  Experimental apparatus and measurement technique are described for precision
absorption measurements in sodium - noble gas mixtures. The absolute absorption
coefficient is measured in the wavelength range from 425 nm to 760 nm with 2%
uncertainty and spectral resolution of 0.02 nm. The precision is achieved by
using a specially designed absorption cell with an accurately defined
absorption path length, low noise CCD detector and double-beam absorption
measurement scheme. The experimental set-up and the cell design details are
given. Measurements of sodium atomic number density with 5% uncertainty
complement absorption coefficient measurements and allow derivation of the
reduced absorption coefficients for certain spectral features. The sodium
atomic number density is measured using the anomalous dispersion method. The
accuracy of this method is improved by employing a least-squares fit to the
interference image recorded with CCD detector and the details of this technique
are given. The measurements are aimed at stringent testing of theoretical
calculations and improving the values of molecular parameters used in
calculations.
