Image Processing Reference
Figure4.22 Backscatter x-ray image of a man. (CourtesyofAmericanScienceand
were unknown to science until we observed them with orbiting telescopes. X-
ray telescopes must be located above the atmosphere, since the air mass above
us absorbs nearly all of the dangerous UV, x-ray, and gamma-ray light that shines
down on Earth. Some of these celestial objects such as black holes orgamma-ray
burstersare very bright in the x-ray or gamma-ray bands but are hardly noticeable
at other electromagnetic wavelengths. Other objects, such as our own sun, are much
brighter in the visible band than in the x-ray or gamma-ray bands.
As with radio astronomy, celestial objects can look markedly different in the x-
ray and gamma-ray bands from the way they appear in visible light. For example,
the Sun, imaged in the x-ray region of the spectrum, normally appears quite black,
as shown in Fig. 4.23.
This image was made using x rays at the low-energy end of the x-ray waveband.
These are calledsoftxrays, and they are lower in energy than the x rays used for
medical imaging. The apparent darkness of the Sun in the x-ray band is remarkable,
since the Sun is so bright we can hardly stand to look at it with our eyes for even a
fraction of a second. Yet the outer layer of the Sun that emits visible light is simply
too cold to radiate much light in the x-ray band. It takes magnetic phenomena to
heat the Sun's surface material up hot enough to emit x rays. The bright loop-
shaped structures shown in the image are solar flares. Solar flares are episodic
events that occur when the Sun's magnetic field pops out of the surface in localized