Information Technology Reference
In-Depth Information
4.1
Imaging Sensors
The imaging sensors used in most DDD systems fall into one of these two
categories, depending on the range of the electromagnetic spectrum in which
they operate: (i) visible light (“conventional”) cameras; or (ii) near infrared (NIR)
cameras. The former can provide excellent resolution at relatively low cost, but
depend on appropriate lighting conditions to operate satisfactorily. The latter can
be used—often in addition to conventional cameras—to handle nighttime and other
poor lighting situations.
4.1.1
Visible Light Cameras
The two most popular technologies for imaging sensors used in visible light cameras
are either CCD (Charge-Coupled Device) and CMOS (Complementary Metal-
Oxide Semiconductor). Both of the sensors perform essentially the same function,
namely the conversion of light into electrical signals which can be further encoded,
stored, processed, transmitted, and analyzed by specialized algorithms.
In a CCD sensor, every pixel's charge is transferred through a very limited
number of output nodes (often just one) to be converted to voltage, buffered, and
sent off-chip as an analog signal. An analog-to-digital converter turns each pixel's
value into a digital value. CCDs use a special manufacturing process to create the
ability to transport charge across the chip without distortion. This process leads to
very high-quality sensors in terms of fidelity and light sensitivity.
In a CMOS sensor, each pixel has its own charge-to-voltage conversion, and
the sensor often also includes amplifiers, noise-correction, and digitization circuits,
so that the chip outputs digital bits. These other functions increase the design
complexity and reduce the area available for light capture. With each pixel doing
its own conversion, uniformity is lower, but it is also massively parallel, allowing
high total bandwidth for high speed.
Both of these technologies have their advantages and disadvantages. The CCD
technology is considered to have matured over time but the CMOS is slowly
catching up with them. CCD sensors create high-quality, low noise and high
resolution images. CMOS sensors are usually more susceptible to noise and their
light sensitivity is lower then CCDs. In terms of power consumption, CMOS
technology requires substantially less power. CMOS also leads as a cheaper to
fabricate technology because CMOS chips can be fabricated on just about any
standard silicon product line, which makes them fairly inexpensive.
Driver Drowsiness Detection systems need cameras that can produce high quality
images with high resolution and low noise level which is so far the domain of the
CCD cameras; on the other hand, the chosen imaging sensor should be cheap and
battery efficient, which tilts the scale toward CMOS. The final decision will be a
tradeoff among these pros and cons of each candidate technology.
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