Date: Wed, 26 Nov 1997 08:11:53 -0500 (EST) From: Perry Malouf <pmalouf@access.digex.net> To: cp@opus.hpl.hp.com Message-Id: <aabcdefg4516$foo@default> Subject: converting lumens to footcandles
Mark Pogany wrote:
> How do you convert lumens output into footcandles?
I'll take a stab at answering this one, though every time I
review the material I'm still left with some questions of my
own.
The following two definitions are extracted from Principles
of Optics (sixth edition) by Born and Wolf.
A lumen is a unit of _luminous flux_. One lumen is the luminous
flux emitted within a unit solid angle by a uniform point source
of luminous intensity 1 candela.
A footcandle is a unit of _illumination_. One footcandle is the
the illumination of a surface area of one square foot receiving
a luminous flux of one lumen.
So, if you know the rating of your light source in lumens then
you can find the illumination of an area in footcandles. For example,
if your lamp is rated at 100 lumens, and all that light is focussed on
an area of one square foot then the illumination of that area is 100
footcandles. If you allow all the light to shine on an area of 10 feet by
15 feet (150 square feet) then the illumination of that area is
2/3 footcandles.
You have to be a little careful, since most of the time you can't arrange
for _all_ the light emitted by a source to fall upon the desired area.
Thus the illumination in footcandles will be smaller than the
lumens divided by the area.
I think that's right. Hope it helps.
Regards,
Perry Malouf
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