CMOS Sensors : Brief on CCD Technology

BRIEF ON CCD TECHNOLOGY

CCDs were first invented in 1969 as a way to store data using bubble memory. In 1974, the first imaging CCD was produced by Fairchild Electronics with a format of 100×100 pixels.

CCD imager consists of two main parts: color filter and pixel array

• Color filter

Micro-lenses funnel light onto the photo-sensitive part of each pixel. On their way, the photons pass through a color filter array. The mosaic of these tiny filters captures color information. Color filters enable separate measurement of the red (R), green (G) and blue (B) photons. Color filter filters out wavelengths of unwanted colors and allows only specific colors of light to pass through a pixel sensor. For this purpose, each pixel is covered with a red, green and a blue filter according to a specific pattern, like the Bayer CFA pattern.

Bayer filter uses the sub-mosaic 2x 2 patterns with one red, one blue and two green filters. As human’s eye has greater sensitivity for green light, two green filters are used.

• Pixel Array

The pixel array functions on the principle of the photoelectric effect and pixel sensors are responsible for capturing the intensity of the light passing through. The light intensity data is combined before being converted into an analog voltage signal, which is outputted to an external circuit board to be further processed.

After conversion of incident light into electrons, electron charge is accumulated in the same way as bucket stores water. The pixel charges are read using vertical and horizontal shift registers which act as charge carriers.