Open the file cis-dark.ldev and set the sense and tx voltages to 3.3V and run a single simulation. In this topic we discuss the contributions to the dark current noise source, and the simulation techniques required for the dark current and dark current shot noise calculations. These effects can be offset by careful design optimization through computer simulation. As CMOS pixel sizes continue to decrease, there is a reduction in image signal to noise as well as an increase in cross-talk between adjacent sensor pixels. However, CMOS pixel size reduction is only acceptable without sacrificing image quality. A slightly increased K-Factor may improve linearity at the cost of saturation capacity.The cost of CMOS image sensor pixel-based digital camera systems is being reduced through the use of smaller pixel sizes and larger fill-factors. The K-Factor depends on the camera design. K electrons are required to increase the grey level by 1 DN. This conversion is described by the overall system gain K, measured in digital number (DN) per electron (e –). The maximum SNR (SNRmax) is reached for the saturation irradiance.Ī camera converts the electrons (e –) from the image sensor into digital numbers (DN). SNR increases with the number of photons. The signal-to-noise ratio (SNR) is the ratio between the grey value (corrected for the dark value) and the signal noise. The values measured in a camera can differ from image sensor supplier data, as a camera might use a cover glass or filters.Ħ) Maximum Signal-to-Noise Ratio (SNRmax) The more photons are converted into electrons, the more sensitive to light the sensor is and the more information can be obtained from the image. The conversion ratio, the quantum efficiency (QE), depends on the wavelength. Physical processings within a sensor / a cameraĪn imaging sensor converts photons into electrons. So a high DR is especially important in applications with dark and bright areas in one image or with rapidly changing light conditions. Cameras with a high DR are able to give more detailed image information for dark and bright areas in a single image at the same time. The dynamic range (DR) is the ratio between saturation irradiance and the minimum detectable irradiation. The dark noise together with the photon shot noise and the quantization noise describe the noise of the camera. A lower dark noise is preferred for most applications. Which is called dark noise (measured in electrons). With increasing exposure time and temperature electrons are generated in each pixel without light. The value is determined from the value where SNR is equal 1 (signal is as large as noise).Įven if the sensor is not illuminated each pixel shows a (dark) signal.
It is more significant than only referring to the QE, as the AST combines QE, dark noise, and the shot noise (which is caused by the quantum nature of the photons). You should take the AST into account in very low light applications. This means, the lower the threshold, the more sensitive the camera. The absolute sensitivity threshold describes the lowest number of photons (minimum detectable irradiation) where the camera can differentiate useful image information in a picture from noise. If a pixel is over-exposed it is set to maximum DN and it does not contain useful information.Ģ) Absolute sensitivity threshold (AST) A high saturation capacity allows for longer exposure times. This difference might cause discussion if comparing imaging sensor data and camera data. The value is typically smaller than the full-well capacity.
The saturation capacity actually used for the characterization of a camera is measured differently and directly from camera images. This corresponds to the maximum number of photons which would generate such electrons (saturation irradiance). Think of a pixel as a well and of the full-well capacity as the maximum number of electrons that can be stored in this well. 1) Full-well capacity and saturation capacity