This glossary is meant to provide definitions and explanations of some important X-ray inspection terms you may see on our website. We are constantly working to expand and improve the information available here.
| B | |
|---|---|
| Bremsstrahlung (Braking Radiation) | The term "bremsstrahlung" is retained from the original German term to describe the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. Bremsstrahlung is the continuum component of the X-ray spectrum* and is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The highest energy in the spectrum, that means the shortest wave length, corresponds to the maximum acceleration potential of the electrons. The higher the tube voltage, the higher the energy of the photons, the higher their capability of passing through material. |
| C | |
| Characteristic radiation | The discrete part of the X-ray spectrum*. The electrons suddenly decelerate upon colliding with the material target and knock out electrons from the inner shell of the target atoms. As a result, electrons from higher energy levels fill up the vacancies and X-ray photons are emitted to compensate for the difference in energy between the two shells. The wavelength distribution of the characteristic radiation varies from material to material- different anode materials create more or less high-energy X-rays, thus directly influencing their capability to pass trough material. |
| Contrast | Radiographic contrast describes the differences in photographic density in a radiograph. The larger the difference in thickness or density between to areas of the subject, the larger the difference in radiographic density or contrast, the more visible features become. Contrast is caused by the fact that different parts of the object absorb X-rays differently. How great a difference in radiation absorption is necessary depends on the detector. With two parts of an object with radiation intensities I[A] and I[B], the difference in contrast is defined as 2*| I[A] – I [B] | / ( I[A] + I [B] ). As a rule of thumb, a difference in radiation absorption of 2% (0.5% for a digital detector) is needed in order for an image intensifier to produce a visible image. Moreover, contrast depends on the wavelength of the primary radiation. |
| Coolidge, W.D. | Inventor of the Coolidge tube, also called hot cathode tube (1913). It is still used today, since most X-ray tubes used today are based on the design of the Coolidge tube. |
| Cosslett, V.E. | This British physicist was the first to build a microfocus X-ray tube, based on a suggestion of M. v. Ardenne, and to use it in his “Shadow X-ray microscope” in 1951 (Nature 10, 1951, pp 24). |
| D | |
| Detail detectability | The highest amount of detail that can be shown in an image. Defined by the size of the smallest object that can be conveniently viewed, which, for nanofocus and microfocus X-ray tubes, is about half the size of the focal spot*. |
| E | |
| Electromagnetic lens | Electron-optical device used to focus the electron beam by means of a magnetic field. The magnetic lens is a rotationally symmetric electromagnet consisting of a wire coil, a magnetic iron yoke and iron pole pieces. Like optical lenses, magnetic lenses are characterized by focal length and principal planes. |
| F | |
| Filament | Source of electrons in the X-ray tube. A thin tungsten wire (0.1- 0.5 mm) emitting electrons due to thermionic emission when in a vacuum and energized with electric current. |
| Filter | Thin plates made of materials such as iron, copper and aluminium that filter out lower energy (soft) X-rays, for instance to prevent overexposure. |
| Fine focus X-ray tube | An X-ray tube with a focal spot* smaller than 0.5 millimeters. |
| Focal spot | The spot on the target which is struck by the electron beam. |
| G | |
| Gray (Gy) | Energy unit. 1 Gray equals 1 Joule per kilogram. |
| Grid | see Wehnelt electrode |
| I | |
| Image intensifier | Electron-optical device transforming X-rays into optically visible rays, thus producing an amplified radioscopic image that can be captured by a CCD camera. |
| Image resolution | Image resolution has two main contributors: the size of the focal spot and the detector or image chain resolution. For nanofocus™ and microfocus tubes, resolution lies in the range of several hundred of nanometers or in the very low micrometer range. X-ray image resolution is defined as the period length of the finest grid that can be comfortably viewed in the image. This grid may consist of gold structures on a silicon surface, but there are other alternatives as well. With a grid period length of 2 micron, for example, the spaces in between are 1 micron wide each. At the lowest magnification possible, image resolution equals detector resolution and can be as high as several hundred of micrometers. At very high magnifications, the resolution that can be obtained is limited by the size of the focal spot. |
| M | |
| Magnification using software | To be able to magnify the image further by using the software, one image pixel is mapped onto a n x n array of screen pixels. This type of magnification does not provide more information, but allows the user to better view certain object details. |
| Magnification, electron-optical | The ratio of the size of the detector input image and the size of the image on the screen (user interface). This ratio is determined by all optical and electronic imaging procedures of the image chain, provided that one camera pixel is mapped onto exactly one monitor pixel. |
| Magnification, geometric | The (theoretical) magnification in an X-ray image that occurs when the focal spot is assumed to be a point and not an area. Geometric magnification depends on the radiographic setup. For nanofocus and microfocus X-ray systems, geometric magnification is defined by the focus-to-detector (film) distance and the focus-to-object (film) distance. |
| Magnification, total | The product of the geometric* and the electron-optical* magnification. The ratio of the object size as displayed and the actual object size, provided that one camera pixel is mapped onto exactly one monitor pixel. Magnifications of up to 26,000x are possible. |
| Mann, Thomas | As far as we know, the first novelist to describe real time X-ray inspection. In his novel “The magic mountain”(1924), Mann describes how a young man is examined by a physician for tuberculosis by means of X-ray imaging. The patient feels ashamed that the physician was able to look into his chest and considers the whole ordeal “sacrilegious”. |
| Microfocus X-ray tube | Defined as an X-ray tube with a focal spot* smaller than 200 microns. |
| N | |
| nanofocus X-ray tube | Defined as an X-ray tube with a focal spot* smaller than 1 micron (1000 nanometers). |
| Newton, Helmut | As far as we know, the first photographer to use X-ray imaging in art and photography. |
| O | |
| Open X-ray tube | An X-ray tube which can be opened to replace worn out part such as filament* and target*. Unlike closed tubes, open tubes can be operated closer to their physical limits, thus providing higher magnifications and smaller focal spots making them suitable for highest inspection demands (X-ray microscopy) |
| R | |
| Radioactivity | The ability of unstable atomic nuclei to spontaneously emit subatomic particles. This process, consisting of various sub-processes, is called radioactive decay. Radioactive decay results in a loss of mass, which is converted to energy. This energy is released as kinetic energy of the emitted particles. Unlike the production of X-rays, which can be easily stopped by simply switching off the X-ray tube, radioactivity is an inherent property of a substance. |
| Röntgen, W.C. | In November 1895, in the German town of Würzburg, Wilhelm Conrad Röntgen, a German physicist, began observing and further documenting X-rays while exploring the effects of high-tension electrical discharges in evacuated glass tubes. The tube had been covered with cardboard to prevent light from escaping. Suddenly he noticed a faint green shimmering against a fluorescent screen located app. 1 meter away from the tube. The light, coming from the covered tube he had been experimenting with, had travelled through several solid objects before reaching the fluorescent screen. Putting various other objects in front of the generator, he noticed that the outline of the bones from his hand were displayed on the wall. Röntgen used photographic plates to document his discovery. In December 1895, Röntgen wrote a preliminary report On a new kind of ray: A preliminary communication, which he submitted to the Würzburg's Physical-Medical Society journal. In a footnote, he temporarily termed the new rays "X-rays". The news of X-rays revealing internal details of the human body (which Röntgen himself actually considered a mere "by-product" of his discovery), spread like wildfire within the international science community within only a couple of weeks. But Röntgen showed his awareness of the potential benefits of using X-rays for non-destructive testing when stating:.. so I own a photograph ... of a piece of metal the inhomogenity of which becomes perceptible through the X-rays. |
| S | |
| Sealed X-ray tube | Unlike open X-ray tubes, sealed X-ray tubes contain a permanent vacuum and are maintenance free. The trade-off, however, is limited tube lifetime due to the fact that neither target* nor filament can be replaced. This type of tube is usually used for less demanding inspection tasks, since performance is limited in terms of magnification and detail detectability. |
| Sievert (Sv) | The sievert (symbol: Sv) is the ST derived unit of dose equivalent. Unlike the absorbed dose, which reflects the physical effects of radiation (measured in gray), it attempts to reflect the biological effects of radiation. It is named after Rolf Sievert, a Swedish medical physicist renowned for his work in the field of radiation dosage measurement and research into the biological effects of radiation. |
| T | |
| Target | The basic production of X-rays is by accelerating electrons in order to collide with a metal target. We differentiate between two types: transmission and directional targets. |
| Tube current | In the X-ray tube, heating current is applied to the filament, which, once it is heated up, emits electrons. Due to differences in voltage, these are accelerated to the anode, the stream of electrons travelling from the one to the other being the tube current. The filament temperature is an important factor for the intensity of the X-ray output, because the higher the temperature of the filament, the larger the number of electrons that leave the cathode and travel to the anode. An increase in tube current results in a proportional increase in electron intensity. Electrons striking with more energy result in X-rays with more penetrating power. |
| Tube output | Product of tube voltage* and tube current*. Only a small part of the actual tube output is transformed into X-radiation (see X-ray yield*). |
| Tube voltage | The difference in potential between filament and anode in an X-ray tube. The speed at which the electrons travel from cathode to anode is dependant on the tube voltage. At a tube voltage of 100 kV, the electrons travel to the anode with appr. one third of the speed of light. The tube voltage determines the maximum energy of the X-ray spectrum*, since the higher the voltage, the faster, and therefore more energetic, the electrons when they strike the anode, resulting in X-rays with more penetrating power. The photon intensity is approximately proportional to the square tube voltage. |
| Tungsten | Element No. 74. Because of its high atomic number, which makes for a relatively high X-ray yield, and melting point of 3410 °C, tungsten is commonly used as target and filament material. |
| V | |
| Vacuum | A vacuum is a volume of space that is substantively empty of matter so that gaseous pressure is much less than standard atmospheric pressure. It is necessary that the X-ray tube is evacuated because otherwise the electrons would be stopped by air or any other gas. Unlike in closed (or sealed) X-ray tubes, in which the vacuum is permanent, open tube systems have to be outfitted with a vacuum pump for the purpose of creating vacuum during tube warm-up. |
| W | |
| Wehnelt electrode | Wehnelt electrode or grid. Cylindrical electrode enclosing the cathode of an X-ray tube. The Wehnelt electrode has a negative potential with respect to the cathode. Varying the voltage applied to the grid regulates the tube current, which, in turn, affects the intensity of the X-ray photons. |
| X | |
| X-ray film | A photographic film used to generate a visual X-ray image. X-ray films provide very good spatial resolution and contrast, but need long exposures times and need to be chemically processed. Moreover, they cannot be digitally processed. |
| X-ray generator | Supplies heating current and both acceleration and grid voltage for the X-ray tube. It is crucial that grid voltage is smooth and stable to prevent defocussing and chromatic aberrations (unsharpness). |
| X-ray parameters | Tube voltage* and tube current*. |
| X-ray spectrum | The distribution of the energy (wavelength, frequency) of the X-ray photon emerging from an X-ray source. Typically the spectrum of an X-ray tube consists of the continuous bremsspectrum (s. bremsstrahlung*), which is superimposed by the lines of the characteristic spectrum*. |
| X-ray tube | Device for the production of X-rays. An evacuated tube in which a swift electron beam is generated by an electron gun setup and then stopped by an anode. The deceleration of electrons leads to the emission of electromagnetic radiation, namely X-radiation. As opposed to conventional X-ray tubes, where the anode of the electron gun is also the target, in nanofocus and microfocus* X-ray tubes, the electron beam is transmitted trough a hole in the anode where it is then focussed onto a small spot on the target. This way, a very small but bright X-ray source is produced. Depending on the type of tube housing, we differentiate between two types of X-ray tubes: open and sealed tubes. Either one can be outfitted with either a directional* or transmission* type target. |
| X-ray tube, open | See open X-ray tube* |
| X-ray tube, sealed | See sealed X-ray tube* |
| X-ray yield | The X-ray yield is the percentage of tube power* transformed into X-ray radiation. The main part of the tube power is used for warming up the target. An increase in tube voltage results in a linear increase in X-ray yield. Also important in this context is the atomic number of the target material: The higher the atomic number, the better the X-ray yield. Hence, target materials with high atomic numbers, such as tungsten, should be used. Under a tube voltage of 100kv, tungsten provides an X-ray yield of 0.7%. |
| X-rays | X-rays are a form of electromagnetic radiation with a wavelength in the range of about 10-9m (1 nm) to 6 x 10-12m (6 pm), or frequencies in the range of 3 x 1017Hz to 5 x 1019Hz and photon energy between 1.2 keV and 240 keV. The most common way of producing X-rays is by bremsstrahlung* (German for braking radiation). Another type of X-rays is produced by the inner, more tightly bound electrons in atoms (characteristic radiation*). For more information on the generation of X-rays, please see X-ray tube*. |