Jim Elwell wrote:
>
> At 07:38 PM 3 March 2002 -0500, James R. Frysinger wrote:
> >The candela *is* photometrically related. Its definition is absolute
> >only at one frequency (corresponding to ~555 nm in free space). For
> >other frequencies, the luminous efficiency function is used. The CIE
> >standard luminous efficiency function is photonic (daylight).
>
> I am going to forward this post to the "expert" who told me otherwise and
> see what he says. He is an electro-optical engineer in far-infrared
> (cryogenic) radiometry and interferometry, so obviously does not use
> photonic units directly. So I suspect that he fed me bad info -- thanks to
> Jim F. for correcting me.
>
> Jim, it appears to me that you consider "photometric" and "photonic" to
> mean the same thing -- is that correct?
I used "photometric" because that is the term used in the literature
that I checked. You might point your expert to
Parr, Arthur C.,: The Candela and Photometric and Radiometric Units.
Journal of Research of the National Institute of Standards and
Technology, volume 106, number 1, January-February 2001.
By the way, this is available online at the NIST web site
http://www.nist.gov/jres
as well as by hard copy. This particular issue is the Centennial Issue
and it was edited by Barry N. Taylor, who very kindly brought it to my
attention last year.
> Also, do you concur with my "expert" in saying that nits are used because
> displays are distributed rather than point light sources?
I have never heard of nits as a unit of measurement, except as a
measure of the degree of infestation of one's hair by head lice. That's
not meant to be mean, just a statement of fact. It doesn't surprise me
that once again a community has devised their own unit name for
something that they use all the time.
Of course, I'm sure that in the rare event such a pet name makes it
into print, it is defined in terms of SI units in every paper containing
the term. Right? Otherwise, the authors risk failing to communicate
effectively outside their community. I was just thinking about that as I
drove to Columbia yesterday and glanced down to ensure that my speed had
not crept above 70 miphs (or is it now spelled "miffs"?). After all an
ozzie of prevention is worth a lub of cure. ;-)
Jim
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone/FAX: 843.225.6789
