  The theory of the thin vapor layers linear optical properties is presented
for the case of specular reflection of atoms colliding with the walls. The
effects of light absorption and the shift in the resonance frequency are taken
into account by means of self-consistent calculation of the field and
polarization in a gaseous medium. The obtained formulas reproduce the complex
dependence of the spectral line profile on the gas layer thickness and allow
one to determine numerically the exact values of the blueshift.
