Unlike polychromators monochromators are used to select a single wavelength of an emission spectrum. When recording of a larger part of the spectrum is required they are operated in the sequential mode, i.e. the intensity of one wavelength at the time is measured. This can be done in either the hopping mode, jumping from one wavelength of interest, such as an emission line of an atom, to the next. A different mode of operation is the scanning mode where are selected area of the spectrum is sequentially acquired by tuning the monochromator. Monochromators are an interesting option when high spectral resolution and light throughput is required. When designing a polychromator for the simultaneously acquiring large parts of the optical spectrum compromises have be to be made in terms of the lay-out of the optical components. Monochromators need be optimised for only one wavelength at the time. This allows the use of larger gratings, mirrors etc leading to a better optical qualitiy, at the price of lower speed.
A compromise to combine the optical quality of a typical monochromator with the possibility of simultansesouly acquiring segment of the spectrum can be acchieve by replacing the combination of the exict slit and the detector by a array detector, typically a diode array or a CCD-chip.
Author: Thomas Nelis, EMPA Materials Science and Technologoy, Thun, Switzerland.