CHAPTER 12 MEASUREMENTS WITH THE MINI-PAM
considerably with prolonged operation of the halogen lamp,
particularly at the higher intensity settings and when a high AL-
FACT (menu point 16) is used. Therefore, actinic illumination time
(menu point 14: ACT-WIDTH) is limited to 5 min and not more than
2 min are recommended for recordings of LIGHT CURVES, which
involve 8 consecutive illumination periods (see section 12.3.9).
Nevertheless, internal temperature increases of 10-20 °C are not
uncommon, which will cause a decrease of measuring light intensity
in the order of 10-20 %.
Small temperature related decreases of measuring light intensity
also occur when the frequency is switched from 0.6 to 20 kHz. This
is normally the case during actinic illumination with the internal
halogen lamp. The extent of decrease depends on measuring light
setting (menu point 50: MEAS-INT) and the length of the period at
20 kHz. It can be ignored at all measuring light settings for times
below 2 s and amounts to approx. 1 % when a 3 s pulse is given at
ML-setting 12. Hence, the effect on YIELD-determinations can be
considered marginal. During prolonged 20 kHz operation at maximal
ML-setting, the effective intensity drops by 2-3 %. This effect is
reversible within a few minutes after returning to 0.6 kHz.
Changes in measuring light intensity induced by temperature
changes, just like any effect on the sensitivity and selectivity of
fluorescence measurements with the MINI-PAM, can be also
evaluated by monitoring the fluorescence signal of the
FLUORESCENCE STANDARD delivered together with the
instrument. This blue plastic filter (Roscolene Surprise Blue) emits
red fluorescence at an intensity similar to a leaf. As there is no
photochemical energy conversion, fluorescence yield of this sample
is constant during illumination, provided temperature is not
changing. Using the Leaf-Clip Holder 2030-B with the integrated
temperature-sensor, it can be readily shown that continuous actinic
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