Gloss measurement of coated test panels quantifies the specular reflectance of paint and coating films using standardized geometries of 20°, 60°, and 85°, expressed in gloss units (GU). This technical article explains gloss determination in accordance with ISO 2813, ASTM D523, and GB/T 9754, covering measurement principles, sample preparation, instrumentation and calibration, test procedures, reporting requirements, and typical industrial applications in quality control and coatings development.
Technical Overview
Gloss is the visual attribute associated with the capacity of a surface to reflect light specularly, that is, in the mirror direction. Because subjective visual assessment of gloss is inconsistent between observers, instrumental gloss measurement is used to quantify this property objectively. For coated test panels, specular gloss is one of the most widely specified appearance characteristics and serves as a sensitive indicator of film formation, leveling, pigment dispersion, surface defects, and cure quality.
A glossmeter projects a parallel light beam onto the surface at a defined incidence angle and measures the light flux reflected in the specular direction using a photoelectric detector whose spectral response is matched as closely as possible to the photopic luminosity function of the human eye. The result is expressed in gloss units (GU). The scale is anchored to a primary reference standard consisting of highly polished black glass with a refractive index of 1.567 measured at 589.3 nm, which is assigned a gloss value of 100.0 GU at each measurement geometry. Surfaces more reflective than the black glass reference can exceed 100 GU.
Three measurement geometries are specified for paints and varnishes, and the appropriate geometry is selected based on the gloss level of the film. The 60° geometry is used as the universal reference angle for all coating gloss levels. If the 60° value exceeds approximately 70 GU, the 20° geometry shall be used because it provides better resolution and less sensitivity to surface texture on high-gloss films. If the 60° value is below approximately 10 GU, the 85° grazing-angle geometry shall be used because it improves discrimination between matte and near-matte surfaces.
The principal international and national standards for this test are technically harmonized: ISO 2813 (Paints and varnishes — Determination of gloss value at 20°, 60° and 85°), ASTM D523 (Standard Test Method for Specular Gloss), GB/T 9754 (the Chinese national standard corresponding to ISO 2813), and JIS Z 8741 for specular glossiness. These standards define the optical geometry, the reference standards, the calibration procedure, and the reporting requirements. Under controlled conditions, repeatability of a single instrument is typically better than about 1 GU, while reproducibility between different laboratories and instruments is usually within a few GU, which is why consistent sample preparation and instrument calibration are essential.
Testing Samples
Coated test panels are prepared in accordance with ISO 1514 (Paints and varnishes — Standard panels for testing) or an equivalent agreed substrate preparation procedure. The substrate, surface preparation, application method, film thickness, and curing schedule all influence the measured gloss value and must be documented and held constant when comparing results.
Metal Substrate Test Panels
Cold-rolled steel, tinplate, and aluminum panels are the most common substrates for liquid coatings, and typical panel dimensions are in the range of 75 mm × 150 mm to 150 mm × 200 mm. Panels are degreased, dried, and, where required, pretreated or primed before the test coating is applied by spray application or by drawdown with a film applicator bar. Flatness is critical: any curvature or waviness under the instrument aperture changes the effective reflection geometry and produces biased readings.
Glass and Alternative Substrates
Float glass panels provide optical-grade flatness and are preferred for gloss reference measurements, formulation screening, and drawdown films of clear coats. Coated plastic panels, coil-coated metal sheets, paper and paperboard, and wood-based panels may also be tested, provided the surface is sufficiently flat and rigid to ensure full, uniform contact with the instrument aperture. For paper and flexible films, a flat backing plate is used to prevent deflection during measurement.
Coating Application and Conditioning
The coating is applied to achieve the specified wet or dry film thickness, which is verified in accordance with ISO 2808, because gloss can vary significantly with film thickness until full hiding and complete surface formation are reached. Panels are dried or cured under the conditions specified by the product standard or data sheet. Before measurement, panels are conditioned in a standard atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity in accordance with ISO 3270, typically for at least 16 hours. The measurement area must be uniform, free of dust, fingerprints, orange peel peaks, craters, runs, and sags, and large enough to allow multiple readings away from panel edges.
Testing Items
The following gloss-related determinations are routinely performed on coated test panels by accredited third-party laboratories and in-house quality control facilities:
- Specular gloss at 20° for high-gloss films (automotive topcoats, clear coats, appliance finishes)
- Specular gloss at 60° for semi-gloss and general-purpose industrial and architectural coatings
- Specular gloss at 85° for low-gloss, matte, and sheen-class architectural and furniture coatings
- Specular gloss at 45° and 75° for plastics, films, and ceramic-type surfaces in accordance with ASTM D2457 and ASTM C346
- Reflection haze index for high-gloss surfaces in accordance with ASTM E430 and ISO 13803, characterizing milky halos adjacent to specular highlights
- Distinctness-of-image (DOI) evaluation in accordance with ASTM D5767 for appearance-critical Class A automotive surfaces
- Gloss retention after accelerated weathering (for example, xenon-arc exposure per ISO 16474-2 or ASTM G155), calculated as the percentage of initial gloss remaining after a defined exposure period
Testing Methods
Instrumentation and Calibration
Glossmeters used for this test shall comply with the geometric, spectral, and receptor-aperture requirements of ISO 2813 or ASTM D523, with source and receptor angles controlled to within fractions of a degree. Calibration is performed using a certified working standard, typically a polished black glass or ceramic tile, whose value is traceable through an unbroken chain to a primary standard held by a national metrology institute. Immediately before use, the instrument is verified against the working standard and zeroed using a black light trap or matte black cavity. Calibration is rechecked at defined intervals, typically every one to two hours of continuous use, and whenever the ambient temperature changes noticeably, because calibration tile values are temperature-dependent. The working standard itself is cleaned only with the methods recommended in its certificate, as scratches or contamination on the tile directly transfer error to every measurement.
Measurement Procedure
The standard test procedure is as follows:
- Condition the coated panels at 23 ± 2 °C and 50 ± 5 % relative humidity, and allow the instrument and calibration tile to equilibrate at the same temperature.
- Gently remove loose dust with a clean, dry, lint-free cloth or clean air; do not use solvents unless it has been verified that they do not alter the film surface.
- Verify the instrument zero and calibrate against the certified working standard at the geometry to be used.
- Determine the appropriate geometry: measure at 60° first; use 20° if the value exceeds about 70 GU, and 85° if the value is below about 10 GU.
- Place the instrument aperture in full, flat contact with the film, avoiding panel edges, defects, and previously measured spots.
- Take a minimum of three, and commonly five, readings at different positions distributed over the panel surface.
- On potentially anisotropic surfaces such as brushed or roll-coated films, repeat readings with the instrument rotated 90° to assess directional differences.
- Record the individual readings, their mean, and the spread, together with the instrument identification, geometry, and calibration status.
Data Processing and Reporting
The arithmetic mean of the replicate readings is calculated and reported as the gloss value of the panel, normally to the nearest 0.1 GU, together with the individual readings and the maximum and minimum values. If the spread between readings exceeds the internal acceptance limit, for example due to film non-uniformity, leveling defects, or surface contamination, the individual values are reported and the panel uniformity is investigated. A complete test report identifies the test standard used (ISO 2813, ASTM D523, GB/T 9754, or EN 13523-2 for coil-coated metals), the measurement geometry, the substrate and surface preparation, the application method and dry film thickness, the drying and curing conditions, the conditioning atmosphere, the instrument type and serial number, and the calibration reference. Gloss retention results additionally state the initial gloss, the gloss after exposure, the exposure standard and cycle, and the percentage retention.
Applications
Gloss measurement of coated test panels is a fundamental quality control and product development tool across essentially every sector of the coatings industry, wherever surface appearance is specified or where gloss change signals degradation:
- Automotive OEM and refinish coatings: batch conformance of topcoats and clear coats, evaluation of leveling additives, and appearance qualification of 20° gloss, haze, and distinctness of image.
- Coil coating and pre-painted metal: routine 60° gloss control of prepainted strip in accordance with EN 13523-2, and gloss retention monitoring for façade and cladding products.
- Powder coatings: verification of cure windows, overbake effects, and batch-to-batch appearance consistency for furniture, appliance, and general industrial finishes.
- Architectural coatings: classification of finishes into matte, eggshell, satin, semi-gloss, and gloss categories at 85° and 60° geometries.
- Wood and furniture coatings: development and control of matte-to-high-gloss lacquer systems and evaluation of matting agent performance.
- Weathering and durability studies: gloss retention as one of the primary degradation metrics after artificial weathering or outdoor exposure, used to rank resin systems, stabilizer packages, and pigment technologies.
- Failure analysis: investigation of low-gloss defects such as poor leveling, blush, solvent popping, contamination-induced craters, and under- or over-cure.
FAQ
What is a gloss unit (GU)?
A gloss unit is the conventional unit of specular gloss. It is defined so that highly polished black glass with a refractive index of 1.567 at 589.3 nm has a value of 100.0 GU at each of the 20°, 60°, and 85° geometries. The scale is not bounded at 100; very reflective surfaces such as metallic or mirror-like films can read well above 100 GU.
Which measurement angle should be used for my coating?
Measure first at 60°. If the result is above approximately 70 GU, re-measure at 20°, which gives better discrimination among high-gloss surfaces. If the result is below approximately 10 GU, re-measure at 85°, which gives better discrimination among low-gloss, matte surfaces. This decision rule is specified in ISO 2813, ASTM D523, and GB/T 9754.
Are results from ISO 2813, ASTM D523, and GB/T 9754 directly comparable?
The geometries and reference-standard definitions of these standards are harmonized, so results are generally comparable when instruments are properly calibrated and samples are prepared identically. Small differences can arise from aperture tolerance differences between instruments and from sample preparation variables, so the test standard used should always be stated in the report.
Why do gloss readings sometimes differ between laboratories?
The most common causes are differences in panel flatness and film application, different dry film thickness, incomplete or different cure, surface contamination, calibration tile condition, ambient temperature differences, and instrument drift. Controlling these variables and conditioning panels in a standard atmosphere significantly improves inter-laboratory agreement.
Why might measured gloss be lower than the specification?
Frequent causes include insufficient film thickness or hiding, poor leveling due to application conditions, surface contamination or craters, under-cure or overbake, excessive or poorly dispersed matting agent, pigment flocculation, and measurement on a textured or non-flat area of the panel. Microscopic surface roughness scatters light away from the specular direction and directly lowers the GU value.
Can gloss be measured on curved or textured surfaces?
Standard gloss measurement requires a flat surface in full contact with the instrument aperture; curvature or texture biases the reading and reduces repeatability. For textured appearance effects such as orange peel and image clarity, dedicated appearance instruments measuring distinctness of image and reflection haze in accordance with ASTM E430 or ASTM D5767 are more appropriate than GU values alone.
How is gloss retention calculated after weathering?
Gloss retention is the gloss value measured after a defined exposure period divided by the initial gloss value measured before exposure, multiplied by 100 and expressed as a percentage. Both measurements must be made at the same geometry on the same panel location, following the same calibration procedure, and the exposure method, such as ISO 16474-2 or ASTM G155, must be reported along with the retention value.