Near ir LEDs are also cheap and easy to power, with its what's in remote controls. Low pressure sodium lights aren't. I remember using them in postgrad labs and

Near ir LEDs are also cheap and easy to power, with its what's in remote controls. Low pressure sodium lights aren't. I remember using them in postgrad labs and it wasn't fun. Alternatively you could use a solid state laser or narrow band LEDs if you need the band to isolate to be in the visible spectrum too.