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 20 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.
Definition of SNR
Traditionally, SNR has been defined as the ratio of the average signal value to the standard deviation of the background:
However, when presented with a high-contrast scene, many imaging systems clamp the background to uniform black, forcing to zero, artificially making the SNR infinite. In this case a better definition of SNR is the ratio of the average signal value to the standard deviation of the signal :
which gives a meaningful result in the presence of clamping.
The line data is gathered from the arbitrarily defined signal and background regions and input into an array (refer to image to the right). To calculate the average signal and background values, a second order polynomial is fitted to the array of line data and subtracted from the original array line data. This is done to remove any trends. Finding the mean of this data yields the average signal and background values. The net signal is calculated from the difference of the average signal and background values. The RMS or root mean square noise is defined from the signal region. Finally, SNR is determined as the ratio of the net signal to the RMS noise.
Polynomial and coefficients
- The second order polynomial is calculated by the following double summation.
- Which can be written...
- Computer software or rigorous row operations will solve for the coefficients.
Net signal, signal, and background
The second-order polynomial is subtracted from the original data to remove any trends and then averaged. This yields the signal and background values[clarification needed]:
- = average signal value
- = average background value
- = number of lines in background or signal region
- = value of the ith line in the signal region or background region, respectively.
- = value of the ith output of the second order polynomial.
Hence, the net signal value is determined by:
RMS noise and SNR
The SNR is thus given by
- Coefficient of variation
- Minimum resolvable contrast
- Minimum resolvable temperature difference
- Optical transfer function
- Signal transfer function
- ISO 12232: 1997 Photography – Electronic Still Picture Cameras – Determining ISO Speed here
- Mazzetta, J.; Caudle, Dennis; Wageneck, Bob (2005). "Digital Camera Imaging Evaluation" (PDF). Electro Optical Industries. p. 8. Retrieved 28 March 2010.
- Aboufadel, E.F., Goldberg, J.L., Potter, M.C. (2005).Advanced Engineering Mathematics (3rd ed.).New York, New York: Oxford University Press
- Test and Measurement World (2008). SNR. In Glossary and Abbreviations.http://www.tmworld.com/info/CA6436814.html?q=SNR
- ISO 15739:2003, Photography -- Electronic still-picture imaging -- Noise measurements: specifies methods for measuring and reporting the noise versus signal level and dynamic range of electronic still-picture cameras. It applies to both monochrome and colour electronic still-picture cameras.
- ISO 12232:2006, Photography -- Digital still cameras -- Determination of exposure index, ISO speed ratings, standard output sensitivity, and recommended exposure index: specifies the method for assigning and reporting ISO speed ratings, ISO speed latitude ratings, standard output sensitivity values, and recommended exposure index values, for digital still cameras. ISO 12232:2006 is applicable to both monochrome and colour digital still cameras. It revises ISO 12232:1998.