Sensor Noise

Noise in a camera has a strong influence on the maximum responsivity and the dynamic range of a camera. Reasons for sensor noise are quite different and are explained below:

• Temporal noise

The expression temporal noise summarises all noise sources that have an influence on the temporary progress of a pixel value, i.e. from image to image. Temporal noise is made up from photon, dark current, readout and quantization noise described below. It tends to be the dominant noise source found in CCDs as the charge of each pixel is shifted many times during readout.

• Photon noise

A source for sensor noise is the light itself. The photon flux that strikes the sensor is not even, but Poisson-distributed, causing the so called photon noise or shot noise. In addition it limits the maximum signal-to-noise ratio negatively.

• Dark current noise

Dark current noise is created by electrons that emerge through thermal processes in the pixel. The number of electrons increases with increased exposure time and temperature. Dark noise can be reduced by cooling the sensor, which removes inherent energy from the sensor, thus reducing the number of rogue electrons.

• Readout noise

This type of noise occurs when charge (electrons) is converted into voltage (for CCD sensors also when charge is transferred through the shift registers or pixels). Along with the sensor design, the extent of read out noise is connected to the readout speed. Typically noise increases with higher clock rates. There is a clear balance between speed and noise which should be considered when selecting the right camera.

• Quantisation noise

A/D-conversion can also be a cause of noise as a voltage (continuous values) is converted into a digital value (discrete values).

• Spatial noise

The term spatial noise describes spatial variations of the intensities of different pixels when illuminated with homogeneous light. The causes of spatial noise include FPN and PRNU noise as described below. Spatial noise tends to be more dominant in CMOS sensors as the pixels are read out through different readout circuits.

• Fixed pattern noise FPN (offset noise)

FPN is defined as the peak-to-peak difference between the minimum and maximum measured values for all active pixels in the array. As this should be measured with the sensor in darkness, it is also called dark signal non-uniformity (DSNU).

• Pixel response non-uniformity PRNU (gain noise)

Pixel response non-uniformity is described as deviations between pixels with a fixed gain applied. This is caused by the fact that individual pixels have different sensitivity curves.