Scientist Dr. Engel describes the principle as follows: “When optical materials are exposed to light, and especially UV light, this light is partly absorbed. In some cases, luminescence – i.e. a distinct brightness – is observed. We can attribute this characteristic luminescence to very specific and unwanted impurities.“
Additional elements or impurities in optical materials are already decisive in ranges of less than millionth ppm – a detection limit that until now has been reserved for chemical analyses. However, there have always been serious disadvantages. For one thing, this method is time- and cost-intensive, and for another, it cannot be performed without damaging the samples. This means that the samples in question cannot be used after the analysis.
Fluorescence spectroscopy, in contrast, offers distinct advantages. In concrete terms, this means a representative material sample is taken from a production batch to determine the fluorescence. For this purpose, it is exposed to the light of a 450-watt xenon lamp in the sample chamber of the spectrometer. This kind of excitation source emits light both in visible and in UV ranges. Because of the generally low level of impurity of optical glasses and crystals, a light source of extremely high intensity must be used in order to produce the luminescence. Optical components filter out the characteristic lines from the light emitted from the xenon lamp. The light absorbed in the sample and converted into fluorescent rays is dispersed in a second monochromator to provide the instrumental evidence.
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