The sensitivity of the resonance-fluorescence method for the measurement of very-low sodium-vapor densities (<10<sup>10</sup> atoms/cm<sup>3</sup>) has been studied. Light from a sodium spectral lamp is sent through a sodium-containing vessel, whose temperature can be varied. The vapor density has been measured in the region of 47–146 °C. At 47 °C, the vapor density is about 10<sup>7</sup> atoms/cm<sup>3</sup> . The measurements were performed with both a photon-counting device and a lock-in amplifier. The experimental results have been normalized to absolute density data given in the literature. Fitting our points to the equation log<i>N</i> = <i>a</i> + <i>b</i> / <i>T</i> yields the values <i>a</i> = 23.715 and <i>b</i> = -5257.7 <i>K</i>.
F. C. M. Coolen, L. C. J. Baghuis, H. L. Hagedoorn, and J. A. van derHeide, "Measurement of very-low sodium-vapor densities by means of optical resonance fluorescence," J. Opt. Soc. Am. 64, 482-484 (1974)