Optical plankton (or particle) counters work in a similar way to Video Plankton Recorders (VPRs), allowing a stream of ocean water to pass by a recording device that can count the number of plankton-sized particles in the water. When calibrated with the speed of the vessel towing the OPC, scientists can calculate the amount of water sampled to determine the number of organisms per volume of ocean water. In addition to counting the number of particles of different sizes, laser OPCs can record an image of the silhouette of particles passing through it, allowing for species identification of certain larger planktonic organisms such as krill and large copepods. Like VPRs, OPCs are generally housed inside a towed vehicle such as the Moving Vessel Profiler (MVP) or attached to trawls, etc., to allow for a more thorough investigation of the marine environment.
A diagram of an Optical Plankton Recorder , showing the beam of light (in red) that shines across the path of water flowing through the center of the instrument, recording a simple image of any plankton in that water. (Gulf of Maine Area Program - GoMA)
A diagram of a Laser-OPR , here shown housed within its carriage, a "moving vessel profiler." (Gulf of Maine Area Program - GoMA)
An example of the image generated by a Laser-OPC, showing the silhouette of numerous planktonic organisms. (Gulf of Maine Area Program - GoMA)