Interesting. Seen SOX under SOX before, but never tried it with SON.

Thanks, Yes, I have often played with low pressure sodium lamps too and it fascinates me that low pressure sodium looks like a yellow fog under its own illumination. It is even more mind bending when you realise this is resonance, with the light causing absorption and re-emission just as a bell will start to ring quietly if subjected to the same sound as its natural frequency.

In the SON/T lamp the sodium is at a higher pressure and low-pressure double lines from the lamp fall totally within the broadened absorption band of the HPS lamp thus absorbing all the incident light and appearing black.

I’m sure Max can extend this explanation as I think absorption broadening ends up in a more complex explanation.