## 1 Introduction  
Non-destructive testing (NDT) is globally progressing in the world of material inspections. The purpose of these tests is to inspect the structural integrity of objects without damaging the product itself. Until today, extra products needed to be produced to put them through destructive tests. Those non-destructive evaluations provide a swift, reliable and cost-effective manner to investigate objects without harming their purpose [1; 2]. There exist two types of infrared thermography based on the source of the evaluated temperatures. Every object exceeding absolute zero (0K) emits infrared radiation. In passive thermography, the measured temperature differences are solely dependent on the emissivity and the temperature of the object to be inspected. Active thermography depends on exciting the test object using an external source in order to measure the temperature change between the heating and cooling phase. Multiple excitation sources exist in active thermography based on different heating techniques such as halogen lamps, flash lamps, laser heating ultrasonic excitation, microwaves, eddy currents excitation, etc. In order to choose the right source to perform infrared measurements, one should consider the advantages and disadvantages of each source. Laser heating makes it possible to heat only small regions. Halogen lamps on the other have the advantage to heat the specimen in a cheaper and safer way. Halogen lamps are widely used in infrared thermography because of the cost and broad wavelength spectrum. A disadvantage of using halogen lamps is the large heating region of the object to be inspected. One could try to minimise the heated region by using a slit allowing only a part of the heating energy to leave the reflector of the heating source. With the aid of a slit, the rays outside the desired heating area are blocked and a loss of possible heating energy occurs. One can see that the amount of blocked rays should be minimised. Additionally, diffraction causes the available rays to diverge out of the aperture further reducing the heating energy density. Figure 1 displays two preliminary set-ups that are used in our lab to minimise the heated region. Designing an optimised reflector could offer a safe and cost-effective heating source when combined with a halogen tube lamp. It could be a great substitute for using laser excitation and could facilitate the further development of Dynamic Line Scan Thermography. To increase the temperature difference between the heating and cooling down of the object, the available heating power of the halogen lamp needs to be concentrated in a small region. Concentrating the power enables the possibility to deliver more excitation power in a small time period. This results in a powerful heating. The heating source approximates rather flash excitation than long step heating. Several studies are performed in order to focus electromagnetic radiation in order to produce heat. Almost all these applications are powered by sunlight and use parabolas in order to focus the radiation in a single point [3; 4; 5]. According to the reflecting property of a parabola, rays originating in the focal point of the parabola will be reflected parallel to each other. [PERSON] et al. [6; 7] investigated the possibility to use parabolic and elliptical mirrors to generate local heating of high-strength steels. [PERSON] and [PERSON] [8]  
Figure 1: Two experimental set-ups are displayed as they are used in our lab: (**Left**) cylindrical reflector constructed to concentrate the radiation in a narrow line. This heater is used in dynamic measurements as a hand-held heater. Starting from a hollow tube a slit was milled with the desired width in order to create the reflector shape. (**Right**) empirical test-setup for Dynamic Line Scan Thermography on a conveyor belt. A wooden plate wrapped in aluminium foil was mounted underneath a patio heater. In the middle of the plate, a slit with desired width and length was cut in order to create the reflector shape.