How is gloss measured?

glossmeter (also gloss meter) is an instrument which is used to measure the specular reflection (gloss) of a surface. Gloss is determined by projecting a beam of light at a fixed intensity and angle onto a surface and measuring the amount of reflected light at an equal but opposite angle.


How to measure gloss

There are a number of different geometries available for gloss measurement each being dependant on the type of surface to be measured. For non-metals such as coatings and plastics the amount of reflected light increases with an increase in the angle of illumination as some of the light penetrates the surface material and is absorbed into it or diffusely scattered from it depending on its colour. Metals have a much higher reflection and are therefore less angularly dependant.

Many international technical standards are available that define the method of use and specifications for different types of glossmeter used on various types of materials including paint, ceramics, paper, metals and plastics. Many industries use glossmeters in their quality control to measure the gloss of products to ensure consistency in their manufacturing processes. The automotive industry is a major user of the glossmeter with applications extending from the factory floor to the repair shop.


The construction of a gloss meter

A typical glossmeter consists of a fixed mechanical assembly consisting of a standardised light source that projects a parallel beam of light onto the test surface to be measured and a filtered detector located to receive the rays reflected from the surface, Figure 1. The ASTM Method states that the illumination should be defined such that the source-detector combination is spectrally corrected to give the CIE luminous efficiency, V(l), with CIE illuminant SC[i].

A parallel-beam specular reflection instrument.

Figure 1. A parallel-beam specular reflection instrument.

A number of instruments are commercially available that conform to the above standards in terms of their measurement geometry. The instruments are calibrated using reference standards that are usually made from highly polished, plane, black glass with a refractive index of 1.567 for the Sodium D line, and these are assigned a gloss value of 100 for each geometry[ii].

Choosing the correct angle for gloss measurement

Measurement angle refers to the angle between the incident and reflected light. Three measurement angles (20°, 60°, and 85°) are specified to cover the majority of industrial coatings applications. The angle is selected based on the anticipated gloss range, as shown in the following table.

Gloss Range 60° Value Notes
High Gloss >70 GU If measurement exceeds 70 GU, change test setup to 20°
Medium Gloss 10 - 70 GU
Low Gloss <10 GU If measurement is less than 10 GU, change test setup to 85°

 

For example, if the measurement made at 60° is greater than 70 GU, the measurement angle should be changed to 20° to optimise measurement accuracy. Three types of instruments are available on the market: 60° single angle instruments, a combination of 20° and 60° and one type that combines 20°, 60° and 85°.

Two additional angles are used for other materials. An angle of 45° is specified for the measurement of ceramics, films, textiles and anodised aluminium, whilst 75° is specified for paper.

 

Understanding Gloss units

The measurement scale, Gloss Units (GU), of a glossmeter is a scaling based on a highly polished reference black glass standard with a defined refractive index having a specular reflectance of 100GU at the specified angle. This standard is used to establish an upper point calibration of 100 with the lower end point established at 0 on a perfectly matt surface. This scaling is suitable for most non-metallic coatings and materials (paints and plastics) as they generally fall within this range. For other materials, highly reflective in appearance (mirrors, plated / raw metal components), higher values can be achieved reaching 2000 Gloss Units. For transparent materials, these values can also be increased due to multiple reflections within the material.


Glossmeter Standards

Comparison of standards for gloss measurement
Standard 20° 60° 85° 45° 75°
High Gloss Medium Gloss Low Gloss Medium Gloss Low Gloss
Coatings, plastics and related materials Ceramics Paper
ASTM C346 X
ASTM D523 X X X
ASTM C584 X
ASTM D2457 X X X
BS3900 D5 X X X
DIN 67530 X X X
DIN EN ISO 2813 X X X
EN ISO 7668 X X X X
JI Z 8741 X X X X X
TAPPI T480 X


Glossmeter Calibration

Each glossmeter is setup by the manufacturer to be linear throughout its measuring range by calibrating this to a set of master calibration tiles traceable to NIST (National Institute of Standards and Technology).

In order to maintain the performance and linearity of the glossmeter it is recommended to use a checking standard tile.  This standard tile has assigned gloss unit values for each angle of measurement which are also traceable to National Standards such as NIST.  The instrument is calibrated to this checking standard which is commonly referred to as a ‘calibration tile’ or ‘calibration standard’.  The interval of checking this calibration is dependent on the frequency of use and the operating conditions of the glossmeter.

It has been seen that standard calibration tiles kept in optimum conditions can become contaminated and change by a few gloss units over a period of years. Standard tiles which are used in working conditions will require regular calibration or checking by the instrument manufacturer or glossmeter calibration specialist.

A period of one year between standard tile re-calibration should be regarded as a minimum period. If a calibration standard becomes permanently scratched or damaged at any time it will require immediate recalibration or replacement as the glossmeter may give incorrect readings.

International standards state that it is the tile that is the calibrated and traceable artefact  not the gloss-meter, however it is often recommended by manufacturers that the instrument is also checked to verify its operation on a frequency dependent on the operating conditions.


Advances in gloss measurement

The glossmeter is a useful instrument for measuring the gloss of a surface.  However, it is not sensitive to other common effects which reduce appearance quality such as haze and orange peel.

Haze: Caused by a microscopic surface structure which slightly changes the direction of a reflected light causing a bloom adjacent to the specular (gloss) angle. The surface has less reflective contrast and a shallow milky effect

Orange Peel: An uneven surface formation caused by large surface structures distorting the reflected light.

 A surface with orange peel and haze

A surface with orange peel and haze

Two high gloss surfaces can measure identically with a standard glossmeter but can be visually very different.  Instruments are available to quantify orange peel by measuring Distinctness of Image (DOI) or Reflected Image Quality (RIQ) and Haze.


Glossmeter Applications 

The glossmeter is many industries from paper mills to automotive and are used by the producer and the user alike.

Examples include:

  • Paints & coatings
  • Powder coatings
  • Additives
  • Inks
  • Plastics
  • Wood coatings
  • Yacht manufacture
  • Automotive manufacture and re-finis
  • Aerospace
  • Polished stone and metals
  • Glass manufacture
  • Consumer electronics
  • Anodised metals

More information the theory of gloss meters is available here: