Abstract: A security thread detector assembly for reading a security thread carried by a security document and coded such that each of a succession of segments along the thread represents a digital value, the particular digital value being represented by the particular characteristic of the respective segment comprises a head having a multiplicity of parallel reading channels having a track pitch which is substantially less than the length of a segment and being disposed to read the document such that the thread is substantially broadside to the channels, whereby all the segments are sensed substantially simultaneously and each segment is represented by a plurality of samples.