Image Processing Reference
In-Depth Information
UltravioletImaging
The phenomena seen with ultraviolet imaging tend to be complimentary to
phenomena seen with near-IR imaging. UV light has both higher-energy photons
and shorter wavelengths relative to VIS and near-IR light. These two differences
account for the very different appearance of many materials when imaged with UV,
VIS and near-IR imaging systems.
The applications for UV imaging can be divided into three classes: imaging
visibly transparent materials due to their UV opacity; imaging tiny structures or
texture on surfaces that are not apparent to visible light; and imaging ultraviolet
light sources that have little or no emission in other bands such as the visible or
near-infrared. This section includes examples of each of these classes.
Consider photon energy and how it makes UV behave differently from IR.
Relative to visible light, UV light tends to be either absorbed or reflected from
the very top surface rather than penetrating into a given material, especially if
the material is composed of organic molecules. This is in contrast to near-IR and
SWIR light, which tends to penetrate into these same types of materials deeper than
visible light and much deeper than UV light. The difference is the energy of the
photons in these wavebands and the way they interact with materials. UV photons
are more energetic than visible-light photons. They are therefore more likely to
be absorbed by many materials, as the higher photon energies stimulate quantum-
state transitions in both atoms and molecules that result in the UV photons being
absorbed. Shortwave infrared light does not have the right photon energies to be
absorbed in the same way—it takes more material to absorb it. Many materials
composed of organic molecules are prone to absorb UV and transmit near-IR.
Some inorganic materials like window glass will absorb UV and transmit VIS
and near-IR. Still other inorganic materials like quartz have such tightly bound
electrons in their structure that they transmit UV, VIS and near-IR light. Metals
are in a class by themselves. They have very loosely bound electrons, so there
is little penetration into their surface by UV, VIS or near-IR light, or indeed any
wavelength of light shorter than x rays.
The shorter wavelength of UV light is apparent when one reflects it off of
surfaces with imperfections on them. This phenomenon is important to optics
manufacturers who are very concerned with the surface polish on glass lenses and
other optical materials. Surface scratches and stains may not be readily apparent
to visible-light inspection methods. The scratches show up because of the shorter
wavelength, while the stains can show up because they are organic molecules that
absorb UV photons. A nice example of scratches showing up appears in Fig. 1.25,
which shows a CD jewel case in both the visible band and in the near-UV band.
There are two reasons the images are different: the plastic is slightly more reflective
in the near-UV band than in the visible band, so the surface is already a little easier
to see, and the tiny scratches on the surface scatter the near-UV light waves. The
scratches are too small to readily scatter visible light waves—the waves tend to
“average out” the surface scratch, as though they are sampling a larger area of the
surface on both sides of the scratch. One sees this when working with infrared
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