The device has a sensitive range extending from LN to LF. A measured distance is assigned to one of three range bands Z1, Z2, Z3 by means of two boundaries PA, PB. These boundaries are set by dividing the sensitive range into 16 portions by means of points PAi. The further boundary PA is set first, to be one of these hints PAi. The range between LN and PA is then redivided into 16 portions by means of new points PBi. The nearer boundary PB is then set to be one of these new points PBi. This enables the closer boundary to be set more accurately. <IMAGE>
