A underwater light meter is used to measure how much visible light is penetrating to various depths. As Russell lowers the light meter into the water (above left), Sharon Zsebe records the measurements from the data logger (above right). Matching light measurements are taken simultaneously from another light sensor mounted on the ship's railing (below) to correct for varying incomming light levels due to changing atmospheric conditions, for example, passing clouds. In addition, based on the direction of the sunlight, the port or starboard side is chosen to minimize the effects of the boat's shadow in the water.
Light sensor mounted on ship's railing

Related web links:
LiCor Data Logger