Editing Tunable diode laser absorption spectroscopy (section)

=== Concentration measurement ===
The basic principle behind the TDLAS technique is simple. The focus here is on a single absorption line in the absorption spectrum of a particular species of interest. To start, the wavelength of a [[laser diode|diode laser]] is tuned over a particular absorption line of interest and the intensity of the transmitted radiation is measured. The transmitted intensity can be related to the concentration of the species present by the [[Beer-Lambert law]], which states that when a radiation of [[wavenumber]] <math> (\tilde{\nu}) </math> passes through an absorbing medium, the intensity variation along the path of the beam is given by,<ref>See Bernath, Peter F. (2005), C7§6 p.272–274.</ref>

:<math>I(\tilde{\nu}) = I_{0}(\tilde{\nu}) \exp(-\alpha(\tilde{\nu})L) = I_{0}(\tilde{\nu}) \exp(-\sigma(\tilde{\nu})NL)</math>
where,
<!-- The \! is to keep the formula rendered as PNG instead of HTML. Please don't remove it.-->
:<math> I(\tilde{\nu}) </math> is the transmitted intensity of the radiation after it has traversed a distance <math> L </math> through the medium,
:<math> I_{0}(\tilde{\nu}) </math> is the initial intensity of the radiation,
:<math> \alpha(\tilde{\nu}) = \sigma(\tilde{\nu})N = S(T)\phi(\tilde{\nu}- \tilde{\nu}_{0}) </math> is the absorbance of the medium,
:<math> \sigma(\tilde{\nu}) </math> is the absorption cross-section of the absorbing species,
:<math> N \!</math> is the [[number density]] of the absorbing species,
:<math> S(T) \!</math> is the line strength (i.e. the total absorption per molecule) of the absorbing species at temperature <math> T </math>,
:<math> \phi(\tilde{\nu}- \tilde{\nu}_{0}) </math> is the lineshape function for the particular absorption line. Sometimes also represented by <math> g(\tilde{\nu}- \tilde{\nu}_{0}) </math>,
:<math>\tilde{\nu}_{0}</math> is the center frequency of the spectrum.