A Compound refractive lens (CRL) is a series of individual lenses arranged in a linear array in order to achieve focusing of

X-rays An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 Picometre, picometers to 10 Nanometre, nanometers, corresponding to frequency, ...

in the energy range of 5-40 keV. They are an alternative to the KB mirror. For all materials the real part of the

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

for X-rays is close to 1, hence a single conventional lens for X-rays has an extremely long focal length (for practical lens sizes). In addition, X-rays attenuate as they pass through a material so that conventional lenses for X-rays have long been considered impractical. The CRL gets its reasonably short focal length, on the order of meters, by using many lenses in series, hence reducing the curvatures of each lens to practical levels. Absorption in the lens is still a challenge, however, and lenses are usually made from low-atomic-number materials such as

aluminium Aluminium (aluminum in American and Canadian English) is a chemical element with the symbol Al and atomic number 13. Aluminium has a density lower than those of other common metals, at approximately one third that of steel. I ...

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to form mi ...

, or

lithium Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense solid el ...

. CRLs were first demonstrated in the mid-1990s by a group of scientists at the

ESRF The European Synchrotron Radiation Facility (ESRF) is a joint research facility situated in Grenoble, France, supported by 22 countries (13 member countries: France, Germany, Italy, the UK, Spain, Switzerland, Belgium, the Netherlands, Denmark, ...

. They drilled holes in an aluminium block, and achieved focusing in two dimensions. For X-rays a concave lens focuses the X-rays because the index of refraction is slightly below unity. In a CRL of this type the walls between the cylindrical holes act as concave lenses for X-rays traveling perpendicular to the axis of the drilled cylinders. In contrast, for visible light the index of refraction is larger than unity and focusing is done with a convex lens. Scientists associated with the ESRF synchrotron have done much of the CRL's subsequent development, notably the parabolic CRLs pioneered by the Aachen group under Lengeler. Their signature material is beryllium: a group at the Advanced Photon Source APS demonstrated the same lenses in lithium. These lenses have a direct counterpart in visible light. The saw-tooth lens is a unique optical scheme suggested and demonstrated by Cederstrom. It approximates a parabolic lens much as a numerical computation on a grid approximates a smooth line, with a series of prisms that each deflect the X-rays over a minute angle. Lenses of this type have been made from silicon, plastic, and lithium. To address the challenge with absorption in the lens, each prism in the saw-tooth lens can be exchanged for a column of smaller prisms, hence removing phase shifts of 2π that do not contribute to refraction but add absorption. This scheme is similar to the approximation of a conventional parabolic lens by a

zone plate A zone plate is a device used to Focus (optics), focus light or other things exhibiting wave character.G. W. Webb, I. V. Minin and O. V. Minin, “Variable Reference Phase in Diffractive Antennas”, ''IEEE Antennas and Propagation Magazine'', ...

. The relatively simple manufacturing of the saw-tooth refractive lens and the prism-array lens make them usable also outside of research and both have been suggested for applications in medical

x-ray imaging Radiography is an imaging technique using X-rays, gamma rays, or similar ionizing radiation and non-ionizing radiation to view the internal form of an object. Applications of radiography include medical radiography ("diagnostic" and "therapeut ...

References

External links

*{{cite web, title=Refractive X-ray lenses, periodical=, publisher=, url=http://x-ray-optics.de/index.php/en/types-of-optics/refractive-lenses, url-status=, format=, access-date=2019-11-21, archive-url=, archive-date=, last=Arndt Last, date=, year=, language=, pages=, quote= X-ray instrumentation