Outdoor Lighting and Crime - Amper

saturation problem, parts of these discrepancies are doubtless due to difficulties of matching
areas and populations between space and ground measures. Another possible reason is that
the cities brighter than Las Vegas might have had snow cover, which would make these cities
appear brighter to the satellites. Las Vegas itself did not have snow cover.
Appropriate online or hard copy historical weather data could not be found for Clermont-
Ferrand. This city is on high ground, however, and the satellite observation was made on
1997-01-13, ie during the northern hemisphere winter. Ciudad Juarez has snow only on about
2 days a year, but has 53 icy days/year (Ciudad Juarez 2001). The satellite observation was
on 1997-01-11. Wunderground (2002) gave details for the day: the minimum temperature
was 4.0°C, no snow event was recorded and no snow depth was given. It is possible that
some frost was on the ground.
Wunderground (2002) also gave details for the day of observation for the Canadian cities in
Table 3, except Trois Rivières. They had thick snow cover recorded with the exception of
Montreal, which did have a snow event on the day, however. Minneapolis also had thick
snow cover.
An attempt is made here to estimate what effect the presence of snow might have had on the
satellite measures. Newly fallen clean snow has a visible total reflectance of over 90% (eg
Leeds 2003). Subsequent contamination of the surface, eg by dust and soot, can reduce this to
as low as 20% (Klein, Hall and Nolin 2000) or even lower if the snow turns to dirty ice. The
reflectance of the typical built environment is about 10%. At first sight it might be thought
that snow cover could increase the satellite measure by up to nine times but this is without
taking account of important factors such as reduced reflectance of snow in the near infrared
(ie within the DMSP OLS sensor spectral response region), and the waste light radiated above
the horizontal by illuminated signs, internally lit windows and fixed inadequately shielded or
poorly aimed luminaires. Snow may also discourage or prevent the use of motor vehicles and
thereby reduce the total amount of light radiated by vehicle lights.
Assume that the upward waste light ratio (UWLR) for streetlights and other public lighting is
0.15. (Typically it is zero for full-cutoff types, 58 less than 0.15 with semi-cutoff types and
more for mercury vapour ‘flower pots’, globes and wall packs.) Assume that the UWLR for
all advertising signs, floodlights, decorative lighting and external windows is 0.5, and for all
vehicle lights, 0.1. Denote the total source flux in lumens as S for street and path lights, A for
signs etc. and V for vehicle lights. Then the total direct upward flux TDU from these sources
is
58 Lamps shielded and mounted so that no geometrically direct rays from the face of the
luminaire are emitted above the horizontal are called ‘fully shielded’ (FS). Some countries
specify this as limiting the horizontal intensity in any azimuth to less than (integer) 1 candela
(cd) per 1000 lumen (1 klm). The Illuminating Engineering Society of North American
(IESNA) specifies an additional requirement, that the light intensity does not exceed 100
cd/klm at 80 degrees above the nadir in any azimuth. Compliant luminaires are called ‘cutoff’
or preferably ‘full-cutoff’ (FCO) (IDA 2002b, section 9.16).
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