Rationale
Specular gloss is a critical parameter for evaluating the optical properties of a material's surface, defined as the ratio of the specular flux of a material to the specular flux of a standard plate at a specific angle of incidence, typically expressed as a percentage or gloss unit (GU). Its core principle is based on Fresnel's law of reflection, and the gloss value G can be passed by the formulaG = (Rs / R0) × 100calculation, of whichRsis the reflected light flux of the specimen,R0The reflected luminous flux of the standard plate. Standard plates typically use black glass with a refractive index of 1.567, and its theoretical gloss value at each angle is defined as 100 GU.
Scope of application:
The choice of gloss measurement angle depends primarily on the gloss level of the material's surface. The sensitivity to gloss changes varies from angle to gloss, so it needs to be matched according to the material properties to ensure the accuracy and distinction of the measurement.
20° angle (high gloss surface): The angle of incidence is large, and it has a high ability to distinguish between high-gloss surfaces (usually materials with a 60° angle measurement higher than 70 GU). It is suitable for polishing metal, high-gloss paint, plastics and some ceramic products.
60° angle (universal angle): As the most commonly used measuring angle, it is suitable for most medium-gloss surfaces (measurements between 10 and 70 GU). This angle balances sensitivity with range of application and is commonly used for routine inspections in quality control.
85° angle (low gloss surface): Smaller angle of incidence and more sensitive to low-gloss or matte surfaces (usually materials with an angle measurement of less than 10 GU at 60°). It is commonly used to measure matte coatings, paper, fabrics, and partially rough surfaces.
Instrument selection
When choosing a measurement angle, consider the material's gloss range, industry standard requirements, and measurement purpose. Here are the key selection criteria:
Basis 1: The gloss range of the material。 Initial evaluation was first performed using a 60° angle. If the measured value is higher than 70 GU, it is recommended to supplement the 20° angle measurement to improve the resolution. If the measured value is less than 10 GU, an 85° angle is recommended for more accurate data.
Basis 2: Follow relevant standards and specifications。 Different industries have clear regulations on the measurement angle. For example, furniture coatings often use a 60° angle, while automotive high-gloss finishes may require a 20° angle. Laboratories should give priority to the selection based on the technical standards of the field to which the product belongs.
Basis 3: Measurement purpose and data comparability。 In the process of research and development, it may be necessary to comprehensively analyze the surface characteristics from multiple angles. while production line quality control pays more attention to the consistency of data at a fixed angle. When comparing across institutions, the angle and instrument calibration method must be unified.
Notes:
To ensure reliable measurement results, the following aspects should be noted:
The instrument needs to be calibrated regularly with a certified standard plate, and the calibration frequency depends on the use environment. When measuring, the surface of the specimen should be clean and flat, and the probe should be tightly fitted to the surface to avoid ambient light interference. For curved or textured surfaces, multiple point measurements should be selected and the average value should be calculated. In addition, temperature and humidity may affect the surface condition of some coatings, and it is recommended to measure under standard environmental conditions.
Summary
In the specular gloss measurement, the 20°, 60° and 85° angles are for high, medium and low gloss materials, respectively, forming a complementary measurement system. The selection is based on the gloss level rooted in the material itself, the specific requirements of industry codes, and the actual goals of the measurement task. Correct angle selection and standardized operation are the basis for obtaining reliable, comparable data that aids in material development, process optimization, and quality control.
References
ASTM D523 - Standard Test Method for Specular Gloss
ISO 2813 - Colored paints and varnishes - Determination of mirror gloss
GB/T 9754 - Colored paints and varnishes - Determination of mirror luster of colored paints and paints without metallic pigments