TY - GEN A1 - Ziegler, Mathias T1 - Locating subsurface defects with structured heating using high-power VCSEL-laser arrays N2 - Among the photothermal methods, full-field thermal imaging is used to characterize materials, to determine thicknesses of layers, or to find inhomogeneities such as voids or cracks. The use of classical light sources such as flash lamps (impulse heating) or halogen lamps (modulated heating) led to a variety of nondestructive testing methods, in particular lock-in and flash-thermography. In vertical cavity surface emitting lasers (VCSELs), laser light is emitted perpendicular to the surface with a symmetrical beam profile. Due to the vertical structure, they can be arranged in large arrays of many thousands of individual lasers, which allows power scaling into the kilowatt range. Recently, a high-power yet very compact version of such a VCSEL-array became available that offers both the fast timing behavior of a laser as well as the large illumination area of a lamp. Moreover, it allows a spatial and temporal control of the heating because individual parts of the array can be controlled arbitrarily in frequency, amplitude, and phase. In conjunction with a fast infrared camera, such structured heating opens up a field of novel thermal imaging and testing methods. As a first demonstration of this ansatz, we chose a testing problem very challenging to conventional thermal infrared testing: The detection of very thin subsurface defects perpendicularly oriented to the surface of metallic samples. First, we generate destructively interfering thermal wave fields which are then affected by the presence of defects within their reach. It turned out that this technique allows highly sensitive detection of subsurface defects down to depths in excess of the usual thermographic rule of thumb, with no need for a reference or surface preparation. T2 - 19th International Conference on Photoacoustic and Photothermal Phenomena CY - Bilbao, Spain DA - 16.07.2017 KW - Thermografie KW - Laser KW - Zerstörungsfreie Prüfung KW - Thermography KW - Non-destructive testing PY - 2017 UR - https://opus4.kobv.de/opus4-bam/frontdoor/index/index/docId/41829 AN - OPUS4-41829 AD - Bundesanstalt fuer Materialforschung und -pruefung (BAM), Berlin, Germany