Published on the Italian Gemological Review, issue N°1, May 2017
UV-Vis-NIR (Ultra Violet-Visible-Near Infrared) Spectroscopy allows to analyse the interaction between matter and electromagnetic radiation in the Ultraviolet, Visible and Near Infrared wavelength range.
The spectral range of these instruments, depending on the model, can go from 200 nanometers (nm) of the C-Ultraviolet until 1100 nm of the Near Infrared and it’s quite broader than the visible region (400-700 nm) available by using the optical spectroscope.
The theoretical principles, which will not been elaborated in this article, refer to the optical spectroscopy: every single material subjected to radiation reacts in a specific way by absorbing portions of such radiation. If these absorption bands are presented as a chart, they can provide an effective material identification method. Absorption and transmission are directly linked to the chemical composition and crystal structure of the gem. About the visible spectral range, when comparing directly observed spectra we are familiar with to the ones recorded by spectrophotometers, it is important to underline that the sensitivity of human is not constant for the whole visible range. The sensitivity of the human eye is at maximum at about 550 nm and gradually decreases towards both longer and shorter wavelength directions. This fact leads to reduced perception of the bands closer to the visible range ends, while the CCD sensors equipping the modern UV-Vis-NIR units perform in a far more efficient way.