About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays. The rest of the energy is released as heat. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface. This will slowly darken the tube and was thought to degrade the quality of the X-ray beam. Vaporized tungsten condenses on the inside of the envelope over the "window" and thus acts as an additional filter and decreases the tube's ability to radiate heat. Eventually, the tungsten deposit may become sufficiently conductive that at high enough voltages, arcing occurs. The arc will jump from the cathode to the tungsten deposit, and then to the anode. This arcing causes an effect called "crazing" on the interior glass of the X-ray window. As time goes on, the tube becomes unstable even at lower voltages, and must be replaced. At this point, the tube assembly (also called the "tube head") is removed from the X-ray system, and replaced with a new tube assembly. The old tube assembly is shipped to a company that reloads it with a new X-ray tube. The X-ray photon-generating effect is generally called the bremsstrahlung effect, a compound of the German bremsen meaning to brake, and Strahlung meaning radiation. The range of photonic energies emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses. Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation. The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time. Heat released Heat is produced in the focal spot of the anode. Since a small fraction (less than or equal to 1%) of electron energy is converted to X-rays, it can be ignored in heat calculations. The quantity of heat produced (in Joule) in the focal spot is given by : being the waveform factor = peak AC voltage (in Volts) = tube current (in mili Amperes) = exposure time (in seconds) Heat Unit (HU) was used in the past as an alternative to Joule. It is a convenient unit when a single-phase power source is connected to the X-ray tube. With a full-wave rectification of a sine wave, =, thus the heat unit: 1 HU = 0.707 J 1.4 HU = 1 J