There are widely used incandescent light bulbs which produce light with a filament such as tungsten filament heated by flowing an electric current through it. Incandescent light bulbs have various advantages, for example, (a) they are inexpensive, (b) they show superior color rendering properties, (c) they can be used with any operating voltage (they can work with either alternating current or direct current), (d) they can be lightened with a simple lighting implement, (e) they are used worldwide, and so forth. However, efficiency of incandescent light bulbs for conversion from electric power to visible light is about 15 lm/W, which is lower than that of fluorescent lamps (conversion efficiency is 90 lm/W), and therefore they impose larger environmental loads.
Patent document 1 discloses that multiple microcavities (holes) are formed on the surface of filament to prevent radiation of lights having a wavelength of 700 nm or longer and control luminous intensity distribution, in order to enhance the conversion efficiency into visible light.
Further, Non-patent document 1 discloses a microstructure formed on the surface of filament, and the physical effects of the microstructure, i.e., radiation-enhancing and radiation-suppressing effects for a part of infrared lights.