Image Processing Reference
Near-IR light can penetrate through thin paper as well. Figure 1.15 shows four
images of a paper label on cardboard in four different wavebands. Note that the
penetration improves as the wavelength of light gets longer. There is less scattering
of the light rays off the fibers in the paper label.
As with the whiteout and ink in the preceding altered document example, the
paper label becomes increasingly opaque in the UV band, as shown in Fig. 1.16.
The near-UV image is 365 nm and the shortwave UV image is 254 nm. The
shortwave UV image shows some staining on the paper under the numeral 3.
Forensics photographers use various invisible-light wavebands to uncover
evidence at crime scenes or on crime victims. Sometimes the evidence is difficult
or impossible to see with the naked eye, and sometimes the invisible-light band
reveals information that is not otherwise apparent. Figure 1.17 shows a visible-
light image of a polo-style shirt with a bullet hole in it.
What is not apparent in this VIS image is the ring of gunshot residue around the
hole, indicating the bullet's entry point. In many cases where a person has been shot
Figure1.15 Four views of a paper label on a cardboard box: (a) visible: 400-750 nm;
(b) NIR: 830-1100 nm; (c) SWIR: 1400-1700 nm; (d) SWIR: 2000-2500 um. (Courtesy of
Figure1.16 Two views of a torn paper label with printing underneath: left—365 nm; right—