Premise of research. The encrusting arboreal lichen Dirinaria picta (Sw.) Schaer.ex Clem. (Physiaceae) is commonly found growing on tree trunks in Thailand and globally in the tropics and subtropics. Trebouxia sp. is its chlorophyte symbiont. Pulse amplitude modulation (PAM) techniques have been used to measure photosynthesis of a tropical lichen as well as its cultured symbiont. PAM technology has not been used to full advantage in lichens.Methodology. Absorptance of the thalli and glass fiber disks impregnated with cultured Trebouxia was measured with a leaf absorptance meter. PAM fluorometers measure the light reactions of photo-organisms to estimate gross photosynthesis (Pg; mol O2 m−2 s−1) from the measured electron transport rate through PSII (4 electrons per O2), provided that the absorptance of the photo-organism is known. Photosynthesis-versus-irradiance curves were measured with a blue-diode PAM machine and fitted to the waiting-in-line function.Pivotal results. The absorptance of moist active l...