Three-angle gloss meter for evaluating coatings with different gloss levels

Measurement principle

Gloss is a key metric for evaluating the optical properties of a surface, defined as the ratio of reflected to incident luminous flux on a surface, typically expressed as a percentage or unit of gloss. In the coating industry, gloss directly affects the appearance quality and visual experience of products. The three-angle gloss meter enables a quantitative evaluation of surface gloss by measuring the intensity of specular reflection light at a specific angle of incidence. Its core measurement principle is based on the Fresnel formula, when light is directed from medium 1 to medium 2, the reflectance R is related to the angle of incidence θ and the refractive index n of the two media. For the coated surface, it can be simplified to an approximate formula: R(θ) ≈ [ (n2 - n1) / (n2 + n1) ]², where n1 is the refractive index of air and n2 is the refractive index of the coating material. The instrument is typically configured with three measuring angles: 20°, 60°, and 85° to accommodate coatings with different gloss ranges: a 20° angle is recommended for high-gloss surfaces, a 60° angle for medium-gloss and an 85° angle for low-gloss surfaces.

Sample preparation

In this study, five common industrial coating samples were selected, covering different gloss grades such as high-gloss, semi-glossy, and matte. The sample substrate is a uniform specification of cold-rolled steel plate with sizes of 150mm×100mm. Coating preparation strictly follows relevant process standards, including surface pretreatment, uniform coating application, curing and drying, and other steps to ensure that the sample surface is flat and free from visible defects. All samples were measured after 24 hours of conditioning at a standard temperature of 23°C±2°C and a relative humidity of 50%±5%. Before measuring, use a microfiber cloth to clean the surface of the sample to avoid dust or fingerprints affecting the measurement results.

Measurement method

Measurement is carried out using a three-angle gloss meter that meets international standards. The instrument is calibrated with a standard calibration plate to ensure measurement traceability. Each sample surface is measured in five different positions, and the position distribution is in accordance with the center and four corners principle. The gloss values at 20°, 60° and 85° angles were recorded at each position, and the arithmetic average was used as the gloss result of the sample at the corresponding angle. Keep the ambient light stable during the measurement process to avoid direct light interference. After data acquisition, the standard deviation of each set of data is calculated to evaluate the uniformity of the coating surface.

Analysis of results

The measurement data showed that the three-angle measurements of the coatings with different gloss levels showed significant differences. The value of high gloss coating is higher at 20° angle, but relatively low at 85° angle. Matte coatings, on the other hand, measured at an angle of 85° are relatively high. This confirms the need for multi-angle measurements to comprehensively evaluate the gloss properties of coatings. The specific data is summarized in the table below:

Further analysis suggests that a single angle measurement may not accurately reflect the true visual appearance of the coating. For example, some coatings containing special effect pigments will show varying gloss at different viewing angles, which may be missed by measuring at 60° angles alone. In addition, the microscopic roughness of the coating surface affects the scattering of light, resulting in different reflected light intensity distributions at different angles. Through the comparison of three-angle data, a deeper understanding of the surface structure characteristics of the coating can be obtained.

Notes:

Coating gloss measurement is influenced by a variety of factors. Surface cleanliness is a critical factor, and micron-level dust can cause measurement deviations. The temperature and humidity of the measurement environment should be stable, as the refractive index of the coating material may vary with the environment. The calibration status of the instrument directly affects the measurement accuracy, and it is recommended to calibrate and verify it before each use. The operation technique should also be standardized to ensure that the measuring head fits closely with the sample surface and avoids ambient light leakage. For curved or small-sized samples, special measuring accessories should be selected and the specificity of the measurement conditions should be indicated.

Conclusion

The three-angle gloss meter provides a comprehensive and reliable technical means for evaluating coatings with different gloss degrees. Multi-angle measurements allow for more accurate characterization of the optical properties of coatings, especially for materials with large gloss spans or special visual effects. In practical applications, the appropriate measurement angle should be selected according to the gloss range of the coating, and combined with multi-angle data for comprehensive evaluation. Standardized sample preparation, strict environmental control, and correct operating methods are the basis for ensuring accurate and reliable measurement results. Future research can further explore the correlation between gloss and other surface performance parameters to improve the coating quality evaluation system.

References

International Organization for Standardization. Coatings and varnishes - determination of gloss. ISO 2813.

American Society for Testing and Materials. Standard method for measuring specular gloss. ASTM D523.

Measurement technology of optical properties of coating surface. Journal of Materials Testing.