Dear Maleki:

"Maleki" <maleki_m_@hotmail.com> wrote in message
news:931ypfdp10v5.1q0crew6br0oi.dlg@40tude.net...

On Tue, 24 Aug 2004 16:34:54 -0700, N:dlzc D:aol T:com
(dlzc) wrote:

Dear Maleki:

"Maleki" <maleki_m_@hotmail.com> wrote in message
news:6f9d6noyclue$.15fitfbm8l4u9$.dlg@40tude.net...

Why did SI adopt "luminous intensity" as one of the 7 base
physical quantities? What is exactly meant by it anyway?

It corresponds to a number of photons per unit area. No constraint on
wavelength.

Number of photons or the energy of them? What is it
"exactly".

Note that I was wrong about the "per unit area". I also believe that I was
wrong about the count of photons.

URL:http://physics.nist.gov/cuu/Units/current.html
<QUOTE>
The candela is the luminous intensity, in a given direction, of a source
that emits monochromatic radiation of frequency 540 x 10^12 hertz and that
has a radiant intensity in that direction of 1/683 watt per steradian.
<END QUOTE>

The units are:
[power/steradian] of photons with a specific energy. This equates to a
number of photons per unit time, per steradian. Yet it is also simply an
amount of power power steradian. This site seems to feel that it should
simply be a power-based measurement:
URL:http://www.electro-optical.com/whitepapers/candela.htm

Historically, it was simply a power-based measurement, from a black-body.

Also, in SI, mass is defined in a way irreproducible
elsewhere. It just refers to a chunck of "platinum-iridium"
sitting somewhere in Paris, withous saying how many atoms of
each ingredient is involved. I don't understand.

The only "absolute" measure of mass is the mole of Carbon-12. But this

is

not the basis of the definition of mass. The current standard of mass

is

as you noted, via transfer standards to the artifact in Paris.

Mole itself was defined by SI using that mass in Paris.

I don't think so, directly. It was *established* that 1 mole of C-12 would
have 12g of mass.

So
it is not reproducible elsewhere independetly. The SI was
supposed to be an effort to end this kind of dependencies.
Standards should be "accessible", in their own words.

It is. For a fee to the standards agency. Just like the standards. (And
I don't much like it either.)

Just
how do you make a duplicate of that chunk of mass somewhere
else independently? A mole of C-12 is 12/1000 times that
mass in Paris. But without that mass, say in Delhi, what do
you have?

Agreed. A real pain to count out a mole of Pt-Ir... ;>)

David A. Smith

First we have to agree what _radiant_ (N.B.) flux is in this context.
It is, in fact, simply the total power emitted by a source, invariably
measured in watts.

Then, we have to understand what _luminous_ flux is: simply a
frequency weighted version of radiant flux. That's all there is to
it.

The weighting function is _motivated_ by the frequency
sensitivity of the human eye, but we can think of it as simply some
arbitrary given function of frequency, so that the unit of "luminous
flux" (the lumen) is an objectively measureable unit.

Next we have to agree what is meant by "intensity" in this context.
Your sticking point is, I take it, "per unit solid angle in a
particular direction". I will concede the addition of the word "unit"
here is unfortunate. This is however an old confusion -- not in
anyway to derogate you, since old confusions are the best confusions,
and I struggled with this one in this context as if I had never met it
anyplace else, but it is just the same semantic problem encountered in
expressing any ratio or derivative as something "per unit" of
something else. We don't mean we have to consider the change over a
complete unit of the denominator!

[...]

What the definition is trying to say is that luminous intensity is
simply the parent extensive quantity (luminous flux) per solid angle.
This intensity in general is a function of the radial direction on the
sphere ("in a particular direction").

As for why it was necessary to include these peculiar units
specialized to human perception and the practical art of lighting
among the "7 base quantities" of SI, I don't know. Must have been a
political thing: maybe the lighting engineers had a powerful lobby?
:-)

Maleki wrote:

On Wed, 25 Aug 2004 04:54:51 +0000 (UTC), Jon Bell wrote:

In article <6f9d6noyclue$.15fitfbm8l4u9$.dlg@40tude.net>,
Maleki <maleki_m_@hotmail.com> wrote:

Why did SI adopt "luminous intensity" as one of the 7 base
physical quantities? What is exactly meant by it anyway?

Luminous intensity is the physical quantity that determines the
"brightness" of a light source as perceived by the eye.

Since when this would constitute enough basis for choosing
it as one of the 7 base quantities? That was my question.

Part of your confusion could be due to the (so-called :)) science of
radiometry/photometry. "Intensity" itself can mean 'per unit area' or
'per unit solid angle', depending to whom you are speaking with.

"Source brightness" is the concept that is desired to be standardized,
but since sources can not only vary in total power and spectral output
but also the direction the power is distributed, one must be rather
pedantic about specifying the physical property of interest. Note that
"brightness" is a photometric quantity in it's own right, but is not
really what the non-specialist refers to.

http://physics.nist.gov/cuu/Units/candela.html

Has some historical background on why the standard is so goofy- there
used to be an honest-to-god standard candle flame. The 1/683 business
has to do with the response of the standard eyebone at green (that's the
540 THz part).

<snip>

Reference:
<http://www.schorsch.com/kbase/glossary/luminous_intensity.html>
and pages linked from it.

At the above site it begins with:

"The luminous intensity is the luminous flux emitted from a
point per unit solid angle into a particular direction."

In other words it doesn't even know how to speak physics,
let alone understanding it or trying to make others
understand anything. I can already see two mistakes in that
one-sentence statement.

I don't understand, the sentence as written makes perfect sense.
Intensity is flux per unit solid angle, not flux per unit area.

The
flux per unit solid angle from all sources save an isotropic radiator
varies with direction.

<snip>

Everything in physics that has to do with a measure of the
quantity of the matter involved, is tied by SI at the base
standards to that lump of mass, and nobody knows how many
atoms of what - how much material - _is_ there in that mass!
What kind of "base standard" is this. It is neither
"accessible" nor "invariable". The moment you make it
accessible (using Avogadro's number) you've redefined that
base standard! What fools one has to deal with.

Perhaps, but it is a better system than using the length of someone's
thumb, or the length of a day. To ask "How do we know the standard
kilogram contains XXX atoms" isn't the point. The standard mass is
unique in that a specific physical object is required for the definition
(as opposed to the meter, second, etc..), although there are efforts to
tie mass to electrical current, I believe.

http://www.npl.co.uk/mass/faqs/kilogram.html

is interesting reading.

--
Andrew Resnick, Ph.D.
Department of Physiology and Biophysics
CWRU School of Medicine