Double Ended High Pressure Sodium

Here’s a curious ‘horticultural’ lamp displaying the interesting UV phosphorescence. The inner PCA tube glow is completely swamped by the outer envelope fluorescing with excitement from the UV being emitted. Not quite sure why they have used a UV blocking jacket considering how little UV is emitted from a HPS lamp after run up. The lamp is double ended HPS and rated at 600W. The arc tube is thinner and about 2cm longer than a Sylvania 600W Growlux lamp I have in comparison. In total, the lamp is over 33cm. Strangely, the lamp is marked 400V but also have 250V 50Hz stamped on it. It claims to be suitable for magnetic or electronic ballasts and is dimmable.
It appears to run fine on the electronic ballast I tested it with, so I don’t know where the 400V rating comes from. The output from my ballast is 400v peak-peak and is square wave. But that infers a 200V supply.