Some functionality such as binning and averaging can be done inside the camera to further reduce shot noise. In bright scenes the contrast detecting ability is dominated by the shot noise of the camera, basically the FWC. So when you want to observe contrast changes in a bright scene, you require a camera with an image sensor that has a higher FWC. an increase in light of 2x causes an increase in video signal of 2x from the sensor). So in metrology systems when saturation behavior needs to be avoided, it is relevant to use the maximum FWC number at which the sensor still has a linear response (e.g. In order to realize this FWC, the sensor is operated in saturation mode. Sensor specifications typically use the maximum FWC number that can be realized in a pixel.
The SNR for higher signal levels is dominated by shot noise. In practice, the Signal to Noise Ratio (SNR) is used. In applications, the noise has to be seen relative to the signal. Noise however should not be considered on its own. In the electron domain this is similar: the standard deviation of the amount of captured electrons in a pixel is the square root of the mean signal level.įor an increasing amount of electrons, the shot noise increases as well. The resulting measurement varies from minute to minute, following a Poisson distribution. The next minute, and the next, and the amount counted is probably not the same. Consider the following example: imagine standing at an overpass above a highway and counting the amount cars passing by in one minute. Shot noise is caused by the arrival process of light photons on the sensor. Shot noise originates from the discrete nature of electrons. So read noise is one of the parameters to mind when you want to observe ‘dark scenes’. These two combined gives the overall sensitivity of the sensor as QE/Read Noise, or the minimum amount of light you can see. Normally the QE is less than 1 (or 100%). A QE of 1 means that every photon generates (in average) one electron. The more electrons in a pixel during the integration period, the higher the output level of the sensor, so the more sensitive the sensor is for that specific wavelength of the light. QE (Quantum Efficiency) is a measure of how efficiently the sensor converts light (photons) to charge (electrons). Of course when talking about sensitivity, QE plays an important role as well as the pixel sensitive area and the use of microlenses. A lower RN therefore results in a more sensitive sensor. Low read noise means that you can see small contrast changes, which is typically present in scenes taken in low light conditions. Read noise is also important in combination with expressing the sensitivity of a camera. A lower read noise means you can see smaller changes in signal amplitude, thus detect details with smaller contrast differences.
The lower the read noise level, the lower the minimum number of signal electrons that can be detected. Read noise basically determines the contrast resolution that the camera is able to achieve. The ADC with CCD image sensors is done outside the sensor and the ADCs for a CMOS image sensor are in each pixel. Note that the build up is different for a CMOS sensor and a CCD sensor. The Read Noise (RN) of the sensor is the equivalent noise level (in electrons RMS) at the output of the camera in the dark and at zero integration time. Read noise is a combination of noise from the pixel and from the ADC. With this, it is also helpful to consider the difference between signal to noise ratio (SNR) and dynamic range. This can help you prioritize the most important camera parameters, such as sensitivity or dynamic range. In this post we will consider temporal noise sources that vary with time including shot noise and read noise. When determining the best metrology camera, one of the first considerations is to determine the dominant noise source.
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