"Greg Mayman" bravely wrote to "Mike Ross" (30 Aug 03  08:30:00)
 --- on the heady topic of "VEHICLE LED'S"

 -=> Mike Ross said to Roy J. Tellason
 -=> about "VEHICLE LED'S" on 08-26-03  22:41.....

 GM> My books lead me to believe that there is no direct conversion
 GM> without other factors coming into it.

 GM> The candela is the SI unit of luminous intensity, or the intesity
 GM> of light falling on the target.

 GM> The lumen is the SI unit of luminous flux, or the amount of light
 GM> emitted by the source.

 GM> For a given amount of luminous flux, the luminous intensity will
 GM> vary as the inverse sqaure of the distance, and as the inverse
 GM> square of the angle of the beam.

 GM> While it is probably more use to know the luminous intensity of
 GM> the beam from an LED or any other directional light source, it
 GM> should be stated as so many candelas at a distance of whatever.

 GM> I wonder what the standard distance is in the case of LEDs...

IIRC the Lumen is related to the Steradian which was used to describe
the area (a square foot?) illuminated on the surface of a sphere with a
given angle at a given distance (1 foot?) by a unit candle but I think
the Candela is relatively recent unit and is what is now typically used
to describe a LED's light intensity. It is a point source type unit. I
think that at a standard angle and intensity there is a unit relation
that can be made between the Lumen and the Candela, if I'm not totally
out to lunch. I dimly recall writing in my notes something about it. Now
where are those notes...!

 Mike
 ****

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