Image Processing Reference
Figure1.17 VIS and NIR images of a cotton shirt showing a GSR pattern. (Courtesy of
Heidi Nichols, Miami-Dade County Medical Examiner's Office)
at close range, there is a distinctive pattern of gunshot residue (GSR). This material
can be a mixture of burned and unburned smokeless powder, and burnt primer.
GSR patterns can be used to match a particular firearm to a victim's article of
clothing. The NIR image (780-1100 nm) shows the ring of GSR because the dark-
colored cotton shirt is much more reflective in the NIR band—the dye pigment is
essentially transparent, and we see a reflection off the cotton itself. Measurements
of the GSR ring's diameter on the shirt compared to lab measurements made with
a gun found at the crime scene can suggest a value for the distance between the
shooter and the victim, with a smaller diameter indicating a closer range.
This phenomenon is very useful in forensics investigation: distracting patterns
or colors disappear or are greatly reduced in contrast when imaged in the near-IR
band. Figure 1.18 shows a tombstone in a Quaker cemetery on Nantucket Island,
Massachusetts, in the VIS and NIR bands. The lichens and discoloration make it
very hard to read the inscription by eye. The near-IR image makes it possible to
more easily read the name of the deceased: Sarah Hamblin. Of course, a much
better way to read all the text is a gravestone rubbing made with a charcoal pencil
and a sheet of paper held against the surface. Rubbing is not always allowed
though, especially on softer marble headstones.
The elimination or reduction of color in an image can be also used for industrial
inspection purposes. Figure 1.19 shows two views of a very loud shirt worn by the
author. The buttons are hard to see in the VIS image, but they stand out very clearly
in the NIR image.