This module interfaces an Apogee PAR sensor to the Growtronix system. The readings are output in µmol/(m²s). PAR stands for Photosynthetically Active Radiation. PAR is defined as 400-700 nm (nano meters) this is the color range of light that plants respond to. In other words the Apogee sensors gives you a light reading that directly correlates to light that the plants can actually use.