In refractive optics X-rays are guided by refraction at surfaces between different materials. This effect is described by Snell's law. For X-rays the difference of the refractive index to one is called index decrement . As the index decrement for X-rays for all materials is only slightly below one, hundreds of well aligned lens elements with very small radii of curvature are needed to form a refractive X-ray lens with a focal length in the range below a meter (fig. 1). For visible light the index of refraction is clearly larger than one for lens materials and focusing lenses are thicker in their centre than at their edges, resulting in their typically biconvex shape. Due to the fact, that the index of refraction is smaller than one for X-rays, focusing lenses are thinner at the centre than at their edges, giving a biconcave shape. A parabolic geometry of the refracting surfaces is well suited to focus X-rays hitting the lens parallel to the optical axis.
Fig. 1: Single lens element with long focal length f1 (top) and CRL with short focal length f2 (bottom)
For thin lenses the focal length f can be calculated as
with the radius of curvature R and the number of lens elements N. When the lens length L is in the range of the focal length, the formula for a thick lens can be applied:
Some typical parameters of CRLs are listed in table 1.
Lens materials used:
Be, Si, SU-8 (resist), Al, Ni, Au
Table 1: Typical parameters of CRLs [VDI 2007b]