Types of X-ray optics: Introduction

There is a wide range of different X-ray optics which all share the same goal: a well defined change of the direction of the incoming X-ray light and a controlled influence on the spectrum of the light. All types of X-ray optics can be classified by the physical effect used. The table below shows the effects influencing the direction or the spectrum of X-rays in a controlled way and the optics using these effects:

physical effect	types of optics using this effect
reflection	(crystal) mirror optics; capillary optics
diffraction	zone plates
refraction	compound refractive lenses
absorption	windows; filters; pin holes and coded mask telescopes

Of course there are many variants of these basic types, listed under the basic types. Each type of optics has its characteristic features, mainly depending on the limitations inherent to the underlying physical effects and those of the manufacturing process. Many optics cover a wide range of applications. Nevertheless it is worth trying to make up a table emphasizing the main individual strengths of the types of optics controlling the direction of X-rays:

optics type	photon energy range	working distance	min. focal spot diameter	imaging or illumination	achromatic behaviour
mirror optics multi layer / crystal mirror optics	0 - 20 keV 0 - 100 keV	>0.1 m >0.1 m	0.03 µm 0.05 µm	imaging¹ imaging¹	yes no
polycapillary optics mono capillary optics	0 - 20 keV 0 - 20 keV	0.002 - 0.2 m 0 - 0.2 m	1 µm <1 µm	illumination imaging¹	yes yes
zone plates	0 - 20 keV	0.001 - 0.1 m	0.015 µm	imaging	no
compound refractive lenses	5 - 500 keV	>0 m	0.1 µm	imaging	no
coded mask telescopes	all²	-	(10 µm)	imaging	yes

¹Good imaging quality is only possible in mirror optics, where each ray undergoes an even number of reflections (only then the Abbé-criterion can be fulfilled approximately)
²photon energy range mainly depends on the detector

These values give a rough estimation and will change with improvements in the fabrication processes.