The visual appearance of paper is a combination of its colour, whiteness and gloss. It plays an important role when customers choose between different paper grades.
Optical homogeneity can also be important in evaluating a paper product. For end users, colourful printed images with correct colours, proper gloss and low disturbing unevenness are important. Opacity is a critical parameter in preventing disturbances by back-side printing or printing from underlying pages. The competition from digital media and rising material and energy prices are forcing the paper and graphical industry to manufacture graphical products as efficiently as possible. Paper optics has a special role, because established simulation and measurement methods (see Optical properties of paper) make it possible to design graphical products for optimum performance.
There is today a better understanding of gloss, as surface evaluation methods have improved. Basic theories can be applied to explain some of the phenomena, but research is still needed to fully understand and describe how a rough surface influences specular reflections. The principles of optics for paper surfaces are discussed in Principles of optics for paper surfaces and the interaction of light and reflectance with rough surfaces is discussed in Specular reflection from a rough surface.
The basic optical properties of the paper surface — light scattering and light absorption — determined with the aid of the Kubelka-Munk theory are very helpful in understanding the different processes in pulp and paper manufacturing. Spectral determination of these properties can provide valuable information on the components and internal structure of paper. Simulation is another successful implementation of the Kubelka-Munk theory. With simulation it is possible to predict the optical properties of different materials used in a specific product.
However, the long history of the Kubelka-Munk theory has exposed a number of shortcomings in its use. By going back to the basics, i.e., the radiative transfer theory, and by applying it to paper optics with fewer restrictions, the potential offered by the Kubelka-Munk theory can be expanded. Using other methods, such as Monte Carlo simulation, the optical responses of the fibre structure and coating layers can be calculated in greater detail than before.
Spectrophotometers are used in the paper industry and have also penetrated the graphical industry. Colour management aiming at the same colours for computer screens, digital cameras, printed images etc. is a global focus area today, involving the research of numerous colour scientists. The paper industry strives to use a colour system recognised by colour management consortia in order to facilitate international colour management work, but different instruments and calibration procedures still need to be considered. Colour management would be easier if some of the testing could be replaced by simulation. The optical interaction between ink and paper is thus an important research topic in modern colour science. Colour management efforts, the access to new theoretical algorithms and the strong evolution of new optical instruments make paper optics a dynamic research area.
Measurement and characterisation of the optical properties of papers and boards are discussed in section Optical properties of paper under the theme Material testing and product properties.