The plastic color difference determination test quantifies the deviation in color between a plastic sample and a reference standard using instrumental colorimetry, with results expressed primarily as ΔE*ab in the CIELAB color space. This test is essential for verifying batch-to-batch color consistency of plastic materials and finished parts, and it is routinely performed by accredited third-party laboratories in accordance with standards such as ISO 11664, ASTM D2244, ASTM D6290, and GB/T 3979. This article explains the testing principles, sample requirements, measured parameters, procedures, and typical industrial applications of color difference testing for plastics.
Technical Overview
Color difference determination is an instrumental method used to quantify how far the color of a plastic specimen deviates from an agreed reference color, such as a master standard plaque, an approved golden sample, or a specified color coordinate. Because human visual judgment of color is subjective and influenced by lighting, observer, and surface finish, objective colorimetric measurement is required for quality control, supplier approval, and dispute resolution.
The most widely used evaluation system is the CIELAB (L*a*b*) color space defined in ISO 11664-4. In this system, color is described by three coordinates:
- L* – lightness, ranging from 0 (perfect black) to 100 (perfect white);
- a* – the red–green axis, where positive values indicate redness and negative values indicate greenness;
- b* – the yellow–blue axis, where positive values indicate yellowness and negative values indicate blueness.
The total color difference between the sample and the reference is calculated as:
ΔE*ab = √[(ΔL*)² + (Δa*)² + (Δb*)²]
where ΔL*, Δa*, and Δb* are the differences in each coordinate. The individual coordinate differences are equally important because they indicate the direction of the deviation (darker/lighter, redder/greener, yellower/bluer), which is critical for color correction in production. For improved correlation with human perception, particularly for saturated colors, the CIEDE2000 color difference formula (ΔE00), standardized in ISO/CIE 11664-6, is increasingly specified as an alternative to ΔE*ab.
Measurements are typically performed under CIE standard illuminant D65 with either the 2° or 10° standard observer, most commonly the 10° observer. Under controlled viewing conditions, a ΔE*ab of approximately 1.0 is generally the threshold of perceptibility; industrial acceptance tolerances typically range from 0.5 to 2.0 depending on the application, color, and surface texture.
Testing Samples
Injection-Molded Plaques and Panels
Flat, opaque plaques molded under controlled conditions are the preferred laboratory sample form. Standardized plaques eliminate surface geometry effects and allow direct comparison between material lots, masterbatch dosages, or processing parameters.
Plastic Pellets and Granules
Raw pellets can be measured directly using a special pellet holder or glass vessel, typically with a large-aperture sphere spectrophotometer. This approach is standardized in ASTM D6290 and is useful for incoming material inspection, although pellet color only approximates the color of the molded part.
Films and Sheets
Transparent and translucent films are measured in transmission or reflectance mode, often with backing material of defined color to standardize the measurement. Multiple layers may be stacked to achieve opacity for reflectance measurement.
Finished Parts and Components
End products such as housings, trims, caps, and panels can be measured directly on flat, representative areas, provided the measurement aperture can make full contact with the surface.
Powders and Compounds
Rotational molding powders, PVC compounds, and masterbatch are pressed into plaques or measured in a powder cell with a defined packing method to ensure repeatability.
Testing Items
Typical parameters determined in a plastic color difference test include:
- Tristimulus color coordinates: L*, a*, b* (and X, Y, Z values where required);
- Coordinate differences: ΔL*, Δa*, Δb*;
- Total color difference: ΔE*ab (CIELAB) and/or ΔE00 (CIEDE2000);
- Yellowness index (YI) per ASTM E313 and ISO 17223, used to detect thermal or UV degradation;
- Whiteness index per ASTM E313 or the CIE whiteness formula;
- Color change (ΔE) after heat aging, UV or xenon-arc weathering per ISO 4892, or chemical exposure;
- Grey scale rating for color change per ISO 105-A02 / GB/T 250, where a visual rating method is required;
- Metamerism index, where samples must match under multiple illuminants.
Testing Methods
Instrumental Spectrophotometric Method
The reference method uses a calibrated spectrophotometer to measure the spectral reflectance (or transmittance) of the specimen from approximately 360 nm to 780 nm. Reflectance data are weighted by the selected illuminant and observer functions, per ASTM E1164 and ASTM E308, to compute the tristimulus values and L*a*b* coordinates. Common measurement geometries include:
- d/8 sphere geometry – diffuse illumination with 8° viewing; may be operated with the specular component included (SCI) or excluded (SCE);
- 45/0 (or 0/45) geometry – directional illumination and viewing, which correlates closely with visual assessment under normal viewing conditions.
SCI measurements include the specular reflection and therefore represent the "body color" independent of surface gloss or texture, whereas SCE measurements exclude the specular component and better simulate what an observer sees on glossy surfaces. The instrument is calibrated before use with a white tile and a black trap (or light trap) traceable to national standards.
Color Difference Calculation
Color tolerances and differences are calculated from the measured coordinates in accordance with ASTM D2244, ISO 11664, GB/T 3979, and GB/T 11186.3. A minimum of three readings at different locations on the specimen are averaged, and the mean values are compared with those of the reference standard measured under identical conditions.
Visual Comparison Method
Where visual appraisal is required, samples are compared against the standard in a standardized light booth under D65, A, and shop-light sources in accordance with ASTM D1729 and ISO 3668. For color change after aging or exposure, the grey scale method of ISO 105-A02 / GB/T 250 assigns a rating from grade 1 (large change) to grade 5 (no change).
Test Conditions and Preparation
Specimens are conditioned prior to measurement, typically at 23 ± 2 °C and 50 ± 5 % relative humidity per ISO 291 / GB/T 2918. Samples must be clean, free of fingerprints and scratches, opaque (or consistently backed), and large enough to cover the measurement aperture. Curved or textured parts should be measured at identical positions on both sample and reference to minimize geometric bias.
Principal Reference Standards
- ISO 11664 series – Colorimetry (illuminants, observers, CIELAB, CIEDE2000);
- ASTM D2244 – Calculation of color tolerances and color differences from instrumentally measured coordinates;
- ASTM E308 and ASTM E1164 – Computing object colors and obtaining spectrophotometric data;
- ASTM D6290 – Color determination of plastic pellets;
- ASTM E313 – Yellowness and whiteness indices;
- ISO 17223 – Determination of yellowness index of plastics;
- GB/T 3979 and GB/T 11186 – Chinese national standards for object color measurement and color difference calculation;
- JIS Z 8722 – Measurement of object color reflectance (Japanese standard).
Applications
Color difference determination is applied across the entire plastics value chain, from resin and masterbatch production to finished part acceptance. Typical applications include:
- Incoming inspection of resins, compounds, and color masterbatch against approved standards;
- Batch-to-batch color consistency control in injection molding, extrusion, blow molding, and thermoforming;
- Automotive interior and exterior trim, where tight tolerances (often ΔE*ab ≤ 0.5–1.0) are required for multi-part assemblies;
- Consumer electronics housings, household appliances, and cosmetic packaging, where brand color identity is critical;
- Evaluation of color stability after UV weathering, heat aging, sterilization, or chemical exposure;
- Root-cause analysis of color defects (for example, masterbatch dosage, residence time degradation, or contamination);
- Sign-off testing and dispute resolution by accredited third-party laboratories.
FAQ
What is a color difference test for plastics?
It is an instrumental test that measures the color coordinates of a plastic sample and compares them with a reference standard, expressing the deviation as ΔL*, Δa*, Δb*, and the total color difference ΔE*ab (or ΔE00) calculated per ASTM D2244, ISO 11664, and GB/T 3979.
What ΔE*ab value is considered acceptable?
It depends on the application and the color. As a guideline, ΔE*ab below 0.5 is considered an excellent match, 0.5 to 1.0 is typical for automotive and premium products, 1.0 to 2.0 is common for general consumer goods, and values above 2.0 to 3.0 are usually visible to the naked eye and rejected. Contractual tolerance limits should always be agreed between supplier and customer.
Can plastic pellets be tested directly?
Yes. Pellets are measured in a dedicated pellet holder or glass vessel with a large-aperture instrument in accordance with ASTM D6290. However, pellet color is influenced by scatter and only approximates the final molded-part color, so molded plaque confirmation is recommended for critical colors.
What is the difference between SCI and SCE measurement?
SCI (specular component included) excludes the influence of surface gloss and texture, representing the intrinsic material color, while SCE (specular component excluded) better reflects how the part appears visually. The required mode must be specified because the two modes can yield different ΔE values on the same textured or glossy part.
How should samples be prepared for the test?
Samples should be conditioned at 23 ± 2 °C and 50 ± 5 % relative humidity, cleaned, flat and opaque (or uniformly backed), and at least as large as the measurement aperture. Sample and reference must be measured under identical illuminant, observer, geometry, and aperture settings.
Why is color difference testing important for plastics?
Color deviation is often the first visible sign of process drift, material degradation, or masterbatch error. Quantitative ΔE data enables objective quality control, reduces subjective disputes over color matching, and supports validation of color durability after weathering or thermal aging in regulated industries.
