2.6 Spectrometer

2.6 Spectrometer

There are many kinds of spectral measurement devices ,for example, spectroscopes for human eye observation of the spectrum, spectrometer to record spectral reflectance, monochro meter to read a single narrow band, spectro photometer for photometry, spectro radiometer for measurement of spectral radiation etc. However, in this section only optical spectrometers are of interest.

Figure 2.6.1 shows a classification of spectrometers, which are divided mainly into dispersing spectrometers and interference spectrometers. The former utilizes prisms or diffraction gratings, while the latter the interference of light. (1) Dispersing spectrometer
A spectrum is obtained at the focal plane after a light ray passes through a slit and dispersing element as shown in Figure 2.6.2. Figure 2.6.3 and Figure 2.6.4 are typical dispersing spectrometers ; Littnow spectrometer and Czerny - Turner spectrometer respectively.

(2) Twin beam interference spectrometer
A distribution of the spectrum is obtained by cosine Fourier transformation of the interferogram which is produced by the inference between two split rays. Figure 2.6.5 shows Michelson interferometer which utilizes a beam splitter.

(3) Multi-beam interference spectrometer
The interference of light will occur if oblique light is incident on two parallel semi-transparent plane mirrors. A different spectrum is obtained depending on incident angle, interval of the two mirrors and the refraction coefficient.