ABSTRACT
Tunable diode laser absorption spectroscopy (TDLAS) uses the absorption phenomena to measure species concentration and temperature. When light permeates an absorption medium as shown in Figure 6.1, the molecular concentration is in proportion to the strength of the transmitted light according to the Lambert Beer’s law. Atomic or molecular concentration is related to the amount of light absorbed, as in the following formula:[6.1],[6.2]
I I nλ κ
α
/ exp( )
exp( )
0 = −
= −
(6.1)
Here, I0 is the input laser light intensity, Iλ the transmitted light intensity at wavelength λ, κ the absorption coef—cient, n the species number density, and the path length. The absorption coef—cient κ is related to the line strength of the absorption transition. Thus, by measuring the attenuation of light that permeates an absorption medium containing atoms or molecules, their concentration can be ascertained. As seen in Equation (6.1), the absorption Iλ/I0 is not directly proportional to the species number density n. The absorbance ln(I0/Iλ) is often de—ned as
α λ= ln( / )I I0 (6.2)
The absorbance is directly proportional to the species number density n and also the path length . Cautious use of the absorbance ln(I0/Iλ) is necessary because log10(I0/Iλ) is usually used in analytical chemistry. Using the natural logarithm instead of the common logarithm to de—ne the absorbance is rather limited in the TDLAS applications.
