Since the UV-Vis range spans the range of human visual acuity of approximately
400 - 750 nm, UV-Vis spectroscopy is useful to characterize the absorption,
transmission, and reflectivity of a variety of technologically important
materials, such as pigments, coatings, windows, and filters. This more
qualitative application usually requires recording at least a portion of
the UV-Vis spectrum for characterization of the
optical or electronic properties of materials.
The light source is usually a deuterium discharge lamp for UV measurements and a tungsten-halogen lamp for visible and NIR measurements. The instruments automatically swap lamps when scanning between the UV and visible regions. The wavelengths of these continuous light sources are typically dispersed by a holographic grating in a single or double monochromator or spectrograph. The spectral bandpass is then determined by the monochromator slit width or by the array-element width in array-detector spectrometers. Spectrometer designs and optical components are optimized to reject stray light, which is one of the limiting factors in quantitative absorbance measurements. The detector in single-detector instruments is a photodiode, phototube, or photomultiplier tube (PMT). UV-Vis-NIR spectrometers utilize a combination of a PMT and a Peltier-cooled PbS IR detector. The light beam is redirected automatically to the appropriate detector when scanning between the visible and NIR regions. The diffraction grating and instrument parameters such as slit width can also change.
Most commercial UV-Vis absorption spectrometers use one of three overall
optical designs: a fixed or scanning spectrometer with a single light beam
and sample holder, a scanning spectrometer with dual light beams and dual
sample holders for simultaneous measurement of P and Po, or
a non-scanning spectrometer with an array detector for simultaneous measurement
of multiple wavelengths. In single-beam and dual-beam spectrometers, the
light from a lamp is dispersed before reaching the sample cell. In an array-detector
instrument, all wavelengths pass through the sample and the dispersing
element is between the sample and the array detector.