7 TECHNOLOGIES:DIRECT IMAGE SENSOR A Color Sensor Delivering Pictures with Unique Purity

The evolution of color-handling in photography

In 1907, the Lumière brothers of France amazed the world by launching the first commercial color photography process. Known as Autochrome, the process used a filter consisting of grains colored red, blue and green (the primary colors of light). The filter was spread over a glass plate, and the colors were recorded horizontally. Made from grains of potato-starch, this RGB filter was rather crude, yet even by today's standards, the color photographs it produced are amazingly vivid, considering that they were taken a hundred years ago.

In later years, color film photography evolved by a method in which three layers of photosensitive material were stacked vertically, and processes using a horizontal orientation, like the Autochrome process, were not developed any further. But time changes everything, and now that digital photography has taken over from film as the mainstream technology, horizontally-oriented color-handling has once again become the standard approach.

Conventional digital cameras use monochrome sensors.

Apart from the SD series and the DP series, almost all the digital cameras on the market use monochrome sensors only capable of capturing light intensity. Because these sensors do not capture color data, a color filter with a mosaic of pixels for the three primary colors - red, blue and green (RGB) - is mounted on top so that color data can be represented. But each light-sensing photodiode has a one-color filter, which means that each pixel can only capture one color, and data for the other two colors is discarded.

Until this stage, of course, as in the Autochrome process, the RGB color “particles”, or pixels, are recorded unmodified, forming the photo. A color interpolation process known as demosaicing is therefore performed in the latter stage of the image processing, and this restores the colors lost by individual pixels. This interpolation process basically consists of guessing the missing colors by analyzing the neighboring pixels, and adding those missing colors back in.

Post-processing the image leads to a loss of detail

Having been continuously improved over an extended period, this image-processing method has matured to a certain extent, so the color interpolation is now performed fairly accurately. But because colors are interpolated from neighboring pixels, the subtle color nuances of the original subject are lost.

Conventional digital cameras using color filter arrays also generate color artifacts - colors not found in the original subject - during the demosaicing processing. This is due to the action of the color filter (generally a Bayer filter), which tries to regulate the color distribution if the subject contains too much detail (high-frequency areas).

A conventional digital cameras using a Bayer color filter has yet another filter, known as an optical low pass or blurring filter, interposed between the lens and the sensor, in order to suppress color artifacts. The optical low pass filter acts on the images resolved at a high level by the imaging lens, its job being to eliminate any detailed elements likely to generate color artifacts (high-frequency areas above a certain level), immediately before they reach the sensor. So it can effectively suppress the generation of color artifacts. However, the downside is that it reduces the resolution of the image.

The Foveon X3® captures the very feeling in the air.

Have you ever looked at an image generated by a digital camera and noticed something, well... unnatural, about it? The edges may be strongly emphasized, and the image may look reasonably nuanced, but there's definitely something wrong. Right?

Images produced by Sigma's SD series cameras, and by the DP series, have what's been called an “emotional quality”. The emotion comes with a level of image quality that only the Foveon X3® direct image sensor can deliver. Image quality of a clarity and exquisiteness easily outclassing that of conventional digital cameras. This level of image quality reproduces the scene you shot, right down to the feeling in the air. It's only possible in a vertical color-capture system that does not require color interpolation, and an image-processing system that does not require an optical low-pass filter.

A conventional image-sensor, on the other hand, fudges the colors, and even cuts out high-frequency areas. To compensate, the sharpness processing is ramped up to give some overall nuancing and a general impression of high resolution. This explains the tendency to generate images that, as a whole, have an unnatural feel. The colors can be adjusted to some extent in post-processing, but the detailed data previously lost cannot be recovered. The breathtaking image quality delivered by the Foveon X3®, which reproduces pure, rich data and nothing else, has to be seen to be believed.

Discards none of the original light and color. And adds none either.

The SIGMA DP's Foveon X3® direct image sensor utilizes the special properties of silicon, which is penetrated to different depths by different wavelengths of light, to successfully achieve full-color capture for the first time ever in a single-pixel site configuration. No color filter is required. Like modern color film cameras, it uses a method that captures all the colors vertically.

Because it does not need color interpolation or a low-pass filter, the Foveon X3® produces images that are sharp right from the start. Therefore, sharpness processing in the latter stages of the image processing - creating edges and emphasizing contours - can be reduced to a minimum. This is why reviewers have evaluated the images captured by the Foveon X3® as having a truly nuanced, sharp feel and praised them as very natural and demonstrating superior image quality.

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