Apogee quantum sensors measure photosynthetically active radiation. The MQ-200 features a handheld meter to display and store measurements with a separate sensor head connected by a lead. The sensor head features a fully potted, domed-shaped design making it fully waterproof, weatherproof and self-cleaning. The head can be held by hand, mounted to a leveling plate, or attached to an optional sensor wand for taking measurements in hard to reach places. Photosynthetically active radiation (PAR), or photosynthetic photon flux density (PPFD), is the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. Gardeners, greenhouse managers, growth chamber users, and salt-water aquarists measure PAR to ensure optimal specimen health.