Pulsed Illumination Synchronization for Global Reset Rolling Shutter Imagers
Global Reset rolling shutter imagers eliminate motion blur when pulsed light fires strictly within the dark global integration window under controlled ambient light.
Readout Noise
Electronic Baseline
The unwanted electronic signal added to the generated charge carriers during the process of transferring, amplifying and converting the analogue signal of a photodetector into a digital value represents the fundamental floor of sensor performance. This contribution, termed readout noise, arises from the internal amplifiers, analog-to-digital converters and signal routing lines of the sensor. Unlike thermal noise, it is independent of the integration time and is generated anew during each readout cycle.
It is a critical specification for selecting sensors for low-light imaging.
Sensor Architecture
Sensor design dictates the magnitude of this electronic contribution. In charge-coupled devices, the charge is transferred to a single output amplifier, whereas in complementary metal-oxide-semiconductor sensors, each pixel has its own amplifier. The variation in these tiny amplifier circuits contributes to the overall readout noise of the array.
Manufacturers use correlated double sampling to measure and subtract the reset voltage of each pixel. This technique removes a large portion of the low-frequency noise from the signal.
Measurement Limitation
Dynamic range is restricted by this baseline noise when the sensor operates under low-light conditions. When the signal generated by incident light is of the same order as the readout noise, the two cannot be distinguished, and the measurement fails. High-speed readouts generally increase the noise level because the amplifier bandwidth must be wider to accommodate the higher pixel rate.
Industrial inspection systems must often choose between high frame rates and low noise floor.
Mitigation Technique
Calibration is achieved by taking bias frames at the shortest possible exposure time. This measurement captures the electronic noise of the readout electronics. The resulting standard deviation of the pixel values defines the noise floor.
Stable supply voltage and proper shielding prevent external interference from increasing this noise during operation.