Gloss meters became popular in the 1970s. German equipment manufacturers started to mass-produce gloss gauges and standards at affordable prices that were suitable for use in production lines. These instruments were based on laboratory instruments used in the paper industry at that time. Due to the evolution of technology and more stringent esthetic demands, the tolerances of gloss unit (GU) specifications were significantly reduced. Some instrument manufacturers claim an accuracy of their gloss meters to within 0.1 GU, and application specifications often state a bandwidth of just 1 GU as acceptance criteria. Are these criteria realistic? In order to answer this question we will have to look at the definition of gloss as stated in the international (ISO and ASTM) standards and as measured by the reference laboratories.
To define a unit like gloss, we need to define some reference points. The defined gloss reference points in the standards are a 0 GU and a 100 GU reference. The 0 GU reference is a full, dull matte surface with zero reflection. For 100 GU the reference is based on black or quartz glass. The refractive index, a derivate of the change in angle when light passes from one media into another at a specific wavelength, is used to calculate the reflectivity of a surface. This is done using the Fresnel equation. This seems pretty robust, but there is a flaw in the definition of the used wavelength. ASTM and ISO do not agree on the wavelength of the spectral line to be used for the reference of 100 GU. ISO uses the d-line (helium) and ASTM refers to the D1-line (sodium). This is in reality a difference of only 1.7 nm, but in terms of standardization is an important deviation that may lead to unexpected results.