Re: Watts, lumens and hogwash

Hi folks,

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 
proportional font.

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 
each curve.

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
 |                                          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