All light bulbs, generally called lamps, produce a thermal spectrum of light with a specific color temperature. This spectrum is a broad, featureless mixture of colors that peaks at a wavelength determined by the temperature or equivalent temperature of the light source. The spectrum of incandescent and tungsten-halogen lamps is determined by filament temperature, but that in fluorescent lamps and some high-intensity discharge lamps is the result of secondary emission from the phosphor coatings.

Because the color temperature of light emitted by lamps is much cooler than that emitted by the sun (5800 C), the light appears redder than sunlight, and very little blue light is present. Although it might not seem logical, the visual "coolness" and blue content of the light increases with its Kelvin (absolute) temperature. The typical color temperatures of incandescent lamps are 2800 K, those of tungsten-halogen lamps are 3000 K, and for cool white fluorescent lamps, they are 4100 K. However, color temperature is an even higher 5000 K from fluorescent lamps manufactured specifically to simulate daylight.