Civil Engineering Reference
In-Depth Information
illumination typically increases quality in manufacturing and manufacturing yield. It is ill
advised to cut down on the illumination to save a few pennies.
The efficiency of a light source is measured in lumens per watt (1m/W). As illustrated
in Table 4.3, some light sources are very efficient whereas others are less efficient. But
there is an important trade-off, namely, the color rendering of the light. The color
rendering index (CRI) is a measure of how colors appear under a light source as
compared with daylight. A perfect CRI score is 100. The main concern is that the color of
the light may distort perception. Low-pressure sodium light, which is intensely yellow,
makes faces look grey and should not be used indoors. It is mainly used for outdoor
lighting, but even in this situation it is difficult to, say, find a car in the car-park because
all colors look similar. Measures of light source efficiency and color rendering are also
presented in Table 4.3.
TABLE 4.3
Efficiency of Light Sources and
Their Color Rendering Index (CRI)
Type
Efficiency
(lm/W)
CRI
Comments
Incandescent
17-23
92 The least effective but most commonly used light
source
Fluorescent
50-80
52-8 Efficiency and color rendering vary
9
considerably with type of lamp
Coolwhite Deluxe
89
Warmwhite Deluxe
73
Mercury
50-55
45 Very short lamp life
Metal halide
89-90
65 Adequate color rendering
High pressure
sodium
85-125
26 Very efficient, but poor color rendering
Low pressure
sodium
100-180
20 Most efficient, but extremely poor color rendering;
used for roads
The maximum value of the CRI is 100.
Adapted from Wotton (1986).
Incandescent light produces the best color rendering, so that faces look natural, but its
efficiency is only 17-23 lm/W, which makes this light expensive to use. Fluorescent
lights have fair to good color rendering. The best color rendering is obtained with the
Coolwhite Deluxe source, which has more red colors in the spectrum and looks more
natural. The light efficiency varies quite a lot (50-80 lm/W).
The other light sources (mercury, metal halide, and high-pressure sodium) have fairly
poor color rendering and should not be used in manufacturing plants or offices. They are
more appropriate in environments where there are few people (e.g., in warehouses,
shipping and receiving, and outdoors) (Wotton, 1986; Boyce, 1988).
