The PhotoBiology Light Source is a main component of the O2k-PB-Module and provides an external light source. It consists of one LED and one photodiode mounted on the PBLS head protected by a PMMA plastic cover. The light intensity can be regulated from 0 to 2750 µmol·s-1·m-2 The integrated photodiode provides real-time measurement of the light intensity allowing for continuous adjustment to the desired value.