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.

Conventional (also called analog, plain-film or projectional) radiography is a fundamental diagnostic imaging tool in the detection and diagnosis of diseases. X-rays reveal differences in tissue structures using attenuation or absorption of x-ray photons by materials with high density (like calcium-rich bones).
Basically, a projection or conventional radiograph shows differences between bones, air and sometimes fat, which makes it particularly useful to asses bone conditions and chest pathologies. Low natural contrast between adjacent structures of similar radiographic density requires the use of contrast media to enhance the contrast.
In conventional radiography, the patient is placed between an x-ray tube and a film or detector, sensitive for x-rays. The choice of film and intensifying screen (which indirectly exposes the film) influence the contrast resolution and spatial resolution. Chemicals are needed to process the film and are often the source of errors and retakes. The result is a fixed image that is difficult to manipulate after radiation exposure. The images may be also visualized on fluoroscopic screens, movies or computer monitors.
X-rays emerge as a diverging conical beam from the focal spot of the x-ray tube. For this reason, the radiographic projection produces a variable degree of distortion. This effect decreases with increased source to object distance relative to the object to film distance, and by using a collimator, which let through parallel x-rays only.
Conventional radiography has the disadvantage of a lower contrast resolution. Compared with computed tomography (CT) and magnetic resonance imaging (MRI), it has the advantage of a higher spatial resolution, is inexpensive, easy to use, and widely available. Conventional radiography can give high quality results if the technique selected is proper and adequate. X-ray systems and radioactive isotopes such as Iridium-192 and Cobalt-60 for generating penetrating radiation, are also used in non-destructive testing.

See also Computed Radiography and Digital Radiography.

(CR) Computed radiography is an imaging technique that uses similar equipment to conventional radiography except that films are replaced by imaging plates. An imaging plate contains photostimulable storage phosphors, which store the radiation level received at each point in local electron energies. The imaging plate is placed under the patient in the same way as conventional film cassettes. After x-ray exposure, the imaging plate is run through a special scanner to read out the image. The digital image can then be processed to optimize contrast, brightness, and zoom. Computed Radiography can be seen as halfway between film-based conventional technology and current direct digital radiography.

(DR) Digital radiography uses a special electronic x-ray detector, which converts the radiographic image into a digital picture for review on a computer monitor. The digital image is then stored and can be post processed by changing the magnification, orientation, brightness, and contrast. Digital radiography (also called direct radiography) is a progressive development of computed radiography (CR).
These advantages can lead to fewer 'recalls' (repeated x-ray images) including a lower radiation dose than analog or conventional radiography. DR and CR systems use no chemicals to process the x-ray images and the hazardous materials and waste associated with film development are eliminated.

Advantages of digital radiography compared with conventional radiography: