The spectrum of electromagnetic radiation emitted by a source is an emission spectrum. One way of producing electromagnetic radiation is by heating a material until it glows, or emits light. For example, a piece of iron heated in a blacksmith's furnace will emit visible light as well as infrared radiation (heat). Similarly, a light bulb uses electrical current to heat a tungsten filament encased in an evacuated glass bulb. The Sun is a source of radiation in the infrared, visible and ultraviolet regions of the electromagnetic spectrum. Radiation produced by a thermal source is called black body, or incandescent radiation. Spectra such as these, in which the intensity varies smoothly over the distribution range, are called continuous spectra.
Atoms that have been heated (typically by a high-energy source such as an electric spark or a flame), will also emit electromagnetic radiation. However, if there are only a few atoms present so that they do not collide with one another, such as in a low-pressure gas, the excited atoms will emit radiation at only a few specific wavelengths. For example, a vapor of neon atoms in a glass tube excited by an electrical discharge produces the familiar red color of neon lights by emitting light of only red wavelengths. In contrast to a continuous spectrum, atomic emission spectra generally exhibit high intensity at only a few wavelengths and very low intensity at all others; such discontinuous spectra are called discrete spectra.