An object, e. g., a paper surface, is termed white when the illumination intensity and the absorption capacity of the surface are independent of the wavelength. Deviations confer a more or less pronounced color shade on the surface.
In the paper industry, a special process is used to characterize the brightness because this is one of the most important optical properties of paper. The determi¬nation of the reflectance factor (ISO brightness) is based on ISO 2470 (1999). For this test, a filter is used which has an intensity maximum at a wavelength of 457 nm. The reflectance (blue component) measured in a reflectometer under specified conditions is known as brightness. It is expressed as a percentage of the brightness of a white standard.
Another optical property of paper is its transparency, i. e., a measure of its light transmittance. It is calculated from the reflectance factors R0, Rw, and R(w), which are determined in accordance with DIN 53 147 (1993). The reflectance factor of the individual sheet on a completely black background is R0, R w is the reflectance factor of the individual sheet on a white background, and R(w) is the reflectance factor of the white base.
Most white papers and paperboards, and therefore also secondary fiber materi¬als, currently contain an optical brightener. Brightness measurements for such materials depend on the relative proportion of UV radiation in the illuminant used for the determination. The standard test methods for ISO brightness have not defined the standard illuminant for use in the determination. The relative amount of UV has also not been defined. As a result, widely different R457 reflectance factors exist for the same kind of fluorescent material. The problem has recently been solved. A revised ISO method, ISO 2470, states that the UV radiation of the illumination must correspond to the relative amount of UV in the standard illumi¬nant C when measuring fluorescent objects.
The opacity is a measure of light-tightness. It is defined in ISO 2471 (1998) as the ratio of the reflectance factor R0 to the reflectance factor R’ . Both reflectance factors are determined in accordance to DIN 53 145 (2000). R0 is measured as the reflectance factor of an individual sheet on a completely black background and R’ as the reflectance factor of an “infinitely” thick stack of the same paper.