The Dynamic Range (DR) of an imager gives an indication of the imager’s ability to resolve details in dark areas as well as details in light areas in the same image. It indicates what is the largest signal that can be detected versus the smallest signal that can be detected.
Mathematically it is defined as :
DR = 20·log(Ssat/?read)
- DR : dynamic range [dB],
- Ssat : saturation signal of the sensor [DN or V or e–],
- ?read : noise in dark [DN or V or e–].
Notice that :
- the saturation level and the noise in dark need to be expressed in the same way : in digital numbers (DN), in Volts (V) or in number of electrons (e–),
- the noise in dark is the total temporal noise contribution of all electronic parts that are included in the readout chain, starting in the pixel.
A few important remarks w.r.t. dynamic range :
- the dynamic range specified for an imager mostly is a single number that should be valid for all pixels. Because the pixels are analog in nature, they all differ (a little bit) from each other, and in principle about 50 % of the pixels will have a lower dynamic range than the specified value and about 50 % of the pixels will have a higher dynamic range than the specified value,
- the noise in dark does not contain any noise related to the exposure of the imager, for instance dark current shot noise. So in reality the noise present in the output signal will always be higher than the one used in the calculation of the dynamic range because of the presence of dark-current shot noise. Moreover, dark-current related noise source are strongly dependent on the integration time,
- in normal operation of an imager, its so-called junction-temperature of the sensor is always higher than room temperature at which the dynamic range is specified. Temperature has a serious impact on the noise level and consequently on the dynamic range,
- fixed-pattern noise is not included in the definition of DR. The argumentation is that fixed-pattern noise can be easily cancelled, but any correction or cancellation of fixed-pattern noise may increase the level of the temporal noise and will reduce the dynamic range again.
In conclusion : the DR specified and its value found in data sheets is a number that only has a theoretical value, it can never be reached by all pixels in real imaging situations, because it is an average number for all pixels. Very often it can even not be reached by any of the pixels, because it does not take into account any exposure, neither any temperature effects or fixed-pattern noise issues.