the document 5 is preferably moved parallel to the longitudinal direction of security thread 1 so as to form an average value over a prearranged portion of the length of the security thread 1 the measurement of the degree of coherence permits a reliable determination of the presence of the security thread 1 even if the security thread 1 is embedded in a diffusely scattering medium instead of the contrast u03bc it is also possible to measure intensity correlation of the second order g sup 2 which is also a measure for the degree of coherence the arrangement shown in fig8 consists of a source or rays 8 39 whose wavelength u03bb is adjustable a ray detector 32 and a signal processing circuit 33 the document 5 is positioned between the ray source 8 39 and the ray detector 32 the ray 2 penetrates the document 5 and is scatttered in a preconceived manner at the security thread 1 the source of rays 8 39 can be implemented for example by means of a dye laser or a light source having a gap and a sky filter the ray detector 32 sorts out a narrow bundle of rays 34 from the scattered radiation and.