Outdoor Lighting and Crime - Amper

uildings, monuments, structures, symbols and advertising signs. The subjects of Boyce et al.
(2000) were less fearful when there was less glare. Shielding can be an effective solution to
glare from luminaires, but generally not for glare from illuminated objects. The solution
therefore is to minimise floodlighting, or to do away with it altogether. This is generally a
better approach than the currently popular fix of adding light to the shadows while leaving the
cause of the problem untreated.
Full-cutoff (FCO) luminaires are designed to have a relatively small glare effect when the
lamp is seen at about 10 degrees or less above the horizontal, an extremely common situation.
Their geometrical cutoff is horizontal. Even less glare is possible by having geometrical
cutoff angles lower than horizontal (eg 10 or 20 degrees down) although generally some light
will still be emitted up to the horizontal or higher (eg Kramer 2001) because of geometrical
constraints, manufacturing imperfections and diffraction of light.
FCO light distribution can be obtained by using a suitably shaped light shield with a polished
obtuse conical reflective skirt to intercept and redirect the near horizontal light from a semicutoff
luminaire. This light then adds useful illumination where it is needed instead of being
wasted. 91 In turn, this may allow a lower power lamp to be substituted, resulting in a useful
decrease in lighting costs. Whether as a retrofit or already incorporated in replacement
luminaires, FCO shielding is a highly effective way of reducing glare from outdoor light
fittings, including streetlights, pedestrian lights, floodlights and sports lights. The majority of
streetlights made by the largest manufacturer of them in the USA are now FCO types (eg Di
Sora 2000, Fleming 2002).
7.2.2 Cloud cover and ambient light
Artificial skyglow visible from the ground consists of light scattered downwards by
atmospheric constituents. The amount of light involved is usually much smaller than the
upwelling mix of waste and used light. But when there is cloud in the sky, the cloud
undersurface and cloud volume above it is illuminated by the upwardly directed light, a
substantial part of which is scattered back towards the ground. The amount so redirected is
dependent on variables such as the area containing the various sources of waste light, the type
and height of the cloud and the proportion of sky covered by the cloud.
Excessive upward waste light can be detected by observing isolated cloud patches seen
against the moonless night sky. If the cloud is brighter than the sky, the waste is excessive.
Clouds should appear black at night, even above large cities. Compared with looking for the
Milky Way from downtown, this is an easier test for waste light.
In the suburbs of a typical medium-sized city, illumination by skyglow on a clear moonless
night may be obvious only within an unlit park or backyard. The ambient light can be
91 For semi-cutoff luminaires with typical cant angles, the upwardly directed unused light is
likely to be in the range of 2% to 30% of the total output, and some of this would be lost in
the redirection process. Offutt (1997) claimed that a commercially available ‘Sky Cap’ can
increase the useful illuminance from roadway luminaires by [or ‘to’?] 115%. Other authors
quote values in the order of 50% increase in the usefully lit area on the ground, which seems
optimistic.
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