Friday, January 08, 2021

Image Sensor Noise Tutorial

Richard Crisp, VP of New Product Development for Etron, continues his image sensor tutorial series. The part 5 title is "Eliminate noise sources in CMOS image sensor designs:"

The following actions can be taken to minimize the noise:

Dark noise components can be reduced by:
  • Reducing exposure time
  • Reducing operating temperature of sensor
  • Dark fixed pattern noise for non-saturated pixels can be removed by dark subtraction a.k.a. despiking. It involves subtracting a dark frame from the image frame, pixel by pixel.
  • Fixed pattern noise can be removed via a process called flat fielding. The process involves dividing the image frame by a pixel calibration image frame on a pixel-by-pixel basis. The calibration frame is simply a high SNR image of a uniformly illuminated featureless background taken using a focused optical system.
Shot noise and read noise are fundamental limits

The only noise components that cannot be removed from an image with non-saturated pixels are the read noise, the image shot noise and the dark shot noise. If it is feasible to cool the sensor, the dark shot noise can be reduced to arbitrarily low levels so as to be a non-factor.

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