Liu Hongbo, Summers Natalie, Chen Yi-Chun, Løvås Håvard Snefjellå, Johnsen Geir, Koestner Daniel, Sætre Camilla, Hamre Børge
Opt Express. 2024 May 20;32(11):19854-19880. doi: 10.1364/OE.523641.
In situ spectral reflectance initially captured at high spatial resolution with underwater hyperspectral imaging (UHI) is effective for classification and quantification in oceanic biogeochemical studies; however, the measured spectral radiance is rarely used as an absolute quantity due to challenges in calibration of UHI instruments. In this paper, a commercial UHI instrument was calibrated for radiometric flat field response and pixelwise immersion effect to support in situ measurement of absolute spectral radiance. The radiometric and immersion factor calibrations of the UHI instrument were evaluated quantitatively through comparative experiments with a spectroradiometer and a spectrometer. Results show that the immersion factor of the center pixel of the tested UHI instrument was 1.763 in pure water at 600 nm, and the averaged difference in immersion factor between the center and edge pixel of the UHI instrument in the visible light band was only 1∼3% across its half angle field of view of 35° in air. The new calibration coefficients were further used to calculate the spectral radiance of transmitted sunlight through ice algae clusters in sea ice measured by the UHI instrument during an Arctic under-ice bio-optical survey.
利用水下高光谱成像（UHI）最初以高空间分辨率获取的原位光谱反射率，在海洋生物地球化学研究中的分类和定量方面很有效；然而，由于UHI仪器校准存在挑战，所测量的光谱辐亮度很少被用作绝对量。本文对一台商用UHI仪器进行了辐射度平坦场响应和逐像素浸没效应校准，以支持绝对光谱辐亮度的原位测量。通过与光谱辐射计和光谱仪的对比实验，对UHI仪器的辐射度和浸没因子校准进行了定量评估。结果表明，在600nm的纯水中，测试的UHI仪器中心像素的浸没因子为1.763，在空气中35°半角视野范围内，UHI仪器中心像素与边缘像素在可见光波段的浸没因子平均差异仅为1%至3%。在北极冰下生物光学调查期间，新的校准系数进一步用于计算UHI仪器测量的穿过海冰中冰藻群的透射太阳光的光谱辐亮度。
