Fig. 93: Schematic sketch describing the idea of X-ray focusing by a compound refractive lens (CRL). Since (1 - ) is smaller than 1, a collecting lens for X-rays must have a concave shape (a). A compound refractive lens, consisting of a number n of cylindrical holes placed close together in a row along the optical axis, focuses the X-rays at a distance n times shorter compared to a single lens (b). r is the radius of the holes; d is a spacing between the holes; is a wavelength of X-rays; f is the focal distance for a parallel beam; is the decrement of the refractive index of the lens material.