Detection of intruders

Claim 1

AN INTRUDER DETECTION SYSTEM COMPRISING AN ELECTRICAL ELEMENT RESPONSIVE TO LIGHT TO CHANGE ELECTRICAL VALUES THEREOF AND HAVING A SURFACE EXPOSED TO AMBIENT LIGHT, A PARTITION DIVIDING SAID SURFACE INTO TWO ISOLATED PORTIONS, FIRST AND SECOND LAMPS AT THE RESPECTIVE SIDES OF SAID PARTITION FOR EXPOSING SAID SURFACE PORTIONS TO LIGHT FROM SAID LAMPS, A SOURCE OF A.C. REACTANCE MEANS CONNECTED IN SERIES WITH SAID ELEMENT ACROSS SAID A.C. SOURCE, A FIRST DIODE CONNECTED IN SERIES WITH SAID FIRST LAMP ACROSS SAID A.C. SOURCE AND ARRANGED TO CONDUCT IN ONE DIRECTION OF CURRENT FLOW, A SECOND DIODE CONNECTED IN SERIES WITH SAID SECOND LAMP ACROSS SAID A.C. SOURCE AND ARRANGED TO CONDUCT IN THE OPPOSITE DIRECTION OF CURRENT FLOW WHEREBY SAID LAMPS ARE ALTERNATELY ILLUMINATED TO ALTERNATELY EXPOSE SAID SURFACE PORTIONS TO AN EQUAL HIGHER AND LOWER LIGHT INTENSITY, A THIRD LAMP CONNECTED ACROSS SAID A.C. SOURCE IN PARALLEL WITH SAID ELEMENT FOR CONSTANTLY DIRECTING AMBIENT LIGHT OF A PREDETERMINED EQUAL INTENSITY ON BOTH OF SAID SURFACE PORTIONS TO COMPENSATE FOR SUBSTANTIAL CHANGES IN AMBIENT LIGHT WITHIN THE AREA TO BE PROTECTED AGAINST INTRUSION, SAID REACTANCE MEANS HAVING AN OUTPUT UPON BEING CHARGED UNEQUALLY AT OPPOSITE SIDES IN SEQUENCE DUE TO THE PRESENCE OF AN INTRUDER, AND MEANS CONNECTED ACROSS SAID REACTANCE MEANS RESPONSIVE TO THE OUTPUT OF SAID REACTANCE MEANS FOR RENDERING AN ALARM.