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'''X-rays''' (aka Röntgen rays, after their discoverer [[Wilhelm Conrad Röntgen]]) are an [[ionizing radiation|ionizing]] type of [[electromagnetic radiation]] in the frequency range of 3&times;10<sup>16</sup>Hz to 3 &times; 10<sup>19</sup>Hz. They can be divided into the more energetic hard x-rays (3&times;10<sup>18</sup>Hz to 3 &times; 10<sup>19</sup>Hz) adjacent to [[gamma ray]]s, and into soft x-rays (3&times;10<sup>16</sup>Hz to 3 &times; 10<sup>18</sup>Hz), adjacent to [[ultraviolet light]]. 

They are widely used for structural investigations in all parts of [[materials science]], though care has to be exerted for uses on living tissue, since ionizing radiation can cause [[cell damage]]; see [[acute radiation syndrome]].

==Principles of X-ray applications==
===Imaging using X-rays===
The most common use of X-rays is in [[imaging]]. These applications have a source of X-rays, possibly a means of focusing the radiation beam, mechanisms for positioning the source and subject in respect to another, and a device for recording the X-ray beam after it passed through the subject.

X-ray imaging depends on the principle that different materials are more or less opaque to X-rays, and thicker sections of the same material will be more opaque than thinner sections. For example, bone is more opaque than soft body tissue, and thick bone is more opaque than thin bone. In other words, different parts of the subject differently attenuate the X-ray beam. Most often, the image is presented in "negative" form, with whiter parts showing more and the darker parts showing less attenuation.

In a few cases, there is no X-ray source in the system (e.g., [[#X-ray astronomy|X-ray astronomy]]) or the image is constructed not from the differently attenuated X-rays (e.g., [[#X-ray fluorescence spectroscopy|X-ray fluorescence spectroscopy]]).
===Non-imaging X-rays used in evaluating materials===
===X-rays used to affect materials and tissue===
==Medical Applications==
In medicine, the most common use for X-rays is in [[medical imaging]], but there are some applications where the purpose of using the X-rays is to affect tissue, such as [[radiology#radiation oncology|radiation oncology]].

It is increasingly common to have images read at a distance from the imaging equipment, distance ranging from an office to a continent away. A standard method for transmitting these images is [[digital imaging and communications in medicine]] (DICOM).
===Medical diagnosis===
Practical X-ray machines for [[imaging]] internal body parts, at a minimum, an [[#X-ray source]], mechanisms both for positioning the X-ray source and the patient, and a means of  that records the signals that have gone through the body to form an image.<ref name=>{{citation
 | url = http://www.latticesemi.com/solutions/marketsolutions/medical/digitalxraymachinecameras.cfm
 | publisher = Lattice Semiconductor Corporation
 | title = Digital X-Ray Machine and Camera System}}</ref>

While they follow the basic principles described here, more advanced medical X-ray techniques, such as [[X-ray computed tomography]], have sufficient differences and refinements that a sub-article is needed for the details.
===Medical treatment===

==X-ray astronomy==
This category of [[astronomy]] and [[astrophysics]] research is one of the few where no X-ray source is needed and the X-ray instrument is purely passive.
==X-ray diffraction==
{{r|X-ray diffraction}}

==X-ray fluorescence spectroscopy==
X-ray fluorescence spectrosopy is a nonimaging testing methods, applied to nonliving materials, in which the X-rays cause certain atoms in the material to generate, through a special case of the [[photoelectric effect]], energy in a different part of the [[electromagnetic spectrum]] than X-rays. <ref name=>{{citation
 | title =X-ray Fluorescence Spectroscopy (XRF)
 | url = http://www.amptek.com/xrf.html
 | publisher = Amptek}}</ref>

==References==
{{reflist}}