Civil Engineering Reference
In-Depth Information
4.9
MEASUREMENT OF ILLUMINANCE AND LUMINANCE
The distinction between illuminance (also called illumination) and luminance is
important. Illuminance is the light falling on a surface. After it has fallen on the surface,
it is reflected as luminance. Luminance is therefore a measure of light reflected from a
surface. Luminance is also used to measure light emitted from a computer screen. This
may be theoretically incorrect, but for all practical purposes, light from a computer screen
can be treated as having the same properties as reflected light.
To calculate how much luminance can be generated from a surface, one must know
how reflective the surface is. This is specified by measuring reflectance, a number which
varies from 0 to 1. It is practically impossible to achieve a perfect reflectance of 1.0; a
piece of white paper has a reflectance of about 0.85. A non-reflective black surface has a
reflectance of 0.
Measurement units are typically specified in the SI system (the metric system).
Illuminance is measured in lux and luminance in candela per square meter (cd/m
2
), also
called “nits.” These are the preferred measurement units (Boyce, 198 1b).
In the U.S. the “English system” is still used, although the SI system is gaining
ground. According to the English system, illuminance is measured in foot-candles (fc).
One foot-candle equals 10.76 lux, but for practical purposes a conversion factor of 10 is
sufficient. Thus 1000 lux illumination, which would be appropriate for an industrial
workstation, corresponds to 100 fc. In the English system, luminance is measured in foot-
lamberts (fL). One foot-lambert is equivalent to 3.4 cd/m
2
(or 3.4 nits). The measurement
units are illustrated in Table 4.1.
There are simple formulas for converting illuminance to luminance.
For the SI system:
Luminance (cd/m
2
)=Illuminance (lux)×Reflectance)/π
For the English system:
Luminance (fhr L)=Illuminance (fc)×Reflectance
TABLE 4.1
Units for Measuring Illuminance
and Luminance (SI Units are Preferred)
English
SI
Amount of Light Falling on a Surface
Illuminance 1 foot-candle (fc) = 10 lux (Ix)
(or illumination) (or lumen/ft
2
) (or lumen/m
2
)
of Light Reflected from a Surface
Luminance
1 foot-lambert (fL) = 3.4 candela/m
2
(or candela/hr ft
2
)
(cd/m
2
or 3.4 nits)
