In limited amounts, X-rays can be used for imaging, while gamma rays are most certainly deadly in large quantities. Sources of Electro-Optical Radiation Sources of electromagnetic energy can be divided into two broad categories, selective and thermal radiators. Selective radiators emit electro-optical radiation in a narrow band about one or more particular frequencies. Therefore selective radiators produce narrowband radiation. For example, sunlight appears white because it has components covering all frequencies in the visible spectrum and therefore appears as a complete mixture of colors.
Sunlight has components in the ultraviolet and infrared as well.
The electromagnetic energy that comes from any object is known as thermal radiation. It is a physical property of matter that all objects emit thermal radiation from their surfaces in proportion to their absolute temperature. The Stefan-Boltzmann law is based on the ideal source, a black body. The idea of a black body is something that absorbs all of the incident energy upon it. In order to maintain thermal equilibrium, the object will radiate the same quantity of energy away.