Like film cameras, digital cameras create images by reading the light in a scene. But how does the camera translate that brightness information into the colors you see in the final photograph? As it turns out, a digital camera does the job pretty much the same way as the human eye.
To understand how digital cameras - and your eyes - perceive color, you first need to know that light can be broken into three color ranges: reds, greens, and blues. Inside your eyeball, you have three receptors corresponding to those color ranges. Each receptor measures the brightness of the light for its particular range. Your brain then combines the information from the three receptors into one multicolored image in your head.
Because most of us didnt grow up thinking about mixing red, green, and blue light to create color, this concept can be a little hard to grasp. Heres an analogy that may help. Imagine that youre standing in a darkened room and have one flashlight that emits red light, one that emits green light, and one that emits blue light. Now suppose that you point all the flashlights at the same spot on a white wall. Where the three lights overlap, you get white, as shown in Figure 2-2. Where no light falls, you get black.
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| Figure 2-2: RGB images are created by blending red, green, and blue light. |
In the illustration, my flashlights emit full intensity light with no fade over the spread of the beam, so mixing the three lights produces only three additional colors: magenta, cyan, yellow. But by varying the intensity of the beams, you can create every color that the human eye can see.
Just like your eyes, a digital camera analyzes the intensity - sometimes referred to as the brightness value - of the red, green, and blue light. Then it records the brightness values for each color in separate portions of the image file. Digital imaging professionals refer to these vats of brightness information as color channels. After recording the brightness values, the camera mixes them together to create the full color image.
Pictures created using these three main colors of light are known as RGB images - for red, green, and blue. Computer monitors, television sets, and scanners also create images by combining red, green, and blue light.
In sophisticated photo editing programs such as Adobe Photoshop, you can view and edit the individual color channels in a digital image. Figure 2-3 shows a color image broken down into its red, green, and blue color channels. Notice that each channel contains nothing more than a grayscale image. Thats because the camera records only light - or the absence of it - for each channel.
RGB | Red Channel |
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Green Channel | Blue Channel |
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Figure 2-3: An RGB image has three color channels, one each
for the red, green, and blue light values.
In any of the channel images, light areas indicate heavy amounts of that channel's color. For example, the red portion of the left flag appears nearly white in the red channel image, but nearly black in the green and blue channel images. Likewise, the blue flag poles appear very light in the blue channel images. And the white center portion of the left flag is bright in all three channel images; remember, strong amounts of red, green, and blue light produce white.
At the risk of confusing the issue, I should point out that not all digital images contain three channels. If you convert an RGB image to a grayscale (black and white) image inside a photo-editing program, for example, the brightness values for all three color channels are merged into one channel. And if you convert the image to the CMYK color model in preparation for professional printing, you end up with four color channels, one corresponding to each of the four primary colors of ink (cyan, magenta, yellow, and black). For more on this topic, see ìRGB, CMYK, and Other Colorful Acronyms,î later in this chapter. Don't let this channel stuff intimidate you - until you become a seasoned photo editor, you don't need to give it another thought. I bring the topic up only so that when you see the term RGB, you have some idea what it means.
Don't let this channel stuff intimidate you - until you become a seasoned photo editor, you don't need to give it another thought. I bring the topic up only so that when you see the term RGB, you have some idea what it means.
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