END-TO-END PHOTOGRAMETRY FOR NON-PROFESSIONAL PHOTOGRAMMETRISTS

An air/fuel ratio monitoring system in an IC engine, using an oxygen sensor of the concentration cell type which has a laminate comprising an inner electrode layer, a microscopically porous layer of oxygen ion conductive solid electrolyte and an outer electrode layer exposable to exhaust gas and which exhibits a sharp change in output voltage level in response to a change in the air/fuel ratio in the engine across a stoichiometric ratio. To ensure accurate monitoring of the air/fuel ratio even when the engine is operated under conditions which cause a large rise or fall in an average level of the output of the oxygen sensor, the monitoring system has a signal treatment means for producing a variable reference voltage, with which the sensor output is compared, by first adding a definite voltage to or subtracting a definite voltage from the output of the oxygen sensor depending on the result of comparison between the sensor output and the reference voltage and then smoothing the voltage resulting from the addition or subtraction of the definite voltage.