Lighting Systems 331
on it per unit area. The unit of illumination is the lux (lumens per square
meter) or footcandle (lumens per square foot).
We see only light that is reflected. Reflected light is measured in
candelas per square meter of surface (or footlamberts). The reflection fac-
tor is the percent of light reflected from a surface expressed as a decimal.
Therefore, the light reflected from a surface is equal to the illumination of
the surface times the reflection factor. The brightness of a surface that re-
flects one lumen per square foot is one footlambert.
Units of light measurement are given in both English and International
Systems of Units (metric) units. This can become somewhat confusing.
Table 13-2 summarizes the most common terms used for light measure-
ment.Table 13-2. Lighting Terms
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Lighting Unit
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Quantity Definition Symbol SI English Definition of Unit
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Luminous Ability of source I Candela (cd) Approximately equal to the
intensity to produce light luminous intensity produced
(candlepower) in a given by a standard candle
directionLuminous flux Total amount of φ Lumen (lm) Luminous flux emitted by a 1
light candela uniform point sourceIllumination Amount of light E Lux Footcandle One lumen equally distributed
received on a (lx) (fc) over one unit area of surface
unit area of
surface (density)Luminance Intensity of light L ed/m^2 ed/ in.^2 A surface reflecting or
(brightness) per unit of area (foot- emitting light at the rate of 1
reflected or lambert) candela per unit of projected
transmitted from area.
————————————————————————————————Incandescent Lighting
Incandescent lighting is a widely used method of lighting, used for
many different applications. The construction of an incandescent lamp is
shown in Figure 13-3. This lamp is simple to install and maintain. The
initial cost is low; however, incandescent lamps have a relatively low ef-
ficiency and a short life span.