Lumens vs. Colour temp?

stonato(z6a - Toronto)March 8, 2005

Does anybody know which of the two is more important when shopping for lights. I recently purchased daylight bulbs at 6500K w/ 2300 lumens. However I saw other cool bulbs w/ 3300 lumens and 5000K. Which is more important...Colour temp or lumens?



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jkirk3279(Z5 SW MI)

Well, Color is everything.


But if you were to somehow get tubes designed to make only GREEN light, the plants would starve. Plants can't directly use green light. There's a theory that they can IN-directly use it, but I haven't got the data on that yet.

Now, that's unlikely, but it's possible to get a bright tube, lots of lumens, that outputs a lot of that light as green and therefore useless to plants.

Specialty grow lights exist that leave out all but blue and red light and therefore look purple.

In practical terms, most folks use a mix of red and blue by using one cool white and one warm white tube, and some get more lumens by overdriving.

Your first choice, 6500K w/ 2300 lumens, is pretty bluish. Your second choice, 3300 lumens and 5000K, is brighter but less blue and more red.

Try one of each. I like warm whites on the theory that plants need infra-red more than they need blue light.

Here is a link that might be useful: Lights

    Bookmark   March 8, 2005 at 4:12PM
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shrubs_n_bulbs(z8/9 UK)

Plants can grow using only green light. They won't starve. Green light isn't useless to plants, it is just less useful than certain other wavelengths. Depending on the plant, green light is 20%-80% as useful as the best red or blue light. That's not a theory, its a fact.

Hard to say which lamp is best without knowing more details. about the tubes Brightness is certainly important, probably the most important single factor, its just that lumens isn't really the best way to measure it. 3300 is a lot more than 2300 so that lamp is almost certainly better, but possibly not. Are the lamps even the same wattage?

Colour temperature is also not a reliable measure of how useful a bulb is. Higher numbers mean somewhat bluer light but it is just an average and you can't tell much without a spectral output graph. Luckily, a lot of bulbs have very similar spectra so we can sometimes compare, but there are some bulbs that are distinctly better than an average bulb of the same temperature.

A mix of 6500K and 5000K lamps is a little pointless. The 6500K has slightly more blue and slightly less red but not enough to care about. A warm white bulb would be between 2700K and 3000K and the difference would be worthwhile.

So, to answer your question, tell us what bulbs you have and we'll opine on them :)

    Bookmark   March 8, 2005 at 4:58PM
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stonato(z6a - Toronto)

thanks for the info. I have a Philips F40/DX/ALTO for my daylight 6500k (40 watts). My other is a Sylvania Design 50 (40 watts). I know this is also a cool white but it's a $7 bulb. What makes it so special? I think I should take your advice and get a warm bulb to accompany my daylight.


    Bookmark   March 8, 2005 at 7:54PM
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shrubs_n_bulbs(z8/9 UK)

All becomes clear.

The Design series of bulbs are high CRI bulbs for good colour rendition. High CRI isn't necessarily important for plants but these bulbs often have better spectrum in the far red, where a normal triphosphor would have a sharp drop just before the photosynthetic peak. The low lumens are because the light is spread into the red and blue instead of being in a big green spike, but there is actually just as much total light being emitted as the 3300 lumen bulb. In this case the Design 50 bulb is no better than a standard Daylight Deluxe which should be available for less money.

The Alto series of bulbs are modern triphosphor bulbs designed for long life, good lumen maintenance (the light doesn't dim as the bulb gets old), and marketed as "green" because of low mercury content and high lumen efficiency. The lamps have a typical triphosphor spectrum with a big peak at 611nm in the red, two blue peaks near 420nm, and a green one around 540nm. Nothing special for plants, but the long life without losing brightness may be helpful.

The Sylvania bulb is probably the better one initially, but the Philips might actually perform better after the first few thousand hours. A Daylight Deluxe bulb would probably be as good as either for less money, and a cool white virtually as good, at least until they started to dim after a few thousand hours. Overall, not a huge difference between any of these bulbs. You can grow under just these bulbs if you want, or mix with warm whites.

    Bookmark   March 9, 2005 at 6:43AM
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stonato(z6a - Toronto)

Once again Shrubs..great help.
My Daylight is actually a Deluxe. I went out last night and bought a Philips Warm White @ 3000K and 3300 lumens (40 watts). Would you suggest I mix this with the Daylight or with the Design? Or better question...I still have all my reciepts..what configuration would you recommend if I had to start over?


    Bookmark   March 9, 2005 at 11:58AM
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shrubs_n_bulbs(z8/9 UK)

I've just realised you would be referring to canadian dollars when you said you paid $7. That seemed high to me because I was thinking of US dollars. I have no idea what an appropriate canadian dollar price would be, maybe $7 is about right. Both your daylight bulbs are available by the case for around $3 US, although you might have to buy several to get the best price. I believe Home Depot had them at two for $6 last month.

Like I said before, not much to choose between the bulbs. If the price is the same, go with the Philips just for longer life.

You never said what you wanted to grow? Seedlings? Pot? Aquarium plants? It might be worth splashing out for a better bulb if you want to grow something fancy.

    Bookmark   March 9, 2005 at 2:35PM
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I used to grow under a mix of Cool white and Daylight deluxe bulbs. I was pleased with the results, but if your plants will be more than 8-12" away, your plants won't be terribly happy. I've since switched over to 85W compact fluoros in 6500K.

I personally put more stock in light output and less in light coloration, but I like to stick with 6500K or as close to that as possible.


    Bookmark   March 9, 2005 at 5:02PM
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stonato(z6a - Toronto)

Is it the general consensus that a warm bulb and cool bulb should be used for optimum results?


    Bookmark   March 10, 2005 at 11:03AM
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ChicagoMike(z5 IL)

I've been researching this today and found that typically, cooler lights are better for green growth as they mimic the spring and summer lights and warm lights are better to encourage blossoming as they encourage fall light. So if I'm just interested in starting seedlings and getting tomato plants ready for spring planting, it seems that I should stick to all cool.


    Bookmark   March 16, 2005 at 7:30PM
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stonato(z6a - Toronto)

That's interesting CMike. I think next year I'll find the real truth and do the experiment myself. Perhaps use a more difficult seed, under same conditions, but change the colour temp.


    Bookmark   March 16, 2005 at 11:56PM
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