Partial Coherence and Spectral Brightness at X-Ray Wavelengths

Abstract

Microscopic examination of physical and biological systems requires short wavelength radiation which can be focused to very small dimensions. Indeed it is possible in the limit to both “see” and “write” features approaching the radiation wavelength λ, and furthermore to identify particular chemical elements through their characteristic electronic transitions. Sources of radiation which can deliver a substantial flux of photons to very small dimensions are said to be of high brightness. Sources of high brightness radiate a large photon flux (photons/second), from a small area · solid angle product (ΔA · ΔΩ). If, in addition, the photons are emitted within a spectrally narrow region, Δλ the radiation is said to be of high spectral brightness. The focusability of such radiation is, however, set by additional attributes of spatial coherence and, of course, by available optics [1]. The emphasis in this paper is on radiation which is not only of high spectral brightness, but which has additional attributes which permit it to be focused to dimensions approaching that set by its finite wavelength, λ. With a near perfect lens [1] such radiation could be focused to dimensions approaching the Rayleigh limit, 1.2 Fλ, where F is the lens F number. Spatially coherent radiation, capable of being focused to wavelength limited dimensions, is often referred to as “diffraction limited”. Figure 1 illustrates the concept of source brightness and focusability.