Incandescent light bulb granted a Hungarian patent (No. 34541) for a tungsten filament lamp that lasted longer and gave brighter light than the carbon filament. Tungsten filament lamps were first marketed by the Hungarian company Tungsram in 1904. This type is often called Tungsram-bulbs in many European countries. Filling a bulb with an inert gas such as argon or nitrogen slows down the evaporation of the tungsten filament compared to operating it in a vacuum. This allows for greater temperatures and therefore greater efficacy with less reduction in filament life. In 1906, William D. Coolidge developed a method of making "ductile tungsten" from
Incandescent light bulb is 683 lm/W for monochromatic green light at 555 nanometers wavelength, the peak sensitivity of the human eye. The luminous "efficiency" is defined as the ratio of the luminous efficacy to the theoretical maximum luminous efficacy of 683 lpw, and, as for luminous efficacy, is of two types, radiant luminous efficiency (LFR) and source luminous efficacy (LFS). The chart below lists values of overall luminous efficacy and efficiency for several types of general service, 120-volt, 1000-hour lifespan incandescent bulb, and several idealized light sources. The values for the incandescent bulbs are source efficiencies and efficacies. The values for the ideal