It's a question of temperature. Hotter filament gives more efficient and whiter light (the black body radiation has more visible and less infrared), but the ho

It's a question of temperature. Hotter filament gives more efficient and whiter light (the black body radiation has more visible and less infrared), but the hotter filament doesn't last as long (faster evaporation rate). It's perfectly possible for end users to use a dimmer switch to make incandescent lamps last much, much longer at the expense of less light and a "warmer" colour. Lifespan is very, very sensitive to the temperature.