30 Display characterization
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30.1 Display characterization overview
Metrics for assessing display quality.
Gamma
spatial uniformity (Mura)
The single-word metric you are likely recalling is Mura.
The term Mura (村) is a Japanese word meaning non-uniformity or unevenness and is the standard industry term used to describe noise-like, spotty, or cloudy variations in brightness or color (non-uniformity of pixel intensities) across a flat panel display.
Mura: The Noise-Like Non-Uniformity What it is: Mura is a visible defect characterized by blotchy, often irregular patterns of varying brightness or chrominance (color) on a screen that is supposed to be displaying a uniform color (like a mid-grey or white background).
The “Noise-Like” Aspect: While Mura is technically a fixed-pattern noise (it stays in the same place), it often presents as a large, low-frequency, cloudy variation that can look like noise or dirt, especially on OLED and LCD screens.
The Cause: Mura is typically caused by manufacturing issues, such as:
In LCDs: Non-uniform cell gap, impurities, or inconsistent backlight brightness (Light Source Non-Uniformity, or LSNU).
In OLEDs: Variations in the thickness or efficiency of the organic layers across the panel.
Significance: It is one of the most critical metrics for display quality, as it directly relates to user experience. Displays are typically tested for Mura using a uniformity test (or “flat field” test) and are measured using statistical methods like standard deviation or by modeling the Noise Power Spectrum (NPS), which assesses the spatial frequency of the noise pattern.