Incandescent bulbs produce illumination by running an electrical current through a tungsten filament. The filament is heated, typically between 2,000 and 3,000 Kelvin, and the inert gas inside the bulb reacts with the tungsten to produce a spectrum that radiates in the visible light region. This visible spectrum is what causes the filament to illuminate, although the peak of the intensity is in the infrared region. The spectral plot of a 2254K incandescent bulb can be seen in Figure 5-1.

Figure 5-1 2254K Incandescent Bulb