CCD matrices
The CCD sensors (Charge Coupled Device : charge-transfer systems) refer to a semiconductor architecture in which the charge is transferred via a storage area. Most sensors operating in the visible region uses a CCD architecture to move the charge packet. They are commonly known as CCD matrices. Their architecture consists of three basic functions, in addition to the creation of the charge:
charge collection,
charge transfer,
conversion of the charge into measurable voltage.
As matrices operating in the visible are monolithic systems, the generation of the charge is often regarded as the initial function of CCD sensors. The charge is created in a pixel (pixel is the short term for “picture element”: it is the smallest piece of image) proportionately to the level of incident light in this site.
Attention :
The aggregation effect of all the pixels produces a sampled representation of the continuous scene.
The technology on which CCD sensors are based is the MOS (Metal Oxyde Semiconductor) capacitor.
Attention :
The capacitor is called a “gate”.
Charge packets are sequentially transferred from gate to gate until they are measured at the detection node. In most systems, the generation of charges is produced by photoelectric effect (Fig2) at a MOS gate (also called "photogate"). For some systems, including interline transfer systems, photodiodes generate the charge. After generation, the transfer of the charge to the conversion node (conversion of the charge into voltage) occurs in the MOS capacitors for all systems.
Remarque :
Although CCD matrices are in the public domain, their manufacture is complex. The number of steps that are necessary to their production can vary between ten and a hundred depending on the complexity of their architecture.
The systems can be described functionnaly according to their architecture (“frame transfer”, interline transfer, ...) or according to their application. To minimize costs, the complexity of the matrix and electronic processing, the architecture is typically chosen for a specific application (“ ASIC : Application Specific Integrated Circuit ” ).
Exemple :
Astronomical video cameras typically use full frame matrices, while video systems generally use interline transfer. Finally, the separation between professional TV, camcorder users, artificial vision and scientific or military applications is becoming more ans more tenuous with the advances in technology.