Lock-in thermography (LIT) is a non-destructive and fast electrical characterization method of large-area samples, and it has high spatial resolution. Detection of Joule heating in a biased device enables local structures to be imaged without the influence of heat broadening in a short acquisition time. Furthermore, because of its wide field of view, typically from sub-millimetre to sub-centimetre, one LIT image can characterize local structures in large samples. For instance, this method has been used to check for local structural failures in large-scale semiconductor devices such as integrated circuits and solar cells. In this talk, we present LIT imaging of carbon nanotube (CNT) network paths in a centimetre-scaled CNT composite at micrometre resolution. These results strongly suggest that LIT is a fine resolution tool for quickly visualizing local structures in conductive materials having large areas. Furthermore, the electrical characterization of a large CVD graphene sheet by the LIT technique is also presented. It is shown that LIT successfully visualizes the difference in electrical resistance caused by local defects as inhomogeneous thermal radiation.