The color of light emitted by the primary LED distinguishes the BLUE from the RED version of the MINI-PAM-II fluorometer (Fig. 1). The BLUE version (MINI-PAM-II/B) possesses a blue LED emitting maximally around 475 nm which is replaced by a red LED emitting maximally around 655 nm in the RED version (MINI-PAM-II/R). Both versions have a second LED providing far red light for specific excitation of photosystem I.

The second difference between the two versions is the spectral window for fluorescence detection. The BLUE version detects fluorescence at wavelengths > 630 nm, whereas the RED version detects fluorescence at wavelengths > 700 nm (Fig. 2).

BLUE or RED Version?

  • Its extended range for fluorescence detection makes the BLUE version more sensitive than the RED version. In samples with high chlorophyll content, a large part of the short wavelength fluorescence, which potentially can be detected by the BLUE version, is reabsorbed by chlorophyll. Hence, in such samples the sensitivity of the BLUE version is only slightly better than that of the RED one.

  • The MINI-PAM-II can be used to investigate cyanobacteria. Cyanobacteria often absorb poorly in the blue. Therefore, in studies of cyanobacteria the RED version is normally preferred over the BLUE version which yields low signal to noise ratios with cyanobacteria.

  • The blue actinic light source of the MINI-PAM-II/B excites the broad short wavelength band of the major light-harvesting complex of photosystem II in higher plants and green algae (LHC II). Red light of the MINI-PAM-II/R excites the comparably minor long-wavelength band of the LHC II. Hence, if LHC II excitation is important, the BLUE version may be advantageous.

  • Blue is absorbed by blue light photoreceptors which can stimulate responses like stomatal movement and chloroplast relocation in higher plants. Chloroplast relocation can affect the fluorescence signal by changing the efficiency of light absorption. This effect is difficult to distinguish from other fluorescence quenching mechanisms. Choosing the RED version excludes such blue light effects.