Emission spectrum of a sodium street light

I took a photograph of an orange sodium street lamp through a diffraction grating. If you view a white light through a diffraction grating you see a full rainbow spectrum. In my picture, the street lamp is at the top of the picture and the spectrum is vertically below it. The yellowish blob is very prominent and that means it has a high intensity in this spectrum. But above it are smudges of green, turquoise and purple; below is a hint of red. All of these colours are separated by gaps. It is not a continuous spectrum. The lamps work by firing a beam of electrons through a thin sodium vapour. The electrons collide with the sodium atoms. This causes the electrons within the sodium atoms to gain energy and jump several electron levels. They fall back to their original level and emit photons of light as they do so. These are the colours that we see. Since there are fixed levels there are gaps in the colour scheme that we see. A better photograph is towards the bottom of this page http://www.chemistryland.com/CHM107Lab/Exp7/Spectroscope/Spectroscope.html