NDT GLOSSARY — Visual Testing
(Entries adapted from the Nondestructive Testing Handbook, third edition: Vol. 9, Visual Testing. Columbus, OH: American Society for Nondestructive Testing (2010).
blind spot: Portion of the retina where the optic nerve enters, without rods and cones and hence insensitive to light.
blister: Discontinuity in metal, on or near the surface, resulting from the expansion of gas in a substance zone. Very small blisters are called pinheads or pepper blisters.
cocoa: Debris (usually oxides of the contacting metals) of fretting wear, retained at or near the site of its formation – a condition easily identified during visual tests. With ferrous metals, the debris is brown, red, or black, depending on the type of iron oxide formed. For this reason, ferrous debris is called cocoa or, when mixed with oil or grease, red mud.
defect: Discontinuity whose size, shape, orientation or location (1) makes it detrimental to the useful service of its host object or (2) exceeds an accept/reject criterion of an applicable specification. Some discontinuities do not exceed an accept/reject criterion and are therefore not defects.
etch crack: Shallow crack in hardened steel containing high residual surface stresses, produced in an embrittling acid environment.
fluorescence: Phenomenon of absorption of electromagnetic radiation and its reemission at a lower energy (longer wavelength). In visual testing, fluorescence is typically a response to ultraviolet radiation.
gray level: integer number representing the brightness or darkness of a pixel or, as a composite value, of an image comprised of pixels.
jaeger eye chart: Eye chart used for near vision acuity examination.
kinetic vision acuity: Vision acuity with a moving target. Studies indicate that 10 to 20 percent of visual efficiency can be lost by target movement.
mottle: Apparently random positioning of metallic flakes that creates an accidental pattern.
replica: Piece of malleable material, such as polyvinyl or polystyrene plastic film, molded to a test surface for the recording or analysis of the surface microstructure.