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The Foveon X3 sensor is a digital camera
image sensor An image sensor or imager is a sensor that detects and conveys information used to make an image. It does so by converting the variable attenuation of light waves (as they pass through or reflect off objects) into signals, small bursts of curr ...
designed by Foveon, Inc., (now part of
Sigma Corporation is a Japanese company, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan. Although Sigma produces several camera models, ...
) and manufactured by Dongbu Electronics. It uses an array of photosites that consist of three vertically stacked
photodiodes A photodiode is a light-sensitive semiconductor diode. It produces current when it absorbs photons. The package of a photodiode allows light (or infrared or ultraviolet radiation, or X-rays) to reach the sensitive part of the device. The packag ...
. Each of the three stacked photodiodes has a different
spectral sensitivity Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal. In visual neuroscience, spectral sensitivity is used to describe the different characteristics o ...
, allowing it to respond differently to different
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s. The signals from the three photodiodes are then processed as
additive color Additive color or additive mixing is a property of a color model that predicts the appearance of colors made by coincident component lights, i.e. the perceived color can be predicted by summing the numeric representations of the component colo ...
data that are transformed to a standard
RGB color space An RGB color space is any additive color space based on the RGB color model. An RGB color space is defined by chromaticity coordinates of the red, green, and blue additive primaries, the white point which is usually a standard illuminant, and t ...
. The X3 sensor technology was first deployed in 2002 in the
Sigma SD9 The Sigma SD9 is a digital single-lens reflex camera (DSLR) produced by the Sigma Corporation of Japan. The camera was launched at the Photo Marketing Association Annual Show on February 18, 2002. It was Sigma's first digital camera, and was t ...
DSLR A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor. The reflex design scheme is the primary difference between ...
camera, and subsequently in the SD10, SD14, SD15, SD1 (including SD1 Merrill), the original mirrorless compact
Sigma DP1 The Sigma DP1 was a high-end compact digital camera introduced by the Sigma Corporation. It featured a 14-megapixel Foveon X3 sensor (2652 × 1768 × 3 layers), a fixed 16.6 mm F4.0 lens (28mm equivalent), a LCD and a pop-up flash. It was the first ...
and
Sigma DP2 The Sigma DP2 is a high-end compact digital camera introduced by the Sigma Corporation. It features a 14-megapixel Foveon X3 sensor (2652 × 1768 × 3 layers), the same sensor used in its predecessor, the Sigma DP1 and in the Sigma SD14 DSLR, a f ...
in 2008 and 2009 respectively, the Sigma dp2 Quattro series from 2014, and the Sigma SD Quattro series from 2016. The development of the Foveon X3 technology is the subject of the 2005 book ''The Silicon Eye'' by
George Gilder George Franklin Gilder (; born November 29, 1939) is an American investor, author, economist, and co-founder of the Discovery Institute. His 1981 book, '' Wealth and Poverty'', advanced a case for supply-side economics and capitalism during the e ...
.


Operation

The diagram to the right depicts how the Foveon X3 sensor works. The image on the left shows the absorption of colors for each wavelength as it passes through the
silicon wafer In electronics, a wafer (also called a slice or substrate) is a thin slice of semiconductor, such as a crystalline silicon (c-Si), used for the fabrication of integrated circuits and, in photovoltaics, to manufacture solar cells. The wafer serv ...
. The image on the right shows a layered sensor stack depicting the colors it detects at each absorption level for each output pixel. The sensor colors shown are only examples. In practice, the color attributes of each output pixel using this sensor result from the camera's
image processing An image is a visual representation of something. It can be two-dimensional, three-dimensional, or somehow otherwise feed into the visual system to convey information. An image can be an artifact, such as a photograph or other two-dimensiona ...
algorithms, which use a matrix process to construct a single RGB color from all the data sensed by the photodiode stack. The depth of the silicon wafer in each of the three sensors is less than five
micrometer Micrometer can mean: * Micrometer (device), used for accurate measurements by means of a calibrated screw * American spelling of micrometre The micrometre ( international spelling as used by the International Bureau of Weights and Measures; ...
s that creates a negligible effect on focusing or
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
. However, because the collection depth of the deepest sensor layer (red) is comparable to collection depths in other silicon
CMOS Complementary metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSFE ...
and CCD sensors, some diffusion of electrons and loss of sharpness in the longer wavelengths occurs.


Use

The first digital camera to use a Foveon X3 sensor was the
Sigma SD9 The Sigma SD9 is a digital single-lens reflex camera (DSLR) produced by the Sigma Corporation of Japan. The camera was launched at the Photo Marketing Association Annual Show on February 18, 2002. It was Sigma's first digital camera, and was t ...
, a
digital SLR A digital single-lens reflex camera (digital SLR or DSLR) is a digital camera that combines the optics and the mechanisms of a single-lens reflex camera with a digital imaging sensor. The reflex design scheme is the primary difference between ...
launched in 2002. It used a 20.7 × 13.8 mm, 2268 x 1512 × 3 (3.54 × 3 MP) iteration of the sensor and was built on a Sigma-designed body using the
Sigma SA mount The Sigma SA-mount is a lens mount by the Sigma Corporation of Japan for use on their autofocus single-lens reflex and mirrorless cameras. It was introduced with the '' SA-300'' in 1992. Originally, the SA-mount was a dual-bayonet mount with i ...
. The camera was followed in 2003 by the improved but technically similar
Sigma SD10 The Sigma SD10 is a digital single-lens reflex camera (DSLR) manufactured by the Sigma Corporation of Japan. It was announced on October 27, 2003, and is an evolution of the previous SD9 model, addressing many of the shortcomings of that camer ...
, which was in turn succeeded in 2006 by the
Sigma SD14 The Sigma SD14 is a digital single-lens reflex camera produced by the Sigma Corporation of Japan. It is fitted with a Sigma SA mount which takes Sigma SA lenses. The camera was announced on August 29, 2006 with a "teaser" advertising campaign an ...
, which used a higher-resolution, 2640 × 1760 × 3 (4.64 × 3 MP) sensor. The SD14's successor, the
Sigma SD15 The Sigma SD15 is an updated version of Sigma SD14 DSLR produced by the Sigma Corporation of Japan and featuring the improved TRUE II image processing engine, but with the same image sensor as its predecessor. As such, the SD15 features the 4.7 MP ...
, was released in June 2010 and used the same 2640 × 1760 × 3 sensor as the SD14. The
Sigma SD1 The Sigma SD1 is a digital SLR camera produced by the Sigma Corporation of Japan. The camera uses a Foveon X3 sensor, which comprises 3 layers of 4800 x 3200 pixels (46 megapixels), giving much higher chromatic resolution than the equivalent Bay ...
was released in June 2011 with a new 23.5×15.7mm APS-C 4800 × 3200 × 3 (15.36 × 3 MP) sensor developed for the professional market. In 2004,
Polaroid Corp. Polaroid may refer to: * Polaroid Corporation, an American company known for its instant film and cameras * Polaroid camera, a brand of instant camera formerly produced by Polaroid Corporation * Polaroid film, instant film, and photographs * Pola ...
announced the Polaroid x530, a compact camera with a 1408 × 1056 × 3, 1/1.8-in. sensor. The camera had a limited release in 2005 but was recalled later in the year for unspecified image quality problems. Sigma announced a prototype of its Foveon-based compact camera in 2006, the
Sigma DP1 The Sigma DP1 was a high-end compact digital camera introduced by the Sigma Corporation. It featured a 14-megapixel Foveon X3 sensor (2652 × 1768 × 3 layers), a fixed 16.6 mm F4.0 lens (28mm equivalent), a LCD and a pop-up flash. It was the first ...
, using the same 14 MP sensor as the SD14 DSLR. A revised version of the prototype was exhibited in 2007, and the camera was eventually launched in spring 2008. Unlike the Polaroid x530, the DP1 had an
APS-C Advanced Photo System type-C (APS-C) is an image sensor format approximately equivalent in size to the Advanced Photo System film negative in its C ("Classic") format, of 25.1×16.7 mm, an aspect ratio of 3:2 and Ø 31.15 mm field d ...
-sized sensor with a 28mm equivalent
prime lens In film and photography, a prime lens is a fixed focal length photographic lens (as opposed to a zoom lens), typically with a maximum aperture from f2.8 to f1.2. The term can also mean the primary lens in a combination lens system. Confusion be ...
. The camera was revised as the DP1s and the DP1x. In 2009, the company launched the DP2, a compact camera using the same sensor and body as the DP1 but with a 41 mm-equivalent f/2.8 lens.


Comparison to Bayer-filter sensors

The operation of the Foveon X3 sensor is different from that of the
Bayer filter A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camco ...
image sensor, which is more commonly used in
digital cameras A digital camera is a camera that captures photographs in digital memory. Most cameras produced today are digital, largely replacing those that capture images on photographic film. Digital cameras are now widely incorporated into mobile device ...
. 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 A demosaicing (also de-mosaicing, demosaicking or debayering) algorithm is a digital image process used to reconstruct a full color image from the incomplete color samples output from an image sensor overlaid with a color filter array (CFA). It is ...
, an interpolative process in which the output pixel associated with each photosite is assigned an
RGB The RGB color model is an additive color model in which the red, green and blue primary colors of light are added together in various ways to reproduce a broad array of colors. The name of the model comes from the initials of the three addi ...
value based in part on the level of red, green, and blue reported by those photosites adjacent to it. However, 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 "Jaggies" is the informal name for artifacts in raster images, most frequently from aliasing, which in turn is often caused by non-linear mixing effects producing high-frequency components, or missing or poor anti-aliasing filtering prior to samp ...
") associated with the process are not seen. The separate anti-aliasing filter commonly usedThough its use is almost universal with Bayer sensors in digital cameras, it is not absolutely necessary. Kodak once produced two digital cameras, the DCS Pro SLR/n and DCS Pro SLR/c (''Digital Photography Review,'
Kodak DCS Pro SLR/c Review
June 2004, Retrieved March 3, 2007) using Bayer sensors without such a filter. However
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s were produced when photographing very fine detail Retrieved March 3, 2007.
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.Microlenses are commonly used in all types of image sensors in digital cameras; in Bayer-filter sensors, microlenses allow the area of the optical image being averaged (i.e., integrated) per sample to approach 25 percent for red and blue, and 50 percent for green, resulting in very little anti-aliasing. For Foveon X3 sensors, the area being averaged can approach 100 percent for each color, resulting in a significant anti-alias filter effect. On the other hand, the method of color separation by silicon penetration depth gives more cross-contamination between color layers, meaning more issues with color accuracy.


Light gathering and low-light performance

The Foveon X3 photosensor can detect more photons entering the camera than a mosaic sensor, because each of the color filters overlaying each photosite of a mosaic sensor passes only one of the primary colors and absorbs 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 a greater light-gathering ability, the individual layers do not respond as sharply to the respective colors; thus color-indicating information in the sensor's raw data requires an "aggressive" matrix (i.e., the removal of common-mode signals) to produce color data in a standard
color space A color space is a specific organization of colors. In combination with color profiling supported by various physical devices, it supports reproducible representations of colorwhether such representation entails an analog or a digital represent ...
, which can increase color noise in low-light situations.


Spatial resolution

According to
Sigma Corporation is a Japanese company, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan. Although Sigma produces several camera models, ...
, "there has been some controversy in how to specify the number of
pixel In digital imaging, a pixel (abbreviated px), pel, or picture element is the smallest addressable element in a raster image, or the smallest point in an all points addressable display device. In most digital display devices, pixels are the smal ...
s 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 A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camco ...
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), which amounts to approximately 3.4 million three-color pixels. However, it has been advertised as a 10.2 MP camera by taking into account 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 A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camco ...
camera produces a larger native file size via
demosaicing A demosaicing (also de-mosaicing, demosaicking or debayering) algorithm is a digital image process used to reconstruct a full color image from the incomplete color samples output from an image sensor overlaid with a color filter array (CFA). It is ...
. 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 demosaicing and the separate anti-aliasing filter are both commonly used to reduce the occurrence or severity of color
moiré pattern In mathematics, physics, and art, moiré patterns ( , , ) or moiré fringes are large-scale interference patterns that can be produced when an opaque ruled pattern with transparent gaps is overlaid on another similar pattern. For the moiré ...
s that the mosaic characteristic of the Bayer sensor produces. The effect of this filter blurs the image output of the sensor which produces a lower resolution than the photosite count would seem to imply. This filter is mostly unnecessary with the Foveon X3 sensor and is not used. The earliest camera with a Foveon X3 sensor, the
Sigma SD9 The Sigma SD9 is a digital single-lens reflex camera (DSLR) produced by the Sigma Corporation of Japan. The camera was launched at the Photo Marketing Association Annual Show on February 18, 2002. It was Sigma's first digital camera, and was t ...
, showed visible luminance moiré patterns without color moiré. Subsequent X3-equipped cameras have less aliasing because they include micro-lenses, which provide an
anti-aliasing filter An anti-aliasing filter (AAF) is a filter used before a signal sampler to restrict the bandwidth of a signal to satisfy the Nyquist–Shannon sampling theorem over the band of interest. Since the theorem states that unambiguous reconstruction ...
by averaging the optical signal over an area commensurate with the sample density. This 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," said Norman Koren. 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. At low
ISO speed Film speed is the measure of a photographic film's sensitivity to light, determined by sensitometry and measured on various numerical scales, the most recent being the ISO system. A closely related ISO system is used to describe the relations ...
, it is even similar to a 7.2 MP Bayer sensor. With the introduction of the
Sigma SD14 The Sigma SD14 is a digital single-lens reflex camera produced by the Sigma Corporation of Japan. It is fitted with a Sigma SA mount which takes Sigma SA lenses. The camera was announced on August 29, 2006 with a "teaser" advertising campaign an ...
, the 14 MP (4.7 MP red + 4.7 MP green + 4.7 MP blue) Foveon X3 sensor resolution is compared favorably by reviewers to that of 10 MP Bayer sensors. For example, Mike Chaney of software 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. A visual comparison between a 14 MP Foveon sensor and a 12.3 MP Bayer sensor shows Foveon has crisper details.


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, chroma noise in particular. Another noted higher noise during long exposure times.This observation is consistent with a comparison of the images, displayed in ''Digital Photography Review,'' taken by the Sigma SD10
see here
with those taken approximately contemporaneously of the same scene by the Bayer sensor-equipped Nikon D70
see here
/page15.asp. Both retrieved March 6, 2007.
However, these reviewers offer no opinion as to whether this is an inherent property of the sensor or the camera's image-processing algorithms. With regards to the Sigma SD14, which uses a more recent Foveon X3 sensor, one reviewer judged its noise levels as ranging from "very low" at ISO 100 to "moderate" at ISO 1600 when using the camera's
Raw image format A camera raw image file contains unprocessed or minimally processed data from the image sensor of either a digital camera, a motion picture film scanner, or other image scanner. Raw files are named so because they are not yet processed and the ...
.


Sample images

Sigma's SD14 site has galleries of full-resolution images showing the color produced by the Foveon technology. The 14 MP Foveon chip produces 4.7 MP native-size RGB files; 14 MP
Bayer filter A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camco ...
cameras produce a 14 MP native file size by interpolation (i.e., demosaicing). Direct visual comparison of images from 12.7 MP Bayer sensors and 14.1 MP Foveon sensors show Bayer images are superior on fine monochrome detail, such as the lines between bricks on a distant building, but the Foveon images are superior in color resolution.


See also

*
Color filter array In digital imaging, a color filter array (CFA), or color filter mosaic (CFM), is a mosaic of tiny color filters placed over the pixel sensors of an image sensor to capture color information. The term is also used in reference to e paper devices ...
*
Bayer filter A Bayer filter mosaic is a color filter array (CFA) for arranging RGB color filters on a square grid of photosensors. Its particular arrangement of color filters is used in most single-chip digital image sensors used in digital cameras, camco ...
* CYGM filter *
RGBE filter In digital photography, the RGBE filter is an alternative color filter array to the Bayer filter (GRGB). It similarly uses a mosaic of pixel filters, of red, green, blue and "emerald" ("like cyan" according to Sony), and so also requires demosaic ...


Notes


References


External links

* Richard F. Lyon and Paul M. Hubel
Eyeing the Camera: into the Next Century
10th Color Imaging Conference: Color Science, System and Applications. IS&T and SID, Springfield, Va, USA, 2002. P. 349–355.
Foveon X3 technology pageDPReview Foveon X3 prototype previewSample Sigma/Foveon photosSample Polaroid x530/Foveon photos

Sigma DP1
{{DEFAULTSORT:Foveon X3 Sensor Digital photography Image sensors Color filter array