  We report on a new optical implementation of primary gas thermometry based on
laser absorption spectrometry in the near infrared. The method consists in
retrieving the Doppler broadening from highly accurate observations of the line
shape of the R(12) $\nu_{1} + 2 \nu_{2}^{\phantom{1}0} + \nu_{3}$ transition in
CO$_{2}$ gas at thermodynamic equilibrium. Doppler width measurements as a
function of gas temperature, ranging between the triple point of water and the
gallium melting point, allowed for a spectroscopic determination of the
Boltzmann constant with a relative accuracy of $\sim1.6\times10^{-4}$.
