Image Processing Reference
Figure1.43 Composite visible and UV (240-280 nm) image of methanol fires. (Courtesy
of Ofil Ltd., Nes-Ziona, Israel)
sources of solar-blind UV (UV in the 240-280 nm band). Both cameras are aimed
in the same direction and see the same field of view. The UV component of the
image is colored red for clarity. The red blobs cannot be reflected sunlight, since
the UV channel is solar blind.
Corona is a phenomenon observed around sharp tips of electrical conductors in
the presence of a strong electric field. The electric fields ionize gas in the vicinity of
the tips. When the gas is air, a faint blue-violet glow emitted by nitrogen and other
gases in our atmosphere can be seen. The name corona, which means “crown” in
Latin, comes from the crown- or brush-like shape of the discharge from the end
of conductors such as lightning rods during an electrical storm, faulty high-voltage
transmission lines, or ship's masts (sailors call it “Saint Elmo's Fire”). Corona is an
undesirable condition in the world of high-voltage power transmission—it usually
indicates a broken wire or cracked insulator that is causing an intense electrical
field to form and electrically overstress the air. Failure to remediate corona can
have serious consequences, including fires and short circuits. Corona is hard to see
with the unaided eye at night, and during the day it is nearly impossible to see, since
most of the light is emitted in the ultraviolet band. Most power line inspections are
carried out by helicopter, and thus must be done during the day for safety.
Fortunately, there is a way to overcome the visibility problem. Corona in air
emits some light at UV wavelengths shorter than 300 nm. By blocking the sun's
rays with a filter that transmits UV wavelengths only shorter than 280 nm, corona
is greatly enhanced even in bright sunlight, making it possible to detect faults
on power lines before the problem becomes severe enough to cause heating.
Figure 1.44 shows a picture of corona on an insulator stack. The white blobs of light
are corona glow from surface leakage along the surfaces of the dirty insulators.
As with the methanol image, the corona glow in the UV channel is overlaid on a
visible-light image so that the observer can reference the corona glow to the points
of emission in the scene.
An interesting source of UV light is radioactive material stored in water. Spent
fuel rods from certain types of nuclear reactors are stored in deep “ponds” of water.
This enriched uranium undergoes radioactive decay that releases gamma rays.
The gamma rays interact with electrons in water and produce high-energy beta
particles. The beta particles are electrons moving at relativistic speeds. Since the