1. A method for operating a proximity sensor comprising: providing a performance reference defining a relationship between a parameter of a sensor signal and a characteristic of an object to be detected; deriving a function that specifies deviation of performance of the proximity sensor from the performance reference; providing a threshold value for the parameter; thereafter, operating the proximity sensor to produce the sensor signal which indicates whether the object is proximate to the proximity sensor; employing the function to normalize the sensor signal produced by operating the proximity sensor to produce a normalized signal; and comparing the normalized signal to the threshold value to determine whether an object is present.