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A gloss meter (glossmeter) is a handheld or benchtop instrument that measures how shiny a surface is. It shines a light beam of fixed intensity at a fixed angle onto the surface, measures the light reflected at the equal and opposite angle, and reports the result in gloss units (GU). Who uses it: quality-control staff, coatings, plastics and ink laboratories, and core-facility users who need a repeatable number for surface appearance instead of a visual judgement.
What a gloss meter is
Gloss is the specular reflectance of a surface, meaning the share of light that bounces off at the mirror angle rather than scattering. A gloss meter turns that into a single number. The scale is referenced to polished black glass with a refractive index of 1.567, which is assigned 100 GU at each measuring geometry. A matt surface reads near 0 GU, and very highly reflective surfaces can read far above 100 GU (up to roughly 2,000 GU at 20 degrees).
The instrument is used in quality control from goods receipt to final inspection, mainly for paints, coatings, plastics and inks, and also for automotive parts, glass, aerospace components, polished stone and metal, consumer electronics and paper.
How it works
A gloss meter has a light source and a receiver set at matching angles on either side of the surface normal. Light leaves the source, hits the specimen, and only the portion reflected at the equal and opposite angle reaches the receiver. A smooth surface sends most light into the receiver (high GU); a rough surface scatters it (low GU). The measuring angle is called the geometry.
Geometries: 20, 60 and 85 degrees
Which angle should you use? Take a first reading at 60 degrees. If the result is above 70 GU switch to 20 degrees; if it is below 10 GU switch to 85 degrees; otherwise stay at 60 degrees. This selection rule follows the ISO 2813 summary published by Polymer Service Merseburg and is repeated by Qualitest and Wikipedia.
| Geometry | Use when the 60-degree reading is | Typical surface class | Source |
|---|---|---|---|
| 20 degrees | above 70 GU | High gloss; 20 degrees is also more sensitive to haze | Qualitest; Polymer Service Merseburg; ASTM D523 scope (as summarised) |
| 60 degrees | 10 to 70 GU | Semi-gloss or medium gloss; can be used for all coatings | Qualitest; Polymer Service Merseburg |
| 85 degrees | below 10 GU | Matt or low gloss | Qualitest; Polymer Service Merseburg; Wikipedia |
Some instrument guides give 30 GU rather than 10 GU as the lower switching point. Use the cutoff in the standard or specification you are working to, and state the geometry with every result. Values from different geometries should not be compared with each other.
Other geometries
Other angles exist for specific materials. According to Wikipedia’s glossmeter article, 45 degrees is used for ceramics, films, textiles and anodised aluminium (for example ASTM C346 and D2457), and 75 degrees is used for paper (TAPPI T480). The 20, 60 and 85 degree geometries cover the majority of industrial coatings applications.
Standards that define the method
- ISO 2813:2014, specular gloss of paint films at 20, 60 and 85 degrees. The 2014 edition replaced the term “reflectometer value” with “gloss value” in GU, and expressing results as a percentage of reflection is not permitted.
- ASTM D523, specular gloss of nonmetallic specimens at 60, 20 and 85 degrees; it notes that 20 degrees is advantageous for specimens with a 60-degree gloss above 70.
- Material-specific methods listed by Wikipedia include ASTM D2457, ASTM C346, ASTM C584, BS 3900-5, EN ISO 7668, JIS Z 8741 and TAPPI T480; ASTM D4039 and D4449 cover related haze and appearance measurements.
Calibration and sampling
Calibration
As summarised by Polymer Service Merseburg, ISO 2813 describes two ways of setting the instrument. Method A sets zero on a foamed black surface, then checks a high-gloss polished black glass standard. Method B, the preferred method, sets the zero offset with the light off. The instrument’s reading on a standard should agree with the manufacturer’s assigned value to within plus or minus 0.1 GU. Modern instruments perform much of this automatically.
Qualitest advises using a certified calibration tile traceable to a national reference such as NIST, recalibrating periodically (more often in dusty or humid conditions), and treating a dirty or scratched tile as a common cause of bad readings. For the general idea, see what calibration is, and for another instrument-check routine see how to calibrate a thermometer.
Sampling and reporting
The ISO 2813 summary calls for at least five readings at representative points, taken in the agreed direction. If the spread between readings is under 5 GU, report the mean as a whole number. If the spread is 5 GU or more, take additional readings and report the mean of all of them. Before measuring, clean dust, oil and fingerprints from the surface, and avoid edges and textured areas unless your specification says otherwise.
Common mistakes
- Leaving the geometry out of the result, or comparing readings taken at different geometries.
- Reporting gloss as a percentage of reflection instead of GU.
- Measuring a high-gloss sample at 60 degrees only, when 20 degrees is the angle recommended above 70 GU and is more sensitive to haze.
- Calibrating on a dirty or scratched tile.
- Measuring on edges or textured areas, or on a dirty surface.
FAQ
What is a gloss meter?
A gloss meter is an instrument that measures specular gloss: it directs light at a surface at a fixed angle, measures the light reflected at the opposite angle, and reports gloss units (GU) against a polished black glass reference of 100 GU.
What does GU mean?
GU stands for gloss units, the scale used in ISO 2813 and ASTM D523 in which polished black glass of refractive index 1.567 reads 100 at each geometry.
Why does a gloss meter have three angles?
Because one angle does not discriminate across the whole range. Measuring at 20 degrees suits high-gloss surfaces, 60 degrees suits medium gloss, and 85 degrees suits matt surfaces.
Sources
- Qualitest, Gloss Measurement with a Gloss Meter (read 2026-11-08).
- Polymer Service Merseburg wiki, Gloss Measurement, summarising ISO 2813:2014 (read 2026-11-08).
- Wikipedia, Glossmeter (read 2026-11-08).
