## 1 Introduction  
Active infrared thermography (IRT) can be employed to document differences in the structure and material composition of objects. During IRT, an object is thermally excited and the thermal response of the object is measured, on the surface, by an infrared camera. Two main types of IRT are pulse thermography (PT), which is relatively straightforward and easy to use, where the object is thermally excited by a short rectangular pulse and then returns to an equilibrium, and lock-in thermography (LIT), where the object is subjected to continuous thermal excitation by a periodic signal. LIT has a potentially higher signal-to-noise ratio than PT [1, 2], which would mean it could achieve comparable or better results at lower temperature fluctuations. It also allows for a more gradual heating up of the object, reducing thermal shocks [2]. This is of particular interest when inspecting cultural heritage objects, where temperature fluctuations must be minimized in order to prevent degradation. In preventive conservation, often a fluctuation of \(\pm 3\)\({}^{\circ}\)C per 24 h is suggested as a threshold value. Because IRT is a contactless and non-destructive technique that can easily be used in situ, it is of interest to conservators. It is, therefore, becoming more widely adopted in heritage science, a field that recently reported various advances in the development of different imaging modalities [3]. Thus far, the applicability of IRT has been explored to inspect a variety of cultural heritage objects. The studied objects include, but are not limited to, paintings [2; 4; 5; 6; 7; 8; 9; 10; 11; 12], frescoes [13; 14; 15], mosaics [14; 16; 17], marquetries [7; 18; 19], ancient texts [1; 6; 20], ancient bronzes [1; 6; 20], glass-in-lead windows [15; 21; 22] and ceramics [15]. Specifically, for paintings, IRT has successfully been employed to detect several kinds of defects [2; 4; 6; 8; 9; 10; 11; 12; 23] and hidden compositions [4; 5; 6; 7], as well as to visualize the woodgrain in panel paintings [4; 5; 7; 9; 10] and the weave pattern of the canvas [5; 7; 8; 9]. For these purposes, several well-known PT processing methods are employed, such as Principal Component Thermography (PCT) [4; 5; 7; 9], Partial Least Squares Thermography (PLST) [8], Pulse Phase Thermography (PPT) [6; 9; 11], Differential Absolute Contrast (DAC) [10] and Thermal Signal Reconstruction (TSR) [12]. The advantages and disadvantages of the most prominently used methods have been discussed by [PERSON] et al. [23] and [PERSON] et al. [9]. In contrast, LIT was tested to a much lesser extent [2; 7]. Recently, a novel technique called pulse-compression thermography has also been used [2], which also shows promise as a way to minimize heat fluctuations during measurements. Thermal cameras generally operate in the mid-wave infrared (MWIR) spectral range (3-5 um) or the long-wave infrared (LWIR) spectral range (7-14 um). An added value of MWIR cameras is that they can also be used for infrared reflectography (IRR) measurements [6; 7; 13; 24], a routine imaging technique in the museum field, which provides complementary information. In this way, an MWIR camera setup could allow for performing both measurements with the same system, which simplifies image registration between the different imaging modalities. As discussed, there already exist a variety of studies where IRT is used for the evaluation of cultural heritage objects. In these studies, various cameras that operate in different wavelength ranges are used, but the effect of the selected wavelength range on the results is almost never considered [7; 25]. LIT could potentially operate with lower temperature fluctuations than PT, but these techniques have not been compared for the inspection of paintings. In this study, the performance of LIT was compared with that of PPT, a prominently used PT technique that is closely related to LIT. In order to investigate the influence of the spectral range of the camera, measurements were taken with both MWIR and LWIR cameras.