Overview
Anodized tinted layers are widely used in construction, consumer electronics, auto parts and other industries, and their color consistency is a key indicator of product quality and aesthetics. Color aberration evaluation realizes the monitoring of the production process and stable control of product quality by quantifying color differences. As an objective and accurate color measurement tool, the colorimeter is based on the standard chromaticity system established by the International Council on Illumination (CIE), which can convert the subjective perception of color by the human eye into reproducible numerical data, so as to scientifically evaluate the color difference of the anodized coloring layer.
Rationale
Colorimeters are usually based on the CIE L*a*b* (CIELAB) uniform color space. The space represents colors in three dimensions: L* for luminosity, a* for red-green directions, and b* for yellow-blue directions. The chromatic difference ΔE between two colored samples can be obtained by calculating their geometric distance in L*a*b* space. The most commonly used formula for calculation is the CIELAB 1976 chromatic aberration formula:
ΔE*ab = √[(ΔL*)² + (Δa*)² + (Δb*)²]
Among them, ΔL*, Δa*, and Δb* are the differences between the L*, a*, and b* values of the standard sample and the test sample, respectively. In order to be closer to the visual perception of the human eye, more complex chromatic aberration formulas such as CIE94 and CIEDE2000 have been developed, which introduce weighted adjustments for brightness, saturation and hue differences in the calculation.
Preparation
To ensure the accuracy and repeatability of measurement results, preparation before measurement is crucial. First, the surface of the anodized sample to be tested should be clean, dry, and free of scratches and contamination. Secondly, it is necessary to select the appropriate measurement pore size and measurement conditions according to the surface characteristics of the sample (such as gloss and texture). For textured surfaces or uneven coloration, multi-point measurements are required to take averages. Finally, the instrument must be calibrated regularly using the included standard whiteboard and blackboard and ensure that the lighting conditions in the measurement environment are stable and stray light interference is avoided.
Measurement process
The standard measurement process begins with instrument calibration. The colorimeter port is then tightly fitted to the standard color plate (or confirmed standard sample) and its L*a*b* value is recorded as a reference. The sample to be tested in the production batch is then measured in the same way. The instrument can directly calculate and display the color difference value ΔE. In data analysis, it is necessary not only to pay attention to the total color difference ΔE, but also to analyze the positive and negative and magnitude of ΔL*, Δa*, and Δb* components to determine that the color difference is mainly due to the changes in brightness, red-green phase or yellow-blue phase, so as to provide a clear direction for process adjustment.
Influencing factors
The acceptable color aberration tolerance (ΔE threshold) needs to be negotiated between the supply and demand parties according to the specific product requirements, and there is no unified international standard. Tolerances should be formulated taking into account industry practices, product use, and human eye identifiable thresholds. The main factors that affect the measurement of color aberration in anodized layers include:
| Oxide film thickness and uniformity |
| Coloring process parameters (e.g., voltage, temperature, time) |
| Sealing quality and workmanship |
| Sample surface gloss and texture |
| Measuring instrument geometry (e.g. d/8° or 45°/0°) |
| The standard light source used and the angle of the observer |
Notes:
Although colorimeters provide objective data, they still have limitations. First, instrumental measurements may not be completely consistent with human visual assessment in extreme cases. Second, for colored layers with special effects (e.g., high metallic luster, scintillation), conventional colorimeters may not be able to fully characterize their color characteristics. In addition, the repeatability of measurement results is highly dependent on sample preparation, instrument calibration, and operating specifications. Therefore, it is recommended to combine instrument measurements with visual evaluation under a standard light source box to form a more comprehensive evaluation system.
Epilogue
Colorimeters provide accurate and quantitative technical means for the control of color aberration of anodized coloring layers. By understanding its principles, standardizing the measurement process, and formulating reasonable color aberration tolerances in combination with specific products, manufacturers can effectively improve product color consistency and meet the market demand for high-quality appearance. In the future, with the continuous development of color science and measurement technology, evaluation methods will become more accurate and efficient.
References
CIE Publication No. 15:2004, Colorimetry.
ASTM D2244-22, Standard Practice for Calculation of Color Tolerances and Color Differences from Instrumentally Measured Color Coordinates.
ISO/TR 28642:2016, Anodizing of aluminium and its alloys — Methods for specifying decorative and protective anodic oxidation coatings on aluminium.