Image Noise
Image noise is the random variation of brightness or color information in images produced by the sensor and circuitry of a scanner or digital camera. Image noise can also originate in film grain and in the unavoidable shot noise of an ideal photon detector.
Noise [young page 32]
Images acquired through modern sensors may be contaminated by a variety of noise sources. By noise we refer to stochastic variations as opposed to deterministic distortions such as shading or lack of focus.
Charge -coupled device (CCD) cameras where photons produce electrons that are commonly referred to as photoelectrons. Nevertheless, most of the observations we shall make about noise and its various sources hold equally well for other imaging modalities. While modern technology has made it possible to reduce the noise levels associated with various electro-optical devices to almost negligible levels, one noise source can never be eliminated and thus forms the limiting case when all other noise sources are "eliminated". we shall make about noise and its various sources hold equally well for other imaging modalities.
1- Photon Noise
The noise problem arises from the fundamentally statistical nature of photon production.
2- Thermal Noise
Electrons can be freed from the CCD material itself through thermal vibration and then, trapped in the CCD well, be indistinguishable from "true" photoelectrons. By cooling the CCD chip it is possible to reduce significantly the number of "thermal electrons" that give rise to thermal noise or dark current.
3- On-chip Electronic Noise
This noise originates in the process of reading the signal from the sensor
4- KTC Noise
Noise associated with the gate capacitor of an FET is termed KTC noise
5- Amplifier Noise
The standard model for this type of noise is additive, Gaussian, and independent of the signal. In modern well-designed electronics, amplifier noise is generally negligible. The most common exception to this is in color cameras where more amplification is used in the blue color channel than in the green channel or red channel leading to more noise in the blue channel.
6- Quantization Noise
Quantization noise is inherent in the amplitude quantization process and occurs in the analog to - digital converter, ADC.
NOISE MODELING [ Ray page 119]
There are three standard noise models which model well the types of noise encountered in most images: additive, multiplicative, and impulse noise. the statistical behavior of the gray level values in the noise may be considered random variables, characterized by a probability distribution function (PDF) .
an image gets corrupted by noise, which may arise in the process of acquiring the image, or during its transmission, or even during reproduction of the image. Removal of noise from an image is one of the most important tasks in image processing. Depending on the nature of the noise, such as additive or multiplicative type of noise, there are several approaches towards removing noise from an image. The nature of the noise is primarily inherent to the imaging procedure.
The general model of such systems consists of a detector and a recorder.