This a profiling reflectance radiometer or PRR. The PRR is the first instrument we deployed when we arrive on a station. Once calibrated the PRR sinks through the water column measuring light levels. We then use the the CTD to collect water at depths having specific percentages of the surface light intensity (sunlight disappears somewhere between 30 and 100 meters depending on the amount of plankton).
Wendy and Joe are deploying the instrument off the stern, Erin is reeling in the extra cable and Peter is in the lab monitoring the cast profile.