Information detection apparatus and displacement information measurement apparatus

An apparatus for detecting displacement information between two objects based on information from a scale imparting a positional reference, comprising:

• a plurality of probes formed on one of said two objects other than an object having scales formed thereon, said probes being arranged to face respective scales and said probes producing detection signals from the scales;
• a performance test circuit for testing a signal producing condition of said probes;
• and
• a signal processing circuit for selecting a detection signal from at least one of said probes having a predetermined signal producing condition based on the test by said performance test circuit and detecting the displacement information of the two objects in response to the selected detection signal.

A method for detecting information from an information bearing medium, comprising the steps of:

• arranging a plurality of probes to face the medium;
• detecting signals from the medium by the probes;
• testing a signal producing condition of each probe;
• selecting a detected medium signal from at least one of the probes having a predetermined signal producing condition based on the testing of the signal producing condition of the probes;
• and
• detecting the information in response to the selected detected medium signal.