Colour Accuracy and Delta E: Measuring and Specifying Panel Colour

Colour is the parameter buyers specify most loosely and dispute most often, because the words are wide – accurate, vivid, consistent – and the measurement is narrow. A colour specification that names a target, a tolerance and the conditions under which both are measured is enforceable; one that names a colour space is not.

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

Colour accuracy is not colour gamut

Gamut describes the range; accuracy describes the error inside it.

A panel can have a wide gamut and still render the wrong red, because gamut is the boundary of what a display can reproduce while accuracy is how closely a requested colour is reproduced. Marketing emphasises gamut because it is a large number; engineering cares about accuracy because the viewer compares a colour with something. For an industrial product the useful measure of colour quality is the error against a defined target, not the size of the triangle.

Two displays with the same gamut figure can therefore differ visibly, while two with different gamut figures can both be accurate for the applications they serve.

Colour spaces in one paragraph

sRGB, Adobe RGB and DCI-P3 describe different boundaries.

sRGB is the common web and office reference, Adobe RGB extends mainly into green, and DCI-P3 is wider still. A display’s gamut is expressed as the fraction of a reference space it covers, which is why the same panel can be quoted as full sRGB coverage or as a smaller Adobe RGB percentage. The reference space must be stated with the figure; for machine interfaces it is usually sRGB, because the content is authored in it. Colour and appearance definitions are published through the CIE.

When a supplier quotes a gamut figure without a reference space, the number cannot be compared with another supplier’s.

What Delta E actually measures

The distance between two colours in a perceptually uniform space.

A colour difference is expressed as a single number that combines the differences in lightness and in the two chromaticity axes, weighted so that a given value represents a roughly similar perceived difference regardless of hue. As a rule of thumb, values below about 2 are hard to see side by side, values around 3 to 5 are visible to a trained observer, and larger values are obvious. Several formulae exist, so the formula belongs with the number.

A specification should therefore name the Delta E method, the reference white point and the measurement conditions, not simply a threshold.

Instruments: spectroradiometer and colorimeter

One measures the spectrum; the other filters it.

A spectroradiometer measures the spectral power distribution and computes colour from it, which makes it the reference instrument and the slower one. A colorimeter approximates the CIE observer with filters, which makes it fast and inexpensive but sensitive to how well the filter matches the display’s spectrum – a real issue with narrow-band LED backlights. An imaging colorimeter captures the whole screen at once, which is what makes uniformity measurement practical. Instruments are documented by vendors such as X-Rite, and detectors by photonics suppliers such as Hamamatsu.

Whichever is used, its calibration status and measurement aperture belong in the report.

Measurement conditions change the answer

Warm-up, brightness, angle and pattern all matter.

LED output and liquid crystal behaviour change with temperature, so a cold measurement differs from one at equilibrium; the brightness setting changes colour slightly on many panels, particularly at the low end; and colour shifts with viewing angle. A colour measurement must therefore state the warm-up condition, the brightness setting, the viewing angle and the test pattern, typically a full white plus a set of greys.

Measuring at the service brightness rather than at maximum is what makes the figure describe the product the customer actually sees.

Consistency between units matters more than absolute accuracy

Users compare displays side by side, not against a reference.

In a machine with two panels, or a row of instruments in one room, a visible difference between units is noticed immediately even when both are within tolerance against the reference white. Unit-to-unit consistency is controlled by LED binning, panel grading and, where the device supports it, per-unit calibration. CDTech’s notes on LED binning and brightness matching describe the supply-side control.

A useful specification therefore carries two clauses: a tolerance against the target, and a tighter tolerance between units of the same model.

How to write a colour tolerance

Six items make it enforceable.

 

Structure of a colour specification

Item	What to state
Reference	White point, and the colour space the content is authored in
Target	The colour the display must render for a defined input
Tolerance	Delta E limit with the formula and the observer named
Consistency	Maximum difference between units of the same model
Conditions	Warm-up, brightness setting, viewing angle, pattern and ambient light
Measurement points	Centre and edges, with the grid stated

A second tolerance for uniformity across the panel prevents a display that meets the target in the centre from failing visibly at the corners.

Calibration and drift

Colour moves as the backlight ages.

LED output falls and shifts with age, so a display calibrated at manufacture drifts unless the device applies compensation or is recalibrated. Where colour matters, the product needs an internal calibration mechanism, a service procedure with an external sensor, or a defined recalibration interval supported by measured drift data. CDTech’s notes on measuring brightness, contrast and uniformity describe the measurements behind such an interval.

Recording the commissioning measurement is what turns a later drift complaint into an assessable claim.

Incoming inspection and supplier evidence

Record coordinates, and compare lots.

The receiving check measures a white field and one or two greys at defined points and records chromaticity; comparing those values across lots reveals a bin or process change before a customer notices. What to request from the supplier is the specification with its colour tolerance and method, the backlight binning policy, and the drift data behind any lifetime claim.

Where a colour claim is commercially significant, independent verification is available from laboratories such as Intertek, and measurement equipment for a test setup is documented by suppliers such as Keyence. Related material is collected at the display and touch solutions centre.