## 2 The Past: 1981-2010  
In the early 1980's, the UPS technique was first introduced by [PERSON] and [PERSON] in a pulsed version in order to assess the fiber orientation of composites [1]. The beauty of the intriguing patterns made the pioneering authors state [1]: _\"A library of Polar-patterns, stored as 'fingerprints', raises the possibility of laminate identification by pattern recognition\"_. It is unfortunate that in the following years, only one sequel study was performed by [PERSON] and [PERSON] to further explore the capabilities of the ultrasonic polar scan [6].  
It took fifteen years before the technique was investigated again through the work of [PERSON] [7, 8, 9]. He used a modernized scanning system to obtain more accurate and detailed P-UPS experiments. The work of [PERSON] has identified several practical applications of the P-UPS technique  
Figure 1: Schematic of ultrasonic polar scan (UPS) method (**a**). State-of-the-art pulsed ultrasonic polar scan (P-UPS) recordings at \(f_{c}=5\) MHz: aluminum with thickness \(d=0.6\) mm (**b**) and [0\({}^{\circ}\)]s C/E laminate with thickness \(d=1.1\) mm (**c**). “QL”, “QT-H” and “QT-V” stand for quasi-longitudinal wave and quasi-transverse wave with horizontal and vertical polarization respectively.  
for composite materials: (i) estimation of fiber direction; (ii) determination of fiber volume fraction and porosity and (iii) detection of fatigue damage [8; 9]. In addition, he and [PERSON] implemented a numerical procedure for simulating P-UPS images of homogeneous composites [7], in view of bringing the technique to the next level: full quantitative characterization of the elastic properties of composite materials using a mixed experimental-numerical approach. Although the gap between experiment and simulation was not yet bridged, their numerical results did contribute to the physical understanding of the formation of a P-UPS image. They found that the characteristic patterns are (more or less) a representation of critical bulk wave angles, while the global transmission amplitude exposes attenuation properties [8]. Consequently, a P-UPS image may be used for characterizing viscoelastic material properties.  
The ultrasonic polar scan research has been further extended by [PERSON], first as a student of [PERSON] [2; 10; 11] and afterwards as a professor at the Georgia Institute of Technology in Metz [12]. He applied the technique for detecting tension-tension induced fatigue damage in glass fiber composites by tracing shifts in the characteristic fingerprint [10]. In addition, [PERSON] extended the simulation technique towards layered viscoelastic materials having arbitrary anisotropy using a global matrix method [2; 11]. He experimentally implemented a time-of-flight (TOF) version of the P-UPS method and commented on the superior sensitivity (compared with amplitude recording) to the presence of damage features [12]. Here, TOF is defined as the arrival time of the peak amplitude of the transmitted ultrasound pulse. As an illustration, Figure 2 displays state-of-the-art amplitude and TOF landscape of a P-UPS for a [0\"]s C/E laminate [13].  
It is clear from the above results that the UPS technique already had several applications, but it is evenly clear that many capabilities and opportunities still have to be explored.