Image Processing Reference
In-Depth Information
incorrect conclusion that light beyond violet did not exist. Not only does near-UV
light exist, but many animals can see it. The human retina is somewhat sensitive to
near-UV light, but we cannot see it because the crystalline lens of our eye absorbs
light below 400 nm. This is believed to be an evolutionary solution to the problem
of chromatic aberration, a wavelength-dependent focusing of light by a lens that
becomes more apparent at shorter wavelengths. 4 Some people are missing the lens
of their eye, either through accident, disease, or a birth defect. This condition is
known as aphakia, and these people can see into the near-UV, although the thick
eyeglasses needed to focus light on their retinas absorb ultraviolet fairly well. Some
experiments have been carried out on subjects with aphakia using quartz lenses
that have a higher transmission in the UV than conventional glass lenses. These
subjects could read an optometrist's wall chart with 365-nm light alone, something
most people cannot do. 5 People can indirectly detect intense sources of ultraviolet
light as a bluish haze caused by fluorescence within the eye, but this is not the
same as ultraviolet vision, since the blue light that is perceived is visible light, not
ultraviolet.
We can make images using light in the near-IR and near-UV wavebands using
standard cameras, film and filters. The near-IR waveband encompasses the region
of the electromagnetic spectrum between 750 nm and 1100 nm. The cutoff at
1100 nm is the wavelength at which silicon detectors become insensitive. At these
wavelengths, glass is still transparent, which enables us to take photographs with
ordinary camera optics and special film designed to respond to near-IR light (like
the human eye, regular photographic film is insensitive to infrared light and has
little sensitivity to red light, which is why many darkroom safelights are a deep
red). If a filter that allows infrared light to pass but blocks out visible light is placed
over the lens, the resulting picture is a true near-IR image.
Figure 1.4 shows a block diagram of this camera system, which can be used for
near-UV imaging as well. These images can often reveal interesting features that
the eye cannot detect; for example, many different materials that appear to have
the same hue and intensity as each other when imaged in visible light appear very
different from each other in the near-IR.
As is the case with infrared light, ultraviolet light interacts differently with
materials than does visible light, and looking at the world with ultraviolet vision
can allow us to sometimes see things that are invisible to the unaided human
eye. How can we see the world in near-ultraviolet light without removing our eye
lenses, which is an unpleasant prospect? As with near-IR light, ordinary film and
camera optics will function with near-UV light. A regular camera still functions
in the region of the spectrum from a wavelength of 400 nm (where human visual
sensitivity cuts off) down to about 300 nm, where ordinary glass lenses become
opaque. 6
Unlike infrared pictures, which require special film, near-UV pictures
4 T. Goldsmith, “Hummingbirds see near ultraviolet light,”Science207, 786-788 (1980).
5 The 365-nm light comes from a mercury vapor lamp known as a Wood's Lamp. This experiment
was carried out in 1956. Kennedy, D. & Milkman,R.D.Biol.Bull.111, 375 (1956).
6 The antireflection coatings on quality lenses are optimized for visible light, and thus are not ideal
for UV photography.
 
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