3.2 - Analysing Lenses with the XRF Technique
Not all of the components of glass can be detected by measuring transmittance. To verify the presence of some of them - including those of optical importance, such as lead - other analyses must be conducted. The X-ray fluorescence technique (XRF) detects the elements present in a material through the so-called "photoelectric effect".
The material to be analysed is irradiated with a flow of X-rays. When a ray strikes an atom, it displaces an electron close to the nucleus (ionization). Instantaneously, another electron fills the gap created, passing from a higher energy level to a lower one. In the transition, the electron liberates the difference in energy under the form of another X-ray, which is however characteristic of the particular atomic element that has been struck. The X-rays produced in this way can be recognised and counted with a special detector.
The elements present in the irradiated zone can be identified by examining the energy curves of the X-rays emitted. In this way it has been discovered that the glass used for telescope lenses in the early 17 th century was tendentially lead-free and that, moreover, it did not differ greatly from the glass used in other products.
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