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Re: Footcandles

Molly in Iowa wrote: 

> Could somebody out there give me a conversion ratio between footcandles
> and watts per square foot?  Any thoughts would be much appreciated.

I haven't seen any conversion ratios, but I found the attached excerpt on 
the University of Missouri Extension site.   The full text is available at 
http://etcs.ext.missouri.edu:70/publications/xplor/agguides/hort/g06515.html
(barring any typos on my part)


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Lighting Indoor House Plants

Ray R. Rothenberger
Department of Horticulture, University of Missouri-Columbia

<<<text snipped>>>

Low light plants. Plants referred to as low light intensity plants
generally should receive between 50 and 250 footcandles. (A footcandle is
the measurement of the light received by a flat surface one foot from the
point of emission of one international candle.) Under artificial light, a
few plants in this group can be maintained at as little as 10 footcandles.

One way you can estimate the amount of light available is to calculate the
number of watts available per square foot of plant area. Low light plants
should receive between 10 and 15 watts of artificial light per square foot
of growing space. A single fluorescent tube, such as a 2-foot 20-watt tube
or a 4-foot 40-watt tube without any other light provides only enough light
for plants in this category.

Medium light plants. These plants prefer 250 to 3,000 footcandles. Best
growth occurs above 1,000 footcandles unless plants also receive extended
periods of direct sunlight. Give them artificial light in the 500 to 1,000
footcandle range or 15 or more watts per square foot of growing area.

While plants in this group can be held in the 250 to 500 footcandle range,
growth is best with more light. A fixture containing two fluorescent tubes
is sufficient for plants in the low to medium light range. Adjustments in
the number of tubes used may be made if you regulate the distance between
the tubes and plants.

High light plants. These plants generally are not satisfactory for growing
under artificial lights in the home. However, if you want to try, use
special high-intensity lamps. These plants need at least 2,000 footcandles
or 20 watts per square foot of growing area, but should have higher
intensities for best growth and flowering. Fixtures containing three to
four fluorescent tubes are necessary for plants requiring high light.

<<<text snipped>>>

---------------------------------
Marianne Hoppe                           
Montana State University-Northern
Havre, Montana  (USDA Zone 3a)
hoppe@msun.nmclites.edu              
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