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Grex Hardware Item 139: Optoelectronics, LEDs, Photocells, ...
Entered by rcurl on Sat Nov 1 05:29:09 UTC 1997:

For discussion of optoelectronic devices such as LEDs, CCDs, optoisolators,
bar graphs, etc.

9 responses total.

#1 of 9 by rcurl on Sat Nov 1 05:31:24 1997:

What is the precise meaning of "MCD Brightness" or "Luminous Intensity at 10
ma (mcd)", used in specifications for LEDs?

#2 of 9 by scott on Sat Nov 1 15:10:54 1997:

Well, all I know is that M stands for Merced (Mercad?  ??)...

#3 of 9 by rcurl on Sat Nov 1 18:29:36 1997:

I know about candles and lumens and the physics definition of luminous
intensity (lumens/m^2, for example), but what is listed for LED specs
either aren't expressed that way, or are even undefined. 

So what is a Merced or a Mercad (Merced is the river that flows out
of Yosemite Valley, but that isn't too illuminating....)?

#4 of 9 by n8nxf on Tue Nov 4 14:00:45 1997:

Try millcandella or .001 candle.  100 mcd @ 10 ma means that you will
get .1 candle of luminious intensity with 10 ma of current flowing
through the LED.  (I'm not sure if it is millicandella or millicandle.)

#5 of 9 by rcurl on Tue Nov 4 19:58:38 1997:

Thanks - but just to be absolutely clear: a candle (or millicandle) is not
a luminous intensity - it is a total flux. Luminous intensity is (say) 
candles/m^2, or more usually lumens/m^2.  Is the use of "luminous
intensity" in a catalog, and your use of it, just a simple departure from
the meaning of "luminous intensity" in physics, or is there something more
fundamental here? I'd be happy if mcd were just the total light flux
(lumens) and not an intensity (lumens/m^2). 

By the way, for anyone trying to follow this, 1 lumen = 1/4pi candles - it
is the light energy flux to one steradian from a point source of 1.0
candles, and there are 4pi steradian in the whole sphere.

#6 of 9 by n8nxf on Wed Nov 5 22:07:18 1997:

To be honest with you, I'm not really sure.  My H.P. Optoelectronics catalog
does not go into detail on the mcd term or exactly what it is.  Perhaps a bit
of resarch on the web is in order?  I do know that LEDs come not only in
various intensities but also with beams of various cone sizes, shaped by the
plastic lens on the front of the LED.

#7 of 9 by rcurl on Thu Nov 6 06:31:13 1997:

I was incorrect in #5 - "luminous intensity" is equivalent to "luminous flux",
though the matter gets confused when it is expressed as "per steradian". The
reference situation is a "one candle" source at the center of a sphere
with a radius of 1 meter. The luminous intensity or flux is the light
energy by unit (steradian) solid angle, expresses in *lumens*.

What I want is *illumination*, or lumens per square meter (a lux) or
per square centimeter (a phot). 

So, is an LED rated in terms of millicandles total - total flux over 
a sphere - or for some narrow angle in a forward direction. What I
would like (I have determined) is the illumination on the axis, as
a function of distance from the LED.

#8 of 9 by n8nxf on Thu Nov 6 11:01:41 1997:

My guess would be that the rating is for a given area at a given distance.
I say this because the mcd numbers go up as the viewing angle numbers go
down for the blue, Panisonic, LEDs on the back page of the Digi-Key
catalog.  All the other numbers stay constant, only the shape of the LED
package seems to vary.

#9 of 9 by rcurl on Thu Nov 6 15:26:49 1997:

A clue! BUt, only a clue... Yes, the only difference between those
LEDs appears to be the package - the IF (ma) are all listed as the same.
Is, then, that millicandles for the given viewing angle (at some
unspecified distance)?

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