Photomultiplier Detects Ocean Color at Night

Just how blue the ocean is can be a clue to the health of its inhabitants, and now scientists have a way to gauge that color even under dark of night.

A team at NASA's Goddard Space Flight Center and Biospherical Instruments Inc. has developed a miniature, ruggedized photomultiplier tube with an existing silicon photodetector microradiometer to enable round-the-clock observations of ocean color. A microprocessor activates the photomultiplier as light levels decline, enabling uninterrupted measurements across multiple wavelengths from UV to NIR.

The fully assembled OCULLAR prototype is pictured on the right. A legacy photomultiplier tube is shown on the top left. A new silicon photodetector microradiometer is on the bottom left. Courtesy of NASA Goddard.

The prototype Ocean Color Underwater Low Light Advanced Radiometer (OCULLAR) was tested off the coast of San Diego.

“OCULLAR sensors have 14 decades of linear dynamic range, with the added responsivity at the low end of illumination,” said Goddard principal investigator Stan Hooker. “So, OCULLAR sensors have five more decades of response to low light than the human eye.”

When commercialized, Hooker said hybrid OCULLAR instruments would be equipped with seven photomultipliers paired with silicon photodetectors. An eighth photodetector would measure a wavelength useful to ocean color, but difficult to measure with a photomultiplier. The next step is to develop an instrument that can be flown on unmanned aircraft and eventually aboard spacecraft.

The ocean’s color is greatly affected by phytoplankton, which are often the first to be affected by pollution or other changes in their environment. Because different types of phytoplankton contain different kinds of chlorophyll, measuring the color of an area of water allows scientists to estimate the amount and general type of phytoplankton there.

“Ocean color is what we see and is a big part of understanding the ocean’s diversity,” Hooker said. “You can figure out what’s happening in the ocean by looking at the different colors or wavelengths, which then are used to determine what constituents in the water are creating that color.”

For more information, visit: www.nasa.gov