|MLA Citation:||Bloomfield, Louis A. "Question 181"|
How Everything Works 20 Jul 2018. 20 Jul 2018 <http://howeverythingworks.org/print1.php?QNum=181>.
Once the filament is hot, it tends to transfer heat to its colder surroundings. While much of its heat leaves the filament via convection and conduction in the gas and glass bulb, a significant fraction of this heat leaves the filament via thermal radiation. For any object that is hotter than about 500° Celsius, some of this thermal radiation is visible light and for an object that is about 2500° Celsius, about 10% is visible light. The light that you see from the bulb is the visible portion of its thermal radiation. However, most of the filament's thermal radiation is invisible infrared light. While you can feel this infrared light warming your hand, you can't see it. Because only about 80% of the electric power delivered to the bulb becomes thermal radiation and only about 12% of that thermal radiation is visible, an incandescent light bulb is only about 10% energy efficient. Other types of lamps, including fluorescent and gas discharge lamps, are much more energy efficient.