As
light passes through a lens, it is focused on the camera’s image sensor. An
image sensor is made up of many photo sites and each photo site corresponds to
a picture element, more commonly known as “pixel”, on an image sensor. Each
pixel on an image sensor registers the amount of light it is exposed to and
converts it into a corresponding number of electrons. The brighter the light,
the more electrons are generated.
When
building a camera, there are two main technologies that can be used for the
camera’s image sensor:
-
CCD (charge-coupled device)
-
CMOS (complementary metal-oxide semiconductor)
While CCD and CMOS sensors are often seen as
rivals, each has unique strengths and weaknesses that make it appropriate for
different applications. CCD sensors are produced using a technology that has
been developed specifically for the camera industry. Early CMOS sensors were
based on standard technology already extensively used in memory chips inside
PCs, for example. Modern CMOS sensors use a more specialized technology and the
quality of the sensors is rapidly increasing.
3.3.1 CCD technology
CCD
sensors have been used in cameras for more than 30 years and present many
advantageous qualities. Generally, they still offer slightly better light
sensitivity and produce somewhat less noise than CMOS sensors. Higher light
sensitivity translates into better images in low light conditions.
CCD
sensors, however, are more expensive and more complex to incorporate into a
camera. A CCD can also consume as much as 100 times more power than an
equivalent CMOS sensor.
3.3.2 CMOS technology
Recent
advances in CMOS sensors bring them closer to their CCD counterparts in terms
of image quality. CMOS sensors lower the total cost for cameras since they
contain all the logics needed to build cameras around them. In comparison with
CCDs, CMOS sensors enable more integration possibilities and more functions.
CMOS sensors also have a faster readout (which is advantageous 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.
