Physical and Technical Basics

Abstract

First, in Sect. 2.1 the general basics of infrared (IR) thermography are briefly reviewed, which are also applicable to IR camera based lock-in thermography. In Sect. 2.2, the principles of the lock-in technique itself and of its digital realization are described. In Sect. 2.3, the two principal variants of lock-in thermography,which are serially measuring systems and camera-based systems, are introduced and compared. Different strategies to organize the timing of the lock-in correlation in relation to the frame rate of the IR camera are described in Sect. 2.4. Section 2.5 discusses the influence of non-harmonic (square wave) heat introduction, which is standard in electronic device testing, in contrast to the harmonic (sin-shaped) heat introduction mostly used in NDT. A detailed noise analysis is presented in Sect. 2.6, which relates the noise properties of the temperature measurement system to the noise level of the lock-in thermography result. Here, the “pixel related system noise density” is introduced as a universal parameter describing the figure of merit of different lock-in thermography systems. Section 2.7 deals with the problem of an easy and reliable calibration of lock-in thermography measurement systems by using a resistively heated test structure. The detection of free carriers in semiconductors by IR lock-in thermography, which is a novel application of this technique based on non-thermal effects, is introduced in Sect. 2.8