A study on the coloration mechanism of Ao-ike Pond, Aomori Prefecture, Japan: refinement of the image analysis methods

In order to verify the blue coloration model for Ao-ike Pond, Aomori Prefecture, Japan, the authors refined the formulas previously reported for the light intensities in the coloration model. Then, the authors utilized a homography transformation technique to obtain the coordinates on the pond water surface from the digital camera image. Moreover, they experimentally determined the sensitivity factor and white balance factor of a digital camera used. Assuming potential coloration mechanisms including irregular reflection at the pond bottom, molecular scattering by water and Mie scattering by suspended solids, two types of images were analyzed so as to determine model parameters; images taken in early spring when the shading by tree leaves does not exist above the pond surface, and images photographed in early summer containing incident light trajectories brought by the sunbeam’s passage through the leaves above the pond surface. Analytical results obtained by both of the images revealed that the main coloration mechanism is molecular scattering by water but with no contribution of Mie scattering. Thus, the blue color of Ao-ike Pond was judged to be attributed to the nature of water itself.