The heartbeat determination method involves generating light waves with a first wavelength and a second wavelength by a first and second light source. Light waves reflected from the skin tissue and the blood vessels are detected to provide electrical measurement signals. At least one non-coherent power spectrum is calculated based on spectral coherence function that takes into account the dependence between the measurement signals related to the first wavelength and second wavelength. The persons heartbeat is determined by the non-coherent power spectrum.
