The mean Dark Signal that is collected in a pixel during exposure is simply the product of the Dark Current and “t”, the duration of exposure. Image Sensor manufacturers specify the Dark Current of a device in e –/p/s at a particular temperature, from which we can estimate the actual Dark Current at the operating temperature of the device. Dark Shot Noise can be modeled as a Poisson Distribution based on the generation and collection of thermally generated electrons. We apply our now-familiar “faucet and bucket” analogy to Dark Shot Noise and Read Noise. Since Dark Current is a thermal phenomenon, its effect (Dark Shot Noise) can be mitigated by cooling the image sensor. Since it can be modeled as a Poisson distribution, the dark-shot-noise is estimated to be the square root of the number of dark signal electrons that are expected to be captured in each exposure. This temporal variation is referred to as dark-shot-noise. In addition to using up some of the capacity for photoelectrons, there is a temporal variation in the number of dark electrons collected from one exposure to the next. At longer exposures, dark current can fill up more of the full-well-capacity of the “bucket” thereby reducing the available space for photoelectrons. Therefore, this dark current adds uncertainty in the total number of electrons that are measured. When the charge accumulated in a pixel is read out there is no way to tell thermally generated “leakage electrons” apart from photoelectrons. The flow of thermally generated electrons is called dark current because such electrons flow even when there is no light.ĭark current is specified in terms of electrons-per-second ( e –/p/s) for each pixel. Thermally generated electrons are not photoelectrons, but they too collect in the depletion region “bucket”. Thermal energy can also cause mobile electrons to be generated. In the article titled The Photoelectric Effect in Image sensors, the energy from a photon was shown to create an mobile photoelectron. In cameras that are designed for use in less stringent bright-light applications, a camera designer may focus, for example, on reducing the size/cost of the camera while permitting some tradeoffs in read noise.ĭark shot noise results from the fact that even if there is no light at all falling on a pixel, the photodiode “faucet” still has a small flow of “leakage” electrons that are generated thermally. In cameras that are designed for use in low-light scientific imaging applications, everything, from size/enclosure design to circuit board layout is done with the goal of minimizing noise, with read noise being a primary concern. Read Noise also depends very significantly on the camera design techniques. Read noise can depend on the choice of the imaging technology, for example cameras with sCMOS image sensors typically have a lower read noise as compared with most CCD or CMOS cameras.

The temporal variation represented by “read noise” is a function of the imager used, the clock rate and the level of care taken by the camera designer to protect the signal from interference. For this reason, different cameras that use the same imager may have different values for their read noise. Some of these factors are dependent upon the imager, but it is also a function of the design, partitioning and layout of the camera electronics. It includes the temporal variations introduced in the process of transferring charges from each pixel, converting charges to a voltage waveform and then digitizing the waveform. Although it is typically specified as a number of electrons (for example, read noise = 2e –), it is an aggregate of several different non-idealities. Read noise is a term used to describe the temporal variation caused by non-idealities in the measurement process.

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