Flaw detectors are used to detect or measure defects in raw materials or finished products. This product area includes fault detectors, NDT instruments that are used to find faults in electrical, mechanical, optical, or other systems. Examples of faults include an arcing circuit breaker (electrical) and failed roller bearing (mechanical). Because both electrical and mechanical faults produce characteristic sounds, fault detectors may use air or structure-borne acoustic detection techniques. In addition to technology, flaw detectors differ in terms of form factor, applications, and features.

There are many technology choices for flaw detectors. Acoustic emission instruments use AE sensors to monitor in electrical power and other systems. Air and structure-borne technologies also detect acoustic signals. Infrared (IR), nuclear, and beta gauge instruments use the absorption of radiation to measure the thickness or basis weight of webs, sheet materials, or coatings. Eddy current, penetrating radar, and other electromagnetic techniques are used to detect or measure flaws, bond or weld integrity, thickness, and electrical conductivity, and to detect the presence of rebar or metals. Electromagnetic acoustic resonance (EMAR) instruments use a non-contact electromagnetic transducer to impart a signal for resonant acoustic analysis. Magnetic techniques used in flaw detectors include Hall effect and induction. Magnetic particle systems use current flow or an external magnet to magnetize the part.

Technologies for flaw detectors include optical or laser methods, penetrant testing systems, radiographic or X-ray equipment, and ultrasonic (UT) inspection techniques. Optical-based NDT instruments use techniques such as laser shearography, magneto-optical methods, and holographic interferometry to detect flaws and residual stress, or to measure thickness. Penetrant testing systems apply a penetrant to the part via spray or immersion. The penetrant is then pulled into surface flaws by capillary action. Radiographic or X-ray flaw detectors use penetrating X-rays or gamma rays to capture images of the internal structure of a part or finished product. Ultrasonic (UT) inspection techniques are used to detect surface and subsurface flaws, or to measure thickness. Beams of high frequency acoustic energy are introduced into the material and subsequently retrieved. Distance calculations are based on the speed of sound through the material under evaluation.

Specifications for flaw detectors include flaw detection or inspection area, measurement of nonconductive materials, and thickness measurements. Choices for flaw detection applications include aircraft or military inspection, coating flaw or holiday, debonding or bond integrity, general corrosion detection, inclusions, machining or grinding damage, metal detector or rebar absence, manufacturing of metals or webs, packaging defects or leaks, pore or void, pressure vessel flaws or cracks, rail inspection, subsurface crack detection, surface cracks or abrasion, tube or pipeline defects, weld or joint inspection, and wire rope or cable tester. Some flaw detectors are capable of testing components fabricated from nonconductive materials such as ceramics, composites, glasses or polymers. Other NDT instruments can measure thickness in addition to the unit's primary function, flaw detection.