Measuring photosynthetically active radiation (PAR) under the surface of water is useful in a variety of fields, from algal biofuels research to environmental quality. When light passes through a water column it is attenuated based on the thickness of the water column and the turbidity of the water. Measuring PAR underneath the surface of a lake, stream, bay, ocean, pond, or bioreactor can provide an excellent gauge of how much light is available to unicellular and multicellular phototrophic organisms such as algae, aquatic plants, protists, and phytoplankton. This, in turn, can be used as a gauge of the overall productivity of a system, though the specific calculations are often complex and include many other factors.
Upwelling and downwelling radiation are two aspects of underwater PAR that can be useful to researchers and environmental quality investigators. Upwelling radiation is radiation received from below the sensor due to reflectance off a lower surface of some type, while downwelling radiation is a measure of radiation from above the sensor, usually due to sunlight or other external light sources. The combined upwelling and downwelling radiation measurements provide an overall measure of PAR available in the water column.