Description: We propose to develop an active sensor for in situ measurement of the inherent optical properties (IOPs) absorption and backscattering at multiple wavelengths. Multi- or hyper-spectral absorption of particles and dissolved materials is routinely measured in the laboratory and in situ in order to characterize, for example, the quantities and types of phytoplankton based on concentrations of specific absorbing pigments. Similarly, backscattering is employed to estimate the concentration of suspended material. Measurements of absorption and backscattering concurrently, and at multiple wavelengths, are useful as proxies for biogeochemical measurements such as particle composition, concentration of particulate organic carbon, and particle size distribution, as well as for remote sensing calibration and validation. The current state of the art for phytoplankton observation using optical sensors on autonomous platforms relies on linking biomass with optical backscattering and chlorophyll. The ability to quantify phytoplankton using absorption not only overcomes limitations of backscattering and fluorescence-based approaches, but multi-spectral (visible wavelength) measurements of absorption also provide the means to discern the presence of accessory pigments and pigment packaging, ultimately leading to not only improvements in phytoplankton biomass estimates, but also the potential for resolving phytoplankton functional types. Briefly, the proposed sensor emits a collimated beam of light into the water and measures the backscattered light as a radial function from the beam location. An inversion algorithm is then used to convert this backscattered intensity as a function of distance from the beam to the inherent optical properties absorption and backscattering. Multiple source wavelengths are used and the sensor is packaged in a compact, flat-faced geometry easing integration into autonomous platforms.

Benefits: The proposed sensor addresses several NASA interests under the S1.09 "Surface & Sub-surface Measurement Systems" subtopic, including particle characterization, water quality, IOP measurements for calibration and validation of ocean color data, proxies for biogeochemical particle composition, and properties of aquatic environments such as detection of phytoplankton and their functional groups. All aspects of optical oceanography research, including biology, particle analysis, and ocean color remote sensing rely on measurements of these key IOPs. Researchers using autonomous platforms are a key market since absorption is not currently measured on these platforms due to limitations of current absorption sensors (e.g., flow-through design, size, power demand, fouling). The flat-faced design also allows for easy incorporation of shutters or wipers to prevent fouling for moored and other long term deployments such as in ocean observatories. The simple and compact design is highly desired for incorporation into existing profiling CTD rosettes used routinely in oceanography. The proposed sensor measuring absorption and backscattering across a wide range of platforms and scales has wide applicability in the field of ocean optics and ocean biology and biogeochemistry. NASA scientists and NASA-funded researchers in these fields routinely measure in-water IOPs and the target market for this sensor is extremely broad.

Similar to the NASA Applications, the target market for the proposed sensor is extremely broad. Government scientists and agency-funded researchers (many federal agencies including NSF, NRL, ONR, NOAA, as well as state environmental agencies) in ocean science and coastal monitoring routinely measure IOPs, and the ability to incorporate a wider range of measurements (especially including absorption) on a wide range of platforms covering a wide range of scales is a critical need.