Image Processing Reference
document that has had typescript defaced by ballpoint pen ink and correcting fluid
(“whiteout”). The first IR image was taken with a silicon CCD sensor with a black
glass near-IR pass filter—the waveband is 830 to 1100 nm. The second IR image
was taken with an electronic camera with an indium gallium arsenide sensor
(InGaAs for short) that is filtered to image in the 1500-1700-nm waveband. This is
part of the SWIR waveband, because the spectral region from 1100 nm to 3000 nm
is designated as the SWIR (short-wave infrared) band.
It is very interesting to look at this same piece of paper in the UV bands as well.
There the ink and whiteout are highly opaque, especially in the shortwave UV
band, as shown in Fig. 1.14. The near-UV image is 365 nm, and the shortwave UV
image is 254 nm. The whiteout is anything but white in the UV band. UV imaging
is a very effective way to examine a questioned document.
Figure1.13 (a) Visible: 400-750 nm: (b) NIR: 830-1100 nm; (c) SWIR: 1400-1700nm;
(d) SWIR: 2000-2500um. (Courtesy of FLIR)
Figure1.14 Near UV: 365 nm (left) and shortwave UV: 254 nm (right) images of an altered