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RE: PAR, Lumens, Watts, etc
The recent thread on PAR/lumens/watts prompted me to pursue a small
project I was thinking about. When selecting fluorescent bulbs for my
planted aquarium, I couldn't find any consistent quantitative info to
use as selection criteria (besides price...). I had spectral plots
and lumen specs, but how these translate into PAR and other pertinent
measures ? Being limited by space above the tank (and money) to 2 watt/gal,
I wanted to have the most bang for my watts. But with the spectral plot,
lumen and watt rating for a bulb, it is possible to derive such
measures. I already sketched the principles in a former posting, but after
that post I realized that I had at my fingertips all the resources to carry
out the project: just a computer and the appropriate software.
The basic idea is that, from the manufacturer's published bulb spectrum
(in relative units) and the bulb's published lumen output and electrical
power consumption, we can in principle compute other bulb parameters such
as PAR output.
For carrying out the comparisons, I used only spectral curves and bulb data
I was able to get from the web, as well as some web-published photosynthesis
action spectra. I can send/post the data sources if requested. I also had
to write a short computer program to carry out the computations.
I got data just for NO fluorescents, since this is the type of bulb in
which I'm interested. But the methodology is general and applicable to
*any* light source.
For people interested in this, I can send a detailed description of the
computation steps, as well as a discussion of possible error sources
(they would make this posting too long). If anyone has access to spectral
curves of bulbs not listed in here, I'll appreciate to get a pointer to them.
The columns in the table list the following quantities:
Power: the bulb's rated power.
Maximum lumen output; this theoretical value depends only on the bulb's
spectrum and rated power. It is the lumen output that the bulb would have
if all electrical energy input to the bulb were transformed into luminous
Rated lumens: (initial) taken from bulb's specs, except the Triton and P&A,
which are educated guesses.
Efficiency: the ratio between rated lumens and maximum lumens. I was a bit
surprised by the low values. How manufacturers compute fluorescent bulb
efficiencies ? I was expecting values in the range of 0.3-0.5.
PAR: the bulb's output in PAR. The units are just photons/sec, I don't
know what are the appropriate units in which to express PAR measures.
But these figures are OK as far as *relative* comparisons go.
MPAR: Modified PAR (this was the main goal of the experiment !). There has
been some suggestions to use a photosynthesis action spectrum to weight
the PAR measure, in the same way as the eye's photopic response is used to
weight the lumen measure. This is exactly what I did here, using an "average"
action spectrum curve. The "total" column lists the sum of all photons in the
range 400-700nm. But since there are no clues in these figures about the
*relative* amount of red and blue photons, I also computed MPAR in the
400-500 nm range only (blue) and 600-700 nm range only (red). R/B is just
the ratio between the red and blue MPARs.
Bulb Power Max. Rated Effic. PAR MPAR R/B
(W) lumens lumens total blue red
Triton 40 8000 2200 0.28 170 105 68 23 0.34
AX50 40 12000 3600 0.30 200 100 48 36 0.75
PowerGlo 40 8500 2200 0.26 160 100 58 22 0.38
AquaGlo 40 4800 960 0.20 140 94 35 52 1.50
GroLux 40 5900 1200 0.20 140 87 26 53 2.05
SP65 40 11700 3050 0.26 170 87 43 24 0.57
Daylight Dlx. 40 10400 2550 0.25 160 83 39 25 0.65
C50 40 9900 2250 0.23 160 76 26 34 1.32
SunGlo 40 13400 3100 0.23 150 72 31 19 0.61
P&A 40 9200 1900 0.21 150 70 15 42 2.90
FloraGlo 40 12500 2180 0.17 120 57 9 33 3.77
TL950 32 12500 2000 0.16 90 34 8 10 1.23
The table lists the bulbs in decreasing MPAR order. It is roughly also the
PAR decreasing order, but not quite so. But I think the most interesting
result is the blue/red comparison. There is a hint of a correlation of
R/B with MPAR output, in the sense that the highest MPAR bulbs are also
the bluest, and the ones with the least production of PAR photons are also
the reddest. Thus very high PAR (or MPAR) output shouldn't be the only
criterion when seeking for the optimum bulb, if the goal is to have also
a good balance between red and blue.
It is easy, from this data, to compute figures for multi/mixed bulb
configurations, by just adding the individual bulb's measures, weighted
by the number of bulbs of each type in the mix. So it should be easy to
come up with optimum mixes given the constraints of ones' configuration.
And bear in mind: these results are only as good as the manufacturer's
published spectral curves allow them to be. Some curves seem to be quite
accurate and have adequate spectral resolution (AX50, SP65, Daylight Deluxe).
Others have somewhat less detail (TL950, P&A, C50), and others are grossly
smoothed out (the Hagen "Glo" bulbs). I estimate that erors should be a few
percent for the best data, up to 10-15 percent in the absolute values
(much less in R/B) for the worst ones.
And just wondering: why lumens are defined in terms of energy while
PARs are defined in terms of photons ? Are the processes acting in
the eye not driven by photons, as photosynthesis is ?