The dynamic range is an important camera specification related to the minimal and maximal signal that can be measured within a single image. It is most notably influenced by how many photons can be collected by a pixel in the image before saturation occurs (full-well depth) and the noise floor of the camera.
The most straightforward measure of the maximum signal that can be tolerated by a camera is the full-well capacity or depth. The full-well depth characterizes how many electrons can be stored by pixel. If this is exceeded, further exposure to light does not result in a linear increase of the signal; the pixel is saturated and the excessive amount ofwill result in pixel leaking.
On the opposite end, the minimal signal measured by the camera is limited by its noise. For many technologies, the readout noise will be the dominant noise source, although the(i.e. ) will become dominant during long acquisitions. While the readout noise will be used for further demonstrations, the main noise source of an acquisition is highly specific to its unique context.
Once these two limits have been identified, the dynamic range is determined by dividing the full-well depth by the noise floor. For example, a full-well depth of 100 kē and readout noise of 40 ē would lead to a dynamic range of 2500:1. A 12-bit image (4096 levels) would be sufficient to cover this data range.
Both the full-well depth and noise specifications are given in electrons. This is because most imaging devices make use of the photoelectric effect; the generation of electrons following exposure to photons. This conversion occurs at a certain rate characterized by the sensor’s quantum efficiency. Hence a camera with a quantum efficiency of 90% and a full-well depth of 100 kē can be exposed to, on average, 110 000 photons/pixel before saturation. It must be noted that measures like full-well depth and quantum efficiency are averages and can change from pixel to pixel.
However, digital camera images are often presented in Analog-to-Digital Units (ADU) and not electrons. To relate ADUs to photons, a measure of the individual camera unit’s gain, often referred to as k-gain, is used. By using this ratio of ē/ADUs and then balancing the number of electrons with the quantum efficiency, a rough estimate of the number of photons captured per pixel can be obtained.