Image Processing Reference
Figure1.38 From upper left to bottom right: SWIR (2000-2500 nm), SWIR
(950-1700 nm), visible (400-750 nm), and near-UV (350-380 nm) images of a human eye.
(Courtesy of FLIR and Rand Molnar )
These four images of the same eye were taken with two different SWIR cameras,
visible light and near-UV light, respectively.
The white of the eye, known as the sclera, is quite reflective in the visible and
near-UV wavebands, as seen in the bottom images in Fig. 1.38; yet the water
molecules in the sclera surface absorb light in the SWIR band, giving the eye
a very unusual appearance when imaged in the 2000-2500 nm and 950-1700-
nm wavebands. Note that the eyebrows and eyelashes appear white in the SWIR
images, since the pigments that give these hairs their color hardly absorb SWIR
light. Interestingly, all hair colors (even artificial ones) appear uniformly white in
SWIR images, regardless of how dark they appear to the eye. The bottom-right
image in Fig. 1.38 was made with near-UV light using black-and-white film in
a standard 35 mm camera with a Wratten 18A filter that passes near-UV light in
the 350-380-nm waveband but blocks visible light. The eye and the skin around it
have reversed their appearance from the SWIR image. The white of the eye is quite
reflective in the near-UV, while the hair is now quite absorbent. The skin shows a
mottling—this is sun damage similar to that shown in Fig. 1.33.
Figure 1.39 contains three images of the author in many of the same wavebands
as in Fig. 1.38. Note the change in the apparent shade of the blue shirt—the cotton
and pigments in the dye respond differently as the waveband is shifted from long
to short wavelengths. The skin in the near-UV image appears quite dark, almost