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.

Nice experiment! The sodium vapor in HPS lamps has a very significant absorbance, which can even be clearly seen under white light with hot Philips SDWs, in which case the inner volume of the burner appears blueish-green! Since the sodium vapor temperature decreases quickly after the arc switch off, the optical process at play is similar to that happening in SOX lamps. Line and quasi-molecular broadening effects become dominant at high temperature, primarily during the HPS lamp's operation. In that case the net emission spectrum is the result of the broadened optical emission from the arc core filtered by the (less) broadened absorption profile of the cooler outer mantle. That's a really interesting process, well described in the classic book by de Groot and van Vliet (The high pressure sodium lamp, 1986).

I bought an Iwasaki NHT150SDX some time as it also demonstrates this behaviour under white light. I think these lamps run at a higher pressure so exhibit this behaviour well. I guess this is also the case with the Philips SDW. Quite amazing to witness.