In this paper, we propose using a snapshot narrow-band multi-spectral imaging system to capture the spectral images of the ocean surface and reconstruct its spectral reflectance. Specifically, a multi-spectral camera with a set of nine narrowband filters and a monochrome camera were used to capture the spectral images of the ocean surface, and the cubic spline interpolation method was used to reconstruct the spectral reflectance curves from the spectral images. In order to examine the validity of our system, the multi-spectral images of the ColorChecker cards in a light booth were captured, and the spectral reflectance curves of them were reconstructed. The experimental result shows that with the spectral range of 400 to 700 nm and the spectral interval of 10 nm, the average spectral root-mean-square error between the standard samples and the reconstructed samples is 0.0114, the average spectral similarity index between them is 95.87%, and the average CIE1976L*a*b* color difference is 2.48. Furthermore, the real experiment was conducted on the sea area of China; the spectral images of the ocean surface were captured from different zenith angles, and the spectral reflectance curves of them were reconstructed. The experimental results show that the values of spectral reflectance of the ocean surface were influenced by the imaging angles.
In order to capture the standard color images from the ocean surface with a digital camera, we propose a new colorimetric modeling method named “Two-step training and modeling”, so that the color conversion model from the camera RGB space to the CIEXYZ space can be established, and standard color images of the ocean surface can be obtained by the camera. In the theory, the color conversion models using polynomial transforms were introduced, and in the methodology, the colorimetric calibration method of the camera for the ocean imaging was put forward. In the first step experiment, a group of training samples were captured by the camera from the Colorchecker-SG cards illuminated under natural sunlight, and the color conversion models from RGB to CIEXYZ were established by means of different polynomial formulas. In the second step experiment, the supplementary training samples were captured by the camera from the real surface of a certain sea area in China illuminated under natural sunlight, and the color conversion models of different polynomial formulas for the ocean surface image were established. The experimental results show that by using the method of “Two-step training and modeling”, the significant improvement of the color conversion accuracy for the ocean surface imaging can be achieved.
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