# Design and Construction of an LED-Based Excitation Source for Lock-In Thermography
## 1 Introduction  
Due to its flexibility, active thermography is used in many fields of applications [1]. Pulsed, step-heating, and lock-in thermography are among the most commonly applied techniques. A wide range of post-processing, image correction and analysis procedures have been developed [2; 3; 4]. All methods of active thermography have in common that they require an external excitation source. Depending on the nature of the object and the aim of the investigation, an appropriate source has to be chosen. Excitation sources based on the principles of inductive, microwave, resistive, mechanical, and convective heating have been used [5]. However, the most commonly used heat sources are based on optical excitation. The sources can be lasers in the form of spot lasers, line lasers, or laser arrays, while for short and powerful excitation, flash lamps are suitable. In other cases, projectors or halogen lamps in combination with shutters are sufficient. As an alternative, LEDs can be used, which is described in the following. To achieve good results in non-destructive testing, a well-designed excitation source is of central importance, a range of performance criteria are discussed in the community [1]. First, the excitation should have a high degree of spatial homogeneity or at least a well-known radiant intensity distribution. In addition, the source has to be powerful enough to enable thermal measurements with sufficient signal-to-noise ratio. To obtain robust results, a good repeatability is necessary, both over the long and the short term. In case of optical excitation, this includes effects such as lamp aging or self-heating of the lamp during operation. Furthermore, the signal shape of the excitation is important. This includes the pulse duration for flash excitation, the steepness of the step for step excitation, or the harmonic distortion for lock-in applications. For lock-in applications in particular, the response characteristics of the source are relevant, i.e., amplitude and phase response as a function of the lock-in frequency. For optical excitation, an overlap of the emission spectrum with the infrared detection range should be avoided. Lock-in thermography is a technique, which relies on a stable periodic signal to create amplitude and phase images of thermal waves. The technique is typically used for defect detection [6,7] or the measurement of material parameters such as the thermal diffusivity, see, e.g., [8, 9, 10]. Layer thickness measurements are also possible, see, e.g., [11, 12, 13]. Over time, lasers have become more and more attractive as excitation sources for lock-in thermography. They are used in a variety of different setups. An application example of a spot laser in modulated continuous wave mode is given by [PERSON] et al. [14]. In their work, the authors highlight the advantages of laser lock-in thermography such as long-distance energy transmission and highly controllable emission. The importance of being able to modulate the excitation source arbitrarily is highlighted by [PERSON] et al. [15]. They use the arbitrary modulation capability of a diode laser for multiplexed lock-in frequencies to reduce the temperature drift in a lock-in measurement due to DC components. The spatial focus is one of the main advantages of laser lock-in thermography. Spot lasers enable for thermal diffusivity measurements via the slope method [16]. In defect detection, the small beam width and high power of lasers is utilized in laser scanning for high spatial resolution [17,18]. Spatially more extended excitation is available from vertical-cavity surface-emitting lasers (VCSEL) [19]. Using VCSELs, highly controllable excitation up to several kilowatts is available. Other sources that have been investigated include LCD projectors [20] or laser-coupled digital micromirror devices (DMD) [21]. A valuable comparison of common excitation sources for lock-in thermography is given by [PERSON] et al. [22].