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RE: High Intensity Light Question

Tad -

<cd/cm^2> is a unit of luminance (formerly, photometric brightness) and
is a property of a ray [realistically, the average of a bundle of rays].
 It is the differential luminous flux per unit projected area in the
direction of the ray and per unit solid.  Thus this unit is its most
basic form is lumens per cm^2 and per steradian.

<footcandles> is a unit of illuminance, i.e., areal flux density.  The
unit for <footcandles>  is lumens per square foot (incident on an area).

These units are not commensurate, and you cannot convert from one to the
other.  I suspect that you have mixed up units in some manner or other.
If you had adequate information about the geometry of incoming luminance
at a point, you could then calculate the resultant illuminance at that

Robert E. Levin
Corporate Scientist
OSRAM SYLVANIA Development Inc.
71 Cherry Hill Drive
Beverly, MA 01915
(978) 750-1594

>From: 	Tad Blanchard[SMTP:Tad.M.Blanchard.1@gsfc.nasa.gov]
>Sent: 	Thursday, April 02, 1998 10:30 AM
>To: 	Multiple recipients of list
>Subject: 	High Intensity Light Question
>Hi Radsafers,
>About 5 years ago I had to figure out how to convert candela/cm^2 to
>footcandles.  I lost my reference data but seem to remember that 
>one candela/cm^2 approximated 500 footcandles.  
>Does anyone have the formula for this conversion?
>************************** /^\   /^\ ***********************************
>Tad  Blanchard            /__ \ /___\   NASA-Goddard Space Flight Center
>Nat'l Health Svc, Inc          O         Code 205.9, Greenbelt, MD 20771
>Sr Health Physics Tech        / \                    Phone: 301-286-9157
>Assistant RSO                /___\                   Fax:   301-286-1618
>                 mailto:Tad.M.Blanchard.1@GSFC.NASA.gov
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