UNIVERSITY OF HONG KONG

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1 b. Aiming i.e. proper aiming to prevent spill over. K.F. Chan (Mr.) Page 12 of 22 Jan., 2010

2 c. Positioning, e.g. - Wider distribution of floodlights can reduce vertical components reflected upwards. - Avoid lighting objects, like signboards, from below. Illuminating a vertical signboard from below will cause light reflected off the signboard to bounce upwards thus polluting the sky. - Furthermore, in sports stadium floodlighting, the trend is away from tall corner towers to mounting floodlights on or under the roof of the spectator stand this helps to contain the light. K.F. Chan (Mr.) Page 13 of 22 Jan., 2010

3 At Preston North End FC s stadium 25 Thorn floodlights have been installed on each of the stand s 2 45m high corner towers. Each floodlight uses Osram 3kW linear metal halide lamps providing an average horizontal illuminance of 800 lux. The Sporting Life by Patricia HUNT, LD+A August 2005 K.F. Chan (Mr.) Page 14 of 22 Jan., 2010

4 In sports stadium floodlighting, the trend is away from tall corner towers to mounting floodlights on or under the roof of the spectator stand this helps to contain the light. Reading FC s Made Ski Stadium is home to 114 Thorn floodlights mounted on the east and west stand roofs. Each unit houses a 2 kw linear metal halide lamp providing an average horizontal illuminance of 1100 lux over the pitch. K.F. Chan (Mr.) Page 15 of 22 Jan., 2010

5 Up-light spills over into neighbouring areas and pollutes the sky Improperly aimed floodlights spill over into neighbouring area Improperly aimed security lighting still spills over into neighbouring area Proper design and positioning limit spillages yet able to provide security illumination K.F. Chan (Mr.) Page 16 of 22 Jan., 2010

6 c. Reduction of reflection Theoretically by reducing the reflectance of the surfaces on earth, thus limiting reflected upward light at night, light pollution can be reduced. In practice, however, the possibilities to manipulate the reflective characteristics in the real world are very limited indeed. Having said this, wider spacing of outdoor luminaires with wider beam angle will cause less light reflected upwards when compared with installations employing narrow beam angle luminaires at a closer spacing. K.F. Chan (Mr.) Page 17 of 22 Jan., 2010

7 Angle of floodlights and the elevations are very important both in controlling light spill and in reducing reflection. Narrow beam units require closer spacing, narrower beam distribution and may result in poorer vertical illumination for facial identification purpose. Also reflected light from car tops may increase light pollution. Adopted from Lighting for Security by Ted Ace, IESNA journal LD+A Oct 2005 Wider distribution requires fewer poles and provide better vertical illumination for facial identification purpose. Adopted from Lighting for Security by Ted Ace, IESNA journal LD+A Oct 2005 K.F. Chan (Mr.) Page 18 of 22 Jan., 2010

8 d. Reducing air pollution The effect of air pollution should be included here. Although not a lighting technique, air pollution does affect extent of light pollution. Furthermore, inefficient lighting wastes electricity, which in turn increases air pollution. Air pollution often creates haze, or even clouds of smoke and dust; the light from lighting installations partly is scattered in these clouds, and partly absorbed. The net result of air pollution on the sky glow, particularly that of far away observatories can therefore be positive or negative. When the scatter predominates, the sky glow many increase, whereas, when the absorption predominates, the net result may be a decrease of the sky glow. K.F. Chan (Mr.) Page 19 of 22 Jan., 2010

9 e. Using monochromatic light astronomical friendly light sources As is mentioned earlier, the most effective way available is the use of quasi-monochromatic light sources, more in particular the use of low-pressure sodium lamps. These lamps emit a very narrow spectral band in the yellow part of the spectrum. Two advantages are obvious: not other spectral regions are involved, so that observations in other spectral regions either photography or spectroscopy are hardly affected. Secondly, as the yellow line is close to the maximum of the sensitivity of the eye, the luminous efficacy of low pressure sodium lamps is high they are by far the most efficient light sources available at present. Finally, they hardly emit any radiation in the non-visible parts of the spectrum. On these grounds, the application of low-pressure sodium lamps near sites of astronomical observatories is universally recommended. K.F. Chan (Mr.) Page 20 of 22 Jan., 2010

10 f. Filtering the light UNIVERSITY OF HONG KONG A countermeasure against the disturbance of light pollution is to narrow the spectral range of the emitted light. light, but obviously it is less effective. The idea is similar to using monochromatic [Text adopted from Outdoor lighting: Physics, Vision and Perception by Duco SCHREUDER and Lighting the Match, Building Services Journal, July 2002 by Paul HADDLESEY.] K.F. Chan (Mr.) Page 21 of 22 Jan., 2010

11 Spectrum of lamps Florescent halo-phosphate SOX Florescent tri-phosphate SON Tungsten GLS MBF (Adopted from Lighting by D.C. Pritchard) K.F. Chan (Mr.) Page 22 of 22 Jan., 2010