Envelope blackening effects in vacuum incandescent lamps

The evaporation of tungsten from the filaments darkens the envelopes of vacuum incandescent lamps more or less evenly as there is no gas fill to move the tungsten molecules convectively.

[@albear972]

Those are some very weird/unusual wattages for those light bulbs. I have *never* seen 33W or 405W. What were those intended to be used for?

[@randacnam7321]

33W lamps are/were commonly used for signs. 405W lamps are/were for lumen rated incandescent street lighting, where the lamp power is dictated by the design luminous flux and design life. They were available in everything from a 32W 200lm lamp all the way to a 860W 15,000lm lamp. Both of these were designed for 3,000hr.
The longer a lamp's design life, the higher its operating power. The 2,500lm lamp class (one of the most common sizes) would use 175W, 189W, 202W or 205W depending on if it was designed for 1,500hr, 3,000hr, 6,000hr or 12,000hr respectively. The 405W lamp in the video has a 3,000hr design life and has an equivalent 448W 6,000hr lamp.

[@Muonium1]

I had a few cheap Chinese "Edison style" incandescents in a dining room fixture I replaced with LEDs recently and noticed one of the bulbs was substantially more darkened than the others despite being in the same fixture and obviously having the exact same run time. I wondered if the gas fill on the darkened one was faulty and if it was essentially a vacuum. Touching a high frequency handheld high voltage probe to the bulbs in succession with a Geiger counter nearby immediately reveals the answer - only the darkened bulb produced x-rays.

[@AlexanderCrump]

I had a bunch of 56W Philips Street Railway vacuum lamps in service in a chandelier for over a decade and they had darkened a lot. But I love how vacuum lamps ramp up (luminesce) and darken (nigresce) slowly.