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foot-candles, lux, lumens, sunlight, PAR

by Stephen Pushak <teban/>
Date: Mon, 03 Nov 1997

Thanks for the several replies on the previous posting! This gives me enough data for some serious scientific wild a** guessing. I also did some searching in the archives and came up with some interesting facts. I hope the results are interesting.

The proper spelling is foot-candle but this is not used in the APD archives.

Everybody should know that 1 lux is the same as 1 lumen per square meter and the archives are full of data on lumen ratings for various lighting devices. How you convert square meters (an area) to gallons (a volume) is an interesting problem. Anybody care to tackle that one? I'd bet that one meter square of tank base will give you somewhere between 1-5 square meters of plant surface. Remember that a lot of the plant leaves in a crowded tank are not going to get 5% of the light that the tops of the plants are getting. But that's not critical so long as enough of the plant leaves are above the compensation point for photosynthesis. The compensation point is where energy gained equals energy expended.

Microsoft Bookshelf (American Heritage Dictionary of the English Language) defines foot-candle as a unit of measure of the intensity of light falling on a surface, equal to one lumen per square foot.

"Plant chlorophyll absorbs light at wavelengths of 400 to 700 nm. This is termed Photosynthetically Active Radiation (PAR)" - Huebert.

One of the more interesting tidbits of data is contained in the posting "Watts, lumens and hogwash" by Wright Huntley in the Sat, 9 Nov 1996 APD. He gives a table showing Photopic Human Eye-sensitivity Curve (Lumens definition base) vs Plant Growth Spectrum (Growth rate) plotted as a function of spectral wavelength. This graph also illustrates quite dramatically that plants absorb radiation quite uniformly throughout the whole spectrum; not in narrow peaks. He didn't quote a reference for the growth data :-( but said "My plant curve was derived from textbooks and was taken from data on emersed plants." Hopefully this data is well established. If there were more data points at more frequencies, perhaps we would see a little spikiness in the plant growth response vs spectrum.

Pete Mohan wrote: "PAR is expressed in microeinsteins per second per square meter. (umol/m2/s)

For each lamp, 1 uE = X ftc. Use these conversion estimates to compare the PAR output of various lamps. This is the light actually produced, not just what people see.

Gro-Lux X = 5.2
Gro & Sho X = 3.5

The Gro & Sho lamp reads much dimmer than the Gro-Lux using a footcandle meter, but is nearly as bright when PAR is measured. The Gro-Lux just produces more light that we can see.

Cool white (GE) X = 7.3
Deluxe warm white (Philips) X = 5.6"

This indicates that the ratio of lumens to PAR may vary as much as 50% between fluorescent bulbs with different spectra! We assume that the lumens/watt is an accurate measure of efficiency but it only is so far as human eyes are concerned.

I didn't find any PAR comparisons between MH lighting and T-8 fluorescents. The best T-8s had a lumen per watt rating around 90 and MH was a little under 80. MH radiate in a wide spectrum whereas you may need a special bulb to get the same PAR/watt rating from a fluorescent. Maybe we can start an interesting discussion on PAR/watt efficiencies and restart the MH vs FL wrangle. Or maybe not. ;-)

Karen, have you looked at any PAR data sheets as I suspect you may have?

Full tropical sunlight is probably 10,000 foot-candles and 50% of the blue sky is about 500 foot-candles which is about 5% of sunlight. Searching through the archives, I found that aquatic plants saturate photosynthesis (assuming everything else is in perfect supply) at "between 300 and 1000 umol per m2 per second of Photosynthetically Active Radiation (PAR). Anything below 100 umol/m2/s PAR should be considered low light as aquatic plant light compensation points are in the range of 15 to 85 umol/m2/s PAR. Full sunlight on a cloudless, clear day at high noon in the midwestern US is about 2000 umol/m2/s PAR." (quoting Dave Huebert)

If Dave and Paul both have good numbers for US noon sunlight, then the X factor for sunlight is about 5 foot-candles / umol/m2/s PAR. Somebody can probably cross check this number quite accurately.

It looks like a good target is going to be 200-500 microeinsteins/square meter/second (umol/m2/s) during the important part of the day when the plants are going to be photosynthesizing so the 5% of sunlight given by lots of blue sky is going to be pretty close to the 100 umol/m2/s PAR value which is just enough to keep the high plants alive. Certainly full on sunlight is too much so if we can arrange to get a 10% shading factor it would be ideal for low light plants and about 25% shading would be good for high light plants. Remember that the context for the original question is concerning natural light in the tropics i.e. the Philippines.

Dave Huebert mentioned that daily fluctuations may be a concern and I think this would really be true if we were only relying upon blue sky for light. The plants themselves will tolerate several days of low light so long as the overall budget allows them enough light above the minimum compensation point (15-85 umol/m2/s) to stay alive.

Using some other numbers I found in the archives, 1 watt per gallon of new fluorescent lighting is probably close to the 100 umol/m2/s PAR value but this is really stretching it.

QUESTION: do daily and hourly variations in light intensity actually favor plants or algae?

Assume that nutrients are well supplied and the high light levels are well in the mid-range of the compensation point and the saturation point for photosynthesis.

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