Heinz Walz US-SQS, Инструкция по эксплуатации

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CHAPTER 7 TERMINOLOGY
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7 Terminology
PAR: Photosynthetically Active Radiation
Photosynthetically active radiation is usually defined as radiation
between 400 and 700 nm. Radiation can be used energy based (W) or
photon event based (µmol/s). In photosynthesis most effects are de-
pendent on the amount of photons rather than the energy incident on
the system. Therefore, we characterize the sensors in photon event
based units (µmol/s).
Sometimes PAR has been used weighted according to the plant’s
photosynthetic response similar to the unit lux, which is weighted
according to the standard human eye’s sensitivity. We do not use
PAR weighted, ( see norm-curve in Fig. 1), because PAR is expressed
in SI-units, which is not weighted per definition. Furthermore, correct
weighting would depend on the pigmentation of the organism, which
varies between different plants and between different types of phyto-
plankton in particular.
Photon Flux
Number of photons per time interval (Braslavsky, 2007). Unit: s
-1
or µmol s
-1
Photon Irradiance
Number of photons per time interval incident from all upward di-
rections on a small element of surface containing the point of consid-
eration divided by the area of the element (Braslavsky, 2007). The
term “all upward directions” expresses the fact that the receiver is not
a sphere but a flat surface and only beams from one hemisphere take
effect, of which beams not perpendicular to the surface only contrib-
ute with the cosine of the angle between the beam and the surface
normal. Unit: µmol m
-2
s
-1
.