Re: Watts, lumens and hogwash
The recent thread on this subject prods me to finally release an old note I did
on it a while back. Re-reading it, it still sounds reasonable, so here it is.
If you have trouble with the figure, try loading it into an editor without a
Lumens, Lux, Foot-candles and other photometric terms are based entirely on how
the average human eye perceives light. Standard observers were defined by
testing many individuals and averaging the results. CRI, Color Temperature and
a host of other common terms all derive from the original work of the
International Committee on Illumination (ICI, or CIE in the French version),
dating back almost 60 years.
Plants don't *have* human vision, so some judgement is required when trying to
use human-sensitive terms to define what our plants need. Sometimes, the exact
opposite of what we want can come from being too slavish to the desire for
"more lumens per Watt." To illustrate this point, look at the relative spectral
sensitivity curves of Figure 1. Plant growth rate is plotted as xxx, while
human eye sensitivity (Photopic, or daylight adapted) is plotted as ooo. The
vertical scales are adjusted so that there is approximately equal area under
The dip in the green for plants is evident (they *do* reflect more green away)
and the green growth sensitivity is only about 1/4, compared to what our eye
perceives. In the violet and deep red, the plants have thousands of times more
sensitivity than the human eye. Our visual response in the visible blue and red
is only about 10% what it is in green and yellow.
Any sensible phosphor designer will tailor his lamp to match the human eye
curve, *if* the objective is to maximize lumens per Watt. This means that
phosphors wasting any energy in the blue and red are eliminated. The standard
"cool white" flourescent bulb is a superb example of this kind of engineering
raised to a very high art. The spectrum of cw bulbs closely matches the human
scotopic curve, yielding a lot of lumens/Watt, but only mediocre plant-growth
response. Photos taken under cw flourescents end up with a sickly green tint.
Most of us like to actually look at our plant tanks, so the cheaper
"plant/aquarium" bulbs that have big spikes in the blue and red, with almost no
green may give good growth, but should be only of interest to the pot grower or
other esthetically uninvolved user. A magenta glow is not very pleasing in an
underwater scene. [Spectacular for a tank of neons or cardinals, tho.]
The ideal spectrum for combined visual and growth purposes is a broad-band
source. In fact, an ordinary incandescent bulb, operating at a color
temperature of about 3200K, is nearly perfect. The one small flaw is the large
amount of heat it generates, for it doesn't fall off at wavelengths above
750nm, like the plant curve does. Halides are more efficient, so are even
better, but the heat is still a big problem.
Flourescent tubes are available that give pleasing color rendition, but still
provide better growth efficiency than "cool white." I have grown lush Riccia
fluitans under an 8W cool-white tube, for there is nothing that says plants
have *no* response in the visual region. It just *looks* very bright and
doesn't give the best plant growth.
For about four times the plant growth rate at a slight perceived drop in
brightness, either a daylight or GE Chroma 50 will give pleasant colors and
vigorous plant growth. Phillips Schedule 35 and most tri-phosphors work well,
too. None of these tubes are very expensive. I will reserve my thoughts on the
poorly-designed, unreliable, short-lived specialty tubes, called aquarium
bulbs, sold for four to ten times the price of these suggested, well-engineered
Esthetically, I like a 50-50 combination of daylight and Chroma 50. With only
80W (2 40W tubes) in a shop-light fixture, over a 55G tank, the growth-limiting
factor definitely is CO2, not the light. That's only 1.5 Watts/Gallon, far
below the frequently suggested 2-4 W/G. Even with lower lumens or lumens/Watt
ratings, they are the *growth* equivalent of about 6W of "cool white" tubes.
Photopic Human Eye-sensitivity Curve -- o o o (Lumens definition base)
Plant Growth Spectrum -- x x x ( Growth rate vs spectral power density)
1 _ o
| o o
| o o
| o o
.5_ o o
| o o
| o x x
| x o o x x
| x x o x o
| o x o
| x o o
| o o
| o o
0| o o
400 500 600 700 nm
UV Violet Blue Green Yellow Orange Red IR
Figure 1. Eye vs Plant-Growth Curves
I sincerely hope this helps clarify how we look at illumination sources for
both our visual stimulation and our plant growth.
My plant curve was derived from textbooks and was taken from data on emersed
plants. It is closer to our reality than the many studies I saw on algal
growth, which lends itself to lab analysis, but doesn't reflect higher-plant
reality very well.
Lumens are for "looking at..." Watts are energy. Neither is the whole answer.
Use good judgement in selecting the best for your tank.
Wright Huntley (408) 248-5905 Santa Clara, CA USA huntley at ix_netcom.com