Civil Engineering Reference
In-Depth Information
than 3:1 (Illuminating Engineering Society, 1982). But 3:1 is too restrictive, and 10:1 is
more reasonable (Kokoschka and Haubner, 1985). Grandjean (1988) recommends that
the maximum luminance ratio within an office should not exceed 40:1.
Illuminance, luminance, and contrast ratio can be measured with a hand-held
photometer. This device is similar to a camera light meter, except that it provides a direct
readout in lux (or cd/m
2
). A photometer is color-corrected so that it simulates the human
sensitivity to color. Thus, since the human sensitivity to violet and red (at the opposite
ends of the color spectrum) is less than to green and yellow (at the centre of the color
spectrum), the photometer will produce lower values for violet and red than for green and
yellow. Therefore, in determining the luminance one need not be concerned about color,
since the photometer will convert the values to simulate the sensitivity of the human eye.
A good photometer has two different settings: one for measuring illuminance and one
for measuring luminance (Figure 4.9). To measure the illuminance that falls on a surface,
one must consider contributions from a variety of sources: light sources (luminaries),
windows, and wall reflections. The photometer must therefore have a wide angle of
acceptance. The photometer must be cosine-corrected to account for contributions which
are not perpendicular to the photocell on the photometer.
To measure luminance the photometer must have a narrow angle attachment, for
example an attachment with 1 degree of acceptance. This enables precise readings of
adjacent areas with different reflectances. To measure the contrast ratio between two
adjacent areas, two luminance readings are obtained, and the contrast ratio is calculated.
FIGURE 4.9
Use of a photometer for
measuring (A) illuminance and (B)
luminance. (A) Wide acceptance angle
