Charged Coupled Device(
CCD); You have probably heard about CCDs, they are
a type of lightsensor used in scanners, fax machines, digital
still- and video cameras.
The
CCD is made of
silicon, the most used semiconducting
element today. A
CCD sensor is built up off up to several millions photosensitive cells.
When light hits the one of these cells,a release of free electrons in the silicon will occur. By applying
a negative
charge to the connected
electrodes, the free
electrons
in the silicon will be
repeled and the part of the
substrate
that is closest to the
electrodes gets a positive
charge.
This charge is used to find out how much light hit the cell,
the higher the charge, the more light hit. By combining the
measurements of all the cells you'll get a grayscale image of the
light that hit the
array.
A colour CCD works in the same way,
but uses colour
filters to seperate the individual color components. There are two ways to do this:
3-pass scanning:
3-pass scanning uses three individual colour filters(One that lets red light through, one for green, and one for blue).
The image wil be read once trough each filter.
Now we have three
images, one representing the
intencity of each
colour. These are then combined by the
software in to a colour
image.
3 pass scanning was used in some scanners but is not used much any more since it's takes three
times as long to scan a
complete picture.
One pass devices also uses filters but here
they are thin lines fixed to the CCD so they cover one vertical
line of CCD cells each. This means that the first coloum of cells
might have a filter that red can pass through, the next has a filter
that lets green trough, and number three have a filter that lets blue
light pass. This is then
repeated the full length off the array.
As in three pass scanning you get three images, that are mixed by
software
in to one.
This metod only needs to be read once, and is called one-pass.
The
drawback of this type of array is that you only get 1/3
of the horisontal resolusion on the finished picture, but since
the cells in modern CCDs is so small, there is no problems
in compensating for this. This is the way modern CCD
scanners and
cameras
uses.
This is the most used
image sensor at this time but
it has been replaced in cheap
digital cameras by
CMOS
based
sensors.
CMOS Image sensors are cheaper and uses less power, but have lower resolution
and is less
sensitive to light, so CCD is still used in more
expensive,
high resolution cameras.
Addition :
Some color
imaging devices uses
mirrors that are transparent
to one colour
wavelength(
dichroic mirrors) or prisms to split up the colours, and uses one CCD for each
colour.
Basic working principle
Green CCD
__
..
..G
..
Green ./....| Blue CCD
t.mirror /: |
::
::GB
::
_______\.......:/....| red CCD
light / / R |
RGB
Red transparent
mirror
(The prism types works in a similar fashion, by using dichroic
surface reflections
in the glass.)
This means that you can get a higher resolution and sharper
image, but the added CCDs and
optics makes the device more expencive.