Non-uniform sources

In most laboratory sources the excitation temperature decreases towards the boundary of the discharge region. Consequently the absorption in this part of the source is increased and the profile of the emitted radiation is not only appreciably broadened but often shov/s a pronounced dip at the centre of the line. This effect is known as self reversal and it has been studied in detail by Cowan and Diecke (1948). It is particularly important that self reversal is avoided in the lamps used in the resonance fluorescence experiments described in Chapters 15-17, for the strengths of the signals are proportional to the intensity at the line centre frequency 

In the atmospheres of the Sun and stars the excitation temperature also falls from the centre outwards to the boundary of the photosphere. In the outer regions this temperature gradient is responsible for the absorption line spectrum (the dark Fraunhofer lines) which is superimposed on the continuous spectrum of the Sun. By measuring the equivalent widths of these absorption lines (see section 10.4), and by solving the equation of radiative transfer, it is possible to deduce the chemical composition and physical state of the stellar atmosphere. 

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