Image Processing Reference
Figure1.35 Visible (left) and near-UV (right) images of Goneopteryx cleopatra. (Courtesy
of Edward Aicken)
Figure1.36 Visible (left) and near-UV (right) images of coral reefs and fish. (Courtesy of
Prof. Thomas Cronin and the Journal of Fish Biology)
quite different: the water looks much brighter due to near-UV scattering, and the
fish look dark. Ultraviolet vision may serve to make prey—particularly UV-opaque
plankton—visible against a bright background illumination.
Many fish have UV-sensitive photoreceptors in their eyes. 13 These receptors
have peak sensitivity to light with a wavelength of 360 nm, which suggests that
these fish perceive near-UV light as a distinct color, as butterflies and honeybees
do. It is possible that ultraviolet vision may also help certain fish identify mates in
the same manner as butterflies, and this theory is supported by the presence of a
variety of pigments in fish skin that have reflectance peaks in the ultraviolet.
Why is it that materials like varnish and certain colors of oil paint are transparent
to near-IR and SWIR light but not to visible or UV light? We have seen this
phenomenon in the preceding examples of IR and UV imaging, and it is worth
exploring the science behind it. UV light interacts with the electrons in molecules
13 Losey et al.,J.FishBiol.54, 921-943 (1999).