Text 1A5-I

A critical component of spectrophotometers is the device used to select the appropriate wavelength. Several types of wavelength selectors are available, including filters, prisms, grating monochromators, and holographic gratings. The quality of these selectors is described by their nominal wavelengths, spectral bandwidths, and bandpass. Nominal wavelength represents the wavelength in nanometers at peak transmittance. Spectral bandwidth is the range of wavelengths above one-half peak transmittance. It is sometimes called the half power point or full width at half peak maximum (FWHM). The total range of wavelengths transmitted is the bandpass.

The figure below summarizes these characteristics of wavelength selectors and shows the difference in wavelength selectivity between interference and absorbance filters. The numbers 1, 2, 3 and 4 represent, respectively, (1) the wavelength of 550 nm, (2) the transmittance of 52%, (3) the range of wavelengths represented by the side of the rectangle indicated and (4) the range of wavelengths between 530 and 577 nm, as shown in the figure. The letters A and B represent the effect of two different filters, obtained in different experiments.