Foveon X3

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The Foveon X3 CMOS sensor developed by Foveon (now part of Sigma ) uses three sensor elements lying one on top of the other instead of several pixels lying next to one another to record color information with each pixel. The chip is made by the South Korean manufacturer DB HiTek (formerly Dongbu HiTek , previously Dongbu Electronics ).

Absorption properties and structure of a Foveon X3 image sensor for one pixel

Working principle

Light of different wavelengths penetrates silicon to different depths . The average penetration depth of blue light (450–480 nm) is around 1 to 1.8 µm, of green light (520–560 nm) around 2.7 to 3.6 µm and of red light (600–640 nm) about 5 to 7 µm (these depths are statistical mean values, not specific depths in which the photon “gets stuck”). This depth discrimination enables a certain color separation through the construction of a three-layer sensor. The color sensitivity differs considerably from that of the human eye, in particular the range between 550 nm and 600 nm (green → yellow → orange → red) has quite different characteristics for the human eye than for a Foveon-X3 sensor (absorption coefficient changes in this area by about 30 percent). This can be adequately corrected through skillful pre-filtering and signal processing.

Comparison with conventional color digital cameras

System-related advantages

With Foveon X3 sensors, in contrast to Bayer sensors, color moiré is virtually unknown due to the spatially superimposed color sensors. This means you can do without the sharpness-reducing anti-aliasing filters without having to fear color moiré. This results in the two main objective advantages of these sensors: The lack of color moiré and high image sharpness even with a small number of pixels. Further (partly subjective) advantages are a more pleasant display of orange and red colors.

On the other hand, other frequently listed advantages are either not verifiable in practice or verifiably wrong. Today's Foveon sensors cannot capitalize on the lack of light absorption in a color mask; they are far behind in terms of sensitivity. The resolution advantage over Bayer sensors is still not a factor of 3, but rather between 1.5 and 2. Furthermore, neither Foveon nor Bayer sensors provide direct color values. Foveon sensors, however, do not require demosaicing of the missing color subpixels.

Systemic disadvantages

The better spatial contrast resolution of colors of the Foveon-X3 sensor is without a doubt good. The color fidelity sometimes derived from this in the literature, however, has nothing to do with the contrast resolution of colors. Here, these sensors have the disadvantage that you cannot apply matched optical color filters, but rely on the depth discrimination of the semiconductor material used, silicon . This is different and weaker than that of the cones of the human eye; this can only be partially compensated by pre-filtering and post-processing . In practice it turns out that the color purity is overemphasized in places, but reduced in other places. Gray-blue and lavender become bright gentian blue, and leaf green is poorly differentiated and sometimes tends towards a yellowish olive.

The color noise is low at ISO 100 to 400, but increases significantly above ISO 1600 in particular. It consists of large green and purple-colored spots that can be perceived as very annoying. There are two reasons for this: on the one hand, the readout noise of the Foveon sensor is very high; on the other hand, the color separation is low, so that a subsequent increase in the color contrast is necessary in order to obtain images with normal color saturation.

Specification of the number of pixels

Sigma specifies the number of pixels for the Foveon-X3 “direct image sensor”, which, in analogy to the number of pixels for Bayer sensors, result from three times the number of pixels, despite the gradation of the three colors in a pixel. The last 44.25 megapixel sensor was rounded up to 46 MP.

The resolution of cameras with Bayer sensors and the same number of pixels is in fact not achieved with regard to the brightness signals. However, the Foveon X3 is clearly superior to the image sensors with color mosaics in terms of color resolution. The optical resolution that is actually achieved with a camera is often not limited by the image resolution, but by other factors, such as the use of optical low-pass filters and noise suppression methods or by aberrations and focusing errors .

Applications

Currently the chip is only used by Sigma in the digital SLR cameras SD9 , SD10 (both 2268 × 1512 × 3), SD14 , SD15 (both 2652 × 1768 × 3) and the new Sigma SD1 Merrill with a 30 percent larger sensor and 14.7 × 3 megapixels (4704 × 3136 × 3) as well as the compact cameras of the DP1 , DP2 and DP3 series.

The dp2 Quattro presented in 2014 has the newly developed Foveon X3 Quattro sensor. Its size has remained about the same. 5424 × 3616 pixels in the blue layer are only 25 percent for the green and red layers compared to 2712 × 1808 pixels, and so the sensor for these colors is still in the range of the previous cameras below the SD1 such as the SD 15. The other models are also missing The Quattro series, the dp0 , dp1 and the dp3 , use the Foveon X3 Quattro sensor, which consists of four times more blue diodes and is similar in structure to the classic color film, as each layer can capture all the information from the available light. 14-bit channel depth is supported in raw format.

The chip was also used in the Polaroid X530 introduced in 2004 , but the X530 never reached market maturity due to its problems with image processing inside the camera and was withdrawn during the introductory phase.

See also

literature

  • Paul M. Hubel and Markus Bautsch: Resolution for Color Photography in: Electronic Imaging: Digital Photography II, SPIE-proceedings 6069, San Jose, CA, January 2006, paper 6069-22

Web links

Individual evidence

  1. ↑ a b Paul Hubel, Markus Bautsch: Resolution for Color photography (PDF; 279 kB)
  2. ↑ Real-time color imaging with a CMOS sensor having stacked photodiodes
  3. ↑ Sigma DP Quattro with new Foveon sensor. Retrieved April 1, 2016 .
  4. ↑ Sigma dp2 Quattro in the test. Retrieved August 27, 2014 .
  5. ↑ Polaroid X530: Inexpensive digicam with Foveon chip. (No longer available online.) Chip.de, February 11, 2004, archived from the original on May 9, 2011 ; Retrieved September 28, 2010 . Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.chip.de
  6. ↑ Polaroid is recalling digital camera X530. test.de, May 4, 2005, accessed February 1, 2013 .