Kipp&Zonen CM 4 Pyranometer - Manual (Page 15)

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1.2.3 Directional / Cosine response
The measurement of the radiation falling on a plane surface (also
called irradiance or radiative flux) requires two assumptions: that the
surface is spectrally black (that it absorbs all radiation of all
wavelengths) and that it has a 180° field of view. Another way of
expressing these directional properties is to say that the sensor has
to comply with an ideal cosine response. ISO 9060 defines the
cosine response (or directional response) as the range of errors
caused by assuming that the normal incidence responsitivity is valid
for all directions when measuring with a beam radiation whose
normal angle of incidence irradiance is 1000 W/m².
A perfect cosine response will show maximum sensitivity (1) at an
angle of incidence of 0° (perpendicular to the sensor surface) and
zero sensitivity at an angle of incidence of 90° (radiation passing
over the sensor surface). In between 0 and 90 degrees the sensitivity
should be proportional the cosine of the angle of incidence. Figure
1.4 shows the typical curve and the maximum percentage deviation
of a CM 4 pyranometer. The vertical axis shows the deviation from
ideal behaviour, expressed in percents of the ideal value.