Glossimetry
Technique: Gloss measurements
Formal name: Glossmeter
What this techniques measures:
This technique measures the so-called gloss or glossiness of a surface. It indicates how much light, which strikes a surface at a given angle, θ, in the diagram below, is reflected at the same angle, θ. If all of the light is reflected at θ, the surface is considered to be 100% glossy. This is essentially what happens when you see a perfect reflection of something in a mirror at a certain angle. If the surface is roughened, or, for example, dust falls on it, part of the light which strikes the surface at angle θ will be scattered at other angles. One therefore sees less light at the original angle, θ, and one considers the surface to be more matte.
A typical glossmeter is shown in the photograph to the right. A glossmeter works based on the principle of the diagrams shown above left. A narrow beam of light is projected onto a surface at a given angle, θ. A detector measures the intensity of light reflected from the surface at the same angle θ with which it struck the surface. If the surface is "perfectly" glossy, the reflected light will have the same intensity as the incoming light at angle θ. If the surface is rougher, that is, more matte, the detector will measure less light coming in at angle θ.
Tri-gloss meters typically measure at 20°, 60°, and 85° angles measured from the vertical. (Some are also available with 45° and 75° geometries). 85° is thus a raking angle measurement. Glossmeters are available with a single-angle or a combination of two or three angles. Tri-gloss meters, for example, allow you to measure three angles simultaneously. As noted in the examples section below, the choice of angle depends on the matteness/glossiness of the surface to be measured. The 85° angle is used more for matte surfaces, while the 20° angle is generally used for high gloss surfaces. If a glossmeter is to be used for different types of materials and for different research objectives, the tri-gloss meters, though more expensive, are the best choice.
Is the technique quantitative?
This technique is quantitative in the sense that it provides a number. Most glossmeters provide either a percentage from 0 to 100, or a so-called gloss unit (GU) with values from 0 to 1000 or 2000. As a percentage, 100% means "perfect" gloss as in a mirror finish, and lower percentages means that the surface is matter. Gloss units provide more resolution, by dividing the 100% measurement range into 1000 or 2000 steps. The higher GU numbers are glossier than the lower numbers.
Note, however, that the number alone does not mean anything. It can only be used in comparison with another surface. Furthermore, whether something is matte or glossy, and how much so, is a matter of perception. Thus, reference measurements are required. Further, it is recommended that some form of perception testing is conducted along with the gloss measurements. One can then assign measured values of glossiness/matteness to the perception to "quantify" the results of the subject perception tests.
It is also important to measure the gloss of samples several times. The statistics of gloss measurements, average (mean) and standard deviation can be large on real objects and even on specimens. Most meters allow for a statistics mode that calculates the arithmetic mean and standard deviation of a user defined group of measurements. The meters also allow for an automatic comparative analysis as described in the previous paragraph. This is done by establishing a reference point or control and measuring samples.
Examples of chemical substance(s) or physical feature(s) identified:
In industry, gloss measurements are used for high-tech research and industrial instrument components such as high-quality mirrors in scientific instrumentation used in conservation science, to maintaining a consistent appearance for consumer goods, for example, metallic paints for automobiles or matte finishes for wood furniture. Recommendations for the selection of measurement angle for such applications are
- 20° is for high gloss coatings, plastics, and related materials
- 60° is for semi gloss coatings, plastics, and related materials
- 85° is for low gloss coatings, plastics, and related materials
- 45° is for semi gloss ceramics and film
- 75° is for low gloss paper and vinyl
In conservation, gloss measurements are useful for determining changes in appearance due to conservation treatments which are suspected of removing a glossy coating or roughening a surface, for comparing materials used for retouching with the original materials, for studying surface changes due to aging, and for monitoring dust.
Phases it can be used to examine: Solids
Is this technique destructive or non-destructive:
This measurement technique is non-destructive and the meters are often small and portable. However the gloss measurements are contact measurements. The glossmeter has to be placed firmly on the surface to be measured to avoid stray light leaking into the measurement at the bottom edge of the glossmeter. The flat bottom of the glossmeter can therefore physically disturb the roughness, and thus the gloss of the surface being measured. Furthermore, the glossmeter itself can pick up material from the surface, so care must be taken to clean the surface, as well as avoiding contamination of the optics.
Size of sample necessary to use this technique:
The sample size is dependent on the meter and the geometry used. Most glossmeters on the market require a minimum sample size about the size of a glass slide for optical microscopes, 25 x 75 mm. The length is required to accommodate the fact that when measuring at an angle of 85° which is a raking angle based on the diagram above, the light is project on to a very long oval surface of rough 10 x 54 mm. (On the other hand, the light projected onto the surface at 20° is roughly a circle of 10.5 mm in diameter. In any case, the sample must be flat because the bottom of the glossmeter is flat. Anything more than micro-roughness will allow outside light to leak into the measurement area.
There are special glossmeters for the measurement of curved surfaces, for example, automobiles bodies as mentioned above. However, these are specialized, read expensive. One could use the SCI (specular component included) function of a colorimeter. The specular component of reflected light is also used as a measure of gloss.
Examples of how this technique is used in the field of art conservation:
Monitoring of dust in museums, collections and storage: The technique of using gloss measurements was developed by research scientists at the University of East Anglia and the National Trust, both in the UK, for monitoring dust collection in museums and historic homes, see for example [1] [2]. A more user friendly version of the method was developed at the Cultural Heritage Agency of the Netherlands, see [3].
Identification of photograph papers: Gloss measurements were used in a preliminary study of the role of scientific measurements in determining the appearance of various types of photographic papers, see [4]. This study included perception testing.
Determining the effect of conservation treatments on surface condition and appearance of objects
Monitoring of the condition of materials over time (aging), assuming little or no color change of the materials.
Risks associated with using this technique: Given that a glossmeter is a contact instrument, it can, besides changing the roughness of the surface as described previously, pick up material from the surface. In such cases, if the measurement of tacky surfaces is unavoidable, care must be taken to clean the glossmeter surface after each measurement, as well as avoiding contamination of the optics.
Cost to purchase this instrument:
~$500 to $6000 where the inexpensive meters are the low-end, low resolution single angle meters, and the tri-glossmeters are the most expensive. The more expensive meters will usually include the instrument, software, USB cable, and carrying case, and possibly ISO certification.
Annual costs to maintain this instrument:
~$400 to $1000 Most manufacturers recommend factory maintenance every one to two years, in particular for the high-end meters, which includes cleaning of optics, control calibration, firmware and software update, and recertification.
Sample analytical costs: Aside from labor costs, sample preparation is the only analytical cost.
Complementary Techniques: This instrument is often used with other techniques that measure physical characteristics
- Colorimeter
- Dry-film Thickness Gauge
- Surface profilometry
- Visual observation and perception testing
Links to external resources/databases:
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- ↑ D. Ford and S. Adams, "Deposition rates of particulate matter in the internal environment of two London museums", Atmospheric Environment, 22 4901-4907 (2002).
- ↑ S.J. Adams, R. Kibraya and P. Bimblecombe, "A particle accumulation study during the reconstruction of The Great Court", British Museum. Journal of Cultural Heritage 3 2830287 (2002).
- ↑ W. Wei, et al., "Experience with Dust Measurements in Three Dutch Museums, ZKK Zeitschrift für Kunsttechnologie und Konservierung 21 2 261-269 (2007).
- ↑ W. (Bill) Wei and S. Stigter, “Surface Roughness, Appearance, and Identification of AGFA-Gevaert Photograph Samples, Topics in Photographic Preservation, Vol. 17, Photograph Materials Group of the AIC, Washington, D.C., pp. 11-24 (2017).
