here is the spectral output of a Cree XM-L Cool White LED (typical of white LEDs of the same colour temperature).
http://www.canreef.com/vbulletin/pic...pictureid=5004
As can be seen, from a purely spectral point of view, converting all sodium lamps to LED would be disastrous in terms of light pollution.
However this will not be the case, because an LED can be very easily and highly efficiently controlled in terms of optical spread. This will be the key difference, with high optical efficiency luminaires produced, as opposed to the nasty streetlights currently used which throw mercury/HP sodium light all over the joint.
Inevitably there will be back scattering, but IMO, LED will certainly reduce LP overall.
Because of the broad spectrum though, it will be impossible to filter out completely, unlike LP sodium or mercury vapour which produce light at fairly specific wavelengths.
Generally also agree with Sharkbite's post as a fellow electrical engineer
