## 2 DLIT System  
Thermographic cameras use infrared detectors that detect incident infrared radiation which is converted to temperature values. This results in a map of temperature values also known as thermograms. Inspected parts are usually in thermal equilibrium and therefore require the use of a heat source to produce a thermal contrast between any detected regions of interest and the background material. This is known as active thermography. Several options for heat sources exist in inducing the required thermal contrast. In the context of photovoltaic materials, the two main options are the use of a light source and voltage application. Light transfers its energy as heat to the sample whereas the application of the voltage induces the flow of current in a conductor material which in turn results in the generation of heat.  
In both cases, constant application of the heating would lead to lateral heat conduction within the sample. This results in poor spatial resolution even with the use of current commercially available infrared detectors that are capable of achieving sub mm spatial resolution. Modulated heating in combination with lock-in post processing of the data could overcome this limitation, offering excellent spatial resolution that increases with increasing modulation frequency, and significantly improved signal-noise-ratio due to its averaging nature of the lock-in correlation procedure [20].  
The thermography inspection system will be eventually implemented and validated on a roll-to-roll (R2R) electrical testing line at VTT [18; 19]. The test operations are performed by general-purpose electrical test and measurement instruments and layout-specific test fixtures, which can be equipped with up to 400 arbitrarily configurable test pins. The pins are used to provide physical access to the printed structures on the web to be tested, and the typical test applications are measurements of basic electrical parameters, such as resistance, current or voltage. As such, the application of voltage--also known as Dark Lock-In Thermography (DLIT)--was preferred over the use of a light source (Illuminated Lock-In Thermography--ILIT) in this study.  
In the DLIT system, as shown in Figure 1, a periodically modulated voltage is applied to the test sample while being imaged using an infrared thermal camera (FLIR A6751 MWIR F/2.5). This cooled indium antimonide (InSb) camera offers high-speed windowed frame rates (up to 120 Hz), performs a variety of non-destructive testing, and provides 327,680 (640 \(\times\) 512) pixel resolution. Along with thermal images, the camera can also record the synchronized square wave reference signal that is generated using a signal generator (RS Pro RS DGS05 Signal Generator). The in-house developed software comprises data acquisition, data visualization, lock-in post processing and data analysis capabilities through a graphical user-interface (GUI).  
During the thermography test, the camera records the frames of thermal video images and the synchronized heating excitation simultaneously. In the lock-in correlation procedure, the frames are multiplied with the weight factors in two channels. The product of recorded frame and in-phase weight factor is summed over the entire dataset to provide the sine image \(\ I_{SIN}\). On the other hand, the same process is repeated but with a 90\({}^{\circ}\) phase shifted quadrature weight factor resulting in a cosine image \(\ I_{COS}\). The amplitude of the temperature variation as well as the phase can be obtained from the two images produced [20]. The post processing procedure is graphically illustrated in Figure 2.  
Figure 1: DLIT system set-up.  
Figure 2: Lock-in processing of thermal images.