calling cd/m2 ``nit'' when needed

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

> But don't use the "nit".  It has had its day.  The measurement community
> tried for decades to popularize its use; but that didn't happen.  Yes,
> it's simple to say and write; but let's use the official unit of cd/m2
> since that's what people see printed in articles and listed on meter
> scales.
>
> Terry McGowan

Terry, I suspect the reason why this special old name for cd/m2 did not
made its way into SI is, that luminances were difficult to measure and
hence seldom used explicitly. There are so many named units, why trouble
all engineers etc. with another one.

In any case, a text which uses a special name for cd/m^2, should be
properly introduced and perhaps even interspersed by occasional
remembering that ``nit'' is but a name and ``nt'' a symbol for cd/m2.

In my recent Czech text (on the night environment in our largest and
oldest national park) I give hundreds of luminance values. They are
commonly at the millinit, centinit or decinit level. Of course I could
write ``three millicandela per square metre'' or ``about 3 mcd/m2'' or
even ``0.003 cd/m2'' from time to time, but having such a long formulation
twice on a row and dozen times in a paragraph would make it horrible to
read. Even more so with a value like ``one fourth of a millinit'', which
is representative for natural moonless clear sky night luminances in
zenith. Having no name for the unit makes little problem, if the luminance
values are at the level of several (tens of) candles per square metre, but
it becomes troublesome when dealing with natural nighttime values a lot.

However, my experience is that all but most rigorous people simply discard
the m^2 part when speaking about common luminances at the one nit to one
hectonit level, naming the unit wrongly, but conveniently as ``candela''.
No wonder, even if true candela is a basic SI unit of luminous intensity,
this quantity is so seldom used in ordinary language, that people have no
problem when using ``candela'' as a unit of luminance instead.

A widespread analogy is discarding the ``per hour'' part in the unit of
velocity (``the car approached with a fifty-kilometre speed'', a common
formulation in Czech). Fortunately, there is not much risk of being
misunderstood.  (Water transport is more comfortable to speak about,
having the name ``knot'' for nautical mile per hour; it would be nice to
have a ``metric knot''...)

Luminances are the most (or the only?) visible quantities at all. Of
course, we perceive just their ratios, with no need for a unit, having a
difficulty to judge even a decadic order of magnitude. But as the method
to record and visualise absolute levels of luminance easily and accurately
will become entirely common in the future, much more widespread than
measuring illuminances (most families will have a digital camera, few have
a luxmeter), re-introducing a simple name ``nit'' is a must. It won't be a
quick process, regarding that even most (amateur) astronomers don't know
which quantity describes what they see, speaking about nonsenses like
``surface brightness'' etc.

I think we will soon need nits as much as we need pascals now.

jenik