In this work the methods for testing of the optical properties of the paper substrates are presented. Many complex phenomena such as reflection, refraction, diffraction, scattering and absorption occur when the light strikes the paper surface. All these interactions must be under the standard conditions examined, and because of that, the knowledge about the testing methods of the particular optical properties of the paper substrates is very important. The measurements of the reflectance are very important for calculating of the optical paper characteristics. From reflectance factor one can calculate Y-value, brightness and opacity. Within the paper optics, ISO standards and TAPPI methods are dominant procedures from wich every one of them processes one property of pulp, paper or board by using the sources of light and the optical geometry precisely defined. The test method which defines the measurements of the scattering and absorption coefficients by Kubelka–Munk theory is one of the significant methods within the paper optics field examinations. The coloured paper substrates and the equivalent colour spaces, which, under today's conception, should be included in paper optics, are also discussed.