RE: Maximum levels of intrusive lighting

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

> It's seems very difficult to have a norm of intrusive light by public
> lighting under for instance 5 lux. Because the distance between windows
> and the lighting is so small in some cases.

Well, 0.1 lx at the window plane can be achieved even with public
lighting, when faint enough after curfew. Assuming almost no direct light
from luminaires (this can be always achieved with large enough additional
shields), just the light from the lit ground comes to windows.

Let's assume a vast plain with a constant luminance being in front of
windows, and a dark sky above. This is equivalent to getting light from
just half of the hemi-space. Having a whole hemi-space with 0.1 cd/m2, the
illuminance of the window would be 0.31416 lx. With dark sky and just the
bottom half of the space having 0.1 cd/m2, the window illuminance would be
just 0.15 lx. And having a street which is not infinitely broad, the
window illuminance can be easily below 0.10 lx.

So, street luminance (as apparent from the window) of 0.1 cd/m2 is
compatible with having 0.1 lx maximum illuminance at the window. What's
the corresponding street illuminance?

Having an albedo of 15 % (a common one) and a lambertian surface, the
illuminance can be 0.1 cd/m2 * pi sr / 0.15 = 2.1 lx. Whole 2 lx average
street illuminance is compatible with having not more than 0.10 lx window
illuminance!

2 lx average is a plenty of light after curfew, I can't imagine a
situation where it would be not large enough. It's ten times the winter
full moon... Or, comparing it with a common situation in which people read
or wrote at night using a candle, in the past centuries: a table 0.5 m
from a candle which is 15 cm high is illuminated by 1 lx only.

For a two-sided street with super white facades on the opposite side, the
average street illuminance might be needed to go down near to 1 lx, to
keep the window illuminance below 0.10 lx even at the first floor. OK,
this is a limit to obey in such circumstances, not those generous 2 lx.

Let's stress once more: outdoor lighting is not there for cars, but for
people, esp. those without their own headlights. At night, it has to be a
compromise between the need of darkness for most and the need of enhanced
visibility for the eventual (and rare) outdoor pedestrians (enhanced
compared to the visibility offered by natural light at night).

Quite often, the compromise should be switching off the lights altogether.
Like usual in Austrian state Carinthia at late night (Andrej, Martin,
Chris and Pavol witnessed that when returning from the successful night
observation of one of least-polluted skies in Europe on Oct 8 early Monday
morning, after the Bled conference). This is a secure (and the most
climate-friendly) option how to get easily far below 0.1 lx of artificial
window-illuminance component.

jenik