in practice transducer 13 and leds 20 and 22 are adhesively mounted on the patient 39 s extremity and connected to the remainder of the oximetry apparatus by cables alternatively a hood or other mounting means can be used to position the leds and the transducer on the extremity although a shielded cable does not have to be used with the transducer since detector assembly 24 removes 60 cycle interference such a cable can be employed if desired to even further reduce the possibility of interference similarly although the transducer does not have to be shielded from ambient light since the detector assembly is tuned to respond only to pulses of light having the frequencies of oscillators 12 and 14 see discussion below such shielding can be employed if desired to further reduce the possibility of ambient light interference transducer 13 is designed to respond to light having the wavelengths produced by each of led 20 and led 22 most conveniently transducer 13 is a broadband photodetector which is sensitive to for example red and infrared light transducer 13 produces an electrical signal which is representative of the intensity of light impinging on the transducer in the case of light originating from led 20 the electrical signal will consist of essentially square wave pulses at the frequency of oscillator 12 the intensity of each pulse and thus the amplitude of the resulting electrical signal produced by transducer 13 being dependent upon the absorption which the pulse underwent in passing from led 20 to transducer 13 similarly in the case of light originating from led 22 the electrical signal will also consist of essentially square wave pulses but at a different frequency i e the frequency of oscillator 14 in.