Talking about the Foveon X3 Sensor

The Foveon X3 sensor is a CMOS image sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductor and Dongbu Electronics. It uses an array of photosites, each of which consists of three vertically stacked photodiodes, that are organized in a two-dimensional grid. Each of the three stacked photodiodes responds to different wavelengths of light, i.e., each has a different spectral sensitivity curve. This is due to that fact that different wavelengths of light penetrate silicon to different depths. The signals from the three photodiodes are then processed, resulting in data that provides the three additive primary colors, red, green, and blue.

The development of the Foveon X3 technology is the subject of the 2005 book The Silicon Eye by George Gilder.

Operation

The diagram to the right shows how this works in graphic form. Depicted on the left is the absorption of colors of the spectrum according to their wavelength as they pass through the silicon wafer. On the right, a Foveon X3 layered sensor stack in the silicon wafer for each output pixel is shown depicting the colors it detects at each absorption level. The color purity and intensity of blue, green and red depicted for the sensors are for ease of illustration. In fact, the attributes of each output pixel that are reported by a camera using this sensor result from the camera's image processing algorithms that employ a matrix process to construct the single RGB color from the data sensed by the photodiode stack.

Because the depth in the silicon wafer of each of the three layer Foveon X3 sensors is less than five micrometres, it has negligible effect on focusing or chromatic aberration. However, because the collection depth of the deepest sensor layer (red) is comparable to collection depths in other silicon CMOS and CCD sensors, some diffusion of electrons and loss of sharpness in the longer wavelengths occurs.

Utilization

As of March, 2008, the Sigma SD14 digital SLR camera the Polaroid X530, and the Sigma DP-1 compact camera (released in Japan on 3 March 2008) are the only consumer cameras shipping with a Foveon X3 sensor.

The Foveon X3 sensor is also used in the Hanvision HVDUO-5M and HVDUO-10M scientific and industrial cameras, though the sensors in these products, one of which is the same as in the Polaroid x530, are at end-of-life status. It was also used in the Sigma SD9 and SD10 consumer digital SLR cameras. These cameras are no longer in production.

Comparison to Bayer filter sensors – operational differences

The operation of the Foveon X3 sensor is quite different from that of the Bayer filter image sensor more commonly used in digital cameras. In the Bayer sensor, each photosite in the array consists of a single light sensor (either CMOS or CCD) that, as a result of filtration, is exposed to only one of the three primary colors, red, green, or blue. Constructing a full color image from a Bayer sensor requires demosaicing, an interpolative process in which the output pixel associated with each photosite is assigned an RGB value based in part on the level of red, green, and blue reported by those photosites adjacent to it. The Foveon X3 sensor creates its RGB color output for each photosite by combining the outputs of each of the stacked photodiodes at each of its photosites. This operational difference results in several significant consequences.

Color artifacts

Because demosaicing is not required for the Foveon X3 sensor to produce a full-color image, the color artifacts ("colored jaggies") associated with that process are not seen. The separate anti-aliasing filter commonly used to mitigate those artifacts in a Bayer sensor is not required. This is because little aliasing occurs when the photodiodes for each color, with the assistance of the microlenses integrate the optical image over a region almost as big as the spacing of sensors for that color.

Light gathering and low-light performance

Another difference is that more of the photons entering the camera will be detected by the Foveon X3 photosensor than is possible with a mosaic sensor. This is because each of the color filters overlaying each photosite of a mosaic sensor passes only one of the primary colors, absorbing the other two. The absorption of these colors reduces the total amount of light gathered by the sensor and destroys much of the information about the color of the light impinging on each sensor element. Although the Foveon X3 has greater light gathering ability, the individual layers do not respond as sharply to the respective colors. As a result color-indicating information in the sensor's raw data requires "aggressive" matrixing (essentially, removal of common-mode signals) to produce color data in a standard color space, which can increase noise in low-light situations.

Spatial resolution

According to Sigma Corporation, "there has been some controversy in how to specify the number of pixels in Foveon sensors." The argument has been over whether sellers should count the number of photosites, or the total number of photodiodes, as a megapixel count, and whether either of those should be compared with the number of photodiodes in a Bayer filter sensor or camera as a measure of resolution.

For example, the dimensions of the photosite array in the sensor in the Sigma SD10 camera are 2268 × 1512, and the camera produces a native file size of those dimensions (times three color layers). This amounts to approximately 3.4 million three-color pixels. However, it has been advertised as a 10.2 MP camera by taking account of the fact that each photosite contains stacked red, green, and blue color sensing photodiodes, or pixel sensors (2268 × 1512 × 3). By comparison, the dimensions of the photosite array in the 10.2 MP Bayer sensor in the Nikon D200 camera are 3872 × 2592, but there is only one photodiode, or one pixel sensor, at each site. The cameras have equal numbers of photodiodes, and produce similar RAW data file sizes, but the Bayer filter camera produces a larger native file size via demosaicing.

However, the actual resolution produced by the Bayer sensor is more complicated than the count of its photosites, or its native file size, might suggest. The reason has to do with both the demosaicing and the separate anti-aliasing filter commonly used to reduce the occurrence or severity of color moiré patterns that the mosaic characteristic of the Bayer sensor produces. The effect of this filter is to blur the image output of the sensor, As a result producing a lower resolution than the photosite count would seem to imply. This filter is largely unnecessary with the Foveon X3 sensor and is not used. The earliest camera with a Foveon X3 sensor, the Sigma SD9, showed visible luminance moiré patterns, but not color moiré. Subsequent X3-equipped cameras have less aliasing because they include microlenses, which provide an effective anti-aliasing filter by averaging the optical signal over an area commensurate with the sample density, which is not possible in any color channel of a Bayer-type sensor. Aliasing from the Foveon X3 sensor is "far less bothersome because it's monochrome" according to Norman Koren. Therefore, in theory, it is possible for a Foveon X3 sensor with the same number of photodiodes as a Bayer sensor and no separate anti-aliasing filter to attain a higher spatial resolution than that Bayer sensor. Independent tests indicate that the "10.2 MP" array of the Foveon X3 sensor (in the Sigma SD10) has a resolution similar to a 5 MP or 6 MP Bayer sensor, and at at low ISO speed even similar to a 7.2 MP Bayer sensor.

With the introduction of the Sigma SD14, the 14 MP (4.7 MP red + 4.7 MP green + 4.7 MP blue) Foveon X3 sensor resolution is being compared favorably by reviewers to that of 10 MP Bayer sensors. For example, Mike Chaney of ddisoftware says, "the SD14 produces better photos than a typical 10 MP dSLR because it is able to carry sharp detail all the way to the 'falloff' point at 1700 LPI whereas contrast, color detail, and sharpness begin to degrade long before the 1700 LPI limit on a Bayer based 10 MP dSLR." Another article judges the Foveon X3 sensor as roughly equivalent to a 9 MP Bayer sensor. 

Comparison to Bayer filter sensors – noise

The Foveon X3 sensor, as used in the Sigma SD10 camera, has been characterized by two independent reviewers as noisier than the sensors in some other DSLRs using the Bayer sensor at higher ISO film speed equivalents. Another has noted higher noise during long exposure times. However, these reviewers offer no opinion as to whether this is an inherent property of the sensor or the camera's image processing algorithms.

More recently, one reviewer is judging the Sigma SD14 camera with the new 14 MP Foveon X3 sensor to have noise levels ranging from "very low" at the ISO 100 sensitivity equivalent to "Moderate" at the ISO 1600 equivalent using the camera's Raw image format.

Comparison to Bayer filter sensors – actual samples

Sigma's SD14 site has galleries of full-resolution images showing the color produced by the current state of Foveon technology. The 14-MP Foveon chip produces 4.7 MP native-size RGB files; 14-MP Bayer filter cameras produce a 14 MP native file size by interpolation (demosaicing). Direct visual comparison of images from 12.7-MP Bayer sensors and 14.1 MP Foveon sensors show Bayer images ahead on fine monochrome detail, such as the lines between bricks on a distant building, but the Foveon images are ahead on color resolution.

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