CCD CAMERA SENSOR

CCD sensors are analog devices that employ a charge transfer process to capture images. Each pixel in a CCD sensor consists of a photodiode and a potential well, which act as a receptacle for photoelectrons. The speed of image acquisition is limited by the conversion of photoelectrons into signals (voltage) at a single port.

Despite this limitation, CCD is renowned for its ability to produce high-quality, low-noise images, which originally made it a popular choice for various applications, including machine vision and embedded vision systems, before CMOS caught up in technological advancement.

Simplified functional principle of the sensitivity to light .

The inner photoelectric effect is fundamental for the functioning of the sensor.The atoms of the silicon crystal are located on discrete energy bands, the energetically lower is called valence band, the energetically higher is called conduction band. In the basic state, most of the electrons are on the valence band, however they can be transported to the conduction band by means of excitation from the outside.

The energy required for this is 1.26 eV or more. In case of the CCD sensor, this transport can be induced by light, but also by higher heat supply (dark noise of the sensor). (Note: 1.26 eV approximately correspond to the energy of infrared radiation with a wavelength of 1 µm. Light with longer waves can pass through the silicon without absorption, it is virtually translucent from these wavelengths on and insensitive.)

Due to the excitation, free (negative) electrons and positively charged "holes" in the valence band are created at the same time which separate from each other because of the applied voltage. These charges, however, do not immediately flow off to the outside (like in the case of a photo diode in the CMOS sensor), but are stored in the memory cell itself.

By means of the charge shifting described above, the charge is transported through the entire sensor in many small steps towards a central amplifier / A-D converter.

Typical layout of CCD sensors

In general we differentiate several typical CCD sensor layouts which are described subsequently:

of which mainly the so-called "interline transfer sensor" has gained acceptance in machine vision.

CCD Video-Camera Noise Reduction

a technique to identify and measure the prominent sources of sensor noise in commercially available charge-coupled device (CCD) video cameras by analysis of the output images. Noise fundamentally limits the distinguishable content in an image and can significantly reduce the robustness of an image processing application. Although sources of image sensor noise are well documented, there has been little work on the development of techniques to identify and quantify the types of noise present in CCD video-camera images. A comprehensive noise model for CCD cameras was used to evaluate the technique on a commercially available CCD video camera.