1. Apparatus for generating a machine readable signal indicating which one of a set of reference characters corresponds to a visible pattern, including means for storing signals indicative of a set of reference fine n-dimensional array values, each reference fine array value corresponding to one of the reference characters, means for storing signals indicative of a set of reference coarse m-dimensional array values, where m is less than n, each reference coarse array value being derived either from a signal one only of said reference fine array values (in which case it will be designated an unambiguous value), or from any one of a plurality of said reference fine array values (in which case it will be designated an ambiguous value), means for measuring with an optical instrument a workpiece visible pattern in each of n fields to provide a signal indicative of a workpiece fine n-dimensional array value and means for storing said last named signal, means for forming a signal indicative of a workpiece coarse m-dimensional array value from said workpiece fine array value signal, first comparing means for comparing the signal of the workpiece coarse array value with the signals of the set of reference coarse array values to produce a signal indicating the identification of the workpiece pattern with either an unambiguous reference coarse array value or an ambiguous reference coarse array value, means responsive to a signal from said first comparing means indicative of identification with an unambiguous array value to generate a signal indicative of the reference character associated with the identified unambiguous array value, second comparing means responsive to a signal from said first comparing means indicative of identification with an ambiguous array value to compare signals indicative of values of a preselected subset of elements from the plurality of reference fine array values associated with the identified ambiguous coarse array value with the signals indicative of values of corresponding elements of the workpiece fine array, to generate a signal indicative of identification of the workpiece pattern with a particular one of the reference characters associated with the identified ambiguous value.