Image
Sensors
Charge-Coupled Device
(CCD) Image Sensors
The
CCD shifts one whole row at a time into the readout register. The
readout register then shifts one pixel at a time to the output
amplifier.
Until recently, CCD sensors were the
only choice. They capture light in the small-photo sensing elements
on their surface and get their name from the way that charge is read
after an exposure. To begin, the charges on the first row are
transferred to a readout
register. From there, the signals are then fed to
output circuitry that generates the video signal according to the
RS-170 (EIA) standard. Once a row has been read, its charges on the
readout register row are deleted. The next row then enters the
readout register, and all of the rows above march down one row. The
charges on each row are "coupled" to those on the row above, so when
one moves down, the next moves down to fill its old space. In this
way, each row can be read—one row at a time.
In addition to the parallel/serial transfer
CCD, there are two more recent types of CCDs.
Interline transfer CCD
Frame transfer CCD
CMOS Image
Sensors - CCD sensors are created by using
specialized and expensive processes while CMOS sensors are created
using the same process used to make chips for computer processors
and memory – thus making the CMOS sensors less expensive.
There are two basic kinds of CMOS image
sensors: passive and active. The active-pixel sensors reduce the
noise associated with passive-pixel sensors but still have a higher
noise level and are not as sensitive to light (lower dynamic
range) as CCD sensors. However, the CMOS image sensors have been
constantly improving and in the future may outperform CCD sensors.
The advantage of CMOS sensors over CCD sensors is their ability to address
each photo-element independently and have other circuits added to
the same chip, eliminating the many separate chips required for a
CCD. This also allows additional on-chip features to be added at
little extra cost.
Color Image Sensors – Visible light
waves are the only electromagnetic waves we can see. We see these
waves as the colors of the rainbow. Each color has a different
wavelength. Red has the longest wavelength and violet has the
shortest. When all the waves are seen together, they make white
light.
.
To achieve a color image, the light must
be sensed at the three different wavelength ranges known as color – red,
green, and blue. The information from these three measurements can
be combined to simulate the color we see with our eyes.
Color cameras produce a color signal in
one of the following ways: generating an 1-wire composite video by
adding color information to the monochrome video signal (NTSC),
generating a 2-wire S-Video signal (Luminance + Chroma), or
generating a 3-wire RGB signal.
3-CCD vs. 1-CCD Color
Cameras
Three chip color cameras always
contain a prism which divides the incoming light rays into their
red, green and blue components. Each chip then receives a single
color at full resolution (Figure 2)
Figure 2
One chip area scan cameras use
a single sensor that is covered by a color filter with a fixed,
repetitive pattern. Filters with several different patterns are used
but the Bayer color filter is the most common The Figure 3 and 4
show a portion of the Bayer filter. When a color filter is used with
a single sensor, each individual cell in the sensor gathers light of
only one particular color. To reconstruct a complete color image, an
interpolation is needed. The red, green and blue information is
interpolated across several adjacent cells to determine the total
color content of each individual cell.
 
Figure 3
Figure 4
A
variety of sophisticated and well-established image processing
algorithms are available to perform color reconstruction, including
nearest neighbor, linear, cubic, and cubic
spline techniques. In order to determine the correct color for
each pixel in the array, the algorithms average color values of
selected neighboring pixels and produce an estimate of the color
(chromaticity) and intensity (luminosity) for each pixel in the
array. Presented in Figure 5(a) is a raw Bayer pattern image before
reconstruction by interpolation, and in Figure 5(b) is the results
obtained after processing with a correlation-adjusted version of the
linear interpolation algorithm.
One chip line scan cameras use a sensor that has three rows of
cells, a red row, a green row and a blue row. As an area on an
object moves past the camera, the area is examined first by the
cells in the red row, second by the cells in the green row and third
by the cells in the blue row. The information from the red, green
and blue cells is then combined to produce a full color image.
3-CCD Color Camera Advantages:
3-CCD Color Camera
Disadvantages:
-
High
camera cost due to the need for a prism and three sensor chips
-
Large camera housing needed for prism and sensors and therefore a
high weight
-
Typically require expensive, special optics
1-CCD Color Camera Advantages:
1-CCD Color Camera
Disadvantages:
-
For
area scan cameras, an interpolation algorithm must be run to
reconstruct the color
-
Lower then full
spatial resolution (65% of the full horizontal resolution and 80%
of the full vertical resolution when using Bayer filter)
-
For
line scan cameras, spatial correction must be done to combine the
color data from the three sensor rows
When deciding on a three chip
or a one chip camera, you must consider the advantages and
disadvantages of each and determine which type is most appropriate
for your application. Experience shows that in many cases, a one
chip camera is more than adequate and is the cost efficient
solution.
Foveon® X3™ Technology - The Breakthrough in Color Imaging
The Foveon X3 CMOS sensor technology
represents a breakthrough in the detection of color data. A
single CCD color sensors required three pixels to detect the full
range of color and full-resolution color required expensive, bulky
3-CCD cameras. With the X3 technology, sensors can now detect full
color at every pixel. The result is more accurate color detail
without color aliasing, very simple color matrix processing and high
net quantum efficiency.
In the Foveon X3 image sensor, three
photodiodes are formed in every pixel, stacked like the three layers
in color film. This arrangement utilizes the wavelength-dependent
light absorption property of silicon to produce natural filters that
use the incoming light to greatest advantage.
The color resolution of Foveon X3 image
sensors is identical to their monochrome resolution so there is no
need to reconstruct missing color data by complex computation.
Line-Scan Sensors – They can be
manufactured from CCD or CMOS technologies; the only difference is
that line-scan sensors are one photo-element wide. Typical use is in
acquiring information from moving objects – e.g. continuous sheets
or web. Acquiring successive lines of video at the fixed intervals
and storing them into a memory can generate a two-dimensional image
of a moving object. The advantage of a line-scan sensor is
the availability of much higher resolutions - up to 12,000 pixels.
For more information, please contact
High-Tech Digital Technical Support.
310-265-8203
support@high-techdigital.com.
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