Operation Manual

10/ 2010 BENNING LUXMETER B
24
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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 points
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.