The x-ray (or roentgen-ray) spectrum consists of electromagnetic radiation with wavelengths shorter than ultraviolet (UV) and longer than gamma rays. The usual photon energies of x-rays range from 100 electron volt (eV) to 100 keV (wavelengths of around 10 to 0.01 nanometers; or around 100 to 0.1 Angstroms); corresponding to frequencies in the range of 30 PHz to 30 EHz (see Hertz).
The energy distribution (wavelength, frequency) of x-ray photons emerges from the source, the x-ray tube. In a conventional tube, x-rays are generated in two different ways that, together, form a typical spectrum consisting of the bremsstrahlung, which is superimposed by the lines of the characteristic spectrum (in a graph, the curve is shaped like a hump topped by several spikes).

See also Angstrom, Direct Radiation, Secondary Radiation, and Radiation Meter.

X-ray tubes are devices for the production of x-rays. X-ray tubes consist of an evacuated glass vessel and two electrodes. An electrical current with very high voltage passes across the tube and accelerates electrons emitted by thermionic emission from a tungsten filament (cathode also called electron gun) towards the anode target. The electrons collide with the anode and this deceleration generates x-rays (bremsstrahlung).
The high vacuum allows the electron beam an unimpeded passage. The electron beam heats the anode (usually copper), which is cooled by water to prevent melting. A copper target emits x-rays with a characteristic wavelength. Other used metals soften or harden the x-ray beam.
The x-rays pass through a very thin beryllium (Be) foil. This beryllium window absorbs a high amount of the elastically scattered electrons (produced by the target) and allows the radiation to get out of the tube without substantial absorption.
In conventional x-ray tubes, the anode is also the target. In nanofocus and microfocus x-ray tubes, the electron beam is transmitted through a hole in the anode where it is then focused onto a small spot on the target.

See also X-Ray Tube Housing, Fine Focus X-Ray Tube, Transformer, Diode, Digital to Analog Converter and Angular Response.