Multi-angle colorimeter detects color changes in pearlescent coatings.

Due to its unique optical effect, pearlescent coatings are widely used in industries such as automotive, packaging, and decoration. Its color performance depends on the interference and reflection of light by the flake pigment, and the color and luster perceived by the human eye will change significantly at different angles, which is called "different colors with angles". Traditional single-angle chromatic aberration measurement methods are difficult to fully characterize its color characteristics. By simulating the process of the human eye observing from multiple fixed angles, the multi-angle colorimeter provides an accurate and objective technical means for quantifying the color change of pearlescent coatings.

Optical properties of pearlescent coatings

The optical effect of pearlescent paint mainly comes from the directional arrangement of mica, alumina and other flake effect pigments on the surface of the substrate. Light is refracted, reflected, and transmitted multiple times inside these transparent or translucent sheets, creating an interference effect that results in color and scintillation that change with the viewing angle. Its color is a complex function of the combination of light source, pigment, and observer geometry. Using a single measurement angle (e.g., 45°:0°) will lose most of the information about the color change with the angle, resulting in inconsistent measurement results and visual evaluation, which cannot effectively guide production and quality control.

How it works:

Multi-angle colorimeters typically contain a fixed light source and multiple photodetectors that receive reflected light at different geometric angles. Internationally accepted measurement geometries include:

1. Specular Inclusion (SPI or -60°): This angle contains specular reflected light, reflecting the overall gloss of the sample surface.

2. Near mirror angle (e.g., 15° or 25°): Most sensitive to color changes with angles, often used to capture the "shiny" effect of pearlescent or metallic pigments.

3. Basic measurement angle (e.g. 45°): Close to daily observation conditions, it is a common angle for color matching.

4. Away from mirror angles (e.g. 75° or 110°): Sensitive to diffuse reflection and "side-viewing" colors of pigment particles, reflecting the degree of "color shift".

The instrument calculates the color coordinates (e.g., CIE L*a*b*) at each angle by measuring the reflectance data of these angles simultaneously or quickly in sequence.

Quantification of color variations

The key to evaluating the color change of pearlescent coatings is to analyze the differences in color parameters between different angles. Usually one angle (such as 45°) is selected as the datum, and the color difference value ΔE between the other angles and the datum is calculated. The CIE 2000 chromatic aberration formula is generally used to calculate the chromatic aberration ΔE, which takes into account the uniformity of the human eye's perception of the color space, and the formula is as follows:

ΔE₀₀ = √[(ΔL′/k_LS_L)² + (ΔC′/k_CS_C)² + (ΔH′/k_HS_H)² + R_T(ΔC′/k_CS_C)(ΔH′/k_HS_H)]

Among them, ΔL′, ΔC′ and ΔH′ are the differences in brightness, saturation and hue, respectively. S_L、S_C、S_His a weighted function; k_L、k_C、k_His the parameter factor; R_Tis a rotation function. By analyzing the ΔE value of multiple angles, the curve of the difference with the angle can be drawn, and the "color shift" characteristic can be visually displayed.

Data analysis

In addition to the total chromatic aberration ΔE, the following parameters are critical to characterizing the pearlescent effect:

Brightness change (ΔL): Reflects the degree of change in the brightness and darkness of the sample at different angles. Pearlescent coatings usually have the highest brightness at near mirror angles (e.g., 15°).

Saturation change (ΔC): Reflects changes in color vibrancy. At certain angles, the interference color is the strongest, and the saturation reaches its peak.

Hue angle change (Δh): Directly characterizing the visual hue shift of colors "from green to red" or "from blue to yellow" is the core of evaluating "color shift".

With systematic measurements, the following key data sheets can be established:

The data in this table show that the sample is the brightest at the 15° angle and the darkest at the 75° angle, and there is a significant chromatic difference from the reference angle at 45°, which confirms its chromatic characteristics with the angle.

Application:

On the production line, multi-angle colorimeters can be used to quickly inspect batch-to-batch consistency. By setting the L*a*b* tolerance range or ΔE tolerance limit for each angle, it is possible to quickly determine the qualification. In the R&D phase, the technology helps to optimize formulations: by analyzing the effects of different effect pigment types, particle sizes, and additions on the color curves at each angle, it guides the development of coatings with specific color shift effects. For example, increasing the flicker of the near-mirror angle usually enhances the flicker of the near-mirror angle, increasing the ΔL between 15° and 45°.

Conclusion

By simulating the visual process of multi-angle observation by the human eye, the multi-angle colorimeter provides a reliable solution for quantifying the complex phenomenon of different colors with angles in pearlescent coatings. It overcomes the limitations of single-angle measurement, comprehensively captures changes in color, brightness, and saturation with angle, generating objective and reproducible data. This data is fundamental for achieving accurate color matching, rigorous quality control, and efficient formulation development, and is a key technical tool for improving the expressiveness and consistency of pearlescent coating products.

References

1. CIE Publication No. 15:2004. Colorimetry.

2. ASTM E2194-14. Standard Practice for Multiangle Color Measurement of Metal Flake Pigmented Materials.

3. Technical Manual of Coatings and Pigments, Color Measurement Chapter.