Image Processing Reference
In-Depth Information
8
Imaging S
ystems
In earlier chapters, the interior, function, and driving mechanisms of image sensors have
been described. In this chapter, factors having an impact on the quality of image informa-
tion and the signal processing done after sensor output will be discussed to provide an
outline of whole imaging systems.
8.1 Deteriorating Elements of Image Information Quality
In imaging systems, light from self-luminous objects or objects illuminated by a light
source is focused on the image sensor through an optics system such as a lens, and the
optical image information is converted to an electrical image signal, as shown in Figure 8.1.
The image signal is digitized by way of an analog noise reduction circuit and processed
by a digital signal processor (DSP) to make it a color image signal. Image information is
composed of four factors, as mentioned in Chapter 1; the quality of the information trans-
lates directly into image quality. Elements that exert influence on the information quality
of each factor at each step are considered in terms of noise (inaccuracy) and range (limit).
1
In Figure 8.2, the elements of the light source and optics are shown. The stability of the light
source is most important. In the case of light sources that come and go periodically, such as
fluorescent lamps, flicker noise* makes image brightness fluctuate with time, depending on
the frame frequency and exposure period. The second type of noise generated by the light
source, optical shot noise, is universal noise and consists of variance of the photon number
itself, as described in Section 3.4. Light reflected by the object arrives at the lens, where shad-
ing, flare, and modulation transfer function (MTF) exist as intensity noise. Space noise is com-
posed of aberration (distortion), focusing, diffraction phenomenon caused by the iris (Airy
disk), and camera shake. Chromatic aberration is wavelength noise. In an infrared cut filter,
spectral response is wavelength noise. As optical low-pass filters only remove single-wave-
length light, this is the origin of space noise for light of other wavelengths. The major light
intensity noises in image sensors were mentioned in Chapter 3 and are detailed in Figure 8.3.
While the noises described above are involved in the light coming to the sensor, the fol-
lowing are generated in the sensor.
Intensity noise in the sensor is composed of electronic noise and optical noise. Electronic
noise is composed of device noise including transistor noise, to which 1/f noise, random
telegraph noise (RTN), and thermal noise belong, and circuit noise, to which kTC noise,
vertical line noise caused by column circuit variance, noise that was not canceled enough
because of insufficient correlated double sampling (CDS) efficiency, and shading noise
belong. Further, sensitivity unevenness or shading caused by variation of the light focus-
ing effect of the on-chip micro lens (OCL) is intensity noise.
The dynamic range restricts the intensity signal level that the sensor can treat.
*
Eastern Japan is the only area where the frequencies of commercial power and television systems are different.
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