MATERIALS TESTING FOR JOINING AND ADDITIVE MANUFACTURING APPLICATIONS 213 Laser speckle photometry – Optical sensor systems for condi- tion and process monitoring Lili Chen, Ulana Cikalova, Beatrice Bendjus, Laser speckle photometry (LSP) is an innovative, non-destruc- Andreas Gommlich, Shohag Roy Sudip, tive monitoring technique based on the detection and analy- Carolin Schott, Juliane Steingroewer, Dresden, sis of thermally or mechanically activated speckle dynamics Matthias Belting, Aachen, and in a non-stationary optical field. With the development of Stefan Kleszczynski, Duisburg, Germany speckle theories, it has been found that speckle patterns carry information about surface characteristics. Therefore, LSP offers a great potential for the characterization of mate- rial properties and monitoring of manufacturing processes. In contrast to the speckle interferometry method, LSP is very simple and robust. The sample is illuminated by only one la- ser beam to generate a speckle pattern on the surface. The Article Information signals obtained are directly recorded by a CCD or CMOS Correspondence Address Dr.-Ing. Beatrice Bendjus camera. By appropriate optical solutions for the beam path, Fraunhofer Institute for Ceramic typically, resolutions of less than 10 μm are reached if larger Technologies and Systems IKTS Maria-Reiche-Str. 2 areas are illuminated. LSP is definitely a contactless, quick 01109 Dresden, Germany and quality relevant material characterization and defect de- E-mail: [email protected] tection method, allowing process monitoring in many indus- Keywords trial fields. Examples from online biotechnological monitoring Laser speckle photometry, process-monitoring, defect-monitoring, build-up welding, additive and laser based manufacturing demonstrate further poten- manufacturing, biomass tials of the method for process monitoring and controlling. Material testing and material properties and practice, is presented, thus offering an ent laser beam upon an optically rough characterization, which guarantee the innovative approach to optical NDT and surface that results in a boiling grainy im- quality of materials and promote progress monitoring. age [2]. The theory of speckle or dynamic in manufacturing, play a significant role in speckle belongs to the category of statisti- both academic and industrial fields. New Theoretical approach cal optics for which a detailed account of non-destructive testing (NDT) methods the phenomena is given by Goodman [3], and appropriate sensors developed in the Speckle or speckle patterns are interfer- including the mathematical description of industry are used for detecting defects ence patterns produced by a scattering co- speckle properties, for instance the central such as porosities or delaminations as well herent light beam. Scientists have investi- limit theorem of probability theory and as for the characterization of delamination gated this phenomenon since the time of random processes. parameters and material properties re- Newton, but it was only after the invention Techniques based on laser speckle ef- spectively for the description of their and development of the laser that speckles fects, which can be contactless, non-inva- changes in manufacturing processes. have finally come into prominence. Until sive and remote, have become more and Therefore, choosing a suitable measuring now, numerous speckle related applica- more popular in NDT. One of the earliest method is important for fabrication in tions have been found. The dynamic techniques was laser speckle photography. terms of adapting process parameters and speckle is a result of the temporal evolu- Here, speckle patterns are recorded on a avoiding waste. In addition, the sensors tion of a speckle pattern which can be con- photographic emulsion called a photo- should be integrated inline as a control in sidered as the temporal expression of an graphic negative before and after deforma- manufacturing plants. In this paper, Laser interference speckle pattern [1]. It is gener- tion of an object. By using this negative to speckle photometry (LSP), both in theory ated by the incidence of a high-level coher- re-photograph the fringe pattern through 61 (2019) 3 © Carl Hanser Verlag, München Materials Testing 214 MATERIALS TESTING FOR JOINING AND ADDITIVE MANUFACTURING APPLICATIONS illumination by a laser beam, quantity in- change in interference fringes. Therefore, Relying on high speed cameras and easier formation of an object’s deformation can be the defect of an object makes the fringes configurations compared with the experi- obtained by estimating the diffraction look unusual. Speckle interferometry is a mental setups presented above, LSP can fringes. A laser speckle image encodes well-established optical technique for also be used in real time monitoring, and it phase information and phase differences measuring in-plane displacements [10]. In has a high sensitivity to both out-of-plane generated by variations in travel distance this section, three kinds of interferometry and in-plane displacement. Deviating from which are affected by tiny changes in the will be introduced, namely holographic in- other techniques based on speckle phe- surface geometry. If the surface profile is terferometry, electronic speckle pattern nomena which concentrate on the distor- altered by an amount as small as a laser interferometry (ESPI) and shearography. tion of a whole speckle pattern or fringes, wavelength, the speckle pattern is modi- Holography is a technique which can re- LSP is based on measuring the spatial-tem- fied [4]. An algorithm called cross-correla- cord both amplitude and phase information poral dynamics of laser speckles which fo- tion is established to evaluate the images from a light beam reflecting off objects. In cuses on the intensity change of each sin- [5]. It measures the deformation of the ob- interferometry a laser beam is split into gle pixel of a camera sensor. Through a ject surface by associating patterns with two beams, one, called a “reference beam”, specific correlation function (difference the surface that deforms. is used as a reference, the other, the” object autocorrelation), the connection between A second important speckle method, la- beam”, is used to illuminate the object in speckle dynamics and quality states of the ser speckle contrast analysis (LSCAS), is a question. ESPI and shearography use two samples can be established [14-16]. The digital version of the single exposure ver- different optical setups for speckle interfer- basic algorithm of LSP includes the calcu- sion of speckle photography which avoids ometry. ESPI is based on the same princi- lation of thermal diffusivity by applying the main disadvantages of the photo- ples as holographic interferometry but it the thermal transfer equation [14-18] and graphic process. LSCAS is a diagnostic electronically records and processes active thermography for crack detection technique in which the features of scat- speckle interference patterns [11]. The [15]. The algorithms will be introduced in tered coherent light are explored [6]. At visualization is achieved in the form of the next section. In recent years, LSP tech- present, it is alternatively called “laser fringes which approximately represent a nique has been successfully used for the speckle contrast imaging” (LSCI) or “laser displacement of half a wavelength of the hardness calibration of steel samples, the speckle imaging” (LSI). These techniques laser beam. The sensitivity of in-plane dis- evaluation of material porosity of cellular are mainly used in the medical field to de- placement equals the value of half a wave- metals and ceramics, damage fatigue tect blood flow and organic tissues whose length. The features of shearography in- states and stress change during welding local speckle contrast varies according to clude sensitivity to out-of-plane deforma- processes. the velocity distribution of the targets [7- tion and the capability to directly measure LSP represents a promising foundation 9]. The speckle contrast is shown in an strain data in real time but not the dis- for new NDT methods and appropriate sen- equation, which is defined as the ratio be- placement of objects because rigid body sors to be developed by industry to charac- tween the standard deviation of the speckle movement produces displacement as well. terize material properties or describe their pattern and the mean intensity. A formula Shearography was used to detect large changes in the manufacturing process. It is presenting the relationship between vari- cracks in reinforced concrete and the intra- definitely a fast quality relevant material ance of a time-averaged dynamic speckle dos of bridges [12]. Micro-cracks in a glass- characterization and defect detection pattern and the temporal fluctuation statis- fiber reinforced plastic tube stressed by a method, allowing for the monitoring of pro- tics is now given [8]. 0.04 MPa internal pressure can be detected duction processes completely in-line. The A third well-known speckle-method is by shearography as well [13]. sensor system developed comprises the op- laser speckle interferometry. Interferomet- A novel speckle method for NDT is laser tical and electronic components, algo- ric techniques are effective tools for visual- speckle photometry (LSP) [14, 15] which is rithms, the hardware and software. The izing or measuring object deformation. based on the detection and analysis of ther- monitoring process is synchronized with When an object