Numerous spectra were registered after the development of various optical spectrometers in the first half of 19^th century, after the simple theory introduced by Niels Bohr for the hydrogen atomic spectrum, registered by Balmer in 1885.

Different spectroscopic techniques were proposed on the basis of limited frequency ranges depending on the processes that are involved and magnitudes of energy changes associated with these processes as shown in Figure 34.06.

Figure 34.06: Range of frequency, wavelength and energy and techniques involved in analysis.