OPTICAL AND LASER NDT: A RISING STAR

Productivity increase, consumer satisfaction and safety require that manufactured products or structures be thoroughly inspected or characterized during fabrication, after fabrication or in service. Characteristics that deserve evaluation are shape and thickness, material composition, material microstructure and surface and inner defects of various kinds that may affect product integrity (such as delaminations, cracks, porosity, disbonds). These requirements are satisfied by a broad panoply of light and laser-based techniques using technical approaches going far beyond visual inspection. These techniques are photogrammetry, laser triangulation, fringe projection, Moiré, D-Sight, Edge-of-light, Optical Coherence Tomography (OCT) for the evaluation of shape and surface profiles, Laser Induced Breakdown Spectroscopy for composition determination, holography, Electronic Speckle Pattern Interferometry and Shearography for the detection of flaws, laser-ultrasonics for the detection of flaws and microstructure characterization. OCT and another technique called Photon Density Waves can probe transparent or translucent materials. In this presentation, a broad overview of all optics or laser-based NDT/NDE techniques is presented, outlining their present industrial use and future perspectives. Introduction: Light is certainly at the basis of the most widespread NDT technique: visual inspection. This technique is also likely to be the oldest one, dating back to prehistoric times when human beings started to fabricate rudimentary tools and objects. Presently, at the beginning of the third millennium, following the invention of the laser in 1960, the introduction of optical fibers for guiding light, solid state imaging detectors and computers for acquiring and processing complex data and images, light-based techniques occupy a much larger place in the spectrum of NDT or NDE techniques. Light can obviously be used for probing transparent materials but also completely opaque ones, either by measuring surface deformations that have their origin deep inside the probed part or using ultrasound generated and detected by lasers. In this presentation, NDE or NDT is defined broadly as encompassing all the inspection methods and techniques for finding out if materials, manufactured or in-fabrication objects and structures satisfy specifications, are defect-free and safe. Note that a flawed product can be defective in many aspects. First its shape, size or thickness may be out of specifications. Its composition may also be improper or the material may not have the desired microstructure. Its physical properties, particularly its mechanical properties may be insufficient for the intended purpose. Finally its flawed nature may be associated to cracks, disbonds, porosity either produced at fabrication or caused by a degradation process (fatigue, corrosion, erosion, creep...). Light-based techniques, of one kind or the other, are capable of detecting all these out-of-specification or defective features. A key characteristic of all light based techniques is the operation without contact and at a distance. As a consequence they can more easily probe materials and products on the production line when the material is at an intermediate state (e.g. hot) or in a raw state (e.g. molten) and products with complex or odd shapes. Note also that light based techniques can be classified as point techniques in which the object is probed at a single location and an image is obtained by scanning the object or the probe, and full-field techniques in which a whole area of the object is analyzed in parallel at once. Since this subject is very broad, the presented techniques will not be described in detail and essentially only the ones that are currently used in industry or have this potential will be mentioned, leaving aside techniques applicable essentially in the laboratory or used for mechanical engineering studies. Unlike previous reviews that covered sub-fields such as shape determination techniques or speckle techniques, this presentation is very broad and includes essentially all remote light-based techniques for inspection or characterization. Visual inspection: this is the oldest NDT technique but still widely used [1]. It is used in the automobile industry where the visual aspect a car body is an important factor for sale. Specific approaches have been developed to measure in particular paint gloss, reflectance and surface roughness. Systems have also been developed to look at the surface conditions of metal strips (made of steel or aluminum) on a processing line. Visual inspection is also very important in the airline industry, and accounts for most of the inspection tasks in search for cracks and corrosion. Light guiding systems (boroscopes) have been developed for looking in areas where direct eye inspection is not possible. Present systems have benefited from the development of optical fibers technology and are very flexible. Direct eye vision is often advantageously replaced by artificial vision with a video camera using an imaging detector (such as a Charge Coupled Device or CCD). Measuring shape and surface profile: they are many known techniques available for measuring the shape of an object [2]. One is based on photogrammetry (also called stereo vision imaging) and consists in reconstructing the shape from two or more pictures of the object. A point technique called laser triangulation is sketched in Fig.1. In laser triangulation, the surface is illuminated by a narrow beam of light and the image of the light spot on the surface through a lens is tracked by a linear detector. Knowledge of the parameters of the sensor and calibration allows finding the distance between the sensor and the illuminated spot on the surface. Full shape determination requires scanning of the sensor or the object. An extension of laser triangulation is the fringe projection technique (called also structured light). In this full field technique, sketched in Fig.2, a Ronchi grating is projected onto the object. The image of the object modulated by this fringe pattern is recorded by a CCD camera. The deformed fringe pattern provides a mapping of the object elevation in the direction of observation, so shape can be retrieved after analysis. Another technique uses an additional grating (the reference grating) to produce by combination (interference or beating) of these two gratings a Moiré pattern (see Fig.3) requiring less resolution of the camera. A variant of the technique uses the same grating at projection and analysis (shadow Moiré). Other known techniques are based on the measurement of the time-of-flight of short laser pulses, the enhancement of surface deformation by retroreflection (D-sight technique [3]) and the detection of shadow edge deflection by surface slope (Edge-of-light technique [4]). Probing inside transparent or translucent materials: even if a discontinuity in a perfectly transparent material can rather easily be detected by looking through it, finding its depth is not straightforward. Such a capacity is obtained with a relatively novel technique called Optical Coherence Tomography (OCT). This technique is also applicable to translucent materials. OCT is in fact an interferometric technique that uses a broadband source (super luminescent diode) and is based on the principle of white light interferometry. As sketched in Fig. 4, which shows a modern version of this technique using fiber optics, an interference signal is obtained when the pathlengths (or delay times) along the path going to the sample and the reference path are equal. The length of this reference path being scanned, the depth of discontinuities inside the tested specimen can then be measured [5]. OCT is a point technique, so an image is obtained by scanning either the probe or the