The paper investigates the aspects of image processing in vibrothermographic measurements applied to the inspection of composite plate and carbon steel weld test specimens. A carbon epoxy prepreg plate with barely visible impact damage and carbon steel weld test specimens have been tested with use of a handheld ultrasonic excitation device. Due to varying contact pressure and misalignment between the sonotrode and the test piece the intensity of excitation is fluctuating with time. As a solution to this problem the authors propose the image processing technique based on a two dimensional Fourier transform of the infrared images. The technique allows to increase the quality and readability of the results coming from a vibrothermographic measurement with poor ultrasonic excitation.