[Darksky]Re: light trespass threshold

Jan Hollan
Mon, 5 Apr 2004 16:01:13 +0200 (CEST)

Dear Susan,

we have a bullet-proof knowledge, that more than more than one sixth of
Czech population (I wrote the abstracts in a hurry, one third has been
estimate from the first small query, not from the representative one) over
18 years are disturbed by moonlight in their sleep (it's whole one fourth
for old people). Moonlight is never more than 0.02 fc at the outer surface
of a vertical window (it can be up to 0.03 fc for roof windows).

Consequently, 0.02 fc is by no means a safe limit for not-disturbing
imissions. From the first query, it follows that for ninety per cent of
those who are disturbed by moonlight, just the nights around full moon are
a problem. So, 0.005 fc disturbs not much more than 2 per cent of people.
We might set this as a safe limit for window illumination. Tolerating more
than 0.01 fc is completely excluded, if the goal of artificial lighting is
other than a ``natural'' selection of light-tolerant homo sapiens sapiens

> I do not agree with this study:
> http://www.ile.org.uk/documents/guidance-notes-light-pollution.pdf

It's no study at all. The only interesting feature is that ILE assume that
people got more tolerant in cities already... phylo-genesis does not
proceed that fast, however.

> .5 fc is TOO MUCH LIGHT.

We can say almost exactly: it's probably one hundred times the tolerable
value, and, beyond any doubt, more than 25 times the tolerable value, if
the light is as white as the natural one.

Perhaps (no research has been done), it might be just fifty times the
tolerable value if it's HPS light or just twenty times the acceptable
limit if it's LPS light. But it is just an extremely optimistic guess, I'd
say that even for LPS the tolerable illumination of windows is below 0.02
fc. It is something that should be studied, as it might become a strong
argument in favour of using blue-devoid light. If any late-night lighting
at all, then fully shielded LPS as a minimum demand.

> Susan Harder
> http://www.darkskysociety.org/