ULTRA-BLUE LIGHT (PT): Monochromatic blue light of approximately 4300 AU wavelength used to cause certain
dye penetrants to fluoresce.
ULTRASONIC (UT): Pertaining to mechanical vibrations having a frequency greater than approximately 20,000 hertz.
ULTRASONIC ABSORPTION: A dampening of ultrasonic vibrations that occurs when the wave transverses a
ULTRASONIC SPECTRUM; The frequency span associated with elastic waves greater than the highest audible
frequency, generally regarded as being higher than 2.0 x 104 cycles per second, to approximately 109 cps.
ULTRASONIC TESTING: A nondestructive method of testing materials by transmitting high frequency sound waves
UNDERBEAD CRACK: A subsurface crack in the base metal near the weld.
UNDERCUT (RT): A depression or groove adjoining the toe of a weld in a metal object. Appears on a radiograph as a
UNDERCUT (RT): Undercut is a term that is used to describe the excessive radiation intensity that may be found at the
edge of an object. Such undercutting is usually associated with scattered radiation.
UNDER-DEVELOPMENT (RT): Development that is less than that required to produce the optimum results in a
particular radiograph. It may arise from development for too short a time, or at too low a temperature, or from the use
of exhausted developer.
UNDERFILL: Storage of metal so that the true shape is not completely filled.
UNIAXIAL STRESS: A state of stress in which two of the three principal stresses are zero.
UNIDIRECTIONAL: Having one direction only.
UNIDIRECTIONAL VOLTAGE: A voltage of which the polarity, but not necessarily the magnitude is constant.
UNIFORM STRAIN: The strain occurring prior to the beginning of localization of strain (necking); the strain to
maximum load in the tension test.
UNRESTRICTED AREA (RT): Any area to which access is not controlled for purposes of radiation protection.
UNSHARPNESS (RT): Unsharpness is a term used to describe the lack of definition of an edge due to geometric factors
related to the source size and the source-to-film distance.
UPSETTING: Working metal so that the cross-sectional area of a portion or all of the stock is increased.
USE FACTOR (RT): The fraction of the workload during which the useful beam is pointed in the direction under
consideration when designing shielding. Symbol: U.
USEFUL BEAM (RT): All radiation that emerges from a source housing or an X-ray tube assembly through a port,
diaphragm, or cone.
USEFUL DENSITY RANGE (RT): The range of density over which the gradient is adequate for the recognition of
image details. The upper density limit is determined mainly by the brightness available in the film illuminator, and the
lower density limit by the sensitivity required.