Optical lock-in thermography is a non-destructive testing method. The surface of the sample is excited using the thermal waves and the response is recorded by the thermal camera. This image thermal sequence is processed using the lock-in method. Optical lock-in thermography can be used to detect cracks and damages in metal or composite material. Two position modes are used: the reflection and the transmission mode. In this paper, are compared these two modes.  An experiment is carried out on a printed composite plane with the square blinded holes placed in the different depths below the surface. The phase images are created using the lock-in method for both position modes. The results are compared and the advantages and the disadvantage of these position modes find out.

This research was supported by Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic under the contract VEGA No. 1/0141/20 and VEGA No. 1/0510/20.