L202PMS Photometer
Irradian Limited
5
The unit ‘lux’ is a measurement of the amount of light incident on a unit
area, (lumens.m
-2
). Any detector will measure this reliably when
measuring a beam of light perpendicular to the detectors surface,
however, when measuring scattered light or light from an extended
source the sensor must have an accurate response over its 180° field of
view. This response should be proportional to the cosine of the angle of
light incident on the detector, Lambert's Cosine Law. To ensure that the
integration of light from all angles is correct, the cosine diffuser fitted to
the front of the filter ring is profiled such that the angular response of the
detector decreases with cos(i) as the angle between the source and
detector increases from 0 to ±90°.
L202PMS
True Cosine Response
Figure 2
The cosine diffuser fitted to the detector is corrected to match the cosine
response to within ±3.5% up to angles of 75°. This ensures that the
meter accurately reads the light flux density whether it is measuring
scattered light (from an overcast sky) or a point source (a single lamp in
a dark room).
