Image Processing Reference
in a different way from near-IR and SWIR light. The energy of UV photons is
several times higher than near-IR and SWIR photons, and when these photons
interact with a molecule, they tend to impart their energy to electrons in a single
atom within the molecule rather than the molecule as a whole. There tend to be
more excitation modes of molecules in the UV, which is why many molecular
materials such as glass are opaque to certain wavelengths of UV light, yet they
are transparent to visible, near-IR, and SWIR light. A very nice example of this
UV absorption effect can be seen in Fig. 1.37, a comparison of two common
optical materials in various bands of the spectrum. The figure shows a pair of
seemingly identical 2-inch diameter windows imaged in three sub-bands of the
UV. The window on the left is standard BK7 optical glass, while the one on the
right is fused silica, which is transparent down to 180 nm.
These images illustrate why UV imaging below about 330 nm requires optical
materials other than standard glass. However, the conventional wisdom that
near-UV imaging requires fused silica lenses is wrong—BK7 works fine in the
340-400 nm near-UV band, especially if it is not antireflection coated.
Figure 1.38 presents an interesting example of how something commonly
thought of as “white” does not always appear so white in invisible wavebands.
Figure1.37 Images of BK7 glass and fused silica discs in three narrow UV bands.
(Courtesy of David Hayes)