The CCD (charge-coupled device) that converts light to electric charge in today's digital cameras traces its ancestry back to the first black-and-white camcorders. A camera CCD is a single silicon chip that combines a rectangular array of light-sensitive cells with circuitry to process and digitize the image the cells record.
As the camera's shutter exposes the CCD's imaging substrate, each cell builds a charge proportional to the amount of light it receives. The resulting image is read cell-by-cell using a bucket-brigade process that takes 4 to 5 seconds with these cameras. The cells in the bottom row pass their charges to a serial shift register below the array. Cells in the rows above pass their charges down one
row.
Cell by cell, the CCD chip reads the contents of the shift register, converting the number of electrons to an amplified analog signal. Row by row, the analog signal is processed into a digital image that is stored in the camera's memory. During this transfer, information from the cells may be discarded or combined to reduce image file size and resolution. Even today's low-end digital cameras compare each pixel to its neighbors and optimize for contrast, hue, and saturation.
Capturing color images requires color filters (red, green, and blue) because the individual CCD cells measure light intensity, not frequency. The simplest technique dyes the surface of each cell to sensitize it to one of the three colors needed. After early missteps (striped arrays that reduced resolution by two-thirds), Kodak developed a checkerboard mosaic of colored pixels based on experience gained with film chemistry. The resulting Bayer Pattern CFA (color filter array) has more greens than reds and blues (
see figure
), since human visual systems perceive that ratio as "sharper."
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It's All in Your Perception
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Imaging Without Film
