CHAPTER 8 COMPONENTS OF THE MINI-PAM
- Micro-quantum-sensor monitoring PAR
This tiny sensor is unique in monitoring the photosynthetic active
radiation (PAR) at the very spot where also fluorescence is measured
and at which photosynthetic performance is assessed. This function
already is fulfilled, when approximately 10 % of the total measuring
area is occupied by the sensor. The resulting loss in signal amplitude
is small. If wished, the sensor can also be moved out of the
measuring field which is limited by a metal ring of 10 mm inner
diameter. With its tip resting on this ring, even without penetrating
into the measuring field the sensor will accurately monitor incident
light intensity under natural day light conditions, when the leaf-clip
holder is positioned such that light incidence is mainly from the
front.
Essential opto-electronical elements of this micro-quantum-
sensor are a 1.5 mm cross-section diffusing disk; a 0.5 mm diameter
fiber guiding the scattered light to the detector; a filter combination
selecting the photosynthetic active wavelength range between 380
and 710 nm; and a blue-enhanced silicon photodiode. Despite its
small dimensions, the diffuser displays properties of 'cosine
correction', i. e. also light impinging at rather small incidence angles
(e. g. with rising or setting sun) is reliably monitored. Due to the
equalization of leaf and sensor planes, automatically achieved by
fastening the leaf in the clip, the measured effective PAR very
closely corresponds to the PAR at that spot of the leaf where
fluorescence is measured. The micro-quantum-sensor measures
incident photosynthetic radiation in µmol quanta m
-2
s
-1
, i.e. in units
of flux density. Hence, the measured parameter is identical to PPFD
(photosynthetic photon flux density). It is displayed at the end of the
second line of the LCD (...L) when the leaf-clip holder is connected.
The sensor was calibrated against a LI-COR Quantum Sensor (Type
LI-190). The stability of calibration depends strongly on keeping the
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