The authenticity of a document (D) is tested by sensing (14) its translucency throughout an area (12) and correlating the sequence of representations (20 Figure 2) produced, with a sequence of reference representations (18 Figure 2) read (16) from a magnetic recording (10) of an earlier-made sensing. Correlation is carried out by relating successive overlapping parts (L Figure 2) of the freshly-derived and digitized sequence with corresponding overlapping parts (L) respectively, of the digitized reference sequence. The overlapping parts (L) are of the same length (N), and overlap with one another equally, throughout both sequences (20,18), the extent (M) of overlap being such that all individual representations in each sequence (20;18) occur in three successive parts of that respective sequence. Assembler circuits (44,46; Figure 1) supply signals (SUM R, SUM R², SUM RS) in accordance with functions of the successive parts (S;R) of the two sequences, to an arithmetic unit (50) for generation of correlation scores (C) that are then analyzed (56) to produce an indication (42) of the degree of authenticity. A preliminary, independent check on the maxima, minima, and/or average value of the freshly-­derived sequence is carried out by a test unit (38) to signal rejection of the document (D) if the check is not satisfied.