Image Acquisition
Acquiring or digitizing an image
is converting the light that is given off from something,
into a digital representation.
A Charged Coupled Device (CCD) is almost always used for this.
A CCD is an array of light sensitive cells, on a semiconductor wafer.
A-- When the wafer is exposed to light.
The light knocks electrons loose.
The more light the more electrons are knocked loose.
B-- Charged plates, below the wafer have different charges.
The plates attract or repel the electrons and hold them in a "well".
C-- If the charge values on the plates is shifted to the right, the well shifts to the right.
The electrons stay in their wells and are moved.
The electrons are moved into an Analog to Digital Converter (A/D).
The A/D counts the number of electrons and outputs a digital number.
The shifting continues until all of the wells or cells have been converted.
CCDs are used everywhere. Scanners, Video Cameras, Digital Cameras, FAX machines, etc.
Scanners and FAXs use CCDs arrays that are wide and only one pixel high.
Light from a stripe of the image is converted one line at a time.
This is what causes the groaning of many scanners. Movement takes time.
Video and digital cameras use a rectangular array.
An entire image or block is converted at a single time.
Color is accomplished by putting red, green and blue filters in front of the CCD.
In scanners they can have one CCD array which has three filters that are placed in front of the CCD.
This is the common 3 pass scanner. Single pass scanners use 3 arrays.
Video cameras can use a single array with dot filters on it,
or 3 CCDs with separate filters.
Large mosaics of images can be compiled by stitching smaller images together.
As many of NASAs space images.
