The RGB Color Space

through a lens far from its center. The rays do not focus on a point inside the camera, but instead create a cone of light, which looks like a comet tail.

All digitalcameras rely on a color space definition to capture color. This definition, which is part of a processing algorithm, forces light values captured by the camera sensor into a defined color space. Essentially, the camera can only capture specific colors, not the entire spectrum of color. Generally, this is not significant because the human eye cannot really discern colors accurately. There are several color space definitions in use today. Each has its advantages and disadvantages: ■ RGB—Within thecolor model RGB are a number of color spaces, such as Apple RGB, Adobe RGB (1998), and sRGB. Each RGB color space defines color through three axes (R, G, and B), but differ in gamut and other characteristics. RGB can be thought of as three grayscale images (usually referred to as channels) representing the light values of red, green, and blue. Combining these three channels of light produces a wide range of visible colors. The three colors combined generate white, unlike the CMYK color space, which generates black. For this reason, the RGB color space is called an additive color space. ■ sRGB—sRGB is currently the standard color space for cameras and computer monitors. This standard was created by Microsoft and HP in 1996 as a standard for computer monitors and software. The sRGB standard includes three important areas: colorimetric RGB definition, the equivalent gamma value of 2.2, and a set of well defined viewing conditions. This results in a color space that is equal among all devices, from the digital camera to the printer. The only problem is that standardization means much fewer colors are possible. ■ YUV—A television standard used in Europe that enables backward compatibility with black-and-white televisions. A variant called YIQ is used in North American television systems. In addition, all DVDs rely on the YUV/YIQ color space. The engineers who invented the YUV color space needed a way to make color television broadcasts backward-compatible with black-and-white TVs. The color signal they came up with also needed to conserve bandwidth because three channels of RGB data would not fit into the limited broadcast signal space. The YUV color space uses RGB information, but it creates a black-andwhite image (luminance) from the full color image and then subtracts the

three primary colors, resulting in two additional signals to describe color. Combining the three signals back together results in a full color image. Note that in either case, if the chrominance (color information) is ignored, the result is a black-and-white picture. ■ CMYK—Color printers and large offset printers rely on the CMYK color space. This color space matches the color pigments cyan, magenta, and yellow. The color black (the K in CMYK) is included, but not required. The colors C, M, and Y absorb colors on paper, creating black. This differs from the RGB color space, which reflects white when the colors R, G, and B are combined. For this reason, the CMY color space is called a “subtractive model.” The black (K) component is added so that true black can be printed on paper (versus a contrived black from the three colors). As with the RGB color space, every color is represented by three values: C, M, and Y. These values are assumed to be in the 0–255 range. ■ CiéLAB—LAB colormode splits color into three values: ■ L describes relative lightness ■ A represents relative redness-greenness ■ B represents relative yellowness-blueness Adobe Photoshopsoftware uses LAB color as its native color space because LAB color can be converted to another color space without doing damage to the colors’ intensities or hues. Photoshop is just as talented at working with images in the other color spaces, of course. The RGB color space is actually related closely to LAB color. The CiéLAB model was adopted worldwide as the master color space definition in 1991. The color models described here are the most common color spaces. Digital cameras rely on the RGB color space, which has a few issues. One is that the RGB color space is device-dependent. In other words, when the camera captures an image, it may not appear the same on the monitor (another RGB device). This, of course, could cause problems for serious color photographers. The JPEG compression scheme relies on YUV, which is also known as YCbCr. This color space enables the compression of the chrominance (colors) in an image without affecting the brightness. The human eye is more sensitive to brightness changes than color changes, which fits with this color model.