Focal Technologies manufactures a range of Optical Plankton Counters for rapidly counting and sizing zooplankton, fish eggs, and other aquatic organisms. Sizes are determined by measuring the cross-sectional areas of targets as they pass through a calibrated, parallel beam of light. The OPC continuously monitors light attenuance of the water and automatically adjusts the light beam intensity to maintain calibration. An underwater version is available to allow mounting on fixed wings, undulating tow fish, vertical profilers or net samplers, and may be interfaced with CTD’s and other sensors. A miniaturized version is also available and is suitable for limnological and estuarine applications. This mini OPC can be mounted on hand-carried tow bodies or samplers operated from small vessels. A laboratory version is also available and is suitable for handling water from pumps or circulator systems.