80
10. PSI fluorescence - Part of the fluorescence signal contains PSI fluorescence as well
as PSII fluorescence. With Y(II), one is trying to measure variable fluorescence of PSII
in a light adapted state.
PSI fluorescence is not variable, but the low fluorescent signal from PSI does overlap
with PSII. This produces an a small error. This is not a problem for comparing similar
samples, because PSI fluorescence does not change with light intensity temperature or
plant stress.
PAR is photosynthetically active radiation. Radiation on the leaf is measured between
the wavelengths of 400nm to 700nm. PAR sensors and thermisters for measuring
temperature are calibrated to other instruments that are traceable to the NIST. It is
recommended that recalibration should occur every two years. Most modern sensors are
solid state, so drift is minimal.
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