skyglow from various uplights

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

Once again, the base of my claim:

One quantity
  (amount of total uplight)
 and the other one
  (manmade increment to the luminance of the sky)
 are not proportional to each other, if the uplight has varying proportion
of components of different origin, with very different angular
distributions:
  the ``inevitable'' reflected one, from lit terrain,
  and the obsolete direct one, from non-FS luminaires.

Just if the direct component would vanish, then the proportionality would
hold.

Of course we care about uplight, but should never simply add the direct
light component to the reflected one. The direct one has to be mostly
multiplied by 3 to 100 before being added (properly ``weighted'', as it
contributes more to skyglow), if the sum is to have any sense for thinking
about clear sky luminance.

Many lighting experts, if not most, insist that fully shielded fixtures
don't perform well if the task is minimising total (unweighted) sum of all
light going upwards, and say that demanding full shielding may cause
increase of skyglow. If we would accept this dispute, we'd never win.

Our answer should be, that a simple sum of all upward lumens is mostly
irrelevant. Any properly computed contribution (of a lighting
installation) to skyglow, within cities and especially far from cities,
favours fully shielded fixtures tremendously. No sag lens fixtures can
even approach them.

Simple sum of all uplight is a good quantity, BUT:
 just for the purpose of comparing various fully shielded designs
 among themselves.

This is no purely academic issue. IDA had accepted lectures on the amount
of uplight (a simple, unweighted sum of it) on many its meetings, and the
lecturers always assumed a straightforward correspondence between such
quantity and skyglow. It's urgent to declare explicitly, that the IDA
position is different, adhering to the published research papers (by
Pierantonio Cinzano) and common sense.  (However, the IDA position is
currently unclear, as far as I know...)

 merry Christmas,
Jenik

 (as for considering double scatter, I don't expect it might affect the
numbers in my educational figure a lot)