In 1800, luminescence was still an enigma. Of course, the phenomenon itself was known and had been described several times. Phosphorescence has been known of since 1602, when alchemist Vincenzo Casciarolo produced the “Bologna Stone”, a sulphate able to emit light in the dark (Oliver Sacks speaks of it Uncle Tungsten). In 1800, a number of important discoveries were made and a series of major theories drafted to explain it, but in vain: the phenomenon evaded all explanation. When light understood as a wave was applied, a host of unbridgeable theoretical contradictions emerged. And then quantum physics and Albert Einstein arrived on the scene.
The father of the theory of relativity in 1905 proposed a theory that overcame the problems posed by George Stokes' rule, that stated that “through the principle of the conservation of energy, the frequency emitted by a luminescent substance cannot be greater than that of the incident light”. He hypothesised that light was composed of discrete particles of energy (photons or light quanta) and that luminescence was a physical phenomenon consisting of the emission of photons of light that is visible or invisible (to the human eye) by materials excited by causes other than an increase in temperature. According to Einstein, luminescence was a consequence of some materials absorbing given quantities of energy (quanta), returning some of in the form of photons in order to return to the former energetic state.
Having explained the phenomenon, use began to be made of it to exploit and apply its effects in a number of fields, for the purposes of both explanation and production. Thanks to Einstein's theory, it has been possible, for instance, to understand bioluminescence, which is to say the luminescence given off by some creatures. In the firefly, to give just one example, it was noted by Raphael Dubois in 1887 that it is triggered by the reaction between an organic substratum – luciferin – and a catalytic enzyme called luciferase. The mechanism of the reaction remained unknown until the 1910s, when it was established how, excited, the luciferase emits electrons which, returning to a lower level of energy, emit photons to return the system to the same level of energetic equilibrium as before. Since 1975, the enormous potential of bioluminescent systems in clinical chemistry became evident, to replace the conventional techniques such as colorimetry, fluorimetry and radioimmunoassay, offering greater sensitivity, linearity of response and reduced interference.
And in chemistry too, there is no lack of phenomena of luminescence (chemiluminescence). An example of this is the reaction of luminol with hydrogen peroxide and a metallic catalyst: the TV soap CSI shows us what it's used for: this type of reaction is employed in forensics to establish the presence or lack of human blood, given that the iron contained in haemoglobin “lights up” the luminol. In the same way, it is possible to reveal traces of nerve gases and pesticides. In order to discover their presence, it is necessary to irradiate them with a Wood lamp or black light – an invention by William H. Byler – which emits ultraviolet rays, and which is similar to a fluorescent lamp with the difference that the glass tube, called Wood's tube from the name of its inventor, is not lined with fluorescent but with blue or purple powder using nickel oxide. By inducing fluorescence (luminescence that lasts for the time of application of the light source) and phosphorescence (a luminescence that lasts longer than this application), it is also possible to reveal traces of organic matter andfungi, identify hidden symbols and signs in palaeography, verify the authenticity of banknotes, create effects of fluorescence on clothes (bleaching is often done using fluorescent products), eyes and teeth (containing fluorine) in discos.
But let's come back to luminescence. Just to conclude that artists too are interested in it. To mention just one: Paolo Gioli, the master of faces that appear in the dark.
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Illuminated dials
Initially, the luminescence of markers and hands was the consequence of the radioactive emission of Radio-228, which energises the zinc sulphate in the compound and produces an emission of light called radio-luminescence. In the 1990s, new photo- luminescent compounds were created in Switzerland and Japan based on a class of aluminates activated by lanthanides (the 15 chemical elements in the periodic table located between lanthanum and lutetium). These offer a greater intensity of light and duration of luminescence than the earlier sulphate-based ones.
