Signal to noise ratio (imaging)

The signal-to-noise ratio (SNR) is used in imaging as a physical measure of the sensitivity of a (digital or film) imaging system. Industry standards measure SNR in decibels (dB) of power and therefore apply the 10 log rule to the "pure" SNR ratio (a ratio of 1:1 yields 0 decibels, for instance). In turn, yielding the "sensitivity." Industry standards measure and define sensitivity in terms of the ISO film speed equivalent; SNR:32.04 dB = excellent image quality and SNR:20 dB = acceptable image quality.

Traditionally, SNR has been defined as the ratio of the average signal value ${\displaystyle \mu _{\mathrm {sig} }}$ to the standard deviation ${\displaystyle \sigma _{\mathrm {bg} }}$ of the background:

However, when presented with a high-contrast scene, many imaging systems clamp the background to uniform black, forcing ${\displaystyle \sigma _{\mathrm {bg} }}$ to zero, artificially making the SNR infinite. In this case a better definition of SNR is the ratio of the average signal value ${\displaystyle \mu _{\mathrm {sig} }}$ to the standard deviation of the signal ${\displaystyle \sigma _{\mathrm {sig} }}$:

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