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possibilities and more functions. CMOS sensors also have a faster readout (which is advanta-
geous when high-resolution images are required), lower power dissipation at the chip level, as
well as a smaller system size. Megapixel CMOS sensors are more widely available and are less
expensive than megapixel CCD sensors.
3.3.3 Megapixel sensors
For cost reasons, many megapixel sensors (i.e., sensors containing a million or more pixels) in
megapixel cameras are the same size as or only slightly larger than VGA sensors that provide a
resolution of 640x480 (307,200) pixels. This means that the size of each pixel on a megapixel
sensor is smaller than on a VGA sensor. For instance, a megapixel sensor such as a 1/3-inch,
2-megapixel sensor has pixel sizes measuring 3 µm (micrometers/microns) each. By comparison,
the pixel size of a 1/3-inch VGA sensor is 7.5 µm. So while the megapixel camera provides
higher resolution and greater detail, it is less light sensitive than its VGA counterpart since the
pixel size is smaller and light reflected from an object is spread to more pixels.
3.4
Image scanning techniques
Interlaced scanning and progressive scanning are the two techniques available today for reading
and displaying information produced by image sensors. Interlaced scanning is used mainly in
CCDs. Progressive scanning is used in either CCD or CMOS sensors. Network cameras can make
use of either scanning technique. (Analog cameras, however, can only make use of the interlaced
scanning technique for transferring images over a coaxial cable and for displaying them on
analog monitors.)
3.4.1 Interlaced scanning
When an interlaced image from a CCD is produced, two fields of lines are generated: a field
displaying the odd lines, and a second field displaying the even lines. However, to create the odd
field, information from both the odd and even lines on a CCD sensor is combined. The same goes
for the even field, where information from both the even and odd lines is combined to form an
image on every other line.
When transmitting an interlaced image, only half the number of lines (alternating between odd
and even lines) of an image is sent at a time, which reduces the use of bandwidth by half. The
monitor, for example, a traditional TV, must also use the interlaced technique. First the odd lines
and then the even lines of an image are displayed and then refreshed alternately at 25 (PAL) or
30 (NTSC) frames per second so that the human visual system interprets them as complete
images. All analog video formats and some modern HDTV formats are interlaced. Although the
interlacing technique creates artifacts or distortions as a result of ‘missing’ data, they are not
very noticeable on an interlaced monitor.
CAMERA ELEMENTS - CHAPTER 3
