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Photon Multiplication in the Visible Spectrum. Twenty years ago Vavilov put forward the suggestion that photoluminescence with a quantum yield ~ > I might be possible in condensed phases [1]. Several attempts [2][3][4] were made at the time to demonstrate this effect, but they proved unsuccessful [3, 4].In 1963 in studying the elementary mechanisms of ionic crystal radioluminescence II'mas, Lushchik, and Liid'ya detected photoluminescence of activated ionic crystals with ~ > 1 [5, 6]. The effect of transformation of a single quantum of ultraviolet radiation into two or three quanta of visible light was called "photon multiplication in the optical spectrum. " In subsequent studies photon multiplication was detected in many other phosphors [7][8][9][10][11][12][13][14]; in addition, the mechanisms of photon multiplication in solids we re investigated [6,[13][14][15][16].Photoluminescence with a quantum yield ~ > 1 is in principle obtainable in many different ways with the aid of crystal phosphors [14]. We shall not consider all the possibilities. In the cases actually investigated, photon multiplication arises when a single photon produces two electron-hole pairs [5, 6], an electron-hole pair and an exctton [6,17], or an electron-hole pair and an excited impurity center [12, 13]. The first of these methods (which we shall call the "electron-hole" method) and also the third (the "impact" method) are particularly promising. The electron-hole mechanism is universal and can, in principle, occur in any solid capable of recombination luminescence. The impact mechanism has received little attention so far. its realization requires high concentrations of impurity centers and long electron paths in the crystal lattice. Of the photon multiplication mechanisms as yet unrealized it is necessary first of all to take note of the theoretical possibility of sensitization energy transfer from one sensitizer ion to two activator ions [18]. Cascade radiation in crystals activated by rare-earth ions [19] is apparently also of some practical interest. The Use of Photon Multiplication in Light Sources. Modern lighting engineering is based on the extensive use of incandescent, fluorescent, and luminescent lamps. Incandescent lamps have an energy yield p = 0.03-0.04 and a luminous efficiency of 10-20 lm/W. Luminescent mercury daylight lamps have an energy yield p = 0.22 and a luminous efficiency of up to 75 lm/W. Vavilov noted on many occasions that the future of lighting engineering lies in the wide and effective use of luminescent light sources.In all existing luminescent lamps the quantum yield ~? of the luminophor is smaller than unity. It was Fabrikant who first drew attention to the theoretical possibility of producing luminescent lamps without, however, indicating an effective means by which this might be done [19]. On detecting the electron-hole mechanism of photon multiplication in the optical spectrum [5, 6], Lushchik and Fedorov proposed a mercury luminescent lamp with photon multiplication [20]. A neon discharge is best suited...

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