Editing Tunable diode laser absorption spectroscopy (section)

=== Velocity measurement ===
The effect of a mean flow of the gas in the path of the laser beam can be seen as a shift in the absorption spectrum, also known as [[Doppler effect|Doppler shift]]. The shift in the frequency spectrum is related to the mean flow velocity by,

:<math>\Delta\tilde{\nu}_{D} = \frac{V}{c}\tilde{\nu}_{0}\cos\theta </math>
where,
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:<math>\theta</math> is the angle between the flow direction and the laser beam direction.
Note : <math>\Delta\tilde{\nu}_{D}</math> is not the same as the one mentioned before where it refers to the width of the spectrum. The shift is usually very small (3×10<sup>−5</sup> cm<sup>−1</sup> ms<sup>−1</sup> for near-IR diode laser) and the shift-to-width ratio is of the order of 10<sup>−4</sup>.