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There are three primary colors of light—red, green, and blue—so what gives with Sony’s four-color “RGB+E” sensor technology? Should you, as a digital SLR owner, worry about it? As with everything related to digital photography, it’s useful to understand what’s going on, even if it doesn’t apply to your own work. Some day, it might. Who knows?

What Sony is doing is refining the traditional Bayer mosaic pattern by adding a fourth color, a sort of cyanish green, that it calls emerald, ostensibly to match the color response of the sensor more closely to the color perception of the human eye. The problem with a red-green-blue matrix is that human eyes have a quirky response to red at certain frequencies. That can be fixed by applying a blue-green subtractive filter to the red channel, but only at those particular frequencies. The emerald filter over some of the photosites in Sony’s RGB+E sensor provides the information used to make that adjustment.

As a result, the RGB+E sensor has 25% red-sensitive pixels, 25% green-sensitive pixels, 25% “emerald”-sensitive pixels, and 25% blue-sensitive pixels, as you can see in Figure 2.9. The emerald-filtered photosites are close enough in response to the green pixels that, under most conditions, the RGB+E sensor responds like a Bayer mosaic (with 25% red, 50% green (actually green+emerald), and 25% blue). After capture, the four-color image is processed and converted back to a traditional threecolor, RGB image, but one Sony says has more accurate colors.