Wrinkles, Blotches and Bruises: LCD Defect Vocabulary for Buyers and Suppliers

Most display disputes are partly linguistic. A buyer describes a “blotch”, a supplier hears “mura”, an inspector writes “dirt”, and the analysis starts from three different defects. A shared vocabulary that maps everyday words to mechanisms is the cheapest quality tool in a display programme, because it turns a description into a testable classification.

Part of the Optical, Backlight & Panel Technology guide — 30 articles on bonding, cover glass, brightness and panel choice.

Why vocabulary matters in acceptance disputes

The words decide which standard applies.

A defect described as a “dead pixel” falls under the pixel defect classes. The same visible spot described as a “foreign particle in the bond line” falls under the cosmetic standard for area defects. The two have different limits, different inspection methods and different corrective actions, so the first classification decision determines the whole conversation. Terms that map to mechanisms rather than to appearances remove most of the ambiguity.

The practical approach is a one-page glossary in the acceptance standard, with each term linked to a measurement method and a limit.

Wrinkles: lamination and bonding artefacts

A wrinkle is a physical fold or ripple in a layer.

Wrinkles appear in film-based front surfaces, in protective films, and occasionally in the adhesive layer of a bonded stack, and they are visible as fine parallel lines that catch light under grazing illumination. Their mechanism is mechanical: a film applied with uneven tension, an adhesive that flowed unevenly before cure, or a cover glass that deformed during handling. Under a flat white field they can be invisible; under grazing light they are obvious.

Inspection should therefore include an off-axis view with directional illumination, and the acceptance criterion should be expressed in terms of visibility at the intended viewing angle rather than in millimetres alone.

Blotches: mura, contamination and voids

A blotch is a large-area change in luminance or colour.

The three common causes are mura – a structural non-uniformity in the cell or backlight; contamination, such as a particle or a smear trapped in the stack; and a bonding void, where adhesive is missing over an area. Distinguishing them depends on how the defect behaves: mura usually changes with viewing angle, a particle has a defined boundary and often a halo, and a void is visible at the edge of the bond line under grazing light. CDTech’s notes on mura and luminance uniformity and on inspecting bonding quality describe the inspection that separates them.

For acceptance, a blotch is measured rather than described: a luminance deviation limit over a defined grid, plus a size and count limit for discrete defects.

Bruises: pressure marks

A bruise is a cell-gap change caused by mechanical load.

The industry word matches the behaviour: a soft, diffuse dark or bright patch that can change when stress is redistributed, and that either fades over hours or remains permanently depending on how far the cell was deformed. Bruises are introduced during assembly, bonding, packing or mounting, and their location usually points at the mechanical feature responsible. A point load from a screw boss produces a mark in a predictable position on every unit of that design.

Because the mark can appear or grow after shipping or thermal cycling, the inspection stage belongs in the acceptance standard alongside the limit.

Cloudiness and haze

Cloudiness is a loss of clarity rather than a change in brightness.

The common causes are haze from an anti-glare surface, an adhesive layer that has not fully cured or has absorbed moisture, condensation behind the cover glass, and coating wear from cleaning. Cloudiness raises the minimum luminance and reduces contrast, and it is measured with haze or transmission rather than luminance. Haze measurement follows defined methods such as ASTM D1003.

Where cloudiness appears only after cleaning, the specification should name the agents used and require a repeated-cycle test on the production front assembly.

Lines and streaks

A line is usually an electrical or cell defect; a streak is usually optical.

A vertical or horizontal line that persists across content indicates a gate or source driver connection problem, a damaged driver IC or an open trace on the glass. A streak that changes with viewing angle or illumination is more likely an optical artefact in the backlight or the bonding layer. The two are often confused because both appear as a band, and the distinguishing test is whether the artefact moves when the light source or the viewer moves.

Line defects belong in the functional acceptance criteria, because they usually indicate a fault that will worsen rather than a cosmetic variation.

Mapping words to the standards

Three frameworks cover most display defects.

Defect vocabulary and the framework that covers it

Everyday term	Mechanism	Framework
Dead, stuck or bright pixel	Transistor, cell or driver fault at one pixel	Pixel defect classes (ISO 9241-307 family)
Blotch, mura, cloudiness	Cell, backlight or bonding non-uniformity	Cosmetic standard with luminance limits
Bruise or pressure mark	Mechanical load changing the cell gap	Cosmetic standard with inspection stage
Wrinkle or ripple	Film or adhesive deformation	Cosmetic standard, off-axis inspection
Line or streak	Driver, trace or optical artefact	Functional acceptance criteria

Pixel defect classes are described in CDTech’s ISO 9241-307 explainer, and colour and appearance terms come from the CIE publications.

How to record a defect for analysis

Seven fields turn a description into evidence.

• Location on the panel, measured from a datum, not described as “top left”.
• Size, in millimetres, and whether it follows a pixel boundary.
• Appearance on a uniform white field and on a uniform grey field.
• Behaviour at two viewing angles and under grazing light.
• Behaviour when gentle pressure is applied elsewhere on the panel.
• The pattern and illumination used, with a fixed-exposure photograph.
• Whether the defect changes after thermal cycling or a settling period.

Those seven fields are enough for a supplier to reproduce the observation and to identify the layer. Without them, the analysis starts with a guess.

Supplier vocabulary worth knowing

Four terms that appear in factory reports.

Mura, for structural non-uniformity; AOI class or defect code, for the automated inspection category assigned to a unit; boundary sample, for the physical reference that defines the accept and reject limits; and PM (particle or particulate) for contamination. Knowing which term a factory uses for each defect class makes an incoming inspection report comparable with the supplier’s own data, which is what allows a trend to be tracked across lots.

Certificates and process records behind those reports can be checked as described by the International Accreditation Forum, and independent measurement scopes are published by laboratories such as Intertek. Assembly and inspection expectations are published by bodies such as IPC, and CDTech’s own quality chain is summarised at quality assurance and factory compliance.

Building a glossary into the acceptance standard

One page, one column per term.

The glossary should list each everyday term used by the buyer, the mechanism it maps to, the inspection method, the limit and the framework it belongs to. Adding boundary samples for the cosmetic terms and the pixel-class table for the pixel terms makes the standard self-contained, so an inspector at a new site can apply it without a briefing.

The other benefit is contractual: when both parties use the same word for the same mechanism, a rejection is a measurement rather than an opinion. Related material is collected at the display and touch solutions centre.