Foot candles

Gerald F. Deitzer gd3 at umail.umd.edu
Fri Oct 12 13:16:26 EST 2001


I guess that I should get into this discussion, since I do have some credibility
as a photobiologist and have published information on such conversion factors.
Carl Pike is right that you cannot directly convert from foot-candles to
umol/m2/s without knowing the spectral distribution of the light source being
measured.  However, if you know the light source you can fairly accurately
convert from ft.-can. to umol/m2/s.  If you divide your ft.-can. reading
by the
following factors you will get the correct number of photons between 400
and 700
nm (PAR) in micro mol/m2/s:

1. Sunlight = 0.195 micro mol/m2/s/ft.-can (or 5.13 ft.can/micromol/m2/s)
2. Incandescent light = 0.219
3. Cool-White (CW) fluorescent light = 0.137
4. High Pressure Sodium = 0.129
5. Metal Halide = 0.144
6. 3:1 ratio on a wattage basis of cool-white to incandescent (e.g.. 10
x 120W CW
+ 4 x 100W Inc.) = 0.144

Thus, full sunlight at about 10,000 ft.-can. = 1950 umol/m2/s and 500
ft.-can of
CW light = 68.5 umol/m2/s.

I think that Karl Kleiner's calculations below are substantially
correct, but
sometimes he uses the reciprocal for the conversion so that you would
have to
divide rather than multiply.  In any case, use of the above factors is quicker
and easier.  However, it would be dangerous to use an average of all of
them to
get a single factor.  If you (or anyone else) would like other
conversions, I
would be happy to supply them.

Sincerely,

Gerry Deitzer

Karl Kleiner wrote:

> Magaly
>
> I don't know of a direct conversion. What you want to do is convert from
> a photometric unit (illuminance) to a radiometric (irradiance). The
> problem is, most photometric light is in a broader wavelength band than
> that considered for plants (PAR 400-700 nm). Correct or not, here is
> what I have done.
>
> Convert foot candles to lux -> multiply ft candles * 10.8
>
> Convert lux to Watts m2 ->  lux * 2.45 mW m-1/ 1000 mW W-1
>
> Convert W m-2 to umole m-2 s-1 -> multiply W m-2 * constant.
>
> The constant depends upon the light source, but averages to 4.688 for 11
> light sources.
>
> Some specific constants for light in the 400-700 nm range:
>
> Sun & sky, daylight - 4.57
> Blue sky only - 4.24
> Metal Halide - 4.59
> HP Sodium - 4.98
> Cool-white fluorescent - 4.59
> Incandescent - 5.0
>
> Lux can be converted to umole m-2 s-1, by division: Lux/ constant.
> But the constants vary more widely.
>
> Constants for converting light within 400-700 nm.
>
> Sun & sky, daylight - 54
> Blue sky only - 52
> Metal Halide - 71
> HP Sodium - 82
> Cool-white fluorescent - 74
> Incandescent - 50
>
> This is all from:
> Thimijan, Richard W. & Royal D. Heins. (1983) Photometric, Radiometric,
> and Quantum Light Units of Measure: A Review of Procedures for
> Interconversion. HortScience 18(6): I'm missing page #s
>
> >From Techniques in Bioproductivity and Photosynthesis (J. Coombs et al.
> eds.) 1985
>
> Convert Day light, full sun:
>
>         950 W m-2 = 1.36 cal cm-2 min-1 = 95,000 lux
>         of this, PAR 9400-700 nm) is: 1800 umole photons m-2 2-1
>         this is approx 399 W m-2 = 0.572 cal cm-2 min-1 = 42% of total
>         therefore W m-2 (total) = 1.895 umole m-2 s-1 (PAR)
>
> They also have conversions from Blue sky, light and 400W High Pressure
> (Sodium Vapor?) lamp
>
> This latter conversion might be easier.
>
> -Karl
>
> "Rincon-Zachary, Magaly" wrote:
> >
> > Dear Plant-Eders
> >
> > I am in crunch here for time.  I know this question has come up in the list
> > some time ago.  What is the conversion factor from foot candle to moles
> > photons/sq. meter/sec?
> >
> > Thanks
> >
> > Not a shred of evidence exists in favor of this idea that life is serious
> >
> >                                                            --Brendan Gill.
> >
> > Magaly Rincón-Zachary, Ph.D.
> > Associate Professor and Graduate Coordinator
> > Biology Program
> > Midwestern State University
> > 3410 Taft Blvd.
> > Wichita Falls, TX 76308
> >
> > Phone:  (940) 397-4254
> > Fax:    (940) 397-4831
> > E-mail: magaly.rincon at mwsu.edu
> >
> > ---
>
> --
> ______________________________
> Karl Kleiner
> Assistant Professor of Biology
> Department of Biological Sciences
> York College of Pennsylvania
> York, PA  17405
> (717) 815-1754 - Phone
> (717) 849-1653 - FAX
> kkleiner at ycp.edu
> http://goose.ycp.edu/~kkleiner/index.htm
>
> ---




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