Lens coupling efficiency: Derivation and application under differing geometrical assumptions

The calculation of lens coupling efficiency is often performed in the design of lens coupled digital radiographic systems, now currently under development. With these systems, an electronic camera is focused onto a planar scintillator such as a conventional intensifying screen. Historically, this calculation has relied on certain assumptions concerning the emission properties of the scintillator, and primarily this assumption is that the scintillator is either a Lambertian emitter (light is emitted over all angles equally) or a point radiator. Because there now exists new classes of scintillators such as scintillating fiber optic screens and pixelated intensifying screens, it is sometimes necessary to perform lens coupling calculations in the absence of the Lambertian and point source assumptions. In this paper we describe the necessary equations to accurately calculate lens coupling efficiency in the most general of cases. Graphical examples demonstrate the lens coupling efficiency for hypothetical Lambertian scintillating sources, for a rare earth intensifying screen, and for a scintillating fiber optical screen.