Sea Technology

DEC 2012

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Capturing Accurate Colors Underwater With Consumer Digital Cameras COTS Digital Cameras Combined With Spectrometers Aid Color Capture By Derya Akkaynak • Roger T. Hanlon I n 1926, when Charles Martin and models of cameras, and Dr. William Longley capand new ones are being tured the frst color photograph designed continually as underwater, they had to use highnew camera models are ly explosive magnesium fash introduced to the public. powder to provide enough light While digital cameras to the hypersensitized glass plates are highly advanced, for an exposure time of onethey do not capture the twentieth of a second. In those world in a standard way days, Autochrome glass plates so their outputs must were the only means of obtaining be calibrated to obtain color photographs with exposure device-independent times around 12 seconds. color. For scientifc or Before Martin, who was the industrial purposes, it is frst chief of National Geograph- (Top) Comparison of critical that photographs ic's Photo Laboratory, developed the spectral composicontain accurate colors a hypersensitizing solution to coat tion of light underwathat can be reproduced the Autochrome plates, it was not ter (black curves) and by others with no knowlreasonable to take color photo- on land (red curves). edge of the original imgraphs of anything other than still Underwater profles aging system. subjects, and it was practically were measured at impossible to take photos under- specifc dive sites Underwater Color Caliat depths shallower water. But the 2,400-fashbulbbration than 10 meters in strong explosion provided enough Urla, Turkey; Woods Every COTS digital light at 4 meters below the sea Hole, Massachusetts; camera captures images surface that the frst underwater and Kona, Hawaii. in its own RGB (red, color photograph—that of a hog- Spectra are normalgreen and blue) color fsh—was successfully captured ized by their values at space. Transforming 560 nanometers for in the Dry Tortugas. these images to a deIn 2012, there is no need to comparison. vice-independent color discharge a pound of magnesium space is done through fash powder to provide light una transformation matrix. derwater. The sensors of our com- (Right) The general loci of colors that humans see on a daily This matrix, however, is mercial off-the-shelf (COTS) digi- basis on land (red rectangle) and those that have been mea- not unique, and the accutal cameras are sensitive enough sured underwater in Urla, Woods Hole and Kona (black rect- racy of the resulting colors that detailed, colorful photo- angle) plotted on the CIE chromaticity diagram. in the image will depend graphs can be captured using just on the colors chosen to the ambient daylight at shallow waters. For deeper and darkobtain the matrix. For land photography, the transformaer waters, there are many confgurations of strobe systems tion from the camera color space to human color space is that can be purchased, or tailored one-of-a-kind designs usually done through the use of photographic calibration can be contracted to companies for any set of requirements. targets, such as a Macbeth ColorChecker. For underwater Similarly, watertight housings are available for many makes applications, the use of targets designed for land photog- 10 st / DECEMBER 2012 www.sea-technology.com

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