CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
however, that it is not a simple matter to compare absolute
fluorescence values of a sample measured at different times and
under different conditions. While it is almost trivial that the sample
must be in exactly the same position with respect to the fiberoptics
(e.g. in a suitable leaf-clip) and that the same settings of MEAS-INT
(menu point 50) and GAIN (menu point 49) must be used, it is less
obvious that the sensitivity of the fluorometer is affected by
temperature. A 1 °C increase results in an approximately 1 %
decrease in signal amplitude. This is due to the fact that the
efficiency of the light-emitting-diode, which provides the pulse-
modulated measuring light, slightly drops with increasing
temperature. Hence, any internal heating of the fluorometer will lead
to a corresponding decrease of the signal amplitude (see section
12.3.5). To take this aspect into account, the MINI-PAM features
measurement of internal temperature (menu point 35), which is
automatically registered with every YIELD-measurement and stored
in MEMORY.
12.3.5 INT.TEMP (menu point 35)
As outlined in the preceding section, the output of the measuring
light LED is a function of temperature, a feature common to all
solid-state lamps. Intensity decreases by approximately 1 % per °C.
In practice, this has to be accounted for whenever signal amplitudes
are compared. It is of no concern for YIELD-measurements (signal
ratios ∆F/Fm' or Fv/Fm), except for a small local temperature
increase within the LED when pulse frequency is switched from 0.6
to 20 kHz during a saturating light pulse which affects selectively
Fm.
The internal temperature of the MINI-PAM, which is measured
in the optical compartment in the vicinity of the halogen lamp, is
displayed under menu point 35: INT.TEMP. It can increase
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