How High Sensitivity and Advanced Line Scan CCD Image Sensors Work
In high sensitivity and advanced line scan cameras, a CCD image sensor
converts photons (light) into electrons (charge). When photons hit an image
sensor, the sensor accumulates electrons. This is called charge integration.
The brighter your light source, the more photons available for the sensor to
integrate, and the smaller the amount of time required to collect a given
amount of light energy.
The way photosensitive elements (pixels) on CCD image sensors collect
charge has often been compared to wells or buckets filling with water. From
this analogy comes the term "full-well capacity," meaning the maximum charge
(number of electrons) a pixel can hold without "spilling" charge onto
adjacent pixels.
 As
an image sweeps over a line (one TDI stage) of pixels, the pixels collect
charge. At certain intervals, a high sensitivity sensor shifts its collected
charge from one stage to the next, in the same direction as the image
travels. The sensor exposes the line of pixels again, and shifts again.
Multiple stages of exposure are progressively combined to yield an image
much stronger and sharper than a single line could have collected. Finally,
the sensor transfers its aggregate charge to readout registers, which feed
each pixel’s charge from the image sensor into an output node that converts
the charges into voltages.
After this transfer and conversion, the voltages are amplified to become
the camera’s analog output. In digital output cameras, the camera’s
analog-to-digital (A/D) board converts voltages to digital numbers (0-255
for 8-bit cameras). These digital numbers are what the camera outputs as
data to a frame grabber. |
