Standard Terminology for Nondestructive Examinations Part 2

phase-sensitive system—a system whose output signal is dependent on the phase relationship between the voltage returned from a pickup or sensing coil and a reference voltage. phase shift—a change in the phase relationship between two alternating quantities of the same frequency. probe coil—a small coil or coil assembly that is placed on or near the surface of examination objects. probe coil clearance—the perpendicular distance between adjacent surfaces of the probe and examination part; also lift-off. recovery time—the time required for an examination system to return to its original state after it has received a signal. reference coil—a coil or probe, which may be used in conjunction with the appropriate material, to electrically balance a comparative system. rejection level—the value established for a signal above or below which specimens are rejectable, or otherwise distinguished from the remaining specimens. selectivity—the characteristic of an examination system that is a measure of the extent to which an instrument is capable of differentiating between the desired signal and disturbances of other frequencies or phases. sensitivity control—the control in the instrument that adjusts the amplifier gain, and is one of the factors that determines the capacity to detect discontinuities. signal gradient—same as differential readout. signal-to-noise ratio—the ratio of values to signal (response containing relevant information) to that of noise (response containing nonrelevant information). skin depth—see depth of penetration. skin effect—the phenomenon wherein the depth of penetration of electric currents into a conductor decreases as the frequency of the current is increased. At very high frequencies, the current flow is restricted to an extremely thin outer layer of the conductor. (See also depth of penetration.) from a change in the relative motion between the specimen and a test coil assembly. standard depth of penetration (SDP)—see depth of penetration. test coil—the section of the coil assembly that examines the material under examination in a comparative system; the coil used to examine the material in an absolute or differential comparative system. test quality level—see rejection level. three way sort—an electromagnetic sort based on a signal response from the material under examination above or below two levels established by three or more calibration standards. threshold level—the setting of an instrument that causes it to register only those changes in response greater or less than a specified magnitude. threshold setting—the setting of the instrument that causes it to register only those changes in eddy-current response greater than a specified magnitude. DISCUSSION—Sensitivity and threshold settings usually are indicated by arbitrary numbers on the control panel of the testing instrument. These numerical settings differ among instruments of different types. It is, therefore, not proper to translate a numerical setting on one instrument to that of another type. Even among instruments of the same design and from the same manufacturer, sensitivity and threshold settings may vary slightly when detecting the same discontinuity. Therefore, undue emphasis on the numerical value of sensitivity and threshold settings is not justified. transducer—an electromagnetic device for converting electrical energy into magnetic or mechanical energy and vice versa. tubing acceptance standard—a tube used to establish the acceptance level with artificial discontinuities as specified in the applicable product standard. two-way sort—an electromagnetic sort based on a signal response from the material under examination above or below a level established by two or more calibration standards. wobble—an effect that produces variations in coil spacing (operational lift-off) due to lateral motion of the specimen in passing through an encircling coil. speed effect—the phenomenon in electromagnetic testing of which the evidence is a change in the signal voltage resulting Section D: Gamma- and X-Radiologic Testing (RT) Terms The terms defined in Section D are the direct responsibility of Subcommittee E07.01 on Radiology (X and Gamma) Methods. Additional radiologic testing terms are found in Section H. absorbed dose—the amount of energy imparted by ionizing radiation per unit mass of irradiated matter. Denoted by “rad;” 1 rad = 0.01 j/kg. SI unit is “gray;” 1 gray = 1 j/kg. absorbed dose rate—the absorbed dose per unit of time; rads/s. SI unit, grays/s. absorption—the process whereby the incident particles or photons of radiation are reduced in number or energy as they pass through matter. attenuation (X-ray)—reduction of radiation beam intensity caused by the interactions of the beam with the matter through which it passes. accelerating potential—the difference in electric potential between the cathode and anode in an X-ray tube through which a charged particle is accelerated; usually expressed in units of kV or MV. autoradiograph—the image of an object containing a radioelement obtained, on a recording medium, by means of its own radiation. activation—in neutron radiography, the process of causing a substance to become artificially radioactive by subjecting it to bombardment by neutrons or other particles. acute radiation syndrome—the immediate effects of a short term, whole body overexposure of a person to ionizing radiation. These effects include nausea and vomiting, malaise, increased temperature, and blood changes. alphanumeric—term pertaining to both numbers and alphabetical characters, typically used to designate a device capable of handling both types of characters. alpha particle—a positively charged particle emitted by certain radio-nuclides. It consists of two protons and two neutrons, and is identical to the nucleus of a helium atom. amorphous selenium (a-Se) radiation detector array—an array employing a biased amorphous selenium photoconductor that directly converts incident radiation into electrical charge which can then be read to form a digital image. amorphous silicon (a-Si) detector, n—an amorphous silicon (a-Si) radiation detector consists of a glass substrate with an array of photodiodes and thin film transistors; the photodiodes are illuminated by light photons emitted from a scintillator which is irradiated by X-rays or gamma rays, and is in intimate contact with the photodiode array. analog image—an image produced by a continuously variable physical process (for example, exposure of film). analog to digital converter (a/d)—a device that changes an analog signal to a digital representation of the signal. back scattered radiation—radiation which is scattered more than 90° with respect to the incident beam, that is, backward in the general direction of the radiation source. bad pixel—a DDA pixel that does not conform to a specified performance. DISCUSSION—Bad pixel criteria may include, but are not limited to, non-responding, over-responding, under-responding, noisy, nonuniform, non-persistent, or bad neighborhood; non-persistent bad pixels may have flickering or poor lag performance. bad pixel map—a binary image that represents the physical locations of bad pixels on a DDA. betatron—an electron accelerator in which acceleration is provided by a special magnetic field constraining the electrons to a circular orbit. This type of equipment usually operates at energies between 10 and 31 MEV. bit depth, n—the number of bits used to store a value. DISCUSSION—Typically used to describe the pixel value range in digital radiography, for example, a system with a bit depth of 16 provides 216 (= 65 536) possible pixel values. blocking or masking—surrounding specimens or covering their sections with absorptive material. blooming—in radiographic imaging, an undesirable artifact brought about by exceeding the allowable input brightness for the detector, causing signal to leak into adjacent areas, producing a degraded non-homogeneous image artifact near to the high exposure area. blow back—the enlargement of a minified radiograph to its original size by use of an optical direct reader. anode—the positive electrode of a discharge tube. In an X-ray tube, the anode carries the target. cassette—a light-tight container for holding radiographic recording media during exposure, for example, film, with or without intensifying or conversion screens. anode current—the electrons passing from the cathode to the anode in an X-ray tube, minus the small loss incurred by the back scattered fraction. characteristic curve—the plot of density versus log of exposure or of relative exposure. (Also called the D-log E curve or the H and D curve.) aperture—an opening in material, space, or time over which an element is considered to be active. cine-radiography—the production of a series of radiographs that can be viewed rapidly in sequence, thus creating an illusion of continuity. area of interest (AOI)—the portion of the radiograph or digital image that is to be evaluated and interpreted. array processor—a special purpose logical processing device that performs extremely fast mathematical operation on digital arrays. artifact—spurious indication on a radiograph arising from, but not limited to, faulty manufacture, storage, handling, exposure, or processing. collimator—a device of radiation absorbent material intended for defining the direction and angular divergence of the radiation beam. composite viewing—the viewing of two or more superimposed radiographs from a multiple film exposure. compton scatter radiation—the scattered X-ray or gamma ray which results from the inelastic scattering of an incident X-ray or gamma ray on an electron. Since the ejected electron has short range in most materials, it is not considered part of the scattered radiation. computed radiography (CR) (Photostimulable Luminescence (PSL) Method)—a two step radiographic imaging process; first, a storage phosphor imaging plate (IP) is exposed by penetrating radiation; second, the luminescence from the IP’s photostimulable luminescent phosphor is stimulated, detected, digitized, and displayed on an image display monitor. computed tomography (CT)—a nondestructive examination technique that captures radiographic projections of an object at various rotational angles, which are mathematically reconstructed to produce a three-dimensional volume data set or one or more two-dimensional cross-sectional images, where each voxel value represents the attenuation coefficient of the incident radiation at a specific location within the object. constant potential—a method of electrically generating x-rays by placing a constant potential electrical source (voltage and current) across the x-ray tube anode and cathode; the ripple component of the constant potential electrical source is typically less than 2.0 %. contrast-to-noise ratio (CNR)—quotient of the difference of the mean linear pixel values between two image areas (Digital image contrast) and the standard deviation of the linear pixel values. contrast sensitivity—a measure of the minimum percentage change in an object which produces a perceptible density/ brightness change in the radiological image. contrast stretch—a function that operates on the greyscale values in an image to increase or decrease image contrast. corrected digital radiograph—an image whose pixels have been adjusted due to the application of an offset and gain correction, bad pixel correction, or any other type of image correction. definition, image definition—the sharpness of delineation of image details in a radiograph. Generally used qualitatively. densitometer—a device for measuring the optical density of radiograph film. density (film)—see film density. density comparison strip—alternative term for step-wedge comparison film. detector unsharpness (U detector )—two times the Basic Spatial Detector Resolution (SRbdetector ). digital—the representation of data or physical quantities in the form of discrete codes, such as numerical characters, rather than a continuous stream. digital detector array (DDA)—an electronic device that converts ionizing or penetrating radiation into a discrete array of analog signals which are subsequently digitized and transferred to a computer for display as a digital image corresponding to the radiologic energy pattern imparted upon the input region of the device. digital driving level (DDL)—a digital value that when given as input to a display system produces a luminance. digital image—an image composed of discrete pixels each of which is characterized by a digitally represented luminance level. digital image acquisition system—a system of electronic components which, by either directly detecting radiation or converting analog radiation detection information, creates an image of the spatial radiation intensity map comprised of an array of discrete digital intensity values (see pixel). digital image enhancement—any operation used for the purpose of enhancing some aspect of the original image. digital image processing system—a system which uses algorithms to process digital image data. digital magnification (zoom)—any change in the pixel mapping ratio between the captured image and the displayed image, effectively making objects in the image appear larger or smaller. digital radiography—all radiography methods whereby images are in a digital format. DISCUSSION—For example CR, CT, DDA, and digitized film. digitize (for radiology)—the act of converting an analog image or signal to a digital presentation. dynamic range (for radiography)—the span of signal intensity that defines the system’s range of performance. equivalent I.Q.I. sensitivity—that thickness of I.Q.I. expressed as a percentage of the section thickness radiologically examined in which a 2T hole or 2 % wire size equivalent would be visible under the same radiological conditions. equivalent penetrameter sensitivity—that thickness of penetrameter, expressed as a percentage of the section thickness radiographed, in which a 2T hole would be visible under the same radiographic conditions. erasable optical medium—an erasable and rewritable storage medium where the digital data is represented by the degree of reflectivity of the medium recording layer; the data can be altered. exposure, radiographic exposure—the subjection of a recording medium to radiation for the purpose of producing a latent image. Radiographic exposure is commonly expressed in terms of milliampere-seconds or millicurie-hours for a known source-to-film distance. exposure table—a summary of values of radiographic exposures suitable for the different thicknesses of a specified material. fast scan direction (CR)—the laser sweep direction across an imaging plate in a CR scanner. DISCUSSION—Also known as the scan axis; orthogonal to the slow scan direction. film contrast—also called gradient: a quantitative expression of the slope or steepness of the characteristic curve of a film; that property of a radiographic film material which is related to the magnitude of the density difference resulting from a given difference of the logarithmic exposure dose. film density—the quantitative measure of diffuse optical light transmission (optical density, blackening) through a developed film. D 5 log ~I0 /I! where: D = optical density, I 0 = light intensity incident on the film, and I = light intensity transmitted. film speed—a numerical value expressing the response of an image receptor to the energy of penetrating radiation under specified conditions. filter—uniform layer of material, usually of higher atomic number than the specimen, placed between the radiation source and the film for the purpose of preferentially absorbing the softer radiations. fluorescence—the emission of light by a substance as a result of the absorption of some other radiation of shorter wavelengths only as long as the stimulus producing it is maintained. fluorescent screen—alternative term for intensifying screen (b). fluoroscopy—the visual observation on a fluorescent screen of the image of an object exposed to penetrating, ionizing radiation. focal spot—for x-ray generators, that area of the anode (target) of an x-ray tube which emits x-ray when bombarded with electrons. fog—a general term used to denote any increase in optical density of a processed photographic emulsion caused by anything other than direct action of the image forming radiation and due to one or more of the following: (a) aging—deterioration, before or after exposure, or both, resulting from a recording medium that has been stored for too long a period of time, or other improper conditions. (b) base—the minimum uniform density inherent in a processed emulsion without prior exposure. (c) chemical—resulting from unwanted reactions during chemical processing. (d) dichroic—characterized by the production of colloidal silver within the developed sensitive layer. (e) exposure—arising from any unwanted exposure of an emulsion to ionizing radiation or light at any time between manufacture and final fixing. (f) oxidation—caused by exposure to air during developing. (g) photographic—arising solely from the properties of an emulsion and the processing conditions, for example, the total effect of inherent fog and chemical fog. (h) threshold—the minimum uniform density inherent in a processed emulsion without prior exposure. fog density—a general term used to denote any increase in the optical density of a processed film caused by anything other than the direct action of the image-forming radiation. forward scattered radiation—radiation which is scattered less than 90° with respect to the incident beam, that is, forward in the general direction of the radiation source. gamma-radiography—a technique of producing radiographs using gamma-rays. gamma ray—electromagnetic penetrating radiation having its origin in the decay of a radioactive nucleus. geometric unsharpness (U g)3—the penumbral shadow in a radiological image which is dependent upon (1) the radiation source dimensions, (2) the source to object distance, and (3) object to detector distance. graininess—the visual impression of irregularity of silver deposit in a processed film. half-life—the time required for one half of a given number of radioactive atoms to undergo decay. half-value layer (HVL)—the thickness of an absorbing material required to reduce the intensity of a beam of incident radiation to one half of its original intensity. half-value thickness—the thickness of a specified substance which, when introduced into the path of a given beam of radiation, reduces its intensity to one half. image data file—a digital file containing radiological image and text information. image definition—see definition. image processing—a method whereby digital image data is transformed through a mathematical function. image quality indicator (IQI)—in industrial radiology, a device or combination of devices whose demonstrated image or images provide visual or quantitative data, or both, to determine radiologic quality and sensitivity. Also known as a penetrameter (disparaged). DISCUSSION—It is not intended for use in judging size nor establishing acceptance limits of discontinuities. iSRb image—the interpolated basic spatial resolution of the imaging system, which corresponds to the dimension of the smallest feature that can be resolved at a modulation of twenty percent with geometric magnification. NOTE 3—Typical units of resolution measurement are micrometers. iSRb detector—the interpolated basic spatial resolution of a detector, which corresponds to the dimension of the smallest 3 Mathematical derivation of geometric unsharpness is provided in Guide E94. feature that can be resolved at a modulation of 20 % without geometric magnification. NOTE 4—Typical units of resolution measurement are micrometers. indication, n—the response or evidence from a nondestructive examination that requires interpretation to determine relevance. inherent detector unsharpness (U detector )—the blurring of test object features in a radiographic image (primarily due to detector properties), with the object measured adjacent to the detector plane without geometric magnification. DISCUSSION—Equal to two times the SR bdetector. intensifying screen—a material that converts a part of the radiographic energy into light or electrons and that, when in contact with a recording medium during exposure, improves the quality of the radiograph, or reduces the exposure time required to produce a radiograph, or both. Three kinds of screens in common use are: lines and spaces is utilized to determine the maximum density of lines and spaces that can be successfully imaged. The value is expressed in line pairs per millimetre. line pair test pattern—a pattern of one or more pairs of objects with high contrast lines of equal width and equal spacing. The pattern is used with an imaging device to measure spatial resolution. linear accelerator—an electron generator in which the acceleration of the particles is connected with the propagation of a high-frequency field inside a linear or corrugated waveguide. linear digital image contrast—mean linear pixel value difference between any two regions of interest within a digital image. Linear digital image contrast = PV2 – PV1, where PV2 is the mean linear pixel value of region of interest “2” and PV1 is the mean linear pixel value of region of interest “1” on a digital image. (a) metal screen—a screen consisting of dense metal (usually lead) or of a dense metal compound (for example, lead oxide) that emits primary electrons when exposed to X- or gamma-rays. linear pixel value—the numeric value of a pixel in a digital image, which is directly proportional to the radiation dose of the corresponding detector element where a zero value represents the unexposed detector. (b) fluorescent screen—a screen consisting of a coating of phosphors which fluoresces when exposed to X or gamma radiation. (c) fluorescent-metallic screen—a screen consisting of a metallic foil (usually lead) coated with a material that fluoresces when exposed to X or gamma radiation. The coated surface is placed next to the film to provide fluorescence; the metal functions as a normal metal screen. location marker—a number or letter made of lead (Pb) or other highly radiation attenuative material that is placed on an object to provide traceability between a specific area on the image and the part. IQI sensitivity—in radiography, the minimum discernible image and the designated hole in the plaque-type, or the designated wire image in the wire type image quality indicator. keV (kilo electron volt)—a unit of energy equal to 1000 electron volts, that is, the energy gained by an electron or proton moving through a potential difference of 1000 volts in a vacuum. DISCUSSION—A unit of the maximum photon energy of a bremsstrahlung X-ray spectrum, when used to describe an X-ray source. kV (kilo volts)—a unit of electrical potential difference equal to 1000 volts. DISCUSSION—Often used to express the accelerating potential of an electrostatic X-ray source, such as an X-ray tube kVp (kilo volts peak)—a unit used to express the peak voltage of a time varying electrical potential of an X-ray source, such as a rectified X-ray generator system. latent image—a condition produced and persisting in the image receptor by exposure to radiation and able to be converted into a visible image by processing. lead screen—see intensifying screen (a). line pairs per millimetre—a measure of the spatial resolution of an image conversion device. A line pair test pattern consisting of one or more pairs of equal width, high contrast low-energy gamma radiation—gamma radiation having energy less than 200 keV. luminosity—a measure of emitted light intensity. mA (milliampere)—a unit of current equal to 0.001 amperes, used to express the tube current of an X-ray tube. magnetic storage medium—a storage medium that uses magnetic properties (magnetic dipoles) to store digital data (for example, a moving drum, disk, or tape or a static core or film). MeV (mega electron volt)—a unit of energy equal to 1 000 000 electron volts, that is, the energy gained by an electron or proton moving through a potential difference of 1 000 000 volts in a vacuum. DISCUSSION—A unit of the maximum photon energy of a bremsstrahlung X-ray spectrum, when used to describe an X-ray source. micro focus X-ray tube—an X-ray tube having an effective focal spot size not greater than 100 µm. milliamperes (mA)—the technical term is tube current and is defined as the current passing between the cathode and anode during the operation of an x-ray tube, measured in milliamperes (mA) and usually taken as a measure of x-ray intensity. minifocus X-ray tube—an X-ray tube having an effective focal spot size between 100 and 400 µm. MV (mega volt)—unit of electrical potential difference equal to 1 000 000 volts. DISCUSSION—Often used to express the accelerating potential of an electrostatic X-ray source; when used to describe an accelerator-based X-ray source, the accelerating potential that would produce the same electron energy and X-ray spectrum if an electrostatic source were available MVp (mega volts peak)—a term commonly used to express the equivalent electrostatic accelerating potential of an accelerator based X-ray source, see MV (mega volt). DISCUSSION—While not a technically correct term, MVp is used as an analog to kVp in the mega volt X-ray regime. net density—total density less fog and support (film base) density. where: OD = optical density, = light intensity incident on the film, and Io I = light intensity transmitted through the film. optical line pair test pattern—see line pair test pattern. pair production—the process whereby a gamma photon with energy greater than 1.02 MeV is converted directly into matter in the form of an electron-positron pair. Subsequent annihilation of the positron results in the production of two 0.511 MeV gamma photons. pencil beam—a radiation beam which has little divergence, usually created by collimating an intense source of radiation. neutron radiography (NRT)—a process of making an image of the internal details of an object by the selective attenuation of a neutron beam by the object. penetrameter—alternative term for image quality indicator. noise—the data present in a radiological measurement which is not directly correlated with the degree of radiation attenuation by the object being examined. phosphor—any substance that can be stimulated to emit light by incident radiation. non-erasable optical data—a non-erasable, non-rewriteable storage medium where the digital data is represented by the degree of reflectivity of the mediums recording layer. The data cannot be altered. nonscreen-type film (direct-type film)—X-ray film designed for use with or without metal screens, but not intended for use with salt screens. normalized signal-to-noise ratio (SNR N )—the SNR normalized to a prescribed detection area of a 100 micron diameter circle, or an 88.6 micron × 88.6 micron square. DISCUSSION—SNR N is calculated using the measured basic spatial resolution SRb , as measured directly in the digital image and calculated by SNRN = SNR × (88.6 µm/SRb). penetrameter sensitivity—alternative term for IQI sensitivity. photo fluorography—a photograph of the image formed on a fluorescent screen. photostimulable luminescence—the physical phenomenon of phosphors absorbing incident ionizing radiation, storing the energy in quasi-stable states and emitting luminescent radiation proportional to the absorbed energy when stimulated by radiation of a different wavelength. photostimulable luminescent phosphor—a phosphor capable of storing a latent radiological image which upon laser stimulation will generate luminescence proportional to the radiation intensity. pixel—the smallest addressable element in an electronic image. nuclear activity—the number of disintegrations occurring in a given quantity of material per unit of time. “Curie” is the unit of measurement. One curie is equivalent to 3.7 × 1010 disintegrations per second. pixel, display size—the dimensions of the smallest picture element comprising the displayed image, given in terms of the imaged object’s dimensions being represented by the element. object-detector distance (ODD)—the distance between the surface of the source side of the object and the radiation detector. pixel size—the length and width of a pixel. object-detector distance for computed tomography (ODDCT )—the distance between the center of rotation of the object and the radiation detector. object-film distance—the distance between the surface of the source side object and the plane of the recording medium. DISCUSSION—In the case where the recording medium is placed directly in contact with the object being examined, the distance is equal to the thickness of the object. offset correction—the mathematical process of subtracting the response of each pixel, in the absence of ionizing radiation, from an acquired image. optical density—the degree of opacity of a translucent medium (darkening of film) expressed as follows: OD 5 log ~I o/I ! (2) pixel value (PV)—the numeric value of a pixel in a digital image. primary radiation—radiation coming directly from the source. radiograph—any retrievable image produced by penetrating radiation passing through a material being examined. DISCUSSION—This may be hard copy, film, or a digital image. radiographic contrast—the difference in density from one area to another of a radiograph, resulting from the combination of film contrast and subject contrast. radiographic equivalence factor—that factor by which the thickness of a material must be multiplied in order to determine what thickness of a standard material (often steel) will have the same absorption. radiographic exposure—see exposure. radiographic inspection—the use of X-rays or nuclear radiation, or both, to detect discontinuities in material, and to present their images on a recording medium. radiographic quality—a qualitative term used to describe the capability of a radiograph to show flaws in the area under examination. radiographic sensitivity—a general or qualitative term referring to the size of the smallest detail that can be seen on a radiograph, or the ease with which details can be seen. radiography—the art, act, or process of making radiographs. radiological examination—the use of penetrating ionizing radiation to display images for the detection of discontinuities or to help ensure integrity of the part. radiology—the science and application of X-rays, gamma rays, neutrons, and other penetrating radiations. radioscopy—the electronic production of a radiological image that follows very closely the changes with time of the object being imaged. rare earth screens—see intensifying screen. real-time radioscopy—radioscopy that is capable of following the motion of the object without limitation of time. recording media—material capable of capturing or storing, or both, a radiological image in digital or analog form. recording medium—a film or detector that converts radiation into a visible image. region of interest—a defined group of pixels from which measurements or statistics, or both, can be derived. relative digital image contrast—digital image contrast normalized to the average linear pixel value of the two regions of interest in a digital image. representative quality indicator (RQI)—an actual part or similar part of comparable geometry and attenuation characteristics to that of the test part(s), that has known or measurable features, or both, representing the facets of nonconformance for which the test part is to be examined. resolution—a measure of the ability of a radiographic system to separate details between features in proximity. DISCUSSION—Can be described either as a separation in space or a separation in contrast. saturation—the state at which the pixel value no longer increases as a function of dose. scintillators and scintillating crystals—a detector that converts ionizing radiation to light. screen—alternative term for intensifying screen. shim—a material, typically placed under the IQI which is radiologically similar to the object being imaged. signal—the data present in a radiological measurement which is directly correlated with the degree of radiation attenuation by the object being examined. signal-to-noise ratio (SNR)—quotient of mean value of the linear pixel values and standard deviation of the mean linear pixel value (noise) in a given region of interest in a digital image. slow scan direction (CR)—the mechanical transport direction of the imaging plate in a CR scanner. DISCUSSION—Also known as the index axis; orthogonal to the fast scan direction. source—a machine or radioactive material that emits penetrating radiation. source-detector distance (SDD)—the distance between the X-ray focal spot or gamma source and the radiation detector. source-film distance—the distance between the radiationproducing area of the source and the film. source-object distance (SOD)—the distance between the X-ray focal spot or gamma source and the source side of the object. source-object distance for computed tomography (SOD CT ) –—the distance between the X-ray focal spot or gamma source and the center of rotation of the object. spatial resolution—a measure of the ability of a radiographic system to separate geometric details between features in proximity. SRb image—the basic spatial resolution of the imaging system, which corresponds to the dimension of the smallest feature that can be resolved at a specified modulation and geometric magnification. NOTE 5—Typical units of resolution measurement are micrometers. SRb detector—the basic spatial resolution of a detector, which corresponds to the dimension of the smallest feature that can be resolved at a specified modulation without geometric magnification. NOTE 6—Typical units of resolution measurement are micrometers. step wedge—a device with discrete step thickness increments used to obtain an image with discrete density step values. step-wedge calibration film—a step-wedge comparison film the densities of which are traceable to a nationally recognized standardizing body. secondary radiation—radiation emitted by any substance as the result of irradiation by the primary source. step-wedge comparison film—a processed film with defined discrete optical density steps, used to characterize the optical density on a radiograph. sensitivity—see contrast sensitivity, equivalent IQI sensitivity, equivalent penetrameter sensitivity, IQI sensitivity, radiographic sensitivity. storage phosphor imaging plate—a flexible or rigid reusable detector that stores a radiological image as a result of exposure to penetrating radiation. structure noise of DDAs—noise originating from differing properties of the individual detector elements (pixels) in a DDA. structure noise of IPs—noise originating from physical variations in the sensitive layer and surface of an IP, which appears after scanning of the exposed imaging plate as overlaid fixed pattern noise in the digital image. total image unsharpness—the blurring of test object features, in a radiological image resulting from any cause(s). translucent base media—materials with properties that allow radiological interpretation by transmitted or reflected light. transmission densitometer—an instrument that measures the intensity of the transmitted light through a radiographic film and provides a readout of the transmitted film density. subject contrast—the logarithm of the ratio of the radiation intensities transmitted through selected portions of the specimen transmitted film density—the density of radiographic film determined by measuring the transmitted light. system induced artifacts—anomalies that are created by a system during the acquisition, display processing, or storage of a digital image. tube current—the flow of electrons from the filament to the anode target in an X-ray tube; usually expressed in units of milliamperes or microamperes. system noise—the noise present in a radiological measurement resulting from the individual elements of the radiological system. vacuum cassette—a flexible light-tight container that, when operated under a vacuum, holds film and screen in intimate contact during a radiographic exposure. target—that part of the anode of an X-ray emitting tube hit by the electron beam. voxel—the smallest addressable element in a threedimensional digital volume. tenth-value-layer (TVL)—the thickness of the layer of a specified substance which, when introduced into the path of a given narrow beam of radiation, reduces the intensity of this radiation by a factor of ten. window level, n—the center of the numerical range of pixel values that are assigned to the range of displayed luminance, used to adjust the displayed brightness of an image. tomography—any radiologic technique that provides an image of a selected plane in an object to the relative exclusion of structures that lie outside the plane of interest (see tomogram and (CT) computed tomography). window width, n—the numerical range of pixel values that are assigned to the range of displayed luminance, used to adjust displayed contrast of an image. Section E: Leak Testing (LT) Terms The terms defined in Section E are the direct responsibility of Subcommittee E07.08 on Leak Testing Methods. absolute manometer—a manometer whose calibration can be calculated from the measurable physical constants of the instrument and for which calibration is the same for all ideal gases. absolute pressure—pressure above the absolute zero corresponding to empty space, that is, local atmospheric pressure plus gauge pressure. absorption—in leak testing, the binding or incorporation of gas in the interior of a solid (or liquid). accumulation test—a leak test used to detect very small leaks in which gas contained in a component being tested will, if a leak is present, collect for a specified period of time in a closed evacuated chamber into which the component has been placed. At the end of the test period, the chamber is opened to a leak detector which is sensitive to the gas. alkali ion diode—a sensor for halogen gases. (See also halogen leak detector (2).) aperture leak—a leak of such geometric configuration that the length of the leakage path is much smaller than the shortest diameter of the path, so that the leak may be considered the equivalent of an opening in an infinitesimally thin wall. atmosphere (standard)—the pressure exerted by a mercury column 760 mm in height at 0 °C under standard acceleration of gravity; equivalent to 101 325 Pa. atmospheric pressure—the pressure of the atmosphere at a specified place and time (see Table 1). atomic mass unit (amu)—the unit of measure of the mass of a particle (atom, molecule, ion, and so forth), defined as 1 ⁄12 of the mass of carbon-12. The numerical value of the mass of a particle in terms of amu is identical with the older atomic weight. TABLE 1 Composition and Partial Pressures of the Atmosphere Constituent Volume % Partial Pressure, kPa At sea level (atmospheric pressure is 101 kPa): Oxygen 21 (0.21 × 101 =) 21 Nitrogen 78 (0.78 × 101 =) 79 Others 1 (0.01 × 101 =) 1 Total atmospheric pressure, 101 At 3700 m altitude (atmosphere pressure is 64 kPa): Oxygen 21 (0.21 × 64 =) 13 Nitrogen 78 (0.78 × 64 =) 50 Others 1 (0.01 × 64 =) 1 Total atmospheric pressure, 64 audible leak indicator—an accessory to a leak detector which converts the output signal to an audible note whose frequency is a function of the leakage rate. back pressure—same as forepressure. back pressure test—same as pressure-evacuation test. background signal—in leak testing, the steady or fluctuating output signal of the leak detector caused by the presence of residual tracer gas or other substance to which the detecting element responds. backing pump—same as fore pump. backing space—the space between a backing pump (fore pump) and the associated diffusion pump (or other type of pump requiring a fore pump). (See also ballast.) backing space technique—a method of testing for leaks in which the leak detector is connected to the backing space to take advantage of the compression of gas that occurs between the vacuum system and the backing pump, due to the action of the diffusion pump (or other type of pump of high speed relative to its backing pump). bake-out—in leak testing, the degassing of a vacuum system by heating during the pumping process. ballast—in leak testing, a backing space large enough to maintain a low forepressure when the fore pump is temporarily stopped. two sides of the orifice, expressed in volume units per unit time, such as cubic metres per second. cracking—in leak testing, same as dissociation. differential leak detector—a leak detector employing two similar gauge tubes in a bridge circuit with a trap which is selective for the tracer gas between the system and one of the tubes. differential Pirani gauge—a leak detecting device employing two similar Pirani tubes as arms of a Wheatstone bridge. diffusion—in leak testing, the flow of the gas through a substance in which the gas actually migrates through the crystal lattice of the substance rather than through a geometrical leak (molecular diameters versus hole dimension). discharge pressure—in leak testing, same as forepressure. discharge tube leak indicator—a glass tube attached to a system being leak tested, with the glass tube having electrodes attached to a source of high-frequency high voltage, such as a Tesla coil or induction coil, so that changes in the color of the electrical discharge can be observed when a suitable tracer gas (methane, carbon dioxide, alcohol) flows through the leak. dissociation—in leak testing, the breakdown of a substance into two or more constituents. DISCUSSION—Dissociation is sometimes referred to as cracking. Bayard-Alpert ionization gauge—see ionization vacuum gauge. bell jar—a container, open at one end (usually the bottom), which is used as a vacuum chamber or test vessel. drift—in leak testing, the relatively slow change in the background output level of the leak detector due to the electronics rather than a change in the level of the tracer gas. bell jar testing—a test used for detecting leakage from an object completely or partially filled with a tracer gas and placed in a vacuum chamber or bell jar. dynamic leak test—a form of leak test in which some of the tracer gas entering through a leak is continually removed for sensing purposes. bomb test—see pressure-evacuation test. bubble immersion test—a form of leak test of gas-containing enclosures in which a leak is indicated by the formation of a bubble at the site of a leak. dynamic leakage measurement—leakage determined by measuring the tracer gas equilibrium partial pressure while the system is actively being pumped. clean-up—in leak testing, the time required for a leak testing system to reduce its signal output to 37 % of the signal indicated at the time the tracer gas ceases to enter the leak system. Also called clean-up time. clusec—an obsolete unit of flow rate equal to 10-2 lusecs. cold-cathode ionization gauge—see ionization vacuum gauge. concentration ratio—in leak testing, the ratio of the number of atoms (molecules) of a given constituent of a (gas) mixture to the total number of atoms (molecules) in the mixture. For ideal gases, the concentration ratio has the same value as the volume fraction or the partial pressure of the constituent. conductance—in leak testing, the ratio of the throughput (under steady state, conservative conditions) of a gas flowing through a conduit or an orifice to the difference in the partial pressures of the gas at the two ends of the conduit or on the dynamic sensitivity of leak detector—the minimum leak rate that the detector is capable of detecting while the enclosure under test is actively being evacuated continuously under specified conditions. equivalent nitrogen pressure—the calculated pressure that a gauge or another device would indicate if the gas in the device were replaced by nitrogen at the same molecular density. exhaust pressure—in leak testing, same as forepressure. exhaust tubulation—same as pump-out tubulation. flooded system—a system which, while being tested, becomes so filled with tracer gas as to make impracticable further leak testing. flow—same as flow rate. flow rate—in leak testing, (1) the rate at which gas passes a given cross section of a system, determined by the product of the volume passing per unit time and its (partial) pressure at the cross section; (2) a product of the (partial) pressure difference of a gas at the ends of a conduit or across the face of an orifice, and the conductance of the gas for the conduit or orifice. Expressed in pressure-volume per unit time, such as pascal cubic metres per second. fore-line—in leak testing, the line between a fore pump and the pump it backs. fore-line valve—in leak testing, a vacuum valve placed in the fore-line to permit isolation of the diffusion pump from its backing pump. forepressure—in leak testing, the total pressure on the outlet side of a pump measured near the outlet port. Sometimes called the back pressure, backing pressure, outlet pressure, exhaust pressure, or discharge pressure. In discussing the action of a vapor jet, the term forepressure may be used to designate the total pressure of the gas against which the jet impinges. fore pump—in leak testing, the pump that produces the necessary fore vacuum for a pump which is incapable of discharging gases at atmospheric pressure. Sometimes called the backing pump. gauge pressure—difference between the absolute pressure and atmospheric pressure. gas—the state of matter in which the molecules are practically unrestricted by intermolecular forces so that the molecules are free to occupy all space within an enclosure. In vacuum technology, the word gas has been loosely applied to the uncondensed gas and vapor within a vacuum system. halogen—any element of the family of the elements fluorine, chlorine, bromine, and iodine. Compounds do not fall under the strict definition of halogen. However, for the purpose of this standard, this word provides a convenient descriptive term for halogen-containing compounds. Of significance in halogen leak detection are those which have enough vapor pressure to be useful as tracer gases. halogen leak detector—a leak detector that responds to halogen tracer gases. Also called halogen-sensitive leak detector or halide leak detector. (1) The copper-flame detector or halide torch consists of a bunsen burner with flame impinging on a copper plate or screen, and a hose with sampling probe to carry tracer gas to the air intake of the burner. (2) The alkali-ion diode halogen detector depends on the variation of positive ion emission from a heated platinum anode when halogen molecules enter the sensing element. helium bombing—a pressure-evacuation test in which helium is used as the test gas. helium drift—(1) in leak testing with a probe, the drift from a leak or permeable gasket located at some distance from the end of the probe but which is detected by the probe and can mislead the operator into suspecting the area near the probe; (2) a gradual wandering of the output meter needle of the leak detector due to slowly changing helium concentrations (either due to a leak or outgassing) in the detector tube. Expressed in scale divisions per unit time. helium leak detector—a leak detector using helium as the tracer gas. hermetically tight seal—a seal which does not exhibit leakage when dynamically tested with commercially built leak detectors that are sensitive to a gas on the pressure side opposite to the side on which the leak detector is located, or which does not exhibit leakage with any form of liquid test. high vacuum—see Table 2. holding pump—a fore pump used to hold a vapor pump at operating conditions while a roughing pump reduces the system pressure to a point at which the valve between the vapor pump and the system can be opened without stopping the flow of vapor from the nozzles. hood test—an overall test in which an object under vacuum test is enclosed by a hood which is filled with tracer gas so as to subject all parts of the test object to examination at one time. A form of dynamic leak test in which the entire enclosure or a large portion of its external surface is exposed to the tracer gas while the interior is connected to a leak detector with the objective of determining the existence of leakage. hot-cathode ionization gauge—see ionization vacuum gauge. hot-filament ionization gauge—see ionization vacuum gauge. hydraulic pressure test—same as hydrostatic test. hydrostatic test—in leak testing, a pressure test in which the component being tested is filled completely with water or another liquid. Pressure, if required, is then applied to the liquid for the required time and the outside of the component is examined visually for leaks. ideal gas—a gas that obeys Boyle’s law and has zero heat of free expansion (or also obeys Charles’ law). Also known as a perfect gas. in-leakage rate—the combined leakage rate from all existing leaks in a specified evacuated vessel in pressure volume units per unit of time. inlet—the opening, flange, connection, or coupling on a leak detector or leak testing system through which the tracer gas may enter due to a leak in an object under test. inlet flange—see inlet. inlet port—see inlet. inside-out testing—see bell jar testing. ion pump—an electrical device for pumping gas comprising a means for ionizing the gas and a system of electrodes at suitable potentials, and in some cases also a magnetic field, which causes the ions formed to move towards a surface on which they are absorbed or buried. TABLE 2 Degrees of Vacuum Degrees of Vacuum Low Medium High Very high Ultra high Approximate Pressure Range 100 kPa to 3 kPa 3 kPa to 0.1 Pa 0.1 Pa to 0.1 mPa 0.1 mPa to 0.1 µPa 0.1 µPa and less