Re: calling cd/m2 ``nit'' when needed

[Jan Hollan <jhollan@amper....muni.cz>]

Hi Chris,

your quantity is radiance (measured in W/(m2.sr)), an analogy of
luminance. Photon radiance (measured in 1/(s.m2.sr) is another quantity.

Candela should be preferred to lumen, whenever appropriate, as it it the
base unit. Even cd/m2 looks less horribly then lm/(m2.sr).

A simple way of using nits not puzzling the reader is writing
 5 nt (cd/m2), i.e. adding the SI way in parentheses. This is very common
in texts e.g. about LCD monitors (in fact they write 5 nits (cd/m2), but
this is no good habit, either ``five nits'' or ``5 nt'').

There is little hope to include nits into SI: there are just 22 named
units other than the seven basic ones. Becoming a named SI unit is a
privilege. Having lux and lumen together with the basic candela means
photometry got a significant position long ago already. (See ev. a list
within
              Essentials of the SI: Base & derived units
        URL: http://physics.nist.gov/cuu/Units/units.html  ).

I found a nice introduction to luminance (searching how common is the name
nit), within http://www.factbites.com/topics/luminance. It says, among
other,
 ``Photometric brightness is an old and deprecated term for luminance.''

Brightness is hyperlinked there, try it. I'm afraid it had been hardly
ever properly defined in astronomy. I have made a SI-compatible
definition, but in full it's available just in Czech (within
http://astro.sci.muni.cz/pub/hollan/a_papers/si_fot/). In English, I wrote
just a single popular text on astronomical photometry, see ev.
 http://astro.sci.muni.cz/pub/hollan/a_papers/english/

Apart from http://www.schorsch.com/kbase/glossary linked to from
factbites, an outstanding tutorial on radiant/photometric issues is
             Radiometry and photometry in astronomy
        URL: http://www.stjarnhimlen.se/comp/radfaq.html
 -- see how cautiously the authors write about expressing luminance
through an equivalent of star per unit solid angle.

> Lighting engineers should take the trouble to think in units of night
> sky, if so they would realise what their lighting causes.

I cannot disagree more! Astronomy has in fact NO units and even NO
quantities anybody can learn, regarding photometry (just in radiometry,
there is jansky). There is no generally accepted unit ,,mag``. And if you
accept some definition (creating an analogy of decibel, 2.5 mag = 10 dB)
you cannot divide mag by arcsec^2. If it would describe any quantity, then
inevitably that quantity for two arcsec^2 had to be twice larger then per
one arcsec^2 (like 43 instead of 21.5). Similarly, ``dB/m^2'' can serve
just like a concise joke.

Incompatibility of common astronomy language with physics in general and
SI in particular is nothing we should be proud of. I've introduced a
non-conflicting metrology and terminology system into didactics of
astronomy here in Czechia, but just my closest teachers and most cautious
pupils are adhering to it. Most astronomers don't care.
Physics IS difficult even more than SI.

jenik