Subject: Why is there a Na=D absorption line in high pressure Na vapor street light?

Looking at a sodium vapor street light with a spectrometer, the assumption is 
that there will be a major brightness in the yellow band.  For an actual lab 
activity, there are several bands of frequencies giving a more natural 
balanced color rendering with the exception that there is a clear 
dark/absorption line at D.  My assumption is that a cool film of Na collects 
along the inside surface reabsorbing the emission D.  Yes?

Re: Why is there a Na=D absorption line in high pressure Na vapor street light?