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1.3 CAPTURING AN IMAGE | ||
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Image sensors record only the gray scale—a series tones ranging from pure white to pure black. Basically, they only capture a range of brightness.
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| The gray scale contains a range of tones from pure white to pure black. |
How, then do sensors capture colors when all they can do is record grays? The trick is to use differently colored filters on the sensor's photosites to separate out the colors of the light reflected from a scene. On most sensors the filters are red, green, and blue (RGB), but some high end cameras use cyan, magenta, and yellow (CMYK) filters. There are a number of ways these filters are used, including the following:
- Three separate image sensors can be used, each with its own filter. This way each image sensor captures the image in one of the three colors.
- Three separate exposures can be made, changing the filter for each one.
- Filters can be placed over individual photosites so each can capture only one of the three colors. In this way, one-forth of the photo is captured in red light, one-forth in blue, and one-half in green. (Green is given more emphasis because of its importance.)
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| Each pixel on the usual image sensor has red, green, and blue filters intermingled across the photosites in patterns designed to yield sharper images and truer colors. The patterns vary from company to company but the most popular is the Bayer mosaic pattern shown here. Courtesy of IBM. |
When photography was first invented, it could only record black & white images. The search for color was a long and arduous process, and a lot of hand coloring went on in the interim (causing one author to comment "so you have to know how to paint after all!").
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Smiling faces to greet you : mirroring contentment from within. (Title taken from label with hand-colored print.) An unidentified group of six people, two of whom are children (2 families?), standing in front of a possibly newly constructed sod house with a pitched sod roof, stovepipe, two windows and a door showing. With the people is a dog. One of the women is wearing a flat straw hat with a large ribbon. Likely taken in North Dakota. |
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"Fred Hultstrand copy of a photo printed from a glass plate. Glass plate borrowed from Howard O. Berg, Devils Lake, N.Dak. Brought in by Morris D. Johnson, Bismarck, N.Dak."--Back of hand-colored print. Photo likely taken by Job V. Harrison of Rock Lake, N.D. Courtesy of the Library of Congress. |
One major breakthrough was James Clerk Maxwell's 1860 discovery that color photographs could be formed using red, green, and blue filters. He had the photographer Thomas Sutton photograph a tartan ribbon three times, each time with a different color filter over the lens. The three images were developed and then projected onto a screen with three different projectors, each equipped with the same color filter used to take its image. When brought into register, the three images formed a full color image. Over a century later, image sensors work much the same way.
Colors in a photographic image are usually based on the three primary colors red, green, and blue (RGB). This is called the additive color system because when the three colors are combined in equal quantities, they form white. This system is used whenever light is projected to form colors as it is on the display monitor (or in your eye). The first commercially successful use of this system to capture color images was invented by the Lumerie brothers in 1903 and became know as the Autochrome process. They dyed grains of starch red, green, and blue and used them to create color images on glass plates. (As you'll see in the section on printers, a few high-end cameras and all printers use the CMYK system. This system, called subtractive colors, uses the three primary colors cyan, magenta, and yellow (hence the CMY in the name—the K stands for an extra black). When combined in equal quantities, these colors form black.
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RGB uses additive colors. When all three are mixed in equal amounts they form white. When red and green overlap they form yellow, and so on. |
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Konica has an interactive Shockwave applet that allows you to drag cyan, magenta, and yello versions of a photo and see how the colors combine to form a full-color image. Be sure to check it out. |
When three separate exposures are made through different filters, or three separate sensors are used, the three images captured in red, green, and blue are combined to form the full-color image. However, when red, green, and blue filters are placed directly over individual photosites on the sensor, some photosites capture the scene in one colors, and others capture it in others. To generate the final full-color image, a process called interpolation is used. Interpolation uses the colors of neighboring pixels to calculate the two colors a photosite didn't record. By combining these two interpolated colors with the color measured by the site directly, the original color of every pixel is calculated. ("I'm bright red and the green and blue pixels around me are also bright so that must mean I'm really a white pixel.") This step is computer intensive since comparisons with as many as eight neighboring pixels are required to perform this process properly.
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Here the full-color of the center red pixel is about to be interpolated from the colors recorded by the eight surrounding pixels. |
Each of the three colors in an image can be controlled independently and is called a color channel. If a channel of 8-bit color is used for each color in a pixel—red, green, and blue—the three channels are combined to give 24-bit color.
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When an image is open in Photoshop a dialog box shows the red, green, and blue channels so you can select the one you want to work on. The top image in the dialog box is the combined 24-bit RGB. |