Quantitative Emission and Absorption

Abstract

We are ready to begin discussion of quantitative emission and absorption spectroscopy, with the goal of being able to specify emission and absorption as a function of wavelength. Two steps are involved in this treatment. In the first step, a simple form of the equation of radiative transfer will be used to identify a new parameter, known as the spectral absorption coefficient, which will be seen as the governing quantity which characterizes emission and absorption of light, as a function of wavelength. In the second step, the Einstein theory of radiation is employed to show that the spectral absorption coefficient is given simply by the product of the line strength and a lineshape function; the relationship of the line strength to fundamental quantities known as Einstein coefficients is also shown. With these relationships in hand, it will become evident how spectrally resolved absorption (or emission) can be used as a nonintrusive means of measuring a variety of gasdynamic parameters, including: species concentration, pressure, temperature, density, and even flow velocity.