Fw: Speaking of Light and a chuckle.

Brett Dikeman brett at cloud9.net
Sun Jan 20 00:07:12 EST 2002

At 10:34 PM -0500 1/19/02, Kev the Brit wrote:
>Good evening there Mr know it all, fall over yourself to correct me on this

You're assuming I'm going to insist I was right; I wasn't.  Initially
I was also going to say your statement is right, until I found a key
piece of info you missed(and I missed at first too doing more
research after I got your email.)  Read below for more on both.

>"MCSD is a measurement of luminous intensity. It stands for Mean Spherical
>Candelas (formerly known as MSCP or mean spherical candlepower) and is the
>total light output from a lamp measured in all directions.

Ahh, units.  The classic source of so much confusion, mine included
as we'll see.

Originally the unit in question was candlepower, not MSCP, so we're
saying -very- close, but different things(although as I will point
out below, I was wrong on several points in my original email.)

   The distinction is that MSCP would represent the total light
output.  MSCP is just a way of saying "all the light from the bulb",
like you said...Its light output in all directions...whereas CP can
be thought more of as a sample.  If the source is uniform, MSCP can
be extrapolated from the CP sample, so both are reasonably valid to
express the total brightness of a bulb; I'd be willing to bet that
when CP is listed, it's calculated from MSCP.  MSCP, if I understand
things correctly, is 4x a light's CP rating, since the unit of
"spread" in CP is Steradians, and there are 4 Steradians in a sphere.

One could think that since the manufacturer is saying CP, they must
-mean- MSCP.  We both fell for the same trap; I thought the same
thing(CP=MSCP), until I found the relation between MSCP and CP.

This is the kind of stuff marketing people DREAM of.  They can rate
their bulbs in MSCP if others are rating in CP, and slap on a number
4x bigger.  They'll make that larger number in really big print, but
probably won't stoop to saying "4x brighter!" since that's clearly
not the case(though really slimy marketing people could claim they
got confused :-)

Seperately I was wrong on a few points, so here are corrections on my
own statements, if I understand this all correctly and I'm repeating
it back correctly.

I originally said Candlepower involved light cast onto a certain
surface area at 1 meter etc...it is defined not by area, but its by
angle(or better put, beam spread):

"Standard unit of luminous intensity is Candela (cd), also expressed
as Lumen per Steradian (lm/sr) "

I probably transmogrified the definition for a Lumen:

"The luminous flux is a very basic unit of measurement for light. If
a uniform point light source of 1 cd luminous intensity (about the
intensity of a normal wax candle!) is positioned at the center of a
sphere of 1 m radius, then every area of 1 m2 on the inside of that
sphere will receive a luminous flux of 1 lm."

I was wrong about a second item.  One candle is NOT equal to 1 cd, it
is defined as 1.02(further proof scientists are a bunch of wackos.)

Some of the above was taken from:

The rest was from an excellent description of this whole units
mess(along with some food for thought about LEDs that I thought was
interesting) at:


(its originally a PDF, the above is a link through Google's PDF->text
conversion tool.)

On a somewhat unrelated note, listers, when shopping for bulbs for
things like headlights, should note that wattage doesn't equal
brightness; one of the items in the PDF above mentions bulbs are
rated in lumen/watts.  This gets back to Craig's original comment
that everyone has switched over to rating via CP; for the same bulb
code, wattage will be the same, so the only real important
factor(assuming you know your part numbers) is in fact CP(I think?)

Phew.  My head hurts.

"They that give up essential liberty to obtain temporary
safety deserve neither liberty nor safety." - Ben Franklin
http://www.users.cloud9.net/~brett/bdikeman.asc	(PGP Public Key)

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