Overview of the fog phenomenon
In the field of surface coating technology, the visual quality of high-gloss coated surfaces is a key evaluation indicator. When light shines on the surface of the coating, in addition to producing clear specular reflections, some light will be scattered due to microscopic unevenness of the surface, resulting in halos or blurring at the edges of the reflected image, which is often referred to as "fog shadow". Fog shadows can reduce the vividness and clarity of the coating, affecting the visual performance of the final product. Therefore, objective and accurate quantitative detection of fog shadow on the surface of high-gloss coating is of great significance for quality control and process optimization.
How it works:
A fog shadow meter is a precision optical instrument specifically designed to measure scattered light from surfaces. Its core principle is based on the measurement of the spatial distribution of reflected light flux. The instrument typically uses an incident beam of a specific geometric angle to illuminate the sample surface and simultaneously measures the specular direction and the intensity of the reflected light at a specific angle adjacent to a specific angle (usually 2° or less from the specular direction).
The Haze Value (H) is usually defined as the ratio of the intensity of scattered light to the total intensity of reflected light, or is calculated by a specific formula. A common formula for calculation is as follows:
H = (Rs / Rt) × 100%
Among them, Rs Represents the intensity of scattered light measured at a specific small angle (e.g., 2°) from the specular reflection direction, Rt Represents the intensity of reflected light measured in the direction of specular reflection. This value is expressed as a percentage, and the lower the value, the higher the clarity of the surface and the less noticeable the fog shadow phenomenon.
Testing process
Inspection with a fog shadow scanner requires a standardized operating procedure to ensure repeatable and comparable results. Key steps include instrument calibration, sample preparation, measurement, and data analysis.
First, the instrument is calibrated using a standard plate with known reflective properties to eliminate system errors. Samples should be clean, flat, and placed in a stable testing environment to avoid interference from ambient light, vibration, and temperature fluctuations. When measuring, it is important to ensure that the area to be measured is representative, and it is usually recommended to take multiple measurements at different locations to average the value.
The key parameters in the detection mainly include the angle of incidence, the angle of measurement and the spectral characteristics of the light source. These parameters are selected with reference to the relevant technical standards for the specific application field.
Relevant standards
Different industries have developed corresponding technical standards for the detection of fog shadows on coating surfaces, which specify instrument parameters, test conditions, sample handling, and method of representing results. Following uniform standards is the basis for effective comparison and technical communication.
| Standard code | Brief description of the scope of application |
| ASTM D4039 | Surface gloss and fog shadow test of plastics and related coatings |
| ISO 13803 | Determination of paint film fog shadow under 20° mirror gloss of color paint and varnish |
| DIN 67530 | The reflectometer measures the gloss and reflection haze of the mirror surface |
These standards usually specify in detail the angle of incidence (e.g., 20°, 60°), fog shadow reception angle (e.g., 0.3°, 2.0°, etc.), and the calibration requirements of the standard plate, providing a clear technical framework for testing.
Influencing factors
The fog shadow value of the coating surface is affected by a variety of factors. From the perspective of materials and processes, resin type, pigment dispersion, leveling agent effect, curing conditions, and construction environment (e.g., humidity, dust) can affect the microstructure of the coating surface, thereby changing its light scattering characteristics. From a measurement perspective, the curvature of the sample, the characteristics of the substrate, the cleanliness of the instrument, and the stability of the instrument itself can all affect the reading.
When interpreting the test report, it is necessary to combine specific standard conditions and generally accepted technical specifications in the industry. A single fog shadow value needs to be meaningful with clear test parameters. Typically, the report provides information on the average, standard deviation, and reference standard for multiple measurements. Comparing the measurement results with the internal quality control limits or the technical agreement agreed by both supply and demand is the direct basis for judging whether the product is qualified or not.
Applications:
Fog shadow detection technology is widely used in industries with strict requirements for surface visual appearance. For example, in high-end car painting, the fog shadow value is a key indicator for evaluating the vividness and luxury of the body paint; In consumer electronics, it is used to evaluate the texture and clarity of shell coatings; In the furniture and building materials industry, it is used to control the decorative effect of wood paints or metal coatings. By quantifying fog shadows, manufacturers can accurately monitor the stability of the production process, trace the root cause of problems, and guide the development of new formulas or processes, thereby continuously improving the appearance quality and market competitiveness of products.
Summary
The fog shadow meter provides an objective and quantitative detection method for the fog shadow phenomenon on the surface of high-gloss coatings. Its working principle is scientific and the operation process is standardized. Understanding how it works, following relevant technical standards, and considering various influencing factors are key to obtaining reliable test results and effectively applying them to production practices. This technology has a practical supporting role in improving the appearance quality of coated products and realizing refined production management.
References
ASTM International. ASTM D4039 - Standard Test Method for Reflection Haze of High-Gloss Surfaces.
International Organization for Standardization. ISO 13803:2014 - Paints and varnishes — Determination of haze on paint films at 20°.
Deutsches Institut für Normung. DIN 67530 - Reflectometer as a means for gloss assessment of plane surfaces of paint coatings and plastics.