Foveon X3 sensor
Encyclopedia
The Foveon X3 sensor is a CMOS
image sensor
for digital cameras, designed by Foveon, Inc.
(now part of Sigma Corporation
) and manufactured by National Semiconductor
and Dongbu Electronics.
It uses an array of photosites, each of which consists of three vertically stacked photodiodes, organized in a two-dimensional grid. Each of the three stacked photodiodes responds to different wavelength
s of light, i.e., each has a different spectral sensitivity
curve. This difference 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 color
s, red, green, and blue.
The development of the Foveon X3 technology is the subject of the 2005 book The Silicon Eye by George Gilder
.
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.
The results, in terms of color accuracy (metamerism index), were state of the art at the time of their invention.
Because the depth in the silicon wafer of each of the three layer Foveon X3 sensors is less than five micrometre
s, 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.
, a digital SLR
launched in 2002. This used a 2268x1512×3 (3.54×3 MP) iteration of the sensor, and was based around a Sigma-designed body using the Sigma SA mount
. The camera was followed in 2003 by the improved but technically similar Sigma SD10
, which was in turn superseded in 2006 by the Sigma SD14
, which used a higher-resolution, 2640×1760×3 sensor. Sigma announced a successor, the Sigma SD15
, in 2008, although the camera did not go on sale until June 2010. It used the same 2640×1760×3 (4.7×3 MP) sensor as the SD14. As of 2011 it is Sigma's current prosumer digital SLR. In September 2010 the company announced the Sigma SD1
, which uses a new, 4800×3200×3 sensor, and was aimed at the professional market.
In 2004 Polaroid Corporation
announced the Polaroid x530, a compact camera based around a 1408×1056×3, 1/1.8" sensor. The camera received a limited release in 2005 but was recalled later in the year for unspecified image quality problems. Sigma announced a prototype of their own Foveon-based compact camera in 2006, the Sigma DP1
, 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 was based around an APS-C
-sized sensor, with a 28mm equivalent prime lens
. The camera was subsequently revised as the DP1s and the DP1x. In 2009 the company launched the DP2, a compact camera based around the same sensor and body as the DP1, but with a 41mm-equivalent f/2.8 lens.
Foveon X3 sensors were also used in the Hanvision HVDUO-5M and HVDUO-10M, a pair of digital cameras aimed at the scientific and industrial markets.
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.
") 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 microlens
es integrate the optical image over a region almost as big as the spacing of sensors for that color.
On the other hand, the method of color separation by silicon penetration depth gives more cross-contamination between color layers, and therefore more issues with color accuracy especially with the red channel.
, which can increase color noise in low-light situations.
, "there has been some controversy in how to specify the number of pixel
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
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 pattern
s that the mosaic characteristic of the Bayer sensor produces. The effect of this filter is to blur the image output of the sensor, thus 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 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.
A visual comparison between a 14 MP Foveon sensor and a 12.3 MP Bayer sensor shows Foveon has crisper details.
,
and specifically chroma
noise has been noted.
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.
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
.
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.
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
image sensor
Image sensor
An image sensor is a device that converts an optical image into an electronic signal. It is used mostly in digital cameras and other imaging devices...
for digital cameras, designed by Foveon, Inc.
Foveon
Foveon, Inc., is the company that makes the Foveon X3 sensor, which captures images in digital single-lens reflex cameras such as the Sigma Corporation SD9, SD10, SD14 and SD15 as well as in the compacts DP1, DP2 and Polaroid X530....
(now part of Sigma Corporation
Sigma Corporation
is a Japanese company founded in 1961, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan...
) and manufactured by National Semiconductor
National Semiconductor
National Semiconductor was an American semiconductor manufacturer, that specialized in analog devices and subsystems,formerly headquartered in Santa Clara, California, USA. The products of National Semiconductor included power management circuits, display drivers, audio and operational amplifiers,...
and Dongbu Electronics.
It uses an array of photosites, each of which consists of three vertically stacked photodiodes, organized in a two-dimensional grid. Each of the three stacked photodiodes responds to different wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s of light, i.e., each has a different spectral sensitivity
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....
curve. This difference 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 color
Primary color
Primary colors are sets of colors that can be combined to make a useful range of colors. For human applications, three primary colors are usually used, since human color vision is trichromatic....
s, red, green, and blue.
The development of the Foveon X3 technology is the subject of the 2005 book The Silicon Eye by George Gilder
George Gilder
George F. Gilder is an American writer, techno-utopian intellectual, Republican Party activist, and co-founder of the Discovery Institute...
.
Operation
Wafer (electronics)
A wafer is a thin slice of semiconductor material, such as a silicon crystal, used in the fabrication of integrated circuits and other microdevices...
. 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
Image processing
In electrical engineering and computer science, image processing is any form of signal processing for which the input is an image, such as a photograph or video frame; the output of image processing may be either an image or, a set of characteristics or parameters related to the image...
algorithms that employ a matrix process to construct the single RGB color from the data sensed by the photodiode stack.
The results, in terms of color accuracy (metamerism index), were state of the art at the time of their invention.
Because the depth in the silicon wafer of each of the three layer Foveon X3 sensors is less than five micrometre
Micrometre
A micrometer , is by definition 1×10-6 of a meter .In plain English, it means one-millionth of a meter . Its unit symbol in the International System of Units is μm...
s, it has negligible effect on focusing
Focus (optics)
In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is conceptually a point, physically the focus has a spatial extent, called the blur circle. This non-ideal focusing may be caused by...
or chromatic aberration
Chromatic aberration
In optics, chromatic aberration is a type of distortion in which there is a failure of a lens to focus all colors to the same convergence point. It occurs because lenses have a different refractive index for different wavelengths of light...
. However, because the collection depth of the deepest sensor layer (red) is comparable to collection depths in other silicon CMOS
CMOS
Complementary metal–oxide–semiconductor is a technology for constructing integrated circuits. CMOS technology is used in microprocessors, microcontrollers, static RAM, and other digital logic circuits...
and CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
sensors, some diffusion of electrons and loss of sharpness in the longer wavelengths occurs.
Utilization
The first digital camera to use a Foveon X3 sensor was the Sigma SD9Sigma SD9
The Sigma SD9 is a digital SLR camera 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 the first production camera to use the unique Foveon X3 image sensor, which...
, a digital SLR
Digital single-lens reflex camera
Most digital single-lens reflex cameras are digital cameras that use a mechanical mirror system and pentaprism to direct light from the lens to an optical viewfinder on the back of the camera....
launched in 2002. This used a 2268x1512×3 (3.54×3 MP) iteration of the sensor, and was based around a Sigma-designed body using the Sigma SA mount
Sigma SA mount
The Sigma SA mount is a design of lens mount designed by the Sigma Corporation of Japan for use on their single-lens reflex camera designs. The SA mount uses a bayonet mount which is physically similar to the Pentax K mount but uses a flange focal distance of 44 mm, identical to that of the Canon...
. The camera was followed in 2003 by the improved but technically similar Sigma SD10
Sigma SD10
The Sigma SD10 is a digital SLR camera produced 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 camera...
, which was in turn superseded in 2006 by the Sigma SD14
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....
, which used a higher-resolution, 2640×1760×3 sensor. Sigma announced a successor, the Sigma SD15
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 14.7MP Foveon X3 sensor...
, in 2008, although the camera did not go on sale until June 2010. It used the same 2640×1760×3 (4.7×3 MP) sensor as the SD14. As of 2011 it is Sigma's current prosumer digital SLR. In September 2010 the company announced the Sigma SD1
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 , giving much higher spatial resolution than the equivalent Bayer array...
, which uses a new, 4800×3200×3 sensor, and was aimed at the professional market.
In 2004 Polaroid Corporation
Polaroid Corporation
Polaroid Corporation is an American-based international consumer electronics and eyewear company, originally founded in 1937 by Edwin H. Land. It is most famous for its instant film cameras, which reached the market in 1948, and continued to be the company's flagship product line until the February...
announced the Polaroid x530, a compact camera based around a 1408×1056×3, 1/1.8" sensor. The camera received a limited release in 2005 but was recalled later in the year for unspecified image quality problems. Sigma announced a prototype of their own Foveon-based compact camera in 2006, the Sigma DP1
Sigma DP1
The Sigma DP1 is a high-end compact digital camera introduced by the Sigma Corporation. It features a 14-megapixel Foveon X3 sensor , a fixed 16.6 mm F4.0 lens , a 2.5” LCD and a pop-up flash...
, 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 was based around an APS-C
APS-C
Advanced Photo System type-C is an image sensor format approximately equivalent in size to the Advanced Photo System "classic" size negatives...
-sized sensor, with a 28mm equivalent prime lens
Prime lens
In film and photography, a prime lens is either a photographic lens whose focal length is fixed, as opposed to a zoom lens, or it is the primary lens in a combination lens system....
. The camera was subsequently revised as the DP1s and the DP1x. In 2009 the company launched the DP2, a compact camera based around the same sensor and body as the DP1, but with a 41mm-equivalent f/2.8 lens.
Foveon X3 sensors were also used in the Hanvision HVDUO-5M and HVDUO-10M, a pair of digital cameras aimed at the scientific and industrial markets.
Comparison to Bayer filter sensors – operational differences
The operation of the Foveon X3 sensor is quite different from that of the Bayer filterBayer filter
A Bayer filter mosaic is a color filter array 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, camcorders, and scanners to create a color image...
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
Demosaicing
A demosaicing 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...
, an interpolative process
Image scaling
In computer graphics, image scaling is the process of resizing a digital image. Scaling is a non-trivial process that involves a trade-off between efficiency, smoothness and sharpness. As the size of an image is increased, so the pixels which comprise the image become increasingly visible, making...
in which the output pixel associated with each photosite is assigned an RGB
RGB color model
The RGB color model is an additive color model in which red, green, and blue light is added together in various ways to reproduce a broad array of colors...
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 jaggiesJaggies
"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 and/or missing or poor anti-aliasing filtering prior to sampling....
") 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
Aliasing
In signal processing and related disciplines, aliasing refers to an effect that causes different signals to become indistinguishable when sampled...
occurs when the photodiodes for each color, with the assistance of the microlens
Microlens
A microlens is a small lens, generally with a diameter less than a millimetre and often as small as 10 micrometres . The small sizes of the lenses means that a simple design can give good optical quality but sometimes unwanted effects arise due to optical diffraction at the small features...
es integrate the optical image over a region almost as big as the spacing of sensors for that color.
On the other hand, the method of color separation by silicon penetration depth gives more cross-contamination between color layers, and therefore more issues with color accuracy especially with the red channel.
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 colours. Thus 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 spaceColor space
A color model is an abstract mathematical model describing the way colors can be represented as tuples of numbers, typically as three or four values or color components...
, which can increase color noise in low-light situations.
Spatial resolution
According to Sigma CorporationSigma Corporation
is a Japanese company founded in 1961, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan...
, "there has been some controversy in how to specify the number of pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
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
Bayer filter
A Bayer filter mosaic is a color filter array 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, camcorders, and scanners to create a color image...
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
Active pixel sensor
An active-pixel sensor is an image sensor consisting of an integrated circuit containing an array of pixel sensors, each pixel containing a photodetector and an active amplifier. There are many types of active pixel sensors including the CMOS APS used most commonly in cell phone cameras, web...
(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
Bayer filter
A Bayer filter mosaic is a color filter array 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, camcorders, and scanners to create a color image...
camera produces a larger native file size via demosaicing
Demosaicing
A demosaicing 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...
.
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 pattern
Moiré pattern
In physics, a moiré pattern is an interference pattern created, for example, when two grids are overlaid at an angle, or when they have slightly different mesh sizes.- Etymology :...
s that the mosaic characteristic of the Bayer sensor produces. The effect of this filter is to blur the image output of the sensor, thus 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
Sigma SD9
The Sigma SD9 is a digital SLR camera 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 the first production camera to use the unique Foveon X3 image sensor, which...
, 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 low ISO speed
Film 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....
even similar to a 7.2 MP
Bayer sensor.
With the introduction of the Sigma SD14
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 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.
A visual comparison between a 14 MP Foveon sensor and a 12.3 MP Bayer sensor shows Foveon has crisper details.
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 equivalentsFilm 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....
,
and specifically chroma
Chroma
Chroma, the Greek word for color, may refer to:* Colorfulness or chroma, the perceived intensity of a specific color* Chrominance or chroma, one of the two components of a television signal* Chroma, a measure of color purity in the Munsell color system...
noise has been noted.
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.
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
RAW image format
A camera raw image file contains minimally processed data from the image sensor of either a digital camera, image scanner, or motion picture film scanner. Raw files are so named because they are not yet processed and therefore are not ready to be printed or edited with a bitmap graphics editor...
.
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 filterBayer filter
A Bayer filter mosaic is a color filter array 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, camcorders, and scanners to create a color image...
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.