Fig. 10

a The carbon ink crosses imaged at four depths corresponding to each layer using THz imaging. The first layer is on the bottom and gives the strongest contrast; the cross of the fourth layer is still visible but with lower contrast as the THz beam has passed through the preceding three layers and back again before being detected. b THz images of the uppermost cross of made with (from left to right) iron gall ink, carbon ink, red iron oxide, and India ink. This shows that carbon-based inks appear to provide more contrast than metallic-based ink when using THz for imaging