Image Processing Reference
Figure1.4 Block diagram of near-IR and near-UV photographic apparatus.
can be taken with conventional film. Regular Kodak Panchromatic Tri-X film (as
well as many other black-and-white films) are sensitive down to approximately
300 nm in wavelength, and when combined with a filter that passes near-UV light
only, they enable us to take near-UV photographs that are sometimes quite different
from visible-light images. The photographic apparatus is the same as that used for
near-IR imaging, as shown in Fig. 1.4. Professor Robert Wood described the first
near-UV transmitting filter. The filter blocked out visible light and enabled him
to capture the first near-UV images in the 1910s using standard black-and-white
film. 7 One of the earliest known UV images is shown in Fig. 1.5.
If we wish to extend the sensitivity of our photographic apparatus to wavelengths
shorter than 300 nm, we must use special quartz lenses or else a pinhole camera,
which quite literally uses a tiny pinhole instead of a lens. Some types of regular
black-and-white film will still work down to around 250 nm, but special films
become necessary at wavelengths below 200 nm, as regular gelatin photographic
emulsions become very absorbent below that point. The near-UV images presented
in this chapter were all taken with various filters made of special filter glasses.
These filter glasses transmit UV light in various bands but then cut off sharply in
the visible spectrum. There are also a variety of electronic imaging cameras that
are sensitive to near-UV light, such as silicon-intensified target (SIT) and back-
thinned CCD cameras. In 1969, Professor Thomas Eisner and his colleagues used
a near-UV-sensitive video camera to mimic insect vision. 8 Visible-light electronic
sensors can be coated with fluorescent materials that shift the wavelength of the
ultraviolet light into the longer visible-light wavelengths that the array can detect.
7 A.R. Williams, G. Williams,Jour.Biol.Phot.61, 115-132 (1993).
8 T. Eisner et al.,Science166, 1172-1174 (1969).