At higher light levels, another phenomenon appears. Each pixel
has its own analog gain factor, which is directly related to the
physical size of certain capacitors. This phenomenon is called
Pixel Response Non-Uniformity (PRNU). The resulting noise can be
modeled as additive, with a standard deviation proportional to
the signal, i.e., . The SNR in the highlight is approximately
where is a constant which is characteristic of pixel non-
uniformity. This implies that the SNR does not increase further
when PRNU becomes the main source of noise (which could always
happen because PRNU increases faster than photonic noise).
However, sensors always have a saturation value and PRNU is
usually not the dominant source of noise at this value.
Moreover, PRNU is a fix bias and can be compensated for with a
calibration. Color sensitivity is an integral value over the
whole curve characteristic. In particular it depends on the
noise in highlights. However, it also depends on the sensor’s
spectral sensitivities which determine