System and method to determine the heartbeat of a person by a pulse measuring device. Light waves are generated in the direction of the skin by two light sources with a first wavelength (λ1) which is sensitive to the pulse beat and movement of the person, and with a different second wavelength (λ2) which is sensitive to movement, wherein a photodetector detects the reflected light waves. A pulse is determined based on calculation made of at least one non-coherent power spectrum on the basis of a spectral coherence function γ which takes into account the dependence between the measurement signals relating to the first wavelength and to the second wavelength by the formula SNC=(1−γ2)*SAA, where SAA is the average Fourier power spectrum of the signals relating to the first wavelength, and where γ2 is between 0 and 1.
