in the direction of one of the arrows 22 the distance between the radiation source 14 and the fibre pattern 2 is for example approximately 80 mm while the distance between the fibre pattern 2 and the radiation detector matrix 18 is for example approximately 1 mm with an average fibre transverse dimension of 0 04 mm and a fibre pattern surface area of the order of 1 mm 2 fig5 shows on an enlarged scale a part of the cross section of the painting 10 at the position of the recess 12 with the fibre pattern 2 the radiation source 14 projects a radiation beam 16 onto the fibre pattern 2 which radiation is reflected behind the fibre pattern onto a reflecting surface 24 and is intercepted on the radiation detector matrix 18 through parallax phenomena in the case of a reflecting surface 24 two silhouettes of the fibre pattern 2 which are staggered relative to each other will appear on the radiation detector matrix 18 fig6 illustrates the construction of a device similar to that according to fig4 however in fig6 two radiation sources 14a and 14b are used the silhouettes of the fibre pattern 2 obtained on the radiation detector matrix 18 by means of the radiation beams 16a and 16b coming from the radiation sources 14a and 14b respectively can be picked up either in succession or simultaneously if picked up simultaneously a suitable image processing operation handles the separation of the two silhouettes from each other it is also possible to make the radiation sources 14a and 14b each emit radiation of a different wavelength with the result that the separation of the silhouettes is simplified when the.