Gloss Measurement
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Gloss measurement
Fundamentals and international standardisation
Gloss is a visual perception that arises when observing surfaces. The more direct the reflection of light, the more pronounced the perception of gloss.
There is no standardised terminology for describing the gloss of surfaces using terms such as high gloss or matt. According to ISO 2813 [1] and ISO 4816 [2], the terms high gloss, gloss, silk gloss, semi-gloss, satin, matt and dead matt are used for coating materials, whereas for lacquers and paints the terms high gloss, silk gloss, silk matt or semi-matt, matt, dull matt, deep matt or ultra matt and natural matt. In mineralogy, an eight-step classification system – or, where necessary, a more detailed one – is usually used.
The principle of gloss measurement is based on the measurement of directed reflected light. A high-gloss surface reflects incident light only in the main direction of reflection, whereas with matt surfaces, the light is also reflected diffusely in all directions (Fig. 1a). It follows that, generally speaking, the more uniformly the light is scattered, the lower the intensity of the directional component and the more matt the surface appears (Fig. 1b).
| Fig. 1: | Schematic representation of diffuse and direct reflection (a), and different coloured plates with matt (structured) surface and high gloss sections (b) |
To determine gloss, the intensity of a directionally reflected light beam is measured at a defined angle around the angle of reflection and then related to the reflective properties of a black, polished glass standard with a fixed refraction index. The results obtained in this way correlate with the visual impression of gloss. In accordance with international standards, various measurement angles have become established for determining gloss. Table 1 provides an overview of international standards – without claiming to be exhaustive – for different areas of application.
| Table 1: | Overview of international standards for gloss measurement |
| measuring angle | 20° | 60° | 85° | 45° |
|---|---|---|---|---|
| area of application | automotive, paint, coatings and plastics, and ceramics industries | |||
| gloss impression | high gloss | semigloss | matt | semigloss |
| ASTM C 346 [3] | x | |||
| ASTM D 523 [4] | x | x | x | |
| ASTM D 2457 [5] | x | x | x | x |
| DIN EN ISO 2813 [1] | x | x | x | |
| measuring angle | 20° Tappi | 45° DIN | 75° DIN | 75° Tappi |
|---|---|---|---|---|
| area of application | film and paper industry | |||
| gloss impression | high gloss | semi gloss | matt gloss | |
| DIN EN ISO 8254-1 [6] | x | |||
| DIN EN ISO 8254-2 [7] | x | |||
| ISO 8254-3 [8] | x | |||
| DIN EN 14086 [9] | x | |||
Gloss meter
Gloss measurement is influenced by the parameters of measurement angle, aperture size, spectral adjustment and reference refractive index. Spectral adjustment is achieved by applying appropriate filtering to the spectral transmittance (see: light transmission). Figure 2 shows the optical configuration and the optical path of a gloss meter.
| Fig. 2: | Beam path of the gloss meter [1] comprising:
1 Light source 2 Transmitter aperture 3 Transmitter lens 4 Receiver lens 5 Sample surface 6 Receiver aperture 7 Image of the transmitter aperture in the receiver aperture 8 Photodetector 9 Optical axis of the transmitter beam 10 Optical axis of the receiver 11 Normal to the sample surface 12 Measurement direction |
The light from a light source is collimated onto the sample surface at the selected angle, and the reflected light is captured by a lens at the same angle and focused onto a photodetector. The reading on the receiver is proportional to the luminous flux passing through the receiver aperture, and the deviation must not exceed 1 GU.
Calibration of the gloss meter
According to ISO 2813 [1], there are two options available for the calibration of gloss meters. Calibration using Method A is carried out by first determining the light reflection on a foamed black surface (zero standard C according to ISO 2813) as the reference value zero. In the next step, the reflectance is determined on a specified black, polished glass surface (high-gloss standard A according to ISO 2813) based on the calibration. Using these two reference values, the gloss meter is automatically calibrated for all three angles.
Method B is the preferred procedure. Here, the zero value and the offset value are determined by switching off the light sources in the instrument. The reflection is then determined for all three angles on the polished black glass surface specified by the manufacturer (high gloss standard A). In both automatic calibration routines, the adjustment of modern gloss meters required by ISO 2813 is carried out automatically, so that no manual intervention is necessary.
The subsequent inspection is carried out in such a way that the gloss is measured at the three angles on the standardised black glass surface. The result of the gloss measurement must correspond to the values specified by the manufacturer, taking into account the tolerance of ± 0.1 GU specified in the standard.
Performing gloss measurements in accordance with ISO 2813
ISO 2813 is used to determine the gloss value of plastics and coatings, employing three different measurement angles. Figure 3a shows the relationship between the gloss impression of coating samples classified as visually uniform, ranging from matt to high gloss, for the measurement angles of 20°, 60° and 85°, as well as a schematic representation of the three measurement angles (Fig. 3b).
| Fig. 3: | Relationship between gloss perception and gloss value for the three measurement angles used (a), and a schematic representation of the angles (b); adapted from [1] |
Given the non-linear shape of the curves shown in Fig. 3a, the gloss value for each measurement angle can only be specified within a certain range. Based on a measurement taken using a 60° measurement geometry, the relationships listed in Table 2 apply. The measurement geometry must be specified for each gloss value.
| Table 2: | Measurement angle as a function of gloss value for a measurement below 60° |
| gloss impression | gloss value below 60° | measuring angle |
|---|---|---|
| high gloss | semigloss | matt |
| > 70 GU | 10–70 GU | < 10 GU |
| 20° | 60° | 85° |
In accordance with the standard, testing using the 60° geometry is permitted for all coatings; this results in greater precision compared to the 20° geometry, particularly when examining ageing on surfaces that change over time. The gloss measurement is carried out at a minimum of five representative points in an agreed measurement direction. If the measurement deviation between the measured values is less than 5 GU, the mean value is to be stated as the gloss value in whole numbers. Otherwise, further measurements must be carried out and the mean value of all measurements must be stated.
The term ‘reflectometer value’, as used in accordance with ISO 2813, was replaced in the 2015 edition by the term ‘gloss value’ and is expressed in gloss units (GU). According to ISO 2813, it is not permitted to interpret or express the gloss value as a percentage of reflection.
See also
References
| [1] | ISO 2813 (2014-10): Paints and Varnishes – Determination of Gloss Values at 20°, 60° and 85° |
| [2] | ISO 4618 (2023-02): Paints and Varnishes – Vocabluary |
| [3] | ASTM C 346 (1987; reapproved 2018): Standard Test Method for 45-deg Specular Gloss of Ceramic Materials (withdrawn 2023) |
| [4] | ASTM D 523 (2025): Standard Test Method for Specular Gloss |
| [5] | ASTM D 2457 (2021): Standard Test Method for Specular Gloss of Plastic Films and Solid Plastics |
| [6] | ISO 8254-1 (2009-09): Paper and Board – Measurement of Specular Gloss – Part 1: 75° Degree Gloss with a Converging Beam, TAPPI Method |
| [7] | ISO 8254-2 (2016-06): Paper and Board – Measurement of Specular Gloss – Part 2: 75° Gloss with Parallel Beam, DIN Method |
| [8] | ISO 8254-3 (2016-04): Paper and Board – Measurement of Specular Gloss – Part 3: 20 Degree Gloss with a Converging Beam, TAPPI Method |
| [9] | DIN EN 14086 (2003-04): Paper and Board – Measurement of SpecularGloss –45° Gloss with a Parallel Beam, DIN Method |