Sensors Un-Sensored

The Sigma X3 Quattro sensor separates colors vertically in the sensor instead of using a Bayer array.

While it works amazingly well, the Bayer system has its drawbacks. First, because each pixel receives only one primary color, data for the two missing primaries must be obtained through interpolation of data from neighboring pixels using complex proprietary algorithms. This can produce moiré and color artifacts, which must be dealt with, either by using an anti-aliasing filter over the sensor assembly or by the user during postprocessing. (See the article on AA filters, "Do You Use Any Aliases?")

Because each pixel records only one color of light, not all the photons falling on the sensor are used; just half of the green ones, and one-quarter of the red ones and blue ones can be recorded. By using wider-ranging (less specific) colored filters in the Bayer array, sensor manufacturers can allow more light to reach each pixel (at the cost of less precise color rendering); still, at least half of the green light and three-quarters of the red and blue light are lost.

The Bayer system also means that the sensor doesn’t deliver as much resolution as its horizontal-by-vertical pixel count implies because not all the information coming from the lens is recorded at each pixel site.

We should state again that despite these drawbacks, Bayer sensors work very well; pro DSLRs and medium-format cameras have used them for years, with pro image quality. But the drawbacks are there.

As technology has advanced, microlens technology has improved to bring more imaging light to the photosites.

Foveon (used by Sigma digital cameras and now owned by Sigma) offered an alternative to the Bayer sensor, starting with the Sigma SD9 DSLR in 2003. Rather than using colored filters to obtain color data for each pixel, the Foveon X3 sensor stacks three pixel layers and takes advantage of the fact that light penetrates silicon to different depths, depending on wavelength (color). In effect, the top layer records mainly blue light, the middle layer, mainly green, and the bottom layer, mainly red. So each pixel site records light of all three primary colors, there’s less moiré and no Bayer-array color artifacts, and the sensor produces higher resolution than a Bayer sensor of equal horizontal-by-vertical pixel count, in part because there’s no need for the blurring AA filter. The drawbacks have been more noise than Bayer at higher ISOs and slower performance because of all the data that must be processed for each image.

Now, Sigma has announced a new Foveon sensor, the Quattro. Like previous Foveon X3 sensors, the new X3 Quattro features vertical color separation technology rather than colored filters to derive color information. Like previous X3s, the new sensor stacks three pixel layers, the top one recording mainly blue, the middle one, green, and the bottom one, red wavelengths. Where previous X3 sensors had three layers of identical pixel count, the new Quattro features a 4:1:1 ratio—the top layer has four pixels for each pixel in the lower layers. This allowed Sigma to up the pixel count while reducing noise and speeding up processing and writing times, thanks to less total data per image file, all while retaining the essential Foveon assets—each primary color recorded at every pixel site, no moiré and no need for a blurring optical low-pass filter. A new TRUE III processor designed for the Quattro sensor optimizes image quality and speeds performance. (For the record, the 14-bit RAW files output by the new X3 Quattro sensor measu
re 5424×3616 pixels compared to 4704×3136 for the X3 Merrill sensor’s 12-bit files.)

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