Poster Session 4, Thursday, October  6, 11:00–12:40

Poster 60

Pixelwise immersion factor calibration for underwater hyperspectral imaging instruments

Underwater hyperspectral imaging (UHI) is gaining increasing attention as a promising in-situ method with high spatial and spectral resolution for oceanic biogeochemical research. Accurate acquisition of the spectral radiance is crucial for the extraction of spectral reflectance and absorption features of the targets. Immersion factor corrects the radiometric difference caused by the change of refraction and reflection from air-glass-air interfaces in calibration phase to water-glass-air interfaces in deployment phase for an optical instrument sealed in a bulky pressure housing. Unlike a point-sampling radiometer which detects perpendicular incident light, pixels on a UHI camera sensor with different viewing angles possess individual immersion factors. Besides, the materials and the thickness of the optical window together with any assembly errors contribute to different responses between the center and edge pixels. We propose an efficient piecewise method for radiometric calibration of a UHI camera in pressure housing using a small size integrating sphere. Segments of spectra from a 2.5-inch port on a 5.3-inch integrating sphere lit up by a halogen lamp were captured by a commercial UHI camera from Ecotone AS. A spliced spectrum from segments was used as an equivalent of the response of the whole sensor, and a calibrated RAMSES radiometer was used as comparison. Results support the proposed method as an inexpensive alternative to calibrate the UHI camera. With the calibration results in air as prior knowledge, the pixelwise immersion factor of the UHI camera in water was further characterized experimentally.

Hongbo Liu, University of Bergen, [email protected], 0000-0002-2540-9844 

Yi-Chun Chen, University of Bergen, [email protected] 

Camilla Sætre, University of Bergen, [email protected] 

Tristan Petit, Norwegian Polar Institute, [email protected]

Børge Hamre, University of Bergen, [email protected] 

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