Screen Discoloration: Causes, Mechanisms and How It Is Specified

SColour complaints are difficult because the same visible symptom can be a permanent material change, a temperature effect that disappears when the unit warms up, or simply normal variation seen from an angle. Treating all three as defects produces unnecessary rejections; treating all three as acceptable produces dissatisfied customers. The useful work is to separate the mechanisms and then write a colour tolerance that covers the ones that matter.

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

What discoloration means on a display

It is a shift in the relationship between the input signal and the light that leaves the screen.

Discoloration can appear as a uniform tint across the panel, a gradient from one edge, a localised patch, or a colour that changes with viewing angle. Each pattern points at a different cause: a uniform tint suggests the light source or the polariser, an edge gradient suggests bonding or backlight uniformity, a patch suggests mura or contamination, and an angle-dependent shift is usually a panel technology characteristic rather than a fault.

Because the symptom is measured, not seen, the first step is always the same: display a uniform white and a uniform grey field, measure at defined points, and repeat at more than one temperature and viewing angle before deciding what the panel is doing.

Backlight LED binning and ageing

The most common cause of a uniform tint is the light source, not the liquid crystal.

White LEDs vary between production bins in both luminance and chromaticity, and a backlight built from mixed bins will show a slight tint that varies from unit to unit. The same effect appears over time as the LEDs age and the phosphor changes at a rate that depends on drive current and temperature. That is why colour consistency across a fleet is a binning and ageing question, as described in CDTech’s notes on LED binning and brightness matching.

For a buyer, the control is to specify the binning policy and the acceptable colour difference between units, not only the on-axis colour of a golden sample.

Polariser and adhesive ageing

Polymers change colour slowly, and the change is permanent.

Polariser films yellow with ultraviolet exposure and heat, and the optical adhesive used for bonding can shift as it ages, particularly at the edges where moisture and oxygen reach it first. The visible result is a tint that begins at an edge and spreads, or a haze that reduces contrast rather than a hue shift. Because the change is chemical, it does not reverse when the unit cools, which distinguishes it from a temperature effect.

Where a product is installed outdoors or in a hot enclosure, the specification should name the polariser grade and the adhesive, and the qualification programme should include UV and thermal cycling with colour measurement before and after. Substrate and coating properties that set the starting point are documented by glass suppliers such as AGC.

Cell-gap variation and mura-related colour shift

A local patch of different colour is usually a cell or bonding issue.

Liquid crystal transmits different colours depending on the effective cell gap, so a localised variation in gap – from a spacer defect, a bonding pressure difference or an inclusion – appears as a colour patch rather than a brightness patch. The mechanism overlaps with mura: the same physical irregularity that produces a luminance blotch often produces a colour shift as well, which is why the two are usually investigated together. CDTech’s notes on mura and luminance uniformity cover the detection side.

Where the patch appears only after bonding, the bonding process rather than the panel is the likely source; where it is visible in the bare panel, it is a panel-lot question.

Viewing angle and temperature effects that are not defects

Two shifts are normal, and confusing them with defects wastes both parties’ time.

Every LCD changes colour as the viewing angle moves away from normal, and the rate differs by panel technology: TN shifts fastest, IPS and its variants hold colour better off axis, and VA sits between them with different trade-offs. A customer viewing a panel from 45 degrees is seeing the panel’s characteristic, not a defect. The comparison is covered in CDTech’s notes on IPS versus TN viewing angles and colour.

Temperature produces a similar effect that reverses: colour shifts as the panel warms or cools and returns when it stabilises. The way to separate the two from a real defect is to measure at a fixed angle across a temperature range and confirm that the change is reversible.

How discoloration is measured

Measure colour as a number, at several points, angles and temperatures.

A spectroradiometer or a colorimeter gives the chromaticity coordinates and luminance at each measurement point; the difference between two measurements is expressed as a colour difference, commonly Delta E. The measurement geometry matters as much as the instrument: distance, angle, warm-up time and the displayed level all change the result, so the specification should state them. Reference definitions for colour and appearance work are published through the CIE publications catalogue.

Instruments and measurement procedures are documented by colour-measurement vendors such as X-Rite, and detectors used in optical test setups are documented by photonics suppliers such as Hamamatsu. CDTech’s notes on measuring brightness, contrast and uniformity describe the measurement discipline that applies.

Specifying colour tolerance: zone, angle, temperature and age

A single Delta E figure is not a specification; the conditions around it are.

A workable colour specification names the white point and the tolerance, the measurement points (centre, corners, edges), the viewing angles that must be held, the temperature range over which the tolerance applies, and the ageing allowance at end of life. Where the product displays colour-critical content the tolerance tightens; where it displays status information, a looser tolerance with a stricter luminance-uniformity requirement is usually the better trade.

Two additional clauses prevent most disputes: a requirement that colour be measured after a defined warm-up, and a requirement that the specification applies at the brightness setting used in service rather than at maximum.

Defect or normal variation?

The classification decides whether the unit is rejected, sorted or accepted.

A permanent local patch that grows or changes over thermal cycling is a defect. A uniform tint within the declared tolerance is normal variation, and the answer is a binning or sorting decision rather than a rejection. An angle or temperature effect that reverses is a characteristic, and the answer is a specification change if the customer’s requirement was never stated.

Writing that decision rule into the acceptance standard is what allows an inspector to classify a unit consistently, and it is the same logic used for luminance uniformity. LED performance over time is tracked by component suppliers such as ams OSRAM, and display technology trends by research firms such as LEDinside.

Incoming inspection and the evidence to request

Measure the same way every time, and keep the record.

The receiving check is a uniform white field measured at defined points against the boundary sample, with the result recorded as chromaticity coordinates rather than as a judgement. Where a batch deviates, the supplier analysis should state which material or process changed – LED bin, polariser lot, adhesive batch, bonding parameters – and the incoming records for those items should be available.

Rejecting a unit without recording the coordinates makes it impossible to see a trend, and trends are what reveal a process drift before it becomes a batch failure.

Prevention and the supply-chain controls that matter

Binning, material control and change notification.

Colour consistency is maintained by controlling the LED bin used in a backlight, holding the polariser and adhesive grades fixed, and notifying the customer before any of them changes. Those three controls account for most of the difference between two suppliers quoting the same panel, and they are exactly the items that leave no electrical signature when they change.

CDTech’s quality system documents its inspection chain and change control, and the certificate scope behind it is published on the quality certifications page. Related reading: display and touch solutions centre.