[ridgeguy]

It seems like this could be done differently, and perhaps more cost-effectively. Can't give cites right now, but here's a path I'd explore if I were in the field.

I'd pattern the inner surface of the glass envelope with a cube texture - think of taking a cube and pressing a corner normally into a clay surface, then removing the cube. This pattern is a so-called corner reflector, and returns incident light to its source. Figure the cube indentations at about 0.5mm deep, close packed. I'd deposit a dielectric film reflector stack tuned to reflect most infrared radiation onto this surface.

This combination would transmit visible light, but would reflect IR directly back to the filament, reducing the amount of electrical power needed to maintain filament temperature. Glass textural molding and dielectric film deposition are mature technologies. I think this could readily triple incandescent lamp power efficiency, maybe even better.