Many digital photographers do monochrome photography with their regular color digital cameras. This provides a couple of benefits. First, if you shoot RAW, you can process your images to color or monochrome. If you use the camera's monochrome mode, you'll see the images in black-and-white on the LCD monitor, including the tonal-changing effects of built-in or lens-mounted color filters, but you can still process the images to color or monochrome with your RAW converter. (If you shoot JPEGs in monochrome mode, you can't later make the images color.) The second benefit of using your regular digital camera to shoot monochrome is that you can adjust each color channel individually in postprocessing, giving you lots of control over the look of each image.
The drawback? Conventional digital cameras use Bayer-filtered sensors. The photodiodes ("pixels") in image sensors can't detect color; they detect how much light strikes them, but not what the wavelengths are. To obtain color information, most digital cameras use a grid of red, green and blue filters known as a Bayer array (named for the Kodak scientist who came up with the concept) over the sensor, so that each pixel receives light of only one of these primary colors. The missing colors for each pixel are then derived by interpolation, using data from neighboring pixels and complex proprietary algorithms in a process known as demosaicing.
One key advantage of monochrome cameras: no Bayer array on the sensor, thus no interpolation. The Sigma Foveon sensor is full-color, yet due to its unique technology, there's no Bayer array so it has that same advantage.
Sigma's digital cameras—the SD1 Merrill DSLR, the DP1, DP2, DP3 Merrill and the DP2 Quattro compacts—use a unique Foveon sensor, which doesn't employ the Bayer filter grid used by most other sensors. Instead, the Foveon sensor takes advantage of the fact that light penetrates silicon to different depths, depending on wavelength: short (blue) wavelengths penetrate a little; medium (green) wavelengths, deeper; and long (red) wavelengths, still deeper. The Foveon sensor stacks three layers of pixels, the top layer recording mostly blue, the middle layer mostly green and the bottom layer mostly red light. It's more complicated than that, but this simplified explanation makes the point: Unlike Bayer sensors, Foveon sensors record all three primary colors of light at every pixel site, so there's no need for the array, demosaicing or an anti-aliasing filter. The result: better resolution than a Bayer sensor of similar horizontal-by-vertical pixel count provides. A side benefit is that monochrome images are sharper than those from a Bayer sensor of equal pixel count. And unlike the monochrome digital cameras, the Sigma bodies with Foveon sensors provide three color channels, allowing you to adjust each individually for optimum control over the final monochrome image. The drawbacks? There's some light loss (i.e., high ISO performance isn't as good as with monochrome cameras), and the Foveon sensors are all APS-C—smaller than the sensors in the monochrome cameras.