Patent ID: 6912413

Claim:
A method for controlling optical power in a monitoring device intended for determining the amount of at least one light absorbing substance in a subject, the monitoring device comprising emitters for emitting radiation at a minimum of two wavelengths driving means for activating said emitters, and a detector for receiving said radiation at said wavelengths and for producing an electrical signal in response to the radiation, the method comprising the steps of supplying driving pulses from said driving means to the emitters, the pulses having predetermined characteristics determining the optical power of the device, demodulating the electrical signal originating from said detector whereby a baseband signal is obtained; transforming the baseband signal into a frequency spectrum to identify an amplitude and a noise level of the baseband signal, whereby a signal-to-noise ratio of the amplitude to the noise for the baseband signal is obtained; monitoring the signal-to-noise ratio of the baseband signal, and controlling the duty cycle of the driving pulses in dependence on the monitored signal-to-noise ratio.