the brightness of a source is set, there are no clever optical tricks for making a source
brighter. In contrast, other quantities such as power density depend on your distance
from the source or on how the beam is focused. Brightness and spectral brightness
are thus intrinsic properties or source invariants, and we can use them to objectively
compare different X-ray facilities.
1.3.2 Why Is Spectral Brightness Important?
For an X-ray experiment, photons are only of value if you can put them on the
sample. If the photons are from a large source and are diverging in all directions, it is
difficult to focus them back to a small point. Brighter beams are more useful because
they are easier to (a) collect, (b) monochromate, and (c) focus. As we will see in
Chap. 4 on X-ray optics, brightness is especially important for X-rays, because of
severe limitations on X-ray mirrors, lenses, and monochromators. For X-ray spectroscopy, spectral brightness is even more important, because most X-ray spectroscopy is done using one photon energy at a time, so it helps when the source is tuned
to the energy that is desired.
Finally, a couple of warnings about word usage. In some X-ray literature (particularly European sources), the word brilliance is used for what we have defined as
brightness. In some UV-visible optics literature, what we call brightness is referred
to as radiance. However, if Born and Wolf were happy to call it brightness [2], then
so are we.
1.4 The Synchrotron Radiation Revolution
In the second half of the twentieth century, something very special happened to our
ability to produce X-rays—synchrotron radiation sources became available. The
existence of synchrotron radiation (“SR”) had been predicted since the turn of the
century. It was finally seen as visible radiation from a glass synchrotron chamber at
General Electric research (GE) in 1947 (Fig. 1.4) [3]! Thanks to the technological
development of synchrotron radiation sources, the brightness of available X-ray
sources began to double on average approximately every year—a trend noted in the
mid-1980s by Munro and Marr [4]. This exponential rate of increase allowed
enormous improvements in the quality and quantity of X-ray experiments. The
rate of improvement has been faster than Moore’s law—the ~18-month doubling
time for computer chip density and speed that held for ~40 years from 1975. If
airplanes had made the same exponential progress over the last 30 years, we would
now be flying faster than the speed of light! (Fig. 1.5).
1.4 The Synchrotron Radiation Revolution
5
brighter. In contrast, other quantities such as power density depend on your distance
from the source or on how the beam is focused. Brightness and spectral brightness
are thus intrinsic properties or source invariants, and we can use them to objectively
compare different X-ray facilities.
1.3.2 Why Is Spectral Brightness Important?
For an X-ray experiment, photons are only of value if you can put them on the
sample. If the photons are from a large source and are diverging in all directions, it is
difficult to focus them back to a small point. Brighter beams are more useful because
they are easier to (a) collect, (b) monochromate, and (c) focus. As we will see in
Chap. 4 on X-ray optics, brightness is especially important for X-rays, because of
severe limitations on X-ray mirrors, lenses, and monochromators. For X-ray spectroscopy, spectral brightness is even more important, because most X-ray spectroscopy is done using one photon energy at a time, so it helps when the source is tuned
to the energy that is desired.
Finally, a couple of warnings about word usage. In some X-ray literature (particularly European sources), the word brilliance is used for what we have defined as
brightness. In some UV-visible optics literature, what we call brightness is referred
to as radiance. However, if Born and Wolf were happy to call it brightness [2], then
so are we.
1.4 The Synchrotron Radiation Revolution
In the second half of the twentieth century, something very special happened to our
ability to produce X-rays—synchrotron radiation sources became available. The
existence of synchrotron radiation (“SR”) had been predicted since the turn of the
century. It was finally seen as visible radiation from a glass synchrotron chamber at
General Electric research (GE) in 1947 (Fig. 1.4) [3]! Thanks to the technological
development of synchrotron radiation sources, the brightness of available X-ray
sources began to double on average approximately every year—a trend noted in the
mid-1980s by Munro and Marr [4]. This exponential rate of increase allowed
enormous improvements in the quality and quantity of X-ray experiments. The
rate of improvement has been faster than Moore’s law—the ~18-month doubling
time for computer chip density and speed that held for ~40 years from 1975. If
airplanes had made the same exponential progress over the last 30 years, we would
now be flying faster than the speed of light! (Fig. 1.5).
1.4 The Synchrotron Radiation Revolution
5