The Foveon Detector

A new approach to multi-band imaging is provided by FOVEON Inc., which produces two large format detectors, 3.3 and 10 Mpix, that feature a novel technology: each pixel is a stack of three electron collection areas, sensitive to blue, green and red photons. Figure 2 shows a comparison between the architectures of a standard mosaic detector and the Foveon [8].

To better understand how a Foveon detector works, we refer to the silicon photon absorption plot of Fig. 3. The mean distance travelled by a photon inside silicon, before it generates a photoelectron, depends on its wavelength: the blue photoelectrons will be generated near the silicon surface while the red ones in deeper regions. The Foveon pixel is built by means of peculiar ion implants and dopings, which define three depletion regions located progressively deeper in the silicon and therefore suited to collect electrons from blue, green and red light. Three amplifiers and a CMOS multiplexer enable the readout and resetting of this detector with the features of CMOS arrays, such as windowing, rolling shutter and high frame rate live video.

RGBRGBRGB BRGBRGBRG GBRGBRGBR

Figure 2. Comparison of a Foveon detector and a Bayer type detector. In the Foveon direct image sensor there are three layers of pixels. Each pixel detects 1 color sample.
Figure 3. (top)The absorption coefficient and the penetration depth of photons inside silicon at different wavelengths. (bottom) The schematic structure of a Foveon pixel with the 3 different electrons collecting regions.

The peculiar photon absorption and charge collection processes of the Foveon result in three photoelectrical optical bands with peak QE of 35% as shown in Fig. 4. Hereafter, we name these bands red (Rf, ), visual (Vf) and blue (Bf). With large bandpass overlap, the Foveon bands form what is commonly called an "over-sampled colorimetric system", which makes this kind of detector one of the best to render colors in photography [9-10]. Moreover, the stacked structure of independent pixel arrays does not produce "color aliased" images when imaging high contrast regions with sharp edges.

Bf Vf Rf

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Figure 4. The Foveon optical bands as reported by the manufacturer.

2.1 Performance

The manufacture lists the Foveon M10 detector performance as:

• 3 layers of 2200*1500 pixels, square 9.12 microns pixels.

• Readout time <0.25 sec. (rolling shutter), RON <30 e- at

15 MHz.

• Binning, windowing and live video (30 fps at VGA resolution).

• CMOS 3.3V single supply, 3*12 bit ADC on detector.

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