Summary of conversion of footcandles to SI units *smile*

[IAN THUST MURRAY]

Dear netters,

I recently posted the following question:
One of my colleagues is interested in the conversion of light energy from
footcandles to microeinsteins/m/s-2.  I apprecite your help.

Here are the responses, I hope that it is of benefit to you.  It helped
us.
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Subj:	Re: Conversion of footcandles to uE/m/s-2????

footcandle * 10.76391 = lux

lux = lumen/m2
lumen = candela * steradian

candela, defined as the luminous intensity in a given direction of a point 
source "that emits monochromatic radiation of frequency 540 x 10^12 hertz
and that has a radiant intensity in that direction of 1/683 watt per steradian."

-from: Standard Practice for Use of the International System of Units (SI)
(the Modernized Metric System).  ASTM 1989.

Now, asking for microeinsteins--you probably want micromole/m^2/s (not /m/s-2).  
The amount of radian energy in a mole of photons depends on the distribution 
of wavelengths represented.  To go from watts/m^2 to umol/m^2/s, you need to 
have a standard source.  There are approximate empirical relationships for 
"full sun in temperate latitudes", but the actual conversions depend on 
atmospheric conditions at the time of measurement.  For approximate 
interconversions see:  Thimijan, R.W. and R.D. Heins. 1983. Photometric, 
radiometric, and quantum light units of measurement: A review of procedures 
for interconversion.  _HortScience_ 18:818-822.  [cited in Nobel, P.S. 1991.
Physiochemical and Environmental Plant Physiology.  Academic Press, San Diego, 
California.  I don't have a copy handy.  Also from Nobel (1991, p. 201), "at 
noon on a cloudless day with the sun overhead . . . (total irradiance of about 
850 W m^-2; equivalent to about 100 000 lux). . . . is a photon flux density 
of about . . . or 2.0 X 10^-3 mol m^-2 s^-1 (2 000 umol m^-2 s^-1) at the 
earth's surface" ]  

I hope this gets you started.

- Bob Knox
- knox at spruce.gsfc.nasa.gov
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X-News: gnv bionet.plants:1371

I went through this several years ago for a paper I was publishing and looked
everywhere for the appropriate conversion factors but could not find anything,
even in the CRC Handbook. The simplest thing I found was to find a colleage who 
had a light meter that measured microeinsteins and redo the light measurements.
Sorry I can't give you any more help, but if you do get a reply that gives you
the conversion factors, could you e-mail them on to me.
Thanks,
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From:	GW1::"MARDER at agri.huji.ac.il" 15-AUG-1993 

For a rough conversion, use 10 footcandles = 1 uE.m-2.s-1
This is good for fluorescent lighting.

The true value depends on the spectrum of the light since the Einstein
is a measure of photon number and photon energy is inversely proportional to
wavelength.
__
Jonathan B. Marder                 '
Department of Agricultural Botany  |     Internet: MARDER at AGRI.HUJI.AC.IL
The Hebrew University of Jerusalem | /\/ Bitnet:   MARDER at HUJIAGRI
Faculty of Agriculture             |/  \ Phone:    (08 or +9728) 481918
P.O.Box 12, Rehovot 76100, ISRAEL  /     Fax:      (08 or +9728) 467763