Summary: Colour difference between furniture coating films is a quantified colour gap between two panels, measured under a defined illuminant and geometry as ΔE*ab in the CIE 1976 L*a*b* space. In China the measurement and calculation method is unified by GB/T 11186-2025, which replaces the 1989 series. In industry practice, interior wood coating batch acceptance is usually ΔE 1.5–2.0, tightened below 1.0 for demanding orders. The root causes concentrate in colourant batches, film thickness and hiding, gloss differences, substrate and process drift, and metamerism—locked down by master-panel management, instrument measurement, first-article checks and retained samples.
What coating colour difference is
Colour difference is the colour gap between two coating films measured under a defined illuminant, observer field and geometry, expressed as the dimensionless quantity ΔE. In China, film colour measurement follows GB/T 11186-2025 “Methods of colour measurement of paint films”, which integrates and replaces GB/T 11186.1/2/3—1989; it uses the CIE 1976 L*a*b* uniform colour space to compute ΔE*ab and its components ΔL*, ΔC*ab, ΔH*ab, consistent with ISO 11664-4; ASTM D2244 is common in North American trade.
The instrument converts tristimulus values of sample and master panel into L* (lightness), a* (red–green) and b* (yellow–blue); ΔE*ab = √(ΔL*² + Δa*² + Δb*²). A large ΔL* means a lightness mismatch (the most common case), large Δa*/Δb* a hue shift. For pale, low-chroma films the perceptually more uniform CIEDE2000 (ΔE00) is increasingly used; its values are usually slightly smaller than ΔE*ab for the same data—contracts must state the formula and standard, otherwise “is 1.2 a fail?” becomes an argument.
Key data
- GB/T 11186-2025 uses the CIE 1964 10° observer to compute tristimulus values X₁₀, Y₁₀, Z₁₀, with the CIE 1976 L*a*b* system outputting ΔE*ab and its ΔL*, ΔC*ab, ΔH*ab components.
- Spectrophotometers must cover at least 380–780 nm with ±1 nm wavelength accuracy; tristimulus colorimeters serve comparison only and are not for precise absolute colour.
- The standard lists 6 measuring geometries (45°:0°, d:8°, 8:di, 8:de, di:8°, etc.): smooth films may include or exclude the specular reflection; textured films should use specular-included diffuse conditions (8:di or di:8°).
- Illuminant D65 is preferred (average daylight); illuminant A is retained for metamerism assessment; colour-matching booths typically switch between D65 / TL84 / A / CWF.
- Industry tolerance practice: interior wood coating and furniture film batch acceptance ΔE ≤ 1.5–2.0; export and premium orders often tighten to ≤ 1.0; above ΔE 3.0 the difference is obvious at normal viewing distance and usually fails.
- Internal control is usually tightened a further 0.3–0.5 ΔE units below shipping tolerance; master panels are replaced every 3–6 months so the yellowing of the reference itself does not bias results.
ΔE ranges versus perception and typical handling
| ΔE*ab range | Visual perception | Typical handling |
|---|---|---|
| ≤ 1.0 | Almost indistinguishable by eye | Internal target for demanding orders (export furniture, sets used in the same room) |
| 1.0–2.0 | Slight difference visible up close | Common batch-acceptance band for interior wood coatings |
| 2.0–3.0 | Clearly visible at normal distance | Only for appearance-tolerant hidden faces / back panels |
| > 3.0 | Obvious difference | Normal visible parts fail; rework or replace the panel |
Where colour difference comes from: five common causes
Colourant and pigment batches
Batch-to-batch strength variation of organic pigments directly moves b* or a*. Mixing colourant batches of the same code or a drifting tinting-machine calibration is the most common source of batch colour difference—lock the colourant batch first, then talk about the film.
Film thickness and hiding
An under-built base coat lets the substrate show through; on pale panels this appears as lightness and b* shifts. A 10–20 µm film-thickness spread within one batch is enough to produce a measurable ΔE on semi-transparent colours.
Gloss difference read as colour difference
Matting-agent loading, cure level and polishing change gloss; at grazing angles the eye reads gloss mismatch as “a different colour”. Instruments must measure under an agreed geometry, and visual assessment must fix the viewing angle and light source.
Substrate and process drift
Moisture content of the raw panel, veneer batch, sanding grit, UV line dose or hardener ratio all change the final colour via the base tone, penetration and yellowing.
Metamerism
Two panels that match under D65 may “jump” under TL84 or A because the colorants have different spectral curves. Where needed, assess metamerism with illuminant A per GB/T 11186-2025 and make the final judgement under the customer's light source.
Hidden gloss effects
Always compare panels edge-to-edge in the same orientation; rotating one panel 90° can flip a visual judgement without any ΔE change.
Batch consistency: a six-step workflow
- Fix the master panel: seal two identical panels at sign-off (one for the shop, one for the sample cabinet), noting measurement conditions (D65/10°, geometry, formula) and tolerance.
- Unify instruments: same model or same geometry plant-wide, regular white/black tile calibration with records.
- First-article measurement: measure ΔE*ab on each batch's first article (add ΔE00 and A-illuminant data where needed) and record components ΔL*/ΔC*/ΔH* to localise lightness, chroma or hue drift.
- Lock the process: colourant batch, film thickness (record dry film on the GB/T 9286 cross-cut panel of the same batch) and curing parameters written into work instructions and logged at every change.
- Retained samples: keep samples for 2 years or more; on complaint, measure “master panel — retained sample — panel at customer” side by side to locate the responsible stage quickly.
- Renew master panels: rotate every 3–6 months; the new panel must pass at ΔE ≤ 0.5 against the old one to avoid reference drift.
FAQ
What is a reasonable colour tolerance for furniture coatings?
Depends on positioning: interior wood coatings commonly ΔE 1.5–2.0 between batches; export or full sets (table plus chairs in one room) ≤ 1.0–1.2; hidden faces can be looser. Pale and grey-beige tones are visually more sensitive—take the lower end and state the formula (ΔE*ab or ΔE00) and measurement conditions.
What is the difference between ΔE*ab and ΔE00, and which should be used?
ΔE*ab is the CIE 1976 Euclidean-type difference—simple and universal; ΔE00 (CIEDE2000) corrects the non-uniformity of the space and tracks perception better for pale, low-chroma colours. The numbers are not interchangeable; the acceptance document must fix one formula and its standard.
Why does the customer still see a difference when the meter says ΔE is in tolerance?
Three usual reasons: gloss difference amplified visually (colour data excludes gloss), metamerism (the customer's lamp differs from D65), and inconsistent viewing angle/background. Unify the assessment booth and angle; where needed add A/TL84 colour difference and a metamerism index.
Colorimeter or spectrophotometer—which one?
A tristimulus colorimeter outputs colour values only; GB/T 11186-2025 explicitly states it is not for precise absolute colour. A spectrophotometer records the 380–780 nm reflectance spectrum and can compute colour for any illuminant/observer—use it for matching and arbitration.
Is colour difference between panels assembled side by side the same issue as batch-to-batch difference?
No. Within-batch “panel-to-panel” differences come mostly from spray uniformity, film thickness and cure; “batch-to-batch” differences come mostly from colourant batches and process drift. The former is compressed by first-article and in-process control, the latter by colourant locking and the master-panel system.
The master panel has yellowed—has the reference drifted?
Yes. Nitrocellulose and ordinary PU whites yellow slowly, and the master panel ages too. Rotate the master every 3–6 months, store retainers sealed from light, and transition with new-vs-old ΔE ≤ 0.5 to keep the drift negligible.
Last updated: 2026-09-21
References: GB/T 11186-2025 “Methods of colour measurement of paint films” (integrating and replacing GB/T 11186.1/2/3—1989, National Standard Full-text Disclosure System); ISO 11664-4:2019 (CIE 1976 L*a*b* colour space); ASTM D2244 (practice for instrumentally measured colour tolerances); CIE 15:2018 Colorimetry (CIEDE2000 definition).
Kexin New Materials (Guangdong) Co., Ltd.