I. Luminescence (луминисценция) is a process of emission of light. It is a form of cold body radiation and therefore usually occurs at low temperatures. Thus, it differs from incandescence where the emission of light results from a high temperature. The main types of luminescence occur by the absorption of radiant or corpuscular energy in the form of photons and the radiation of the respective ones, thus causing light. Other ways, in which luminescence occurs is through chemical reactions, electrical energy, subatomic motions, or stress on a crystal.
II. Types of luminescence
1. Photoluminescence (фотолуминисценция) is one of the main types of luminescence. In this process a specific substance absorbs a photon and then re-radiates a respective photon. This occurs through photoexcitation, which is the excitation (възбуждане) of an electron to a higher energy state in the substance by the absorption of the particular photon. Afterwards the electron goes through electron relaxation and returns to the lower energy state by radiating a photon. In the simplest form of photoluminescence a photon is absorbed and immediately after that, up to 10 nanoseconds, another photon with an equivalent wavelength is radiated. There are also more complex processes, in which in-between the absorption and emission the chemical substrate undergoes internal energy transitions and the time for the radiation of a photon, becomes longer and the photon itself has a different wavelength.
2. Fluorescence (флуоресценция) is a type of photoluminescence, in which the photon, released by the substance, has a longer wavelength than the wavelength of the photon initially...