LCD Mura Defect: QC Test Method & Acceptance Criteria

Quick Answer (GEO extract block)

Mura is a luminance or chromatic non-uniformity in an LCD that has no sharp edge and does not change with signal content. Shadow mura appears darker than the surrounding area; light mura appears brighter. Because mura is a low-contrast, area-based defect, it cannot be judged by a single pixel test. Incoming QC needs a defined method: dark-room luminance, a fixed viewing angle and distance, a neutral grey field, a specified inspection time, and zone-based acceptance limits tied to an AQL sampling plan agreed with the supplier.

What Mura Actually Is on a TFT LCD

Mura is a local deviation in luminance or colour across an otherwise uniform field. Unlike a dead pixel, it has no hard boundary, so the eye integrates it as a stain, cloud or band. It originates in the panel and module stack: cell-gap variation from spacer or seal geometry, uneven alignment-layer rubbing, backlight light-guide or diffuser artefacts, brightness-gradient from LED pitch, or mechanical stress transferred through the frame and FPC. Because the cause is often mechanical or optical rather than electrical, mura can appear only after assembly, which is why it must be screened at module level rather than assumed from panel data.

Shadow Mura vs Light Mura

The two categories are defined by polarity against the surrounding field, and they behave differently in inspection.

• Shadow mura (dark mura): the region is darker than its neighbourhood. Common sources are cell-gap non-uniformity, local pressure from a boss or gasket, or a light-guide extraction defect. It is usually most visible on a mid-grey field and can intensify when the module is mounted with uneven screw torque.
• Light mura (bright mura): the region is brighter than its neighbourhood. Typical causes are backlight hot spots, LED binning or pitch irregularity, and edge light leakage. It is often most visible on a dark grey or black field.

Some mura is chromatic rather than luminance-based, showing as a green or pink tint. Treat colour mura as its own defect class with its own limit, because it can pass a luminance check yet fail customer perception.

Setting a Test Method Before Setting a Limit

A mura limit is meaningless without a fixed method. Define and document all of the following, and keep them identical between your line and the supplier's:

1. Ambient and luminance: dark-room inspection, typically below 10 lux ambient, with panel luminance set to a defined grey level rather than maximum white.
2. Test patterns: at minimum full white, full black, and one or two neutral greys (for example 30 % and 50 % grey). Shadow mura is usually caught on grey; light mura on black or low grey.
3. Viewing angle: state the angle explicitly, typically normal incidence for the primary judgement, with a secondary check at an oblique angle because TN and some IPS variants shift contrast off-axis. A defect that only appears at 60 degrees should be a separate, looser limit.
4. Distance and time: fix the eye-to-panel distance and the observation time, for example 30 cm and 5 seconds per field, to remove operator variability.
5. Warm-up: allow the backlight to stabilise, since luminance drift in the first minutes can mimic or mask mura.

Acceptance Criteria and Defect Zones

Convert the method into a pass/fail rule using zones and size classes. A workable structure is:

• Zone A (active viewing centre): strictest limit, often no visible mura above a defined contrast threshold.
• Zone B (outer active area): small, low-contrast mura permitted up to a stated diameter and count.
• Zone C (border and non-active area): looser limits, since these regions are usually covered by a bezel.

For each zone, specify maximum diameter, maximum count, and a contrast threshold expressed as a percentage deviation from the local background. Keep the threshold realistic: a limit tighter than the panel's own uniformity specification will simply reject good product. Where the application is safety- or medical-related, tighten Zone A and reduce the permitted count rather than lowering the threshold, which is harder to judge consistently.

Why Sample-Based Screening Beats 100 % Visual Inspection

Full visual inspection of every unit is slow, fatiguing, and inconsistent; operator sensitivity drifts across a shift. A better model is a layered plan: a fast automated or gross visual check on every unit to catch gross mura, plus a sample-based detailed mura audit at a defined frequency using the full method above. The sample audit is what actually controls the supplier, because it produces a measurable defect rate that can be trended. If the rate drifts, escalate with data rather than anecdote. For critical programmes, add a mounting-condition check, since mura that appears only after final assembly will not be caught on a bare module.

What to Specify With the Supplier

Put the following in the drawing or quality agreement, not in email:

• The test method: patterns, grey levels, ambient lux, viewing angle, distance, time, warm-up.
• Zone definitions mapped to the mechanical drawing, with dimensions.
• Acceptance limits per zone: diameter, count, contrast threshold, colour mura limit.
• The sampling plan and AQL. AQL is applied to countable defects; mura is area-based, so define how a mura event is counted as one defect before applying the AQL.
• Reference samples: keep signed golden samples showing the worst acceptable mura, and re-sign them at defined intervals.
• Handling and packaging rules, because pressure during transport can create mura that was not present at final test.

Where the module is used in a regulated end product, the same discipline supports your own traceability. RONEN DISPLAY operates ISO 9001:2015, ISO 13485:2016, IATF 16949:2016 and ISO 14001:2015 systems certified by TÜV SÜD, and can discuss inspection method, zone definitions and sampling plans against your application requirements.

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