Irradian Limited
L203 Photometer
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Illuminance is a measurement of visible light incident on a unit area
(Lumens/m²). A detector will measure this reliably when the light beam is
perpendicular to the detector's surface. Accurate measurements of
illuminance require that the detector should respond to light over a 180°
field of view. More importantly, this response should be proportional to
the cosine of the angle of light incident on the detector. 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°.
CIE Cos-112
True Cosine Response
Figure 2
Irradian's cosine diffusers are corrected to match the cosine response to
within ± 3.5% up to angles of 75°. This ensures that the meter correctly
reads illuminance or visible light flux density whether it is measuring
scattered light (from an overcast sky) or a point source (a single lamp in
a dark room).