Foveon X3 sensor
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The Foveon X3 sensor is a CMOS[1] image sensor for digital cameras, designed by Foveon, Inc. and manufactured by National Semiconductor[2] and Dongbu Electronics.[3] It uses an array of photosites, each of which consists of three vertically stacked photodiodes, that is organized in a horizontal and vertical grid. Each photodiode in the stack is sensitive to one of the three additive primary colors, blue, green and red. To perform its function, the Foveon X3 sensor utilizes the physical property that different wavelengths of light penetrate silicon to different depths.[4]
The development of the Foveon X3 technology is the subject of a 2005 book The Silicon Eye by George Gilder.
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[edit] Operation
The diagram below 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.[5]
Because the depth in the silicon wafer of the each of the three layer Foveon X3 sensors is less than five microns, 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.[citation needed]
[edit] Utilization
As of March 7, 2007, the Foveon X3 sensor is used or has been used only in the Hanvision HVDUO-5M and HVDUO-10M scientific and industrial cameras[6] and the Polaroid X530 compact digital camera, the Sigma SD9 and SD10 digital SLR cameras[7] and the just released Sigma SD14 digital SLR camera.[8] Another compact, the Sigma DP1, has been announced[9] but apparently remains unreleased.
[edit] 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, blue or green. 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 on the level of red, blue and green 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.
[edit] 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[10] commonly used[11] 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[12] integrate the optical image over a region almost as big as the spacing of sensors for that color.[citation needed]
[edit] 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 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. However, the Foveon X3's greater light gathering ability is offset by the matrixing required to reconstruct color information from the sensed raw data,[citation needed] which reportedly results in a Foveon X3 sensor with large photosites being unable to equal the low light performance of more conventional sensors with half the photosite area.[citation needed]
[edit] Spatial resolution
The comparison of the spatial resolution of the Foveon X3 sensor with that of the Bayer sensor has been a point of controversy in photographic circles. The argument has been over whether it is accurate to compare the number of photosites in each sensor's array or to compare the number of Bayer sensor photosites with the total number of photodiodes in the Foveon X3 sensor array. If the former, then the Foveon X3 sensor would appear to produce an image of lower resolution than the Bayer sensor. If the latter, the resolution would appear to be higher.[13]
This, in turn, has generated a controversy about the specifications of cameras that use the Foveon X3 sensor. For example, the dimensions of the photosite array in the sensor in the Sigma SD10 camera are 2268 x 1512 elements and the camera produces a native file size of those dimensions. This amounts to approximately 3.4 million photosites or 3.4 megapixels (MPs), as they are commonly called.[14] However, it has been advertised as a 10.2 MP camera by taking account of the fact that each photosite contains a stacked blue, green and red color sensing photodiode (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 elements, implying an image resolution 1.7 times[15] that of the Foveon X3.
However, the actual resolution produced by the Bayer sensor is more complicated than the count of its photosites might suggest. The reason has to do with the separate anti-aliasing filter commonly used to reduce the occurrence or severity of 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, thus producing a lower resolution than the photosite count would otherwise imply. This filter is unnecessary in the Foveon X3 sensor. Therefore, in theory, it is possible for a Foveon X3 sensor with the same number of photosites as a Bayer sensor and no separate anti-aliasing filter to attain a higher spatial resolution than that Bayer sensor. As a corollary, a smaller Foveon X3 sensor has the capability of equaling the actual resolution of a somewhat larger Bayer sensor. This has been substantiated by independent testing that has determined the 3.4 MP array of the Foveon X3 sensor in the Sigma SD10 produces the resolution of a 5 MP[16] to 6 MP[17] , and (at least at low ISO film speed equivalents) 7.2 MP[18] Bayer sensor.
[edit] 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[19]Another has noted higher noise during long exposure times.[20] However, these reviewers offer no opinion as to whether this is an inherent property of the sensor or the camera's image processing algorithms.
[edit] Notes
- ^ El Gamal, A., Trends in CMOS Image Sensor Technology and Design, Stanford University (2002 or later). Retrieved March 3, 2007. http://isl.stanford.edu/~abbas/group/papers_and_pub/iedm02.pdf
- ^ http://www.national.com/news/item/0,1735,745,00.html Retrieved March 3, 2007.
- ^ http://www.dongbuelec.com/eng/news/inthenews_view.asp?idx=73&gopage=1 Retrieved March 3, 2007.
- ^ El Gamal, supra, p. 2.; A. Rush and P. Hubel, X3 Sensor Characteristics, Foveon X3 Info Page http://www.x3f.info/technotes/X3SensorCharacteristics.pdf (undated but ≥ 2001, based on n. 1), pp. 1-3 Retrieved March 6, 2007.
- ^ Rush and Hubel, supra, pp. 3-5.
- ^ Foveon website http://www.foveon.com/article.php?a=226 Retreived March 7, 2007.
- ^ Ibid.
- ^ http://www.dpreview.com/news/0702/07022008_sd14date.asp. Retreived March 7, 2007.
- ^ Sigma Corporation of America website http://www.sigmaphoto.com/news/news.asp?nID=3281. Retrieved March 7, 2007.
- ^ See, Optical anti-aliasing filter section of anti-aliasing filter
- ^ Though 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, http://www.dpreview.com/reviews/kodakslrc/page2.asp Retrieved March 3, 2007) using Bayer sensors without such a filter. However, significant moiré patterns were produced when photographing very fine detail. http://www.dpreview.com/reviews/kodakdcs14n/page22.asp Retrieved March 3, 2007
- ^ These are extremely small optical lenses covering each sensor (Bayer) or each sensor stack (Foveon X3). They are used to concentrate the light striking the sensor on its photosensitive elements. Concepts in Digital Imaging Technology – Microlens Arrays http://micro.magnet.fsu.edu/primer/digitalimaging/concepts/microlensarray.html Retrieved March 3, 2007. They are commonly used in all types of image sensors in digital cameras.
- ^ Of course, the actual resolution as seen in the output image is affected by other factors, such as the quality of the lens used and the camera's image processing algorithms.
- ^ The terms megapixels and MP are used here as synonymous with the term photosites. There is a dispute about the counting of megapixels as a universal measure of the spatial resolution of image sensors. Nevertheless, these terms are used here because they are ubiquitous in photographic parlance.
- ^ Because it is similar to an area measure, all other things being equal, the differences in resolution between two image sensors varies as the square root of the ratio of the number of their photosites. In this case, sqrt(10.3 million ÷ 3.4 million) rounds off to 1.7.
- ^ Popular Photography & Imaging, Vol. 69, No. 6 (June, 2005), (table on p. 47).
- ^ Digital Photography Review, Sigma SD10 Review, March 2004, http://www.dpreview.com/reviews/sigmasd10/page17.asp Retrieved March 3, 2007
- ^ c|net Reviews, Sigma SD10 http://reviews.cnet.com/Sigma_SD10/4505-6501_7-30588638.html Retrieved March 6, 2007.
- ^ See, e.g., c|net Reviews, Sigma SD10 http://reviews.cnet.com/Sigma_SD10/4505-6501_7-30588638.html Retrieve March 6, 2007 and Steve's Digicams Sigma SD10 review http://www.steves-digicams.com/2003_reviews/sigma_sd10_pg7.html (November 28, 2003) Retrieved March 6, 2007.
- ^ Imaging Resource Sigma SD10 review http://www.imaging-resource.com/PRODS/SSD10/SD10A12.HTM (First posted 10-22-03.) Retrieved March 6, 2007. This observation is consistent with a comparison of the images, displayed in Digital Photography Review, taken by the Sigma SD10 http://www.dpreview.com/reviews/sigmasd10/page14.asp with those taken approximately contemporaneously of the same scene by the Bayer sensor equipped Nikon D70 http://www.dpreview.com/reviews/nikond70/page15.asp. Both retrieved March 6, 2007.
