In the 1970s gloss meters (gloss meters) became popular. German equipment manufacturers began to mass-produce and sell Glossmeters and standards at reasonable prices, suitable for use in production lines. These instruments were based on laboratory instruments used by the paper industry of that era. Due to technological innovations and more stringent aesthetic requirements, the deviation in Gloss Units (GU) has been significantly reduced. Some instrument manufacturers claim that their Glossmeters are accurate to 0.1GU, and application specifications often state that the 1GU range is the acceptance criterion. Are these standards realistic? To answer this question, let's take a look at the definition of gloss in international standards (ISO and ASTM) and how it is measured in reference laboratories.
gloss definition
To define a glossy unit, we need to define some reference points. The gloss reference points defined in these standards are the 0GU and 100GU reference standards. The 0GU reference standard is a completely matte surface with zero reflection. The 100GU reference standard is based on black or quartz glass. The refractive index is the angular change that occurs when light passes at a specific wavelength from one medium to another and is used to calculate the reflectivity of a surface. Calculations can be done by using the Fresnel equations. This may seem like a good idea, but there is a flaw in defining the wavelengths used. The wavelengths of the spectral lines used by ASTM and ISO to define the reference standard for 100GU are different. ISO uses the d-line (helium), while ASTM refers to the D1-line (sodium). This is actually a 1.7nm difference, but is an important deviation in normalization and can lead to unexpected results.
Only two reference points are used in the general case, which is a very weak basis for the standard. Also, the linear relationship between these two points is based on assumptions only. There is no way to verify this assumed linear relationship because no further reference points have been defined and are therefore not available.
Surface roughness
A second unspecified parameter was also found. The surface roughness of the calibration standard is not part of the Fresnel formula, nor is roughness considered in the relevant standards. Some standards specify flatness, what is the confidence level for measuring gloss?
But the roughness is at most specified as "highly polished" (how high is that?). However surface roughness does affect the reflection of the surface. Gloss calibration plates are used by instrument manufacturers to demonstrate linearity over the entire gloss range for products made of the same material, but with varying surface finishes such as roughness and texture.
Strictly speaking, these calibration plates should have the same gloss value as defined by Fresnel formulas and standards. However, when measured by a reference laboratory (NIST or BAM), they give different values. The question then arises, "Does surface roughness affect gloss values?" The answer is clear: yes, it does, and significantly.
Table 1 gives the main parameters used in several global standards. The difference in gloss value due to this difference can be as high as a few gloss units.
Figure 1 shows the relationship between gloss and roughness. The rougher , the lower the gloss. The graph is a graph of gloss versus Ra value (arithmetic mean of surface roughness). All data were obtained from black glass plates of the same refractive index and made of the same material.
lack of unity
The two problems of surface roughness and spectral line wavelength are two systematic problems in the unification of gloss standard plates, which makes it impossible to meet multiple standards at the same time when operating. Glossmeter geometry and design are often adapted to operate over several standard ranges. However, problems may arise when calibrating the Glossmeter against the currently defined standard plates, as shown in Table 1. Even though NIST and BAM maintain the two main reference laboratories, they are not consistent with each other. NIST refers to ASTM standards, while BAM refers to ISO standards.
The reasons for the lack of uniformity among standards are difficult to explain. A search of the literature did not reveal any significant studies on the relationship between gloss/reflection and surface roughness. Some studies have been done on coatings, but are generally limited to haze, distinctness of image, and orange peel—all important parameters for which there are no defined reference standard values. To put it mildly, the glossiness standard is at least debatable.
Several scientific studies have been done on the relationship between light and surface roughness, such as the Beckmann-Kirchhoff scattering theory. But no unified theory can be connected with the Fresnel formula. To scientifically investigate whether the two theories could be combined to create a definitive definitive definition of gloss calibration plates for the determination of harmonized ISO and ASTM absolute values.
In addition to the uncertainty in the definition of gloss standards, there are other problems. There is a serious danger in the fact that everyone wants traceability of their methods. All accredited laboratories are required to have their standards sent in for calibration on a regular basis, traceable to a reference standard. Reference standards should definitely be of higher certainty than calibration standards. This will mean that the reference standards used need to be inspected on a regular basis every year.
Many calibration laboratories accredited to ISO 17025 are able to calibrate user standards, which are provided with each commercially available Glossmeter, for verification with another commercially available Glossmeter. The gloss meter itself is calibrated with BAM certified standards. BAM has an uncertainty of up to ±1 GU over the relevant 100 GU range. Due to this, coupled with the reproducibility and accuracy of the commercially available Glossmeters used for verification, errors of up to a few gloss units can easily be introduced.
The primary reference standard plates for BAM and NIST should be checked based on their refractive index. In the past 15 years, there has never been a cross-check between these two major laboratories. Many aspects of gloss reference standards are less clear and need further clarification, such as the relationship between roughness and gloss, and the assumed linear relationship between spectral line wavelengths and calibration lines.
The current situation was one of the motivations for ISO to revise the ISO 2813 gloss standard . In order to formally promote a standard to the next level of working document, ISO needs to conduct round robin trials or inter-laboratory studies (ILS). A few professional brands of commercially available gloss meters were selected for this study. Studies have shown that the repeatability and reproducibility values stated in the instrument data sheets used in interlaboratory studies (ILS) are not realistic. Furthermore, the data on which these repeatability and reproducibility values are based are lost over time. Based on test results from standard round robin tests, ISO has decided to address these new repeatability (r) and reproducibility (R) levels in the forthcoming edition of ISO 2813, which are currently feasible levels for high-end professional gloss meters (Table 2 ).
in conclusion
ISO took the first step in updating the ISO2813 standard, but that only solved part of the problem. The main problem is the use of reference standards. All data are from the only currently available reference standard and there is no evidence for the assumed linear relationship of quartz glass. It would be desirable to develop and define a range of reliable reference materials that demonstrate gloss reference values over a range of ranges rather than just one or two points. A series of references will also prove or disprove the assumed linear relationship of the calibration line. It seems that in-depth scientific research is needed to define the effect of roughness and wavelength in order to develop a benchmark standard that aligns ISO and ASTM and leaves no issues. Desirably, these benchmarks provide at least ten times the level of accuracy of Glossmeters that are currently commercially available.
A scientific study of the definition of gloss on a global scale would be of great interest to the industry. All specifications concerning gloss should be checked for feasibility and correctness. While gloss is an accepted appearance standard for quality control in many industries, new disclosures on gloss standards require us to rethink our approach to obtaining accurate results.
