Digital indirect detection x-ray imagers with microlens focusing: Effects of Fresnel reflections from the microlens layer

We have been investigating whether a microlens layer placed between the phosphor and the photodetector can improve indirect detection x-ray imagers. Using a simulation study, we analyzed the light collection properties of the proposed imager taking into account Fresnel reflection and transmission properties of the lenses and the screen. A digital x-ray imager combining an 82-mum-thick Gd2O2S:Tb phosphor screen, a fused silica microlens layer, and a 127-mum pixel pitch photodetector (optical fill factor of 57%) were modeled. The light collection for. the prototypes varied from 53% to 69% for lens thicknesses ranging from 10 to 56 mum. The full-width half-maximum (FWHM) of the light spread function ranged from 177-192 mum. 4-8% of the light was reflected back into the phosphor screen when correctly taking into account Fresnel reflections for these prototype imagers. In comparison, 56% of the light was collected and the FWHM of the light spread function was 174 mum for a conventional imager with the screen in direct contact with the photodetector. We observed that the light collection was overestimated by 6-9% but the spread functions were basically unaffected when the Fresnel assumption was not utilized in the simulations.. This study shows that a properly designed microlens layer can more than offset Fresnel losses, thereby producing an improved digital x-ray imager.