Abstract: An optical sensor system (10) in the form of a hand held tonometer probe has an infrared emitting diode (34) and a phototransistor detector (36) positioned to receive light from the diode (34). A rod (18) serves as a variable attenuator to intercept a portion of light from the diode (34) directed at the detector (36). A signal level detecting amplifier (104) is coupled to the detector (36) to receive an output signal from the detector (36) in response to light from the diode (34) received by the detector (36). The amplifier (104) has a non-inverting input (102) and an inverting input (106). The amplifier (104) receives the output signal from detector (36) at non-inverting input (102). A +5 volt reference potential is connected through a variable resistor (110) and a resistor (108) to establish a bias at the inverting input (106) which is approximately equal to signal levels supplied by the detector (36) to the non-inverting input (102).