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of interest, S(λ) is the light spectrum and Xi is the CMF. The values of i represent the X, Y and Z curves, respectively. The RGB values are obtained from the demosaicked raw photographs of patches of interest. Alternatively, if the spectral sensitivity curves of the camera are known, the RGB values can be simulated using the second equation by substituting the spectral sensitivity curves of the camera in place of Xi. Spectral sensitivity curves of a sensor are not usually available through the camera manufacturer and have to be derived manually, for example by using a monochromator, narrowband interference flters or estimation algorithms. With RGB and XYZ values, M is computed according to the frst equation and is then used to transform images from the camera color space to the XYZ space. Images cannot be visualized in XYZ values; they must be transformed to a device-independent RGB space. These transformations are standard and can be found in any colorimetry resource. Accounting for Color Constancy Most underwater photographs look blue or green because light attenuates "In the absence of hyperspectral imagers, spectrometers can be used. This way, images captured by COTS digital cameras can be transformed from the camera space to a device-independent space using colors and light spectra that are representative of a particular habitat." nonuniformly with depth and wavelength. The red components of the spectrum diminish rather quickly with depth. This means that a white slate at depth will photograph as blue-green. One might expect that it should appear to a scuba diver as blue or green, but this does not happen because the human visual system can discount small changes under varying illumination conditions—a phenomenon known as color constancy. For digital photography underwater, it is up to the end user to correct for color constancy. In some applications, users may want to capture the in-situ appearance of features, such as corals and fsh, the way they appear at a certain depth under certain light conditions, and in others they may want to capture the way objects appear under broadband daylight (in which case, strobes can be used to provide artifcial broadband illumination). One way to incorporate color constancy into photographs is by equalizing the RGB values from the image that correspond to a white target, known to have a fat refectance spectrum. Alternatively, it is possible to compute the white point of the ambient light underwater if an irradiance profle has been recorded, and this can be used to determine the appearance of white in the resulting image. Ongoing Development Using site-specifc marine photographic calibration charts under natural light conditions instead of charts designed for land photography enables more accurate color depiction in underwater images. Refectance of features from a particular underwater habitat can be recorded before, during or after the actual photographic study. Downwelling and sidewelling light should also be recorded to reconstruct the ambient light feld and refected light from the subject recorded in the photograph. It is diffcult to do this, however, using a single-channel spectrometer, as mea- Sea Technology Celebrating years of serving the global ocean community 1963 - 2013 12 st / DECEMBER 2012 www.sea-technology.com