Commercial instruments for slit-scanning densitometry, such as Fig. 17, share several features in common [9,53–55,64]. They usu-ally allow measurements in the reflectance mode by absorbance or fluorescence. Different sources are required to cover the entire UV–visible range from 190 to 900 nm. Halogen or tungsten lamps are used for the visible and deuterium lamps for the UV region.High-intensity mercury or xenon arc sources are preferred for fluorescence measurements. A mercury line source is also used for wavelength calibration of systems containing a scanning monochromator. Most instruments employ grating monochromators for wavelength selection and spectrum recording in the absorption and fluorescence excitation modes.