Answer:
Lighter
Step-by-step explanation:
In practice, the lamp is powered by an AC voltage source which is connected in series with an inductive ballast in order to supply an approximately constant current to the lamp, rather than a constant voltage. This helps to provide a stable operation. The ballast is usually inductive rather than simply being resistive to maximize resistive losses.
As this lamp current tending to zero-current point and shadow cast by the sodium flame becoming lighter.