This random variation in photon arrival is imposed by nature and makes it impossible for any image sensor to have zero noise. Thus, if the scene illuminates a portion of a sensor with light that generates an average of 1000 electrons in each pixel during the integration period, the physical nature of this incident light results in noise of approximately 32 electrons RMS. These variations are quantified by calculating the Poisson standard error, meaning that photon noise is the square root of the total number of incident photons. When I say “pixel-to-pixel,” that can refer to both spatial and temporal variations: neighboring pixels in a single frame will exhibit tonal differences despite uniform illumination, or a single pixel exposed to steady illumination will exhibit tonal differences from one frame to the next. Thus, even if a CCD is illuminated by light that appears to be perfectly uniform, pixel-to-pixel intensity variations caused by photon noise will be observed. Illumination and illumination-induced generation of electric charge are quantum phenomena governed by the discrete behavior of photons and electrons. Photons are discrete “particles” of light, and any array of photosensitive elements is subject to the noise-i.e., the random variation-that characterizes the arrival of photons. If we count a certain number of events and apply Poisson statistics, the standard error associated with the phenomenon is calculated as the square root of the count. We use the Poisson distribution to model phenomena that consist of separate, independent events that exhibit unpredictable exact timing but occur at a consistent average rate. In our examination of dark noise, I pointed out that it is governed by the discrete nature of electric charge and follows the Poisson relationship. Pixel readout and frame rate in CCD imaging systems.Sampling, amplifying, and digitizing CCD output signals.CCD types (e.g., full-frame, interline-transfer, and frame-transfer).
We’ll also briefly consider reset noise, which is not a major factor in image quality because it is virtually eliminated by a specialized signal-processing technique.īefore moving on, you may wish to catch up with the rest of this series, which covers the breadth of topics below: In this article, we'll talk about two other major contributors to CCD image quality (or lack thereof): photon noise and read noise. This is an important noise source in CCD applications, and it has a direct influence on system design because it can be effectively controlled by cooling the sensor. In the previous article, we discussed dark noise in CCD sensors, which results from variation in dark current generated by the sensor's semiconductor material.
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