Biomedical Engineering Reference
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these components makes the chip design complex and reduces the available surface area for detection.
With each pixel doing its own conversion, the image's uniformity is lower. However, the chip can be
built to require less off-chip circuitry for basic operation ( Fig. 8.8 ). Because CMOS technologies are
available at low cost, CMOS image sensors can be fabricated at a lower cost than a CCD image sensor.
Compared to a CCD sensor, a CMOS sensor has a lower sensitivity because part of each pixel is
covered with circuitry that filters out noise and performs other functions. The ratio between the sensing
surface area and the total pixel area is called the fill factor. The fill factor of a CMOS image sensor
ranges from 30% to 80%. Because the rest of the pixel area is shielded by transistors and metallic bus
lines, which are optically opaque, these structures absorb or reflect a majority of the incident photons.
These metal layers can also lead to undesirable effects, such as vignetting, pixel crosstalk, light
scattering, and diffraction, causing a higher overall noise level. A CMOS image sensor uses digital
signal processing (DSP) to reduce or eliminate the noise; thus, the dynamics of a CMOS sensor is also
lower than that of a CCD sensor.
8.1.4.2 Digital images
An image captured with an optical instrument is a continuously varying array of shades and color
tones. Images captured on films for instance are referred to as continuous-tone because the shades and
hues blend together without disruption. Continuous-tone images are captured with analog optical and
electronic devices. Optical signals are converted to analog electrical signals or to continuous change of
the chemical properties. In the world of digital systems, these continuous-tone images need to be
converted into the digital format. In this case, the continuous tones of an analog image are divided into
individual intensity values. Similar to the analog-to-digital converting process in electronics, the
FIGURE 8.8
Schematic concept of a CMOS image sensor and the external circuit.
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