Filtered sodium lamp

Didymium glass contains a mixture of praseodymium and neodymium, which is interesting in the fact that it presents a broad optical absorption in the orange part of the visible spectrum. Such optical filter can thus be used to block sodium light, such as in the present case with a GEC-Osram SOX/H 90W, revealing some residual neon light excited in the sodium-dominated discharge. A close inspection of the discharge aspect at the electrodes shows that the anode is on top and the cathode is at the bottom. The cathode regime is recognizable for the plasma excitation inside the beehive electrode, resulting from charge multiplication there (i.e., hollow cathode effect), and the faint excitation of argon (in blue) by highly energetic electrons in the negative glow structure.