Microsecond timing enables the MINI-PAM-II fluorometer to use the same LED as source for PAM measuring light, actinic light and saturation pulses. Measuring light corresponds to μs flashes of constant amplitude, actinic light is quasi-constant light employed to drive photosynthesis, and saturation pulses temporarily saturate primary photosynthesis so that all photosystem II reaction centers are “closed”.
Being a PAM fluorometer, the MINI-PAM-II device records only the fluorescence elicited by measuring light. Fluorescence excited by internal actinic light, saturation pulses or constant external light, like sun radiation, is not measured. Therefore, the MINI-PAM-II determines how environmental factors modulate the efficiency of conversion of measuring light into fluorescence. These “PAM fluorescence data” are required to retrieve information on primary photosynthesis like the photosynthetic efficiency of photosystem II, Y(II).
A second LED in the MINI-PAM-II emits far red light. This LED preferably excites photosystem I but is negligibly absorbed by photosystem II. A special measuring routine uses this far red LED to determine the F0’ fluorescence level which is important to correctly assess the reduction state of photosystem II reaction centers.
In experiments using internal actinic light, the light intensity at sample level can be monitored online using an internal light sensor. This internal sensor must be calibrated against an external light sensor.