Design Aspects - Road Lighting: Fundamentals, Technology and Application - page 193

given by the installation such that the lighting level and quality can never fall below
specification. Here not only the lighting quality aspect plays a role, but cost and
energy aspects as well.
The factors influencing the rate of deterioration are lamp failures, lamp-lumen
depreciation and luminaire fouling. In Chap. 10 “Equipment: Lamps” the lifetime
of lamps has been defined and examples have been given of possible lifetime val-
ues for the different lamp types. Accurate information has to be obtained from the
lamp manufacturer for the actual lamp type and wattage in use. It should be noted
that the actual lamp life is influenced by several factors not controlled by the lamp
manufacturer; for example: type of ballast, ambient temperature, operating voltage,
voltage fluctuations, frequency of the on/off switching, burning position, severity of
mechanical vibrations, and so forth. Variations in lamp life also occur according to
the wattages of the lamps considered. Lamp-lumen depreciation, that is to say the
fall in light output because of lamp ageing, occurs with all lamps. How great this is
at a given moment in the life of a lamp is dependent upon the type and wattage of the
lamp. Again, actual information should be obtained from the lamp manufacturer. As
with lamp life, lumen depreciation is also dependent on practical operating factors,
such as ambient temperature, voltage fluctuations, burning position and severity of
mechanical vibrations.
Another, serious, loss in light output is that brought about by the accumulation of
dirt on the light-emitting surfaces of the luminaires. The severity of the fouling will,
of course, depend on the type of luminaire involved and the amount of pollution
locally present in the atmosphere. Closed luminaires with a high IP factor (see
Chap. 11) afford much better protection against dirt accumulation and also corrosion
than do open luminaires. CIE ( 2003 ) published luminous maintenance factors for
dirt accumulation for different type of luminaires characterized by their IP code, the
type of environment (low, medium and high pollution) and the period the luminaires
are in use after the last serious cleaning operation. The maintenance factor is defined
as the value of the average luminance or illuminance on the plane to be lighted, after
a certain period, relative to the average luminance or illuminance as obtained from
the new installation. Table 13.2 shows part of the CIE maintenance factors required
due to dirt accumulation on luminaires.
In the United Kingdom, measurements of different IP6x luminaires used in differ-
ent pollution-category environments, have been carried out more recently (Sanders
and Scott 2008 ). The conclusion to be drawn from these tests is that high-IP type of
luminaires accumulate less dirt than would be expected from the CIE values. The
values resulting from these tests are given in Table 13.2 under the heading “UK”.
Interestingly, their tests show that luminaires mounted at heights of 8 m or more ac-
cumulate less dirt than do luminaires mounted at lower heights of 6 m and especially
4 m. The Table therefore gives, for high conditions of pollution, one maintenance
value for higher mounting heights and one for lower mounting heights.
The total effect of the various factors influencing the light depreciation experi-
enced by a road lighting installation can easily reduce the lumen output by as much
as 25-40 % at the end of the maintenance interval. By contrast, in pollution-free
areas, a reduction of 15-30 % per year is quite possible. The total effect has to be