UV-Vis Spectroscopy It allows to characterize the absorption or reflection of a material in the ultraviolet (UV) and visible light (Vis) regions of the electromagnetic spectrum. The energy provided by the incident light induces an electronic transition in molecules including chromophores (e.g. double or triple bonds, unpaired electrons, d orbitals, etc..), which can be monitored in an absorption spectrum.
Absorbance/Transmittance spectra. Colored solutions typically absorb in the visible region (400-750 nm), whereas colorless chemicals containing suitable chromophores may absorb in the UV region (200-400 nm). Through the Lambert-Beer law, the absorbance is linearly proportional to the molar concentration of the chromophores. Using calibration curves is often possible to obtain the concentration of the sample from absorption spectra.
Reflectance spectra. These measurements are usually performed on solid (opaque) samples, since light cannot penetrate them. On such samples, incident light is reflected on the surface and can be either reflected symmetrically with respect to the normal line (specular reflection), or scattered in different directions (diffuse reflection). The sum of both is called total reflection.
Near InfraRed (NIR) Spectroscopy Even if it is usually performed with a UV-Vis-NIR spectrometer, it is an infrared spectroscopy technique, which allows to characterize the electromagnetic region from 780 to 2500 nm.l NIR spectra,in specific contests, can be used for film thickness measurements, for chemical characterization of coatings/substrates for optoelectronic applications, and sometimes also for water content monitoring.
Fluorescence Spectroscopy is complementary to absorption spectroscopy and investigates the long-term average fluorescence of a sample when irradiated with UV-Vis light. The fluorescence spectrum is a plot of fluorescence intensity vs excitation wavelength. Such a spectrum allows the determination of the presence of fluorophores and their concentrations.
