Highway tickets treated with ninhydrin.
(a) Ticket after the treatment viewed in the visible region. In this case, there is the black stain on the ticket;
(b) inspection
performed by exploiting the luminescence of the thermal paper (input radiation: 400–640 nm, longpass filter on the emitted radiation:>695 nm);
(c) comparison of the same area viewed in the visible/NIR region.
In the second case, i.e. the figure in the red box, it is possible to see detailed fingermarks. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)