Can you shine a light?

Doing some private research into the suitability of various low-energy lamps, I investigated the current thinking on the specific output of incandescent lamp bulbs.

I found the following in Wikipedia. ‘A standard 100W incandescent light bulb emits approximately 1,700 lumens in north America and around 1300 lumens in 220V areas of the world.’

Does anyone know why this should be, please? Has any research been done into the optimum voltage for the best efficiency of incandescent lamps?

Brian Hammond