Basic principles of image sensors

The “Interline Transfer” matrix consists in a series of photodiodes separated by vertical transfer registers which are covered by an optical metallic shield (Fig. 11).

After integration, the charge generated by photodiodes is transferred to CCD vertical registers in about \(1\text{ }\mu\text{s}\) and so the smear is minimized. The main advantage of interline transfer is that the transfer from sensitive pixels to storage pixels is fast. So there is no need to shutter incident light. This is commonly known as "electronic shuttering". The disadvantage is that it lets less space to active sensors. The shield acts like a Venetian blind that darks half of the information present in the scene. The “area fill factor” can be as low as \(20\%\). (However, we can remedy to this by placing microlenses (Fig. 12) just in front of the sensor surface, in order to redirect the light toward sensitive cells. (HAD sensor: “Hole Accumulated Diode”).

As the interline systems are more often used in mass-produced camcorders, the design of the transfer registers is based on the standard video synchronization. With a \(2 :1\) interlacing system, each field is collected simultaneously and read alternately. It is called “frame integration”. In EIA 170 (formerly RS170), each field is read every \(1/50\text{s}\) (\(1/60\text{s US}\)). This allows a maximum integration time of \(1/25\text{ s}\) for each field.