18 января, 2016 
Pokraskin Contrary to common belief, luminescent materials have been known and used for about 2000 years. Already in ancient times, the Romans used luminescent materials for hair decoration [5.199]; the material used is thought to be CaS [5.200]. Also other luminescent materials were found in nature (e. g. naturally doped willemite, scheelite or calcite crystals). Around 1600, the Stone of Bologna attracted Galilei’s interest. This barite mineral (BaSO4) emits yellow to orange light with long persistence when subjected to sunlight. Although Galilei did not know the physical origin, he definitely excluded mystery as the origin of the phenomenon. In 1671, by heating the mineral with carbon black, Kirchner was able to intensify the luminescence, indicating that not BaSO4, but an impurity-type luminescence of BaS is the origin of the light emission [5.201, 5.202]. One of the first reports on the synthesis of a luminescent material originates from 1603: luminescent BaS was obtained by Casciavolis by reducing minerals in an attempt to obtain gold [5.199]. As in ancient times, luminescent materials in the 1600s were only used for decorative purposes.
At the end of the 19th century, a first major milestone concerning luminescent materials and devices was the realization of gas-discharges and electron beams in evacuated glass tubes by Geissler and Braun as well as the discovery of X-rays by Rontgen [5.203, 5.204]. The visualization of gas-discharges, cathode rays or X-rays represents a first practical use of luminescent materials beyond decoration. Based on these fundamental results, the first luminescent devices were developed only a few years later, including cathode-ray tubes, fluorescent lamps and X-ray intensifying screens (see Section 5.5.5.1) [5.205-5.207]. Thereafter, research on cathode-ray tubes and the relevant phosphors for use in radar screens was very intense during the 2nd World War. In addition, the rapid and successful application of fluorescent lamps for illumination purposes and the use ofX-rays for medical imaging stimulated research on novel luminescent materials in the first half of the 20th century.
The first luminescent devices contained only a single phosphor material. The emitted light was normally not white, but colored. For instance, blue emitting CaWO4, which is a highly efficient phosphor, was used in X-ray intensifying screens, in Braun’s early cathode-ray tubes as well as in the first fluorescent lamps by Edison [5.208]. Lighting became more colorful with so-called Zeon or Neon tubes. Based on Ne/Ar/Hg discharges and in combination with different phosphors and colored glass, differently colored lamps were realized. These lamps were presented by Claude at the World Exhibition 1937 in Paris and used for advertising [5.209]. For illumination purposes, however, emission of white light is required. Based on a single phosphor, generation of white light is possible if emission is more or less continuous over the whole visible spectral range. In line with this, Meyer invented a lamp in 1926 [5.210]. However, technical application did not start until 1938 [5.211, 5.212]. Such singlephosphor lamps containing halophosphates like Ca5(PO4)3(Cl, F):Sb3+, Mn2+ are still in use. Although the physically determined efficiency of such broadband emitting phosphors is high, the lumen equivalent of their emission, taking the sensitivity of the human eye into account, is low. This is especially due to red emission beyond 610 nm, where the eye sensitivity rapidly falls off with increasing wavelength. As suggested in 1971 by Koedam and Opstelten, the use of rare-earth-element-based luminescent materials marked a second major milestone [5.213, 5.214]. Based on line-type f-f transitions, phosphor emission can be narrowed to the visible, resulting in both high efficiency and high lumen equivalent. However, due to line-type emission color rendering (the ability to reproduce all colors in a natural way) is now low. Consequently, at least three phosphors with emission in the blue, green and red spectral range have to be combined. Nowadays, such tri-color phosphor mixtures are used in fluorescent lamps worldwide. Similar considerations regarding lumen equivalent were also of importance in increasing the lumen efficiency of color TV tubes (see Section 5.5.5.1).
Although inherently incomplete, it is of interest to review briefly some of the most important theoretical aspects of luminescence. In general it can be stated, that most of the processes, leading to luminescence, are understood well nowadays. A very
fundamental observation is known as the Stokes shift [5.215]. According to this, the emitted photons generally have lower energy than the radiation used in exciting the luminescence. Seitz and Mott rationalized the occurrence of the Stokes Shift in 1938 and 1939, after they developed the configuration co-ordinate diagram [5.216, 5.217]. Already before the 2nd World War, it was also recognized that luminescence is often due to rather low concentrations (sometimes impurities) of certain metal ions
[5.218] . The relevant optical transitions leading to absorption and emission can be understood within the concept of orbital theory. Here, luminescence of transition metal ions can be explained based on ligand field theory and Tanabe-Sugano diagrams
[5.219] . Luminescence of rare earth ions can be understood, based on transitions between (almost) atomic eigenstates of the system [5.220, 5.221]. Forster and Dexter first described energy transfer between localized centers in luminescent material [5.222-5.224]. Besides orbital theory, semiconductor theory has also contributed to the understanding of radiative transitions: Both band-to-band transitions and transitions involving localized donor and/or acceptor states fit within this framework. Nevertheless, there are also still open questions concerning the theoretical aspects. For instance, the efficiency of luminescent materials is not determined well. Only in the case of excitation with high-energy radiation (e. g. cathode rays), can the maximum energy efficiency be calculated rather accurately, using a surprisingly simple treatment [5.225]. In contrast, loss processes (which do not result in luminescence) cannot yet be predicted well quantitatively.
Today the most important producers of luminescent pigments are Honeywell and Nemoto.
5.5.3
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