Overview
The Coating Freshness Tester is a specialized optical instrument used to quantify and evaluate the imaging clarity of coating surfaces. Its working principle is based on the reflection and scattering characteristics of light on the coating surface, and the objective data characterizing the surface smoothness and optical uniformity are obtained by capturing and analyzing the reflection image of the standard test pattern on the coating surface. This instrument is widely used in automobiles, furniture, electronic product shells, and industrial coatings, providing a key basis for quality control and optimization of coating processes.
Measurement principle
Imaging clarity, often referred to as Distinctness of Image (DOI), is the physical essence of the ratio of specular to diffuse reflected light on the surface of the coating. When a beam of light hits the surface of the coating, the microscopic unevenness of the surface causes the reflected light to scatter. The smoother the surface, the stronger the specular reflection light, and the clearer the image; Conversely, the rougher the surface, the higher the proportion of diffuse reflected light, and the blurred the image.
The meter typically projects a grating composed of alternating light and dark stripes or a specific pattern onto the surface of the coating and receives its reflection through a high-resolution image sensor. By analyzing the sharpness of the edges and the degree of attenuation of contrast in the reflected image, one or more quantification indicators can be calculated. A commonly used evaluation parameter is the freshness value (DOI value), which can be calculated by referring to the following relationship:
DOI ∝ (Cr / C0) × 100%
Among them, Cris the modulation contrast at a specific spatial frequency in the reflected image of the coating being measured, C0is the reference contrast measured on an ideal specular reflector. Higher values indicate better surface imaging clarity.
Technical parameters
The performance of the instrument directly affects the accuracy and repeatability of the measurement results. Key technical considerations include the stability and spectral characteristics of the light source, the imaging quality of the optical system, the resolution and dynamic range of the image sensor, and the robustness of the analysis algorithm. The instrument should be used under controlled ambient light conditions and calibrated regularly with standard plates to ensure that the measurement data meets the requirements of relevant industry standards (e.g., ASTM D5767, JIS K 7374, etc.).
| Measurement parameters | Typical explanation |
| Measurement range | It can cover the brightness range from low to high |
| Repeatability | The range of results from multiple measurements at the same point fluctuates |
| Measure the spot | The size of the instrument's detection area |
| Spatial frequency | Test the density of the pattern lines option |
| Data output | DOI value, image contrast curve, etc |
Application Practice
In practical applications, the vividness of coatings is affected by multiple process factors. The leveling of the coating, the spraying parameters, the curing conditions, and the flatness of the substrate all form different microscopic morphology on the final coating surface, which can change its optical properties. Process monitoring with a coating freshness tester can help identify changes in these factors. For example, measuring at different stages after spraying or baking can track trends in surface conditions and provide data support for adjusting the process window.
It should be noted that vividness is only one of the indicators to evaluate the appearance of the coating, and it needs to be combined with other surface visual indicators such as gloss, fog shadow, orange peel to make a comprehensive evaluation of the overall appearance quality of the coating.
Summary
The coating vividness tester plays an important role in modern coating production and research and development by transforming the subjective perception of surface imaging clarity into comparable numerical data through objective and quantitative methods. Only by correctly understanding its measurement principles, paying attention to the key performance of the instrument, and conducting comprehensive analysis in combination with the actual process can the tool be effectively used to improve the appearance quality and consistency of the product.
References
ASTM D5767-18, Standard Test Method for Instrumental Measurement of Distinctness-of-Image (DOI) Gloss of Coated Surfaces.
JIS K 7374:2007, Plastics — Determination of the distinctness-of-image gloss.
B. R. Hinderliter, S. G. Croll. A review of the relationship between coating surface morphology and optical scattering. Journal of Surface Coating Technology.