Spectral evolution of a sodium lamp

The picture above shows the dramatic evolution of the light emission spectrum of a Philips SDW-T 100W when run from 40 to 300 % of its nominal wattage. To this end I used banks of 0.9 A SOX transformer ballasts coupled to a variac in order to obtain a stable discharge operation over an extremely wide electrical characteristics range. During this test the light color of the lamp changed from orange to blue due to the increasing self-absorption from sodium vapor, which is really extreme at 300 W. The light color turns blue because most of this absorption occurs around sodium's resonant D lines at 589.0 and 589.6 nm, in the orange part of the spectrum. Since the most efficacious part of the spectrum (i.e., around 555 nm) is removed by absorption, the lamp's lumen output consequently drops very significantly as the sodium vapor pressure increases.