What is Full Well Capacity of a Camera?

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- GoPhotonics

Oct 25, 2023

The Full Well Capacity (FWC) of a camera is a parameter that measures the maximum amount of charge (electrons) that a single pixel in the camera's image sensor can hold before reaching the saturation level. In image sensors, each pixel collects and stores charge in response to the incoming light. When photons strike the pixel, they generate electron-hole pairs. The electrons are collected in a potential well within the pixel until the exposure time ends or the well reaches its Full Well Capacity. When the charge in a pixel exceeds the saturation level, the charge starts to fill adjacent pixels, a process known as Blooming. Full well capacity is measured in the units of charge or e-.

Once a pixel's well reaches its Full Well Capacity, it becomes saturated. This means that it cannot collect any more charge, and any additional photons striking that pixel are essentially ignored. Saturation causes highlights in the image to clip or blow out, resulting in bright areas losing detail and appearing as featureless white regions.

The Full Well Capacity is closely related to the physical size of the pixel. Larger pixels can typically hold more charge before reaching saturation, which can be advantageous for low-light imaging and improving the camera's dynamic range. The Full Well Capacity also depends on the quantum efficiency of the sensor, which measures the sensor’s efficiency to convert incoming photons into electrons. Higher quantum efficiency allows a sensor to reach its Full Well Capacity more quickly when exposed to the same amount of light.