cycle however this variation occurs at the frequency of the pulse which is below about 5 hz 300 pulses min and hence far below the 300 hz switching frequency accordingly the attenuation of the red and the infrared light passing through the fingertip may be regarded as substantially constant during each cycle of the switching waveform or during each burst of red or infrared light the intensity i incident fig2 c of the light impinging on photodetector 22 as shown by curve 90 fig2 c will include an ambient component i ambient and a transmitted component i trans the transmitted component i trans represents red or infrared light transmitted from the diodes 18 or 20 to photodetector 22 the magnitude of i trans will of course depend upon the attenuation caused by the fingertip also i trans will vary in accordance with the output i red or i ir of whichever led is operating therefore i trans and i incident will vary in accordance with the 19 2 khz carrier frequency waveform in the condition illustrated by fig2 c the attenuation of red light by the fingertip is relatively mild whereas attenuation of infrared light is more severe therefore during each red burst 86 fig2 a the magnitude i trans of the transmitted light and hence the magnitude of the carrier frequency variation in the incident light will be relatively large during each.