Figure 7.5
Characteristics of standard Ilford filters for the
visible region.range transmission characteristics. It follows also that a very large number of different filters is needed.
As a result, filters are used only in unsophisticated instruments where their rather poor selectivity can be
tolerated. An alternative approach is to separate or disperse the radiation spatially by deflecting it
through different angles according to frequency using a monochromator. This device consists of a
prism or diffraction grating, focusing mirrors or lenses and a slit system. Polychromatic radiation
entering the monochromator is directed on to a narrow entrance slit and thence to the prism or grating.
By rotating the prism or grating, radiation of different frequencies and consisting of sharp images of the
entrance slit can be made to pass successively through a fixed exit slit and on to the detector. If the
output from the detector is fed to a recorder, a continuous trace of the spectrum is obtained.
Alternatively, a photographic film or plate or a series of exit slits and detectors positioned along the
focal plane of the monochromator enables radiation of many frequencies to be detected simultaneously
without rotating the prism or grating. One modern aspect is the use of a prism and diffraction in a
combination in Echelle Optical Systems to provide a two dimensional spectrum with better resolution
(p. 303).
Prism Dispersion
In many spectrometric instruments, the monochromator is based on prism optics (Figure 7.6). This is
particularly true of cheaper instruments. When a beam of polychromatic radiation passes through a
prism, the light is refracted from its original path. The higher the frequency of the radiation, the greater
will be the angle of refraction θ. It should be noted that the angular dispersion, dθ/dν does not vary
linearly with ν. This results in the