We present a new imaging approach to determine porosity in carbon fibre-reinforced polymers by active thermography in the reflection mode. The infrared radiation is excited with help of a semiconductor laser. We use rectangular pulses for the excitation light–either single pulses or a sequence of a couple of pulses–and measure the succeeding temperature transients. These signals are transferred to the frequency domain by means of a discrete Laplace transformation. The evaluation of the thermal effusivity is done by a linear fitting process which gives unequivocal results with comparatively small error bars. The method is fast and robust, and the results compare well with prior experiments carried out with ultrasonic-testing, X-ray computed tomography or other approaches of thermography, where the thermal diffusivity has been determined.