This science education report introduces the technique of pulse amplitude modulated (PAM) fluorescence measurement of chlorophyll for the sensitive, noninvasive and rapid determination of quantum yield of photosystem II (y) in both laboratory and field experiments by the application of a portable chlorophyll fluorometer, the so-called Mini-PAM. In a case study we applied the Mini-PAM for the comparison of Y of various ferns and higher plants adapted to different light intensities on the DLSU campus. Y values were determined for both dark adapted, detached leaves (maximal quantum yield) and plants in the field exposed to natural irradiation (actual steady state Y values), Data show that maximal quantum yield of photosystem II varies in the measured 80 species within a narrow range (0.702 ± 0.009) and is totally independent upon the adaptation of plants to different irradiation during growth. In most cases Y-values of the upper side of the leaves are slightly, but significantly higher than those of the lower sides. However, large differences are observed with various plants when the quantum yield of photosystem II is plotted as function of light intensity. There is linear correlation among those light intensities which cause half maximal inhibition of quantum yield of photosystem II and the actual yields measured at an irradiation of 1000 µmol quanta m-2 s-1 PAR, which corresponds approximately to half full of sunshine. The different shapes of light intensity curves, which are saturation curves, allow the distinction between typical shadow and sun plants and those tolerating relatively broad range of light intensities in between.