Kipp & Zonen PAR lite, Instruction Manual

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SENSOR PROPERTIES
Instruction manual PAR-LITE
11
have higher energy content, the spectral sensitivity of a PAR
sensor has to be low for low wavelengths and higher for higher
wavelengths. This explains the inclination of the spectral
sensitivity curve.
The PAR
LITE has been calibrated for solar radiation under
clear sky conditions. The spectrum under these circumstances
is also drawn in figure 2.
Some plants however are grown under lamps. The spectral
emission of this lamps is mainly in the wavelength range from
500 – 600 nm. This implies that actually the mean spectral
sensitivity of the PAR
LITE in the range of 500 – 600 nm
should be used. This sensitivity can differ from the sensitivity
for daylight, which is also determined by the individual
bandwidth of the PAR
LITE sensor. Deviations can be of the
order of magnitude of
±
5 %. Read also chapter 3, Calibration,
for this matter.
2.3 Directional/Cosine response
The measurement of radiation received by a surface (also
called irradiance or radiative flux) is laid down in two detector
specifications: that the detector has a correct spectral
response and that it has a field of view of 180 degrees.
Another way of expressing the latter directional properties is to
say that the sensor has to comply with the cosine response.
A perfect cosine response will show maximum sensitivity 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
°
the