bulb, you are just wasting worthless RED at the expense of useful BLUE.

The HALOPHOSPHOR, or older style 'greenish' phosphor has a broad peak right on the money in the blue, but then maxed out in the orange and dropped off, just clipping the red. They improved the drab color of the old HALOPHOSPHORS by adding a RED phosphor. They called these DELUXE bulbs.

Unfortunately, by some physics twist, the color-improved DELUXE versions are not as bright as the non-deluxe halophosphors. BUT GET THIS! The RED phosphor used to create the DELUXE bulbs has a peak at 626nm, and is quite WIDE! This is the same phosphor used on many of the DELUXE or COATED versions of a lot of HID lamps.

The spectrum created by this RED phosphor[(Sr,Mg)3(PO4)2:Sn], which peaks at 626nm, provides a useful amount of red, as it is wide and doesn't drop off to half-of-peak until 686nm.

Unfortunately, none of the major lamp manufacturers ever really addressed the poor RED in their plant lights, with the exception of one bulb by Sylvania - the "Standard" GRO-LUX. Sylvania actually went to the trouble to find a RED phosphor, and complemented that with the standard BLUE. creating a kinda-dim, purplish light, which near-perfectly matches the plants chlorophyll absorption.

But as for the beta-carotene, I haven't read many articles where people mention supplying the light to feed these lesser absorption areas, and actually describing their results. I wonder how much this might affect the color of the plant. Like could you beef up the beta-carotine and get an oranger leaf.

I didn't mean to write this much at this time. I started to look up a paste in some of the numbers. Over the last few years, I acquired lists of all of the standard phosphors used in the lamp industry, along with a collection of documents describing the history of their development.

Once you've found out this much, boy can you spot bad hype about bulbs.

Up too late,

    Bookmark   January 18, 2005 at 1:27AM
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john_z(Z 7b Ga.)

Hey Zink! Good to hear from you as you have provided the best info in this forum I've seen yet. I also agree with the technical info you just provided. Like I said once before on another thread, the SPD graphs use lots of artistic license by connnecting peaks with wavy lines that may cover only a few nanometers sometimes. It's like playing "connect the dots". At the same time, the absorption spectra of plant pigments are modified by the solutions they are dissolved in - so we only get a balllpark figure. But at least this gets people on the clue train as to the proportion of wavelengths that are in a lamps's overall spectra. (I have to admit that "cool white" and "warm white" jargon cracks me up. And Kelvin ratings only suggest that the lamp's SPD be carefully looked at).

The combination of tubes I'm using now is the best yet in terms of PAR. I have notable foliage improvement that is dense and lush, as well as flower development in just the past 4 months. They are less than 18" max beneath 960 watts on Intermetro shelves. Determined to use the appropriate spectra as my first consideration, I don't even use the most energy efficient ballasts or T-8s because the lamps I want aren't in sync with them. I'm still with twelve 4' long magnetic ballasts that have their list of deficits. But I get the results: plants that have health and longevity.

I don't own a spectroradiometer yet, but I will definitely choose a LiCor product when I get one. The scientific community seems to favour it internationally. Anyone considering buying spectroradiameter equipment should definately understand the keywords I mentioned then talk with a knowledgeable sales rep about their needs. (To find a good one, I must just nag them unmercifully with technical terms until they turn me over to someone who knows their products. LOL). Have them compare less expensive models with pricey ones. Above all, know your intentions. I wouldn't want one that gives an average reading across the 400-700 nm range. So when I invest in one, I'm going all out, baby. I will get what will put a lamp's SPD and PPFD on a graph so I know what's going on over time. (And by the way, the 500 - 600 nm green and yellow region is part of PAR becauuse some bacteria depend on it for photosynthesis using chlorophyll c, d, and e. The chloroplasts that produce chlorophyll in higher plants were once a free-living bacterium. It has its own DNA different from the rest of the plant - proof of evolution).

Until now I've beeen relying on the the SPD graphs and plant feedback regarding PPFD. Being familiar with photomorphogenesis in plants, In know what to look for in each species as far as physical changes go for the better or worse. Plants are our best feedback system and educators if you know their language.

    Bookmark   January 18, 2005 at 8:16PM
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zink and john- i appreciate your thoughtful replies in this discussion.

so john, what kind of bulbs do you use??

    Bookmark   January 18, 2005 at 8:24PM
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john_z(Z 7b Ga.)

Hi! I'm using GE Chroma 75 (F40T12/C75) that I got from Check for updated prices or see if you can get them cheaper. 5 months ago this was the best deal I could find at a little over $5/tube - but you must buy a minimum of 12 tubes.

The Sylvania Gro Lux Aquarium (F40/GRO/AQ) is a T-12 also. I got them from Also a minimum of 12 at $82.80 plue $5 shipping.

I'll be leaving for Florida tomorrow and will not have my laptop with me. I'll check back with you all next week.

Until then, keep well. John_Z.

    Bookmark   January 19, 2005 at 8:01PM
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john_z(Z 7b Ga.)

OOPS! I spotted an error I made while re-reading this thread. On my first reply I stated that a photometer measures the 400-500 nm area of the spectrum. That's incorrect. It's focus is on the 500-600 nm range of green and green-yellow. Sorry about that.

John Z.

    Bookmark   January 30, 2005 at 2:55PM
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how about light meters that have different settings to correct for different types of lights? does that mean the different settings are reading different wavelength ranges or what?

    Bookmark   January 30, 2005 at 5:12PM
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john_z(Z 7b Ga.)

Sonny, there are photometers with different types of silicon photodiodes and filters that can focus on specific regions of the 400-500 nm range (with the exclusion of other ranges) for specific types of lamps - from what I've read a few years ago. This doesn't function like a spectroradiameter, which includes the 350 - 750 nm range, and even beyond. I would get the photometer info from the manufacturer as to what nanometer ranges are possible with the different diodes and filters. But as far as I know, they eliminate ranges while focusing on others. Does that make sense?


    Bookmark   January 30, 2005 at 8:59PM
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john_z(Z 7b Ga.)

Also, I'm curious what T8 fluorescent lamps you are using. I'll see if I can find the SPD graph for it. Then we can do a comparison between what the plant pigments absorb for photosynthesis and what wavelengths the T8's are emitting to meet that need. All of the SPD graphs that I've looked at for them are very high in essentially useless green & yellow green to meet our visual needs. Photometer readings also vary by 45% depending on the model and what type of diode is in it.


    Bookmark   January 31, 2005 at 7:47PM
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john- if you can find that info, that would be great. the T8 bulbs are the philips daylight deluxe model, 6500K.

    Bookmark   January 31, 2005 at 7:54PM
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john_z(Z 7b Ga.)

Sonny, after a 50-minute search on the Web for your lamp SPD and finding nothing, I contacted a friend in the UK who had the info. She said your tubes have the same SPD as Sylvania Daylight Deluxe (pg. 5 of "Spectral Power Distribution of Sylvania Fluorescent Lamps").

While you are on that Web site, check the 65 K lamps on pages 9 & 14 as examples. Here you will see there are significant differences in the amount of G & Y-G wavelengths in 65K lamps. The higher their "normalized spectral power-W/nm per 1,000 lm" - the higher your foot-candle reading will be. So what! This is irrelevant to photosynthesis in higher plants. As for other wavelengths/colours, you will also see very different proportions of ROY & BV in 65K lamps.

Understand that Kelvin lamp ratings of 50 - 65K are NOT synonomous with natural daylight in terms of wavelength emissions. Natural daylight put on an SPD graph looks nothing like 50 or 65K lamp graphs.

R-G-B make up the primary colours of white light, and their proper portions are needed for good colour rendering on the CRI (index). But the pigments absorbed by our retina are not the same as in plants. They only need the V,B, and R emissions - which would appear as a mauve colour to us; the colour renering or CRI would be lousy, and the "intensity" relatively dim.

Lastly, compare the SPD of Sylvania Gro Lux GRO on pg. 10 of that site. It more closely matches the absorption spectra of chlorophyll a & b, plus the carotinoids. Again, it would be more accurate if this tube had twice the amount of V & B as it does OR & R. But that is why I combine it with Ge Chroma 75.

PS I have 26 Phalaenopsis, Neofinetia falcata, one Cymbidium, and numerous other plants (Bromeliads) in blooms under this spectra. It's the best combo yet for me; yet the foot-candle measurements would say "NO", "IMPOSSIBLE"!!!


    Bookmark   February 1, 2005 at 7:58PM
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john- thanks for the follow-up. what is the wattage and/or lumen output of your current setup?

under my T8 tubes, i found that some plants which are traditionally considered "medium to high light" become dark red and wilty, while other plants that are considered "low light" don't seem to mind at all. any theories as to why that might be?

    Bookmark   February 2, 2005 at 9:10AM
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shrubs_n_bulbs(z8/9 UK)

Zink, your discussion about the improved spectral distribution in the deluxe-type fluorescents is of course correct, but it ignores the decreased efficiency of this type of bulb. The total light emitted per watt is less and the light emitted at the relevant wavelengths (as measured by PAR) is little different from the regular equivalent. Even the Gro-Lux suffers from this problem, with reduced efficiency resulting in a similar PAR figure. Of course, even the definition of PAR is disputed. It is generally calculated assuming a flat distribution photosynthetic spectrum from about 300nm to 700nm. Nothings perfect, I guess.

I've linked to a very good article that many of you have probably seen before. This article has gone to the trouble of weighting the spectral output of many light sources against a photsynthetis action spectrum (called PUR), so it approximates how much "growing light" is given out. On this rating, the Gro-Lux and the deluxe types start to look a bit better, but not radically better than the equivalent regular bulb. It might still be more cost-efficient to go with the simple old adage of getting as much light as possible and not worrying too much about the exact spectrum.

Here is a link that might be useful: Good article

    Bookmark   February 2, 2005 at 5:07PM
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shrubs- you raise a good point that some of my earlier posts were sort of alluding to... that is, with all this research (albeit from a rather limited pool of available information) and fancy equipment, if we find that there is a bulb that is marginally better at growing plants than the rest, is the added time, effort, and cost involved to arrive at this conclusion really worth the benefit?

    Bookmark   February 2, 2005 at 7:01PM
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shrubs_n_bulbs(z8/9 UK)

From a strict cost-benefit calculation, I'm sure that getting a basic fluorescent bulb that is sufficiently powerful is the way to go. Remember that for our usage, the bulbs should really be replaced after about 6 months, which means that the bulbs can end up costing a lot more than the electricity. If you don't do this, you need to pump the power up even more. Using the PUR efficiency rating, the very best fluorescents come in at about 0.7 while the very worst come in at about 0.4. Do your own costing, but I think the price difference can be a lot more than that. But look carefully and some of those bulbs are not the most expensive, although they can be hard to find.

Personally, I am using standard compact fluorescents (2700K and 6500K) that can be passed onto normal domestic use after they have done their stint in the grow box. In strict lumen terms I have a lot of light available for a small area, possibly not the perfect spectrum, but the bulbs are very cheap. Actually, some of the bulbs (Sylania Dulux 2700K) were free since the government here is subsidising them at the moment to encourage more people to switch - now that's cost-effective :)

Maybe I'll come back and recant in favour of special GRO LUX or HID tubes after all my cacti die ;)

    Bookmark   February 2, 2005 at 8:06PM
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Shrubs n Bulbs,

Where do you get the PAR information? I understand what PAR represents, but I never find PAR data listed from any of the manufacturers. Individuals seem to be publishing PAR numbers, and people seem to be quoting those individuals, but I never find where the data actually came from. I'm sure some of you will enlighten me on this.

My method of evaluating a light source, I will present here for critical review:
To begin with, I collected well-drawn graphs, from research, on measured responses to light by chlorophylls and pigments. I that was done to produce those graphs, looking for data I might want to put in some of my files. I download, or bookmark, my info, so I can wade through it later, re-read the research and get a fuller sense of what I am learning.

I have researched the history of the development of ALL of the phosphors used in the lighting industry. I have amassed a collection of technical papers on phosphors, their physics and their common and not-so-common usages. I used spreadsheets to chart every phosphor used by the 3 major lamp manufacturers, their wavelengths and bandwidths. I even charted, and matched, every chemical compound related to the fluorescence process, which Philips has listed on their Hazardous Material Data sheets, for the lamps Philips produces.

I know what wavelengths are emitted by the mercury, and which ones are absorbed and re-emitted at lower energies by the phosphor compounds. Unfortunately, many insignificant mercury emissions are always shown on SPDs, and the are always drawn too wide, misrepresenting their influence. By the way, phosphors do not contain phosphorous. It is because they phosphoresce when stimulated by higher energy photons.

I then match that information to SPD plots. I have collected hundreds of SPD's, for every lamp, from every manufacturer who has them available. I ignore SPDs from retail sales sites. They are often so creative that they seriously mis-inform.

By this process, I have learned what every peak on an SPD represents, and which phosphors are represented on the graph. All manufacturers (the major ones, mostly) have a lamp product which almost exactly matches the composition of itÂs competitorÂs lamps. Aside from a few specialty lamps (GroLux, for instance), every lamp shares the same phosphor mix with itÂs direct rival lamp from a competitor (Chroma50, Colortone50, and Design50 for example).

I can now look at a properly drawn SPD, know what type of lamp it is, and whether or not it will be putting out useful light. It is a no-brainer for me to then judge the usability of 2 different lamps, by comparing their SPDs. Using my Wavelength Comparison method, I found I could take the watts-per-lumen quantity shown at the crucial wavelengths, and multiply that number by the overall lamp watts. I have done this for several lamps, and can ABSOLUTELY tell if one lamp will put out as much useable light, per watt, than another.

Many of you here have downloaded SylvaniaÂs PDF brochure where they show the SPDÂs of their fluorescent lamps. At the end of the brochure they have a list of color ranges, and the relative strength of each lampÂs emission in that range, in Watts-per-1000 lumens. They show, and I have calculated, that the GroLux has and outstanding output at the desired wavelengths, relative to the watts used.

If anyone would want to see it, I can probably post an Excel spreadsheet full of the various phosphors and their parameters on the same site as those OverDrive diagrams. It shows just what phosphors produce what colors, and really explains SPD peaks. If you are really interested in some basics, you must Google these two PDFÂs and download them. YouÂll get the basic halophosphor/triphosphor history and more!

Also, here are a few significant wavelengths. The only phosphors I included here are the tri-color phosphors, the most common ones used today. If this conversation gets serious, I may have to post the sites of a whole mess more of lamp information than this.

254 UV Spike (line)
313 UV Spike (line)
365 UV Spike (line)
405 UV Spike (line)
430 Chlorophyll A
436 UV Spike (line)
445 Eye / Cone
450 Tri-color phosphor (wide band)
453 Chlorophyll B
535 Eye / Cone
544 Tri-color phosphor (tall line)
546 UV Spike (line)
575 Eye / Cone
578 UV Spike (line)
611 Tri-color phosphor (tall line)
642 Chlorophyll B
662 Chlorophyll A

One final note:
The American Association for the Advancement of Science has a research release site, called EurekAlert, where they post new research from all sectors of academia and industry. This is an excellent source for new discoveries. I check it out every day. You can search them for recent and new research.

One final, final note:
John Z, you explain it well. I always thought a good combo would be the GroLux/standard and a Chroma75, or 50. I believe that Sam used the GroLux and the Chroma50.

One final, final, last note, to Shrubs n Bulbs:
I was very familiar with the Ivo Busko website which you posted. I first read his article over a year ago, and found several other sites mirroring it. I re-read that page a couple of months ago and found several glaring errors. I will read it again, and try to find the mistakes of which I speak. I remember he was passionate about his subject, and he researched a lot of stuff. Where does he get his PAR numbers?

I put a light harvest antenna on my TV, but it only wantÂs to work on Sunday

    Bookmark   February 3, 2005 at 1:36AM
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shrubs_n_bulbs(z8/9 UK)

Zink, the PAR numbers come basically from that one article, and a few other scattered sources (which are probably derived from the same place!). I'm pretty sure he calcuated them directly, in the same way as the PUR numbers, which is by averaging the responses at the various wavelengths. A lot of the original data is now not available, at least not at the same URLs. At least he goes to the trouble of defining the metrics that he uses.

I don't disagree with any of your comments about the phosphor responses, they are clearly backed up by the data. Its just that in my case I find it more cost-effective to buy a more powerful bulb for a lot less money. Depends partly on your bulb replacement rate. Lumen maintenance curves are easy enough to find, but I have seen very little about the spectral response changes over time, except that they "get worse" from a CRI point of view which isn't very helpful to us. Which phosphors degrade fastest? Probably the ones we want the most of!

One thing that is very hard to quantify is the actual photsynthetic response to the light. Simply looking at the two chlorophyll peaks, and ignoring the gap in between because the chlorophyll is only 20% as efficient there, is not good enough. The overall photsynthesis in an actual leaf is generally measured by CO2 metabolism. This is the spectral response we are interested in, and it is made up from the combination of the various chlorophyll types in the leaf, as well as several other "helper" pigments like the carotenoids, and the actual cell structure in the leaf which can reflect light several times so that the chlorophyll "gets several goes at it". The net result is that a leaf in real life is far more efficient at using light away from the two big chlorophyll peaks. Iv'e linked to one paper showing this, there are quite a few for different species. Note that the lowest efficiency in the green dip is 60% of the highest efficiency in the red peak. I'm sure you can dig out more papers, the spectrum is a little different in every species.

Also, leaves can only use light up to a certain intensity, varies widely by species, and anything beyond that is just a waste. I wonder if this isn't the source of some people's poor results with the original GRO LUX with very big narrow spikes?

I guess at the end of the day, a strong healthy plant is the true test. So far, so good. Fingers crossed. Touch wood :)

P.S. I've been digging into this recently because of prompting about LEDs. Identifying the best spectrum to use is of paramount importance there because you have so much freedom to mix and match your own spectrum, albeit mostly with a few narrow bands.

    Bookmark   February 3, 2005 at 7:05AM
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john_z(Z 7b Ga.)

Zink, briefly stated - you defend your position quite well! I would love to know the sources for SPD graphs and shun the artistic license used for commercial purposes, being clairvoyantly-impaired. I'm only a "semi-fool" at the mercy of their semi-scientific hype - particularly those targeting "plant lovers". Although the SPD graphs provided are indeed better than nothing, what they give is still "throw the little doggie a bone" - even to those of us not scientifically illiterate.

I'm not concerned about cheaper deals on lamps if it means my intention to provide the best PAR and PPFD for the species under my care are compromised. (ALL compact fluorescent lamps to me are out of the question). The results from moving from GE Chroma 50 to 75 are very noteworthy (in combo with the GRO-LUX) - but my interest is also in the health, appearance, and LONGEVITY of my plants. I'll leave the "throw away and replace" mentality to the decorators and sales personnel of the Interiorscape industry.

Sonny, see what a mess you've started? Just kidding! To answer your question, my 48" GE Chroma 75 lamps are 40 watt and the initial lumens are 1,950; design lumens were not listed. The Osram-Sylvania GRO-LUX Aquarium (same as GRO) is also 48", 40 watt with initial lumens at 1,200. Combined 1:1, I use 4 lamps per growing shelf that measure 48"L and 18" deep. In my years of experience, this is the minimum number lamps that should be used without supplemental natural daylight for even shade lovers.

Without knowing the species and all the environmental and cultural factors, I can only make some suggestions as to why you are seeing red and wilting leaves. It could be any one of these causes or a combination of them. The red pigment is anthocyanin, which is only present in the leaves of some species and is not involved in photosynthesis. It becomes visible when the chloroplasts die that produce the green chlorophyll masking it. Why are chloroplasts dying?

Possibilities include: plants being in insufficient light (PAR wavelengths and/or PPFD) putting the plant at "compensation point". Conversely, plants may be placed above "light saturation point" and the chloroplasts are being zapped by too many photons, especially by the stronger violet & blue wavelengths. (I doubt this is true with the tubes you are using). This can include plants that have not acclimated to the amount of fluorescent light that may normally be okay for them if they have been in a darker position for a week to several weeks beforehand (like at the nursery)! The chloroplasts have to reorient themselves from a horizontal position to capture more PAR to a somewhat vertical position slowly so they are not cooked by too high a PPFD. Don't be fooled by light recommendations for an entire genus - say, Phalaenopsis. Many species and cultivars prove they require more OR possibly less PPFD (number of usable photons) than what is often suggested.

I don't want to start an orchid thread digression on a light forum, but will mention this: the plant can be dehydrating from insufficient watering, or experiencing root deterioration from over-water or poor drainage, and/or possibly too high a fertilizer rate. Many of the "Orchid Mixes" being sold in the U.S.are not mixes at all - just fir bark. It's good for drainage and aeration, but has not moisture holding capacity.

Back to light: There is no standard in the horticulture industry designating "low", "medium", "high" or "bright" light in ft-c or Umol/s-1/m-2 of PPFD. "Bright" can include full sun (6+ hours of unobstructed sunlight), part sun (3-5 hours approx.), filtered afternoon sun, and even "bright" shade. I avoid using "low", "bright" terms because they are ambiguous and subjective. Successfully acclimating full sun plants to even fluorescent lamps is very species dependent. My Adenium obesum produces lots of foliage and flowers in the winter months on my growing shelves, yet other full sun plants are only "okay" during that time, waiting for the yard in spring. Shade lovers may need to be placed in the centre of the tubes where PPFD is the the strongest, or towards the ends according to species or cultivar - and elevated at varying heights.

    Bookmark   February 3, 2005 at 9:20PM
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shrubs_n_bulbs(z8/9 UK)

The Chroma 75 SPD looks almost identical to the Sylvania Daylight Deluxe 6500K. Any comments?

Have you guys seen this page? Actual spectra rather than the manufacturers graphs.

    Bookmark   February 4, 2005 at 8:49AM
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john_z(Z 7b Ga.)

Shrubs n Bulbs, what is the Web site that you found the Chroma 75 SPD on? I printed it off the PDF a while back, but now I cannot find which exact Philips site it is on. How to access their specific info continues to drive me crazy.

I did see the site you included about a year ago, and I think that Nguyen's 2nd & 3rd paragraphs says it all. He is not using a spectroradiameter, but rather instruments that are designed for human perception and interpretation of colour.
However, colour is a subjective experience that does not exist outside of the brain's interpretation of wavelengths. From species to species, perception and interpretion of wavelengths (through the brain via the retina) varies considerably . In fact, your right and left eye may not agree. But on the topic of perception,reptiles can see wavelenghts such as violet and ultra-violet a (UV-A) that are invisible to us, but cats see black, white, and gray. (It reminds me of the song: "Cold-hearted all that rule the night removes the colours from our site. Red is gray and yellow - white. But who's to say which one is right, and which is an illusion." The Moody Blues?). I just had to throw that in. LOL.

If anyone is interested, there is a human eye sensitivity chart on Scroll down to find it. It could also be called the "photometer sensitivity curve", but note that the absorption spectra of the major plant pigments exists outside this curve so dim to us. That was my entire point about photometers and ft-c measurements. Plants don't have eyes or brains.

Little known to most people on this forum is that foot-candle measurements in the U.S. conflict with the accepted international definition. Are American scientists using birthday candles, pillar candles, taper candles, lead based wicks, etc.? Not very scientific is it?

    Bookmark   February 4, 2005 at 9:44PM
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shrubs- THe Daylight Deluxe, to my knowledge, is a Philips product, not a Sylvania product. That is, unless Philips is pimping out its "deluxe" line to other manufacturers/retailers.

Carry on, great discussion!


    Bookmark   February 5, 2005 at 11:19AM
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shrubs_n_bulbs(z8/9 UK)

Sam, I think everyone makes a Daylight Deluxe these days. I have no idea if they are all the same bulb. I was using the Sylvania as an example because I have the output graph for it.

John, GE has some information at its own site, not much but there's a graph for most bulbs. I thought the spectra were interesting because they represent the true width of the spikes and give you an idea of intensity across the whole spectrum, something that is hard to get from a graph. The spectra are obviously weighted towards the yellow/green, look at the intensity of the green spike on all the spectra. Also, I don't think they are well-corrected for total intensity but still interesting.

What's up with the american definition of a foot-candle? Isn't it a lumen per square foot everywhere?

    Bookmark   February 5, 2005 at 2:57PM
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Really? How curious. I think I remember Zink telling me that the Daylight Deluxe was a Philips mercury-free phosphor in 6500K. I wonder if the "deluxe" tag is the new label for mercury-free bulbs.


    Bookmark   February 6, 2005 at 1:53AM
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shrubs_n_bulbs(z8/9 UK)

Sam, I think that the deluxe tag was adopted by most of the main players for their improved-CRI bulbs developed in response to DOE guidelines. I'm not sure of the exact history. You can be fairly confident that any bulb with deluxe in the name will have additional far-red spectrum compared to the regular equivalent, which leads to a better CRI rating. It also leads to a lower lumen (human eye response) efficiency because we perceive tha extra red frequencies as dim, but those frequencies are very useful for plants.

Anyway, GE, Philips, and Sylvania/Osarm all have a range of bulbs using the deluxe tag, all with fairly high CRI numbers. Zink discusses the "deluxe" tag a little in this thread, referring to them as improved halophospor bulbs, but I know that Sylvania's bulb as least is a triphosphor deluxe bulb, created again by adding an improved far red pigment. Philips describe their's as a low-mercury bulb (ALTO series). The phosphor technology is a step beyond simple triphosphor, but the halophosphor/triphosphor distinction is almost a historical curiosity when talking about these advanced bulbs with their multiple proprietary phosphors. I prefer the rare-earth phosphor (REP) designation which better describes all these advanced phosphor combinations.

    Bookmark   February 6, 2005 at 6:04AM
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john_z(Z 7b Ga.)

Shrubs n Bulbs, thank you so much for linking me to the Chroma SPD site! A few disjointed comments and questions here: I'm curious about disputed definitions of PAR, although I haven't run into this yet. Photosynthetically Usable Radiation is a more accurate term to me considering that not all autotropic life forms are in the same Kingdom, and they often have peak absorption at vastly different wavelengths. We usually think in terms of "higher plants", not the needs of cyanobacteria. Does that have anything to do with it? A good site showing this was reached on 8/8/02 by keying in, "Absorption spectra of chlorophylls and bacteriochlorophylls". Voila. The wesite by that exact name appeared as I hope it still exists.

Also, as far as my lamp spectra choice go, I consider the principle absorption pigments in a specific order of % absorption (that of chl-a, b, and beta-carotene). The absorption value of B-carotene in the 500-600 nm range is highly over-rated by even PhDs in the field of horticulture. They never present evidence of their assertions when asked, so I get the impression it is just passed-along misinformation or an uncriticised belief system. While I would never state that the middle region of the visible spectrum is 100% or absolutely useless for photosynthesis, none of the pigments can pass onto the reaction centre any more photons than what they can absorb to begin with. I printed the molar extinction of beta-carotene at each nanometer (and fractions thereof) from the PhotochemCAD site. Beta carotene is not very efficient as an antennae in trapping and channeling photons at 500-523 nm or beyond, but absorbs quite well in the "blue" part of the spectrum. That's why I consider the lamp spectra for the 400-500 and around 630 - 680 nm ranges as priorities.

As for ft-c measurements, your definition agrees with my dictionary: "A unit of illuminance or illumination, equivalent to the illumination produced by a source of one candle at the distance of one foot and equal to one lumen incident per square foot". However, not once have I ever heard the later part of this definition given by any of my horticulture instructors, professors, or by those lecturing on the topic at conferences. It is consistently defined as the amount of light by one candle at a distance of one foot (period), at least in the U.S. Comment on the "lumen incident per square foot" part, and you'll always receive a puzzled look. The very well educated and skillful horticulturists I've met here are landscapers. But mention artificial lighting, SPDs, etc. - they are not informed or really interested in this.

Regarding the international definition, let me quote from a site on photometric units of measurement: "The intensity of electric light is commonly given as so many candlepower, i.e., so many times the intensity of a standard candle. Since an ordinary candle is not a sufficiently accurate standard, the unit of intensity has been defined in various ways. It was originally defined as the luminous intensity in a horizontal direction of a candle of specified size burning at a specific rate. Later the internation candle was taken as a standard; not actually a candle, it is defined in terms of luminous intensity of a specific array of carbon filament lamps. In 1948 a new candle, about 1.9% smaller than the former unit was adopted. It is defined as 1/60 of the intensity of one square centimeter of a black body radiator at the temperature at which platinum solidifies (2,046 degrees K). This unit is sometimes called the new international candle; the official name given to it by the International Commission of Illumination (CIE) is candela".

Zink, since you are collecting SPD graphs, have you seen the ones on These may or may not be of interest to you. Also you may already be familiar with the Joensuu site, if not the address is http:/// or just key in "The Plant Photobiology Notes + P J Aphalo". This is how I would like to see SPDs graphed for those of wanting PAR & PPFD measurement.

I printed and am re-reading your posting on 2/3/05 that is rich in info. I'm always grateful for your sharing. I'll be better able to formulate questions once my brain's absorption capacity is maximised with the bio-chemical from a pot of coffee and some sit-down time on the porch.

    Bookmark   February 6, 2005 at 3:58PM
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shrubs_n_bulbs(z8/9 UK)

Lots of things to discuss here, I'll try and get them all.

Pigment absorption and photosysthesis

I think everything you said it correct. It all varies by plant species, and varies even more if you go beyond the plants, but very roughly: chlorophyll a is most important, followed by chlorophyll b, various carotenoids, various xanthophylls, anthocyanins, and at least a handful of other accessory pigments. It is relatively straightforward to calculate the overall absorption spectrum of a typical plant and there are lots of examples on the net.

But I think it is more valuable to approach the problem from the other side. It is possible to directly measure the effect of different light wavelengths, usually by measuring the output of CO2 from a plant under monochromatic light. This is known as a photosynthesis action spectrum (here's an example). No need for adding up all the pigment absorption spectra, trying to correct for photosynthetic efficiency, energy transfer efficiency, chloroplast arrangements, antennae structures, fluorescence, secondary absorptions, and god knows what else. The graph shows exactly how much use a real leaf is making of each frequency of light. This graph varies for different plant species, at different times of the year, in different temperatures, in different light intensities, and for all I know at different times of the day! But the overall graph shape will be familiar to you. It has peaks of more or less equal height near 400nm and 650nm, dropping very rapidly to near zero at 700nm, dropping more slowly into the UV, and dipping in the yellow/green region to between 30% and 60% of the red/blue peaks. The interesting thing from our point of view is that the yellow-green trough is not as deep as you would expect from looking at chlorophyll absorption or even from including carotenoids, and of course it covers a wide amount of spectrum.


I have seen definitions of PAR covering light wavelengths of 300nm-700nm or 400nm-700nm. What seems consistent is that it weights all wavelengths of light in this range equally, effectively assuming that plants respond equally well to all wavelengths of light within the range. This is clearly not the case, but it does at least make for a clear definition and a relatively easy calculation. I prefer the definition to 300nm since photosynthesis is quite efficient up to 300nm and even beyond, but I suspect the 400nm definition is more widespread.

PUR is defined by weighting each wavelength of light according to its value to a photosynthesising plant, or coral plankton or alga if you have an aquarium. Clearly this is a much more valuable measure, but in practice is almost impossible to calculate. Plenty of photosynthetic action spectra exist in the literature, but which plant should we calculate the information for (aquatic plants have radically different curves from land-based plants, and algae even more different)? And at what intensity of light? And what temperature? Etc, etc. So I think you will find very little published information on PUR lamp ratings. The best thing would be to run up a spreadsheet that you can plug in SPD data and calculate your own PUR figures.

Lumens, foot-candles, and candela

These terms are all well-defined and consistent, but I wouldn't be surprised if some data sources are using obsolete definitions.

Starting with the candela, since it is a fundamental SI unit, defined as "the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian". This definition was adopted in 1979 because the freezing platinum black body stuff was highly impractical. Just to be clear, a steradian is a solid (three dimensional) angle such that at a distance of one unit the area enclosed by the angle will be one square unit.

The lumen is a derived unit being "the luminous flux emitted into unit solid angle by an isotropic point source having a luminous intensity of 1 candela". That is clearly equivalent to a one candela source emitting over one steradian. The tricky thing with lumens is that they have to be "corrected" at different frequencies to account for the phototropic response of the eye. Take a look at this website which has masses of information about this, and loads of other physical concepts, and includes tables for phototropic correction.

Even more simply, the lux is an SI derived unit being "one lumen per square meter". More commonly in the US and still used for historical reasons elsewhere, the foot-candle is one lumen per square foot. One foot-candle is 10.8 lux.

    Bookmark   February 6, 2005 at 6:18PM
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shrubs_n_bulbs(z8/9 UK)

I should have clarified. The definition of a foot-candle as "the amount of light by one candle at a distance of one foot" is completely consistent with the definition as "one lumen per square foot". One candela (assuming this is a one candela candle) emits one lumen per steradian. One steradian covers one square foot at a distance of one foot. Therefore one candela at a distance of one foot is equivalent to one lumen per square foot.

    Bookmark   February 6, 2005 at 6:40PM
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john_z(Z 7b Ga.)

Shrubs n Bulbs, I'm in the process of reading the action spectra site you included, but haven't finish. I really shouldn't comment until I have done so, but knowing me...

As you may be aware, there is more weight put on the absorption spectrum than the action spectrum scientifically. "Measurement problems" (a technology thing) of the later have been the culprit, as a reputable source related to me. I cannot validate this one way or another. But as I said, action spectra closely matches absortpion spectra. This study done in the 1970's clashes with graphs 30 some years later, in terms of "action".

Now, I'm not bullheaded. But beans grown in full sunlight are simply not going to have the action spectum that beans grown under lights will - even if we had the same overall PPFD. Pigments absorb and reflect. I would like to see the action spectra of any plant grown in full sun, vs. grown under incandescent,"cool white" and/or "warm white" with their SPD & action spectra grpahs next to pics of the plant.

What do you think?

With respect, John_Z.

    Bookmark   February 8, 2005 at 7:11PM
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bobb_2002(Z6 S.W. CT)

Sonny -
I have been reading this thread for the last week or so and there is some great information in it. It seems to have digressed into PAR and PUR discussions but as I reread your original post I noticed that the light intensity of your setup doesn't follow the inverse square law. I always suspected that might be true since the law assumes a point source, and what we typically have here is a big source complicated by a reflector and close to the target. So the light doesn't spread out as much as a point source would. But I guess if you stand a mile away it would look like a point source and from that distance would be inverse-square in intensity. Does anyone have any comments on this based on real measurements?

John, Shrubs, and Zink -
As for PAR and PUR, they are certainly important factors in plant growth and should be taken into account when determining the best lamp to use. If you are space constrained you want to maximize the amount of useful light and economics might be secondary, so you could use expensive bulbs and maybe overdrive them. But if you have room for more fixtures you don't need to overdrive, and if you are running a large operation (not me!) then you really want to get the best growth at the best cost. In that case you have to consider the entire cost of running the setup. The link Shrubs posted regarding PAR and PUR and efficiency (PUR/watt) needs to go a step further by computing the cost per PUR including fixtures, bulbs, and electricity.


    Bookmark   February 8, 2005 at 10:30PM
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bobb- i think the reflector does play some part in confounding the law. it's hard to say how the light is being diffused given the use of a reflector as well as the shape/placement of the bulbs perhaps?

    Bookmark   February 8, 2005 at 10:44PM
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shrubs_n_bulbs(z8/9 UK)

Sure John, actual action spectra vary with the conditions. That's a good thing for the plant because it adapts to its growing conditions. Bad thing for scientists because two different experiments can measure the same thing and get two different answers!

    Bookmark   February 9, 2005 at 5:30AM
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john_z(Z 7b Ga.)

Obviously, we are not talking about foot-candle measurements anymore. If we were to continue digressing to support the creation of a new thread, what would the topic be? Come on now, you are all "bright", plant-loving homosapiens!
I want also to know what people on this thread are growing and what their experiences are (for the better or worse) under lights.


    Bookmark   February 10, 2005 at 8:37PM
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shrubs_n_bulbs(z8/9 UK)

OK, back to foot-candle measurements. Here's a quick and dirty way to measure foot-candles without an expensive light meter. Many of us have digital cameras, or even sophisticated film cameras, which have built-in light meters. These can be used to measure in foot-candles, so you too now have a light-meter.

Place a sheet of bright white paper (instructions later on how to do this with an 18% grey card) at the location to be measured. Allow your camera to select the best settings to expose the sheet of paper. You need to know the ISO/ASA speed, the F-stop, and the exposure. Use spot-metering if you can, otherwise make sure that the white sheet fills the frame. Don't get in the way of the main light source! It doesn't matter how far away you stand, so long as the area to be measured fills the meter frame.

S is the ISO/ASA speed
F is the F-stop setting
E is the exposure in seconds

Foot-candles = 6 x F x F / ( S x E )

That is six times the F-stop squared, divided by the speed, divided by the exposure. The six is a bit of an approximation, because camera exposure settings are calibrated against an 18% grey card (or possibly an ANSI standard, or possibly fudged, but who cares) so that the formula would need a number of 20-30. Use 25 instead of 6 if you have an 18% grey card to meter against.

You should only expect your foot-candle number to be accurate to within about 25% because of the finite steps between f-stops and exposure settings, but it is meanignless to be any more accurate than this unless you control the spectral sensitivity. Most light meters are calibrate for human eye photopic (daylight) spectral sensitivity, some for scotopic (night-time) sensitivity, and a few for more specialised sensitivities. Film, and so camera exposure meters, tend to be sensitive to a wider spectrum than human eyes, particularly at the blue end. As we've discussed, plants have a significantly different spectral sensitivity.

    Bookmark   February 11, 2005 at 8:02AM
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john_z(Z 7b Ga.)

Sonny, I just found an SPD graph for a "daylight deluxe" tube, but it did not say if it was Philips, Osram, or GE. I'll try to link it, but if I mess-up - key in either 'Library of Normalized Spectral Plots' (an aquabotanic site)or go straight to (That's a "one" after lightcompare, not an "L"). If you choose to print it off and study it, use landscape and not portrait or the SPD will be chopped-off.

    Bookmark   February 12, 2005 at 3:02PM
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john_z(Z 7b Ga.)

That link didn't work. Let me know if you have found the SPD or not.

    Bookmark   February 12, 2005 at 4:05PM
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john_z(Z 7b Ga.)

Sonny, I just realised that you only have to scroll down to "Results" on the page of the site I attached, and click "Library of Normalized Spectral Plots. The Daylight Deluxe is the 3rd JPEG.


    Bookmark   February 14, 2005 at 7:23PM
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thanks John. appreciate the link!

    Bookmark   February 24, 2005 at 5:22PM
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Interesting thread.

Anyone have any information or experience with Flora Plant light. How similar or different from Gro Lux.

    Bookmark   August 25, 2005 at 5:47PM
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john_z(Z 7b Ga.)

trkl, I'm not yet familiar with the Flora Plant light, but can you tell me who makes it, and whether it is incandescent, fluorescent, etc., plus the wattage? For me it is a matter of first seeing its spectral power distribution, then considering other factors. I keyed in "Flora Plant light + spectral power distribution" on my search engine, but didn't get anything.

    Bookmark   August 27, 2005 at 2:32PM
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shrubs_n_bulbs(z8/9 UK)

I believe this may be the product. My rough calculation says you are buying a $460 shelf system and paying $400 for twelve fluorescents tubes and their fittings, which is about twice what you would pay if you bought your own lights. There is no way of knowing if these lights are good or bad without more information about the product, but they are probably quite suitable for plants.

    Bookmark   August 28, 2005 at 8:00AM
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Hey Shrubs,

I'd bet it is this "Flora" lamp that they are talking about.


    Bookmark   August 28, 2005 at 10:26PM
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Hagen Flora Glo

    Bookmark   September 4, 2005 at 8:10PM
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shrubs_n_bulbs(z8/9 UK)

The Flora Glo spectrum is very similar to the Gro Lux, a pinky light with a lot of red and some blue. The Flora Glo has more orange and less green, otherwise very similar.

    Bookmark   September 5, 2005 at 1:50PM
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Hi Zink, I have just done several days of online research regarding fluorescent lights. During all of my online travels, far and wide, you appear to be the number one expert on the subject. So, if you could please indulge me just a quick question, that would really be appreciated.

My goal is to get as close to the ideal Photopic curve without spending too much money, or having to go to great lengths to actually find the bulbs. So my parameters are:

* good spectrum for plant growth
* easy to find
* inexpensive

My thought is that I should combine several types of bulbs to achieve close t an ideal spectrum. The shrubs I'm growing have a distinct vegetative and flowering cycle. So my plan is to change the light configuration when I switch them over to flowering.

I want to start off with a Blue-shifted light setup. So I am thinking that I will use 3 Daylight Deluxe for the blue part and then a Red-shifted bulb to compliment that.

The Red-shifted bulb is where the question arises.

I am unable to locate a Warm White Deluxe bulb anywhere. They just don't seem to carry them anymore. In fact, I really have very few options for the red shifted spectrum.

The best bulb that I have found, is a GE Soft White from Home Depot. I was concerned at first because it doesn't say "deluxe". The bulb provides 3300 lumens at a temperature of 3000K. But the interesting part is this bulb has an 85 CRI. I have been unable to locate a SPD graph for this particular bulb. However, the high-ish CRI is a tip to me that it is using a wider spectrum than your normal Warm or Soft bulb. Can you comment on whether my hunch is correct here? Or perhaps you have an SPD for this bulb that you could share?

So, my setup will be something like this:

For Vegetative:
3 Daylight Deluxe with a Soft White

For Flowering:
3 Soft White with a Daylight Deluxe.

Zink, in all your wisdom, can you comment on my setup and let me know if there might be an easy-to-find, cost-effective, better solution for filling in the Red and/or Blue spectrum?


And if you want to go ahead and post the spreadsheet with all of that spectral data you have collected, please be my guest!

    Bookmark   October 21, 2006 at 2:05PM
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I checked your footcandle measurements and found them to be essentially correct for the CENTER of the tubes. They fall off the further you go towards the edges. At 6" below the tubes and 6" in from the edges, I measure 350 footcandles.

    Bookmark   May 13, 2009 at 2:36PM
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Wow! this is so much info. I'm not sure I understand a lot of it, but maybe y'all can answer me this: :)

If I'm growing some African Violets, and I need 900-1100 foot candles, are there any T8's that could supply that amount? For instance, would the new T8 that came out, the "phillips plant and aquarium" bulb work? Or do I need two? or should I use an additional bulb?

Which bulbs would be the best color wavelength? Should I just have them 6-9inches away?

    Bookmark   December 13, 2013 at 5:49PM
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