Image Processing Reference
Figure1.34 Visible (left) and near-UV (right) images of Black-Eyed Susans. (Courtesy of
Prof. Tom Eisner )
Many flowers will appear uniformly white in visible light, but they are seen to have
dark regions near the center of the blossom when imaged in ultraviolet light. This
dark color is due to the presence of UV-absorbing compounds called flavanols. 11
Ultraviolet imaging is useful for the scientific identification of flowers that have
nectar guides, as many flower species are difficult to differentiate on the basis
of visible color. Ultraviolet “colors” strongly affect the behavior of moths and
other light-seeking insects, which will fly to light sources covered with ultraviolet-
passing filters, although the filtered sources appear dark to human eyes.
The butterfly is another insect that can see into the ultraviolet. In fact, butterflies
have the widest color sense of any known organism. One Japanese species has five
different photoreceptors—two more than we have—and can see colors from red
to near-UV. 12 Figure 1.35 shows two views of a Cleopatra butterfly (Goneopteryx
cleopatra). The butterfly looks uniformly yellow to our eyes, but interesting
patterns are revealed in the near-UV waveband (350-380 nm). The Cleopatra
belongs to a large family of butterfly species (Pieridae) that to our eyes appear
very similar to one another, generally having a nondescript white or yellow color.
However, the near-UV wing markings shown in this image are an example of
how nature distinguishes this species from other yellow-colored members of the
Pierid family. This is how a Cleopatra flying in a forest can identify a potential
mate from a sea of apparently plain yellow butterflies of various species. Some
butterflies have sexually dimorphic UV wing markings, i.e. the males look different
from the females in the ultraviolet. This is probably an adaptation that helps male
butterflies avoid trying to mate with other males.
Ultraviolet vision is found in marine animals as well. The underwater world
looks very different in the near-ultraviolet waveband. Particulates in ocean water
can scatter near-UV light to such an extent that the water looks foggy, and
consequently marine life is thrown into sharp relief against a background such
as the coral reef shown in the left-hand image in Fig. 1.36. This is a visible image
taken through a filter that passes green light. The right-hand image in Fig. 1.36 is
11 W.R. Thompson et al. “Flavanols: pigments responsible for ultraviolet absorption in nectar guides
of flowers,”Science177, 528-530 (1972).
12 G. Horridge et al.,JournalofComp.Phys.155, 529-542 (1984).