Mura

Mura is a display defect that appears as visible non-uniformity in luminance, color, or both across a screen, typically as faint clouds, patches, streaks, bands, or spots that stay fixed in the same screen location. The term comes from the Japanese word for unevenness or blemish (a rough transliteration of 斑, mura), and it was adopted into English display-engineering vocabulary to describe imperfections of the pixel matrix that are visible during active display use. Unlike a dead pixel or a bright stuck pixel, mura is usually a low-contrast, soft-edged region without a sharp boundary, which makes it hard to detect automatically and easy to perceive on uniform image content such as a solid gray or white field.

Mura affects both LCD and OLED panels, and it is a recurring concern for the Micro-OLED and microLED microdisplays used in virtual reality (VR) and augmented reality (AR) headsets, where the image is magnified close to the eye and small per-pixel variations become easier to see. To counter it, manufacturers apply mura compensation (commonly called demura), which measures each pixel and adjusts its drive signal so the panel renders a uniform image.

Origin of the term

"Mura" is a loanword from Japanese, where it means unevenness, irregularity, or blemish. The Japanese flat-panel-display industry, which led liquid-crystal manufacturing through the 1990s, used the word for the cloud-like brightness imperfections seen on panels, and it entered international display-quality literature in the same sense. The Japanese term is also used in manufacturing and lean-production contexts for unevenness more generally, but in display engineering it refers specifically to spatial non-uniformity of a screen's output.

Appearance and types

Mura is a broad category of defects defined by local non-uniformity rather than by a single shape. Engineering practice commonly groups it by size and form into three classes:

Class	Typical appearance	Notes
Spot mura	A bright or dark blob roughly the size of one or a few sub-pixels, sometimes several adjacent spots	Distinct from a hard point defect; mura spots have soft, low-contrast edges
Line mura	A bright or dark line, often a single sub-pixel wide, running across part or all of the screen, vertical or horizontal	Characterized by contrast, area, position and direction
Region mura	A larger area of uneven brightness with an indistinct shape and poor local contrast	Hardest to detect automatically because it has no clear boundary

Common visible forms include clouding (cloudy patches from uneven illumination), banding (vertical or horizontal brightness lines), and spotting (isolated dark or bright areas). On LCDs, mura that comes from variation in the backlight or from pressure during assembly is sometimes described together with the related dirty-screen effect.

Causes

The causes differ between the two main panel technologies. On LCD panels, where pixels modulate a separate backlight, mura is associated with non-uniform backlight illumination, variation in the thickness of the liquid-crystal layer (cell gap), unevenness in the alignment layer or diffuser, particulate impurities in the cell, and mechanical stress from assembly, mounting pressure, shipping, or heat. Environmental factors such as temperature swings, humidity, and ultraviolet exposure can degrade panel materials and worsen mura over time.

On emissive OLED and microLED panels there is no backlight, and every sub-pixel generates its own light, so mura comes mostly from device-to-device variation in the pixels and their drive circuitry. Small differences in the thin-film-transistor characteristics, in the emissive material, and in manufacturing process steps mean that nominally identical pixels emit slightly different luminance or color for the same input. These variations show up as high-frequency (pixel-to-pixel) and low-frequency (large-area) non-uniformity. For very small emissive pixels, such as those in microLED microdisplays, the per-emitter spread is large enough that uncorrected panels have low manufacturing yield.

Measurement and quantification

Because mura is low in contrast and has no sharp edges, it is difficult to score with simple thresholds, and it is often judged by trained human inspectors viewing a uniform field in a dark room. Quantitative methods aim to predict that human judgment. A standard tool is the SEMU index defined in SEMI D31-1102 (2002), "Definition of measurement index (SEMU) for luminance Mura in FPD image quality inspection," which expresses a mura level as a function of the contrast and the size (major axis) of the region, calibrated to the human eye's sensitivity; the SEMI D31 standard has since been revised. The International Display Measurements Standard (IDMS) published by the Society for Information Display also covers uniformity measurement.

Much of the perceptual basis rests on the just-noticeable-difference (JND) approach, in which the threshold luminance contrast at which a mura region becomes visible is measured against the surrounding background. Mori and colleagues described evaluating luminance non-uniformity in LCDs by observing just-noticeable differences in a 2001 SID International Display Research Conference paper, and Tamura, Satoh and co-workers extended JND-based quantification across different background luminances. Some commercial inspection systems compute JND maps using algorithms based on the Spatial Standard Observer model. Production inspection of small high-resolution panels uses high-resolution imaging colorimeters, which capture luminance and chromaticity at the pixel and sub-pixel level so that discrepancies can be located precisely.

Mura compensation (demura)

Demura, also called pixel uniformity correction, is the process of reducing visible mura by altering the signal sent to individual sub-pixels so that the panel renders a uniform image. A typical external-compensation workflow has three steps: measure each sub-pixel with a high-resolution imaging colorimeter to build a luminance and chromaticity map; calculate per-pixel correction coefficients that normalize the differences; and apply those coefficients to the drive signal of each sub-pixel, usually through a control IC. In its classic LCD form, a camera captures a gray-scale test image, and pixels in over-bright areas are darkened while over-dim areas are brightened until the field looks even.

Compensation can be implemented externally, by sensing pixel characteristics and storing correction data off-panel, or internally, using sub-circuits built into each pixel that compensate for transistor variation on the panel itself; Sony, for example, describes self-compensation circuits integrated into the silicon backplane of its OLED microdisplays. Demura cannot remove every defect: corrections that change a pixel's signal mitigate non-uniformity that originates in the drive characteristics, but a true physical defect, such as a flattened diffuser or a pressure mark in an LCD, cannot be undone by signal adjustment. Because measuring and storing a full per-pixel correction is computationally heavy, research has applied machine-learning methods to speed up the demura calculation; one SID Symposium study reported reducing processing time for a 3.5-inch OLED VR display by 40 percent or more while keeping the same correction quality.

Relevance to VR and AR

Mura matters more in head-mounted displays than in most other products because the panel sits a few centimeters from the eye and is magnified by the headset optics, so a non-uniformity that would be invisible on a phone held at arm's length can be obvious on a near-eye display. The mid-2020s shift in high-end VR from larger TFT LCD panels to compact OLED and micro-OLED microdisplay modules increased the importance of pixel-level uniformity, because emissive microdisplays show mura from per-emitter variation and pack pixels at very fine pitch. A 2025 Nature Reviews Electrical Engineering survey of AR and VR microdisplay technologies treats luminance and color uniformity as a core image-quality parameter for these light engines, and display-metrology vendors note that visible, uncorrected non-uniformity detracts from the viewing experience and can contribute to eye strain on a near-eye panel.

Manufacturers of headset microdisplays build mura compensation into the panel. Sony Semiconductor Solutions states that its OLED microdisplays achieve uniformity through self-compensation circuits that suppress unevenness in each pixel, integrated into the silicon backplane alongside the drive and timing circuitry. Its 1.3-type 4K OLED microdisplay for VR and AR head-mounted displays, announced in August 2023 with sample shipments from November 2023, uses a 6.3 micrometer pixel pitch (much finer than a conventional 40 micrometer design) to reach a 3,552 by 3,840 panel resolution, and Sony describes a variation compensation circuit that delivers uniform luminance at that resolution. Sony micro-OLED panels of this family are used in headsets including the Apple Vision Pro, whose two displays combine a Sony OLED frontplane with a TSMC silicon backplane and pixels about 7.5 micrometers across, according to an IEEE Spectrum teardown. Headset and panel makers also patent VR-specific non-uniformity calibration; published filings describe luminance-based non-uniformity correction and display non-uniformity calibration aimed at the panels in head-mounted displays. Mura compensation is one of several display corrections in a modern headset pipeline, alongside handling of the screen door effect and pixel persistence that also affect perceived image quality on a near-eye panel.

See also

• OLED
• Micro-OLED
• Microdisplay
• Near-eye display
• Screen door effect
• Pixel persistence