10/ 2010
BENNING LUXMETER B
24
4.3 How to measure average illuminance?
Sometimes the average illuminance in a room must be found out.
A luxmeter indicates the illuminance at the point of measurement only, and not
the average illuminance in the space. To find out the average illuminance in an
area at the time, it is necessary to divide the area into a number of equal areas
which should be as nearly square as possible. The illuminance at the centre
of each square is then measured, and the results are averaged. The minimum
number of equal areas required for accuracy can be determined by number n:
L x B
n = -------------------------
(L + B) x H
m
where L is the length, W is the width of the indoor place H
m
and is the height of
the luminaries above the plane of measurement.
The height of working plane is usually 0.85 m for work benches or 0.72 m for
desk top height unless the main plane of the work is known to be some other
height above floor level. If the work is performed down to floor level, then the
floor is taken as the working plane of measurement.
The number of measurement points depends on value n.
n
Minimum number of
measurement points for
uncertainty of ± 5 %
Minimum number of
measurement points for
uncertainty of ± 10 %
n < 1
8
4
1 ≤ n < 2
18
9
2 ≤ n < 3
32
16
3 < n
50
25
Table 2:
Number of measurement point
s
4.4 Calibration of illumination testers
As a general rule silicon diode type meter should be re-calibrated every two
years. Companies conforming to the ISO 9000 standards should have their
meters calibrated every 12 months. It is recommended to recalibrate probe
frequently to maintain specified accuracy.