Ocean Color Underwater Low Light Advanced Radiometer—Ocean Color at Night (OCULLAR)

The OCULLAR activity pairs a miniature and ruggedized photomultiplier tube (PMT) with an existing commercial-off-the-shelf (COTS) silicon photodetector (SiP) microradiometer (co-developed by the PI as part of a prior SBIR) to create a new class of  sensor technology called a hybridnamic multitector. A SiP microradiometer has a spectral responsivity spanning 305-1,640 nm with 10 decades of dynamic range and can be pointed at the Sun, Moon, sky, or sea without saturating. The principal OCULLAR objective is to increase this dynamic range to 14 decades with demonstrated linear calibration. The extra 6 decades of dynamic range from the PMT (2 overlap with the SiP, netting 14 decades) significantly improve the measurement of apparent optical properties (AOPs) under moonlight, which has already been demonstrated to be possible with existing SiP microradiometers, although there are challenges with ultraviolet (UV) channels. Increasing the dynamic range of the sensors will overcome the present limitations and allow unprecedented new research. For example, diurnal processes in the ocean and atmosphere can be studied optically using moonlight as the light source.

The principal objective of the OCULLAR project is to pair a PMT with a SiP microradiometer where their overlapping sensitivities maximize the dynamic range at each wavelength to create a new hybrid sensor class, called a hybridnamic multitector.  Ultimately, hybridnamic multitectors will feature up to 14 decades of dynamic range, spanning the UV to near-infrared (NIR), depending on the photocathode material of the PMT, and will be used to measure the AOPs of the ocean and atmosphere. AOP measurements, by definition, require a light source. Typically, the Sun is used because it provides sufficient flux across an extensive spectral domain for a wide diversity of detectors. Although some measurements have been made using the Moon as a light source, in general, the diurnal cycle of oceanic and atmospheric optical parameters are poorly understood, because of the paucity of instruments that can make high-quality observations in low-light conditions. Similarly, observations in highly turbid conditions are sparse, because signal levels quickly reach the noise level of existing detector systems. The inability to make accurate measurements at low-light levels has restricted the understanding of other important light-based phenomena. This means high latitude processes can be investigated during the substantial time of the year when light levels are restricted to twilight, moonlight, or very low solar elevations. Other areas of new research using OCULLAR sensors include the study of coral spawning, plankton vertical migration, bioluminescence, the onset of the spring bloom under polar ice sheets, characterization of the environment experienced by pelagic predators (e.g., tuna and squid), plus the use of very narrow bandwidths to exploit specific spectral signatures of oceanic and atmospheric phenomena. Although the prototype sensor is anticipated to be deployed into the ocean, recent advances in exploiting microradiometers for airborne applications (e.g., C-AIR flights with Ames Research Center), suggests Ocean Color at Night or Validating the Black Marble using OCULLAR sensors are tractable problems.