Re: Internally illuminated signage

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

I agree with Steve, 200 cd/m2 is surely enough for all purposes. It's a
very generous limit. Anything more means glare and reduced readability, if
you are not viewing the board from within a strongly lit room. Large
signs should have still lower limits, as I'll try to explain.

To get some taste of how much it is, 250 cd/m2 is a common maximum
luminance modern TFT monitors can achieve (this is when they have white
screens, carrying no information at all). This maximum is suitable for
working in daytime. It is far too much at night when working indoors,
therefore the monitors have always some tools for reducing luminance a
lot, by one order of magnitude.

Outdoors, common nighttime luminances of lit surfaces are tenths to units
of candela per square meter. Very comfortable span of luminances is 1:10,
this is a typical one for shadow versus direct sun illuminance. 1:30 is
still OK, representing asphalt in shadow vers. sunlit white concrete. This
can still be captured in one image when photographing. 1:100 may be really
unpleasant, the maxima are becoming to be perceived as glare already.

But 200 cd/m2 is 100x more than a common upper luminance of lit pavement,
which is 2 cd/m2. So it should be an upper limit, anything more becomes a
danger for drivers and pedestrians. Italian road code has a safer upper
limit for signs, 150 cd/m2.

It is possible to obey this limit even for very small signs. Let's have a
5W CFL (emitting 230 lm, as I read from its packaging) inside a mostly
white sign of one square foot, i.e.  of some 0.1 m2. This means there is
an equivalent of up to 2300 lm/m2 available for illuminating the sign from
inside. Assuming inner losses fifty per cent and fifty per cent
transmittance, it reduces specific emissions to one fourth, to about 600
lm/m2. If being lambertian (perfectly diffusing), the sign would obey the
200 cd/m2 limit (divide the specific luminous flux by pi) from all
directions.

200 cd/m2 would be definitely too much if the sign would be large and
viewed from a proximity (hence visible as large angularly, covering a
large space angle). Then it would compromise adaptation of your vision to
another areas, which have at least hundred thousand times lower luminance
(but can have million times lower luminance: illumination of the ground by
natural clear night sky translates to the luminance of ground about 0.0001
cd/m2). 200 cd/m2 luminance is no sufficient upper limit for large signs.

Moon luminance is a nice example, when all readability is lost completely.
Look at rising full moon -- it is so full of details that it might be a
challenge to plot all of them. This is thanks to the filtration of its
light by the air and to the twilight, with the sky and ground being still
quite bright. And then look at it when it is high in the sky -- you barely
notice any darker spots there, your retina is saturated by its glare. No
wonder, you look at a surface, which is fully illuminated by the Sun!
What's a fortune lunar surface is much darker than any terrestrial one
apart from deep clear sea and fresh asphalt roads, so that it's luminance
is about 3 kcd/m2 even if lit by about 100 klx of full sunlight (I
simplify it a bit). Thousands of cd/m2 are _completely incompatible_ with
outdoor night environment, if not meant as a ``light'' to illuminate our
way (Moon has been meant so perhaps...) -- of course, such a light is to
be outside our common field of view to be tolerable (a slight tilt of
head or a visor does the job).

My proposal for sign limits, which would ensure excellent readability and
little pollution is contained within the
  http://amper.ped.muni.cz/light/law/Jan06/lpJan06.html
 proposal for a chapter in any Clean Air Act (pdf and other formats of the
text are available there too). The relevant part reads:

 ``if it is a surface which
   conveys text information or image instructions, its luminous intensity
   is allowed to reach
       two hundred candelas, or
       three hundred candelas for a surface larger than 5 m^2, or
       five hundred candelas for a surface larger than 30 m^2.''

Limiting luminous intensity is more convenient than limiting luminance.

It's a measure taking in account the sign size (large signs should have
lower luminances, from many reasons). And there is no need to measure the
exact size of the sign, a mere guess to which of the categories it belongs
is OK. And a guess of your distance from the sign. Then hold a luxmeter,
point the sensor toward the sign and take a measurement. If the sign is
not the only source around, take another measurement, casting a shadow on
the sensor -- blocking the direct light from the investigated sign. The
difference of the readings is the luminous flux density from the sign.
Multiply it by a square of your distance from the sign and you get the
luminous intensity of the sign (it's one step shorter than what George
wrote already, just pay attention to distance units, use SI ones).

200 cd as a limit for sign luminance is compatible with limits for
unshielded lights (a bare CFL emitting 1800 lm has some 180 cd luminous
intensity at most). It's achievable even for smallest signs. 500 cd for
extremely large surfaces (several hundreds m2) translates to the usual
upper luminance limit for lit surfaces of about 1 cd/m2 (Big Ben in London
achieves this luminance at night, before they switch off its illumination
late at night).

Well, these are my views of the issue, based on many previous versions of
the rules (taking into account both physiology and technology). For future
night environment and night adaptation protection, even more strict limits
might be implemented. However, the limits as given above can be easily
demanded for today's cities and countryside, as there is no discrepancy
between existing best examples and 200 (300 (500)) cd luminous intensity
limits.

jenik

PS. There is a good, but laborious way to study luminances. Lots of
luminance images of ski slopes and their surroundings are contained within
my new report for the largest national park in Czechia. All is in Czech,
but see the colour scale (I use the name nit for cd/m2 there, as in Czech
it is a neutral word) at the right of the report's pages.  The
investigated slopes are lit up to 20x more strongly than obeyed (their
illumination is to be reduced a lot before the next season). Our
recommendation for ski slopes is 0.1 cd/m2 minimum, and 0.2 cd/m2 maximum
(aimed down the hill, excluding any glare). See ev.
  http://amper.ped.muni.cz/noc/krnap/2006/