We have developed a method for determining metal‐atom number‐density integrated over a line‐of‐sight (i.e., the column density) by measuring the optical depth in the wings of the pressure‐broadened resonance lines. This method is capable of monitoring the column densityinsituduring photoelectric absorption measurement of metal vapors in a synchrotron radiation beamline. The present method requires measuring the reduced absorption coefficients of the metal–metal and metal–buffer‐gas collisional quasimolecules &ggr;MMand &ggr;MGand the effective vapor lengthlbefore its use. We thus describe also the procedure for obtaining &ggr;MM, &ggr;MG, andlusing He‐buffered Yb vapor as a specific example.