[ PRNU ] – What is Photo-response Non-uniformity (PRNU) ?


The Photo-response Non-uniformity (PRNU) is a representation of the uniformity of a camera’s response to light, important in some high-light applications.

When light is detected by a camera, the number of photo-electrons captured by each pixel during an exposure is measured, and reported to the computer as a digital greyscale value (ADU). This conversion from electrons to ADUs follows a certain ratio of ADU per electron called the conversion gain, plus a fixed offset value (typically 100 ADU). These values are determined by the Analogue-to-digital converter and Amplifier used for the conversion. CMOS cameras gain their incredible speed and low noise characteristics through operating in parallel, with one or more analogue-to-digital converters per column of the camera, and one amplifier per pixel. This does however introduce a chance for small variations in gain and offset from pixel to pixel.

Variations in this offset value can lead to fixed pattern noise at low light, represented by the DSNU. The PRNU represents any variations in gain, the ratio of detected electrons to displayed ADU. It represents the standard deviation of the gain values of the pixels. Given that the resulting difference in intensity values will be dependent on the size of the signals, it is represented as a percentage.

Typical PRNU values are <1%. For all low- and medium-light imaging, with signals of 1000e- or less, this variation will be insignificant compared to read noise and other noise sources.

Also when imaging high light levels, the variation is unlikely to be significant compared to other noise sources in the image, such as photon shot noise. But In high-light imaging applications requiring very high measurement precision, especially those using frame-averaging or frame-summing, low PRNU may be beneficial.

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