ANALYSIS OF SIGNAL PROPAGATION IN OPTICALLY COUPLED DETECTORS FOR DIGITAL MAMMOGRAPHY .2. LENS AND FIBER OPTICS

An x-ray detector for digital x-ray mammography is under investigation , which consists of a phosphor screen coupled by a demagnifying fibre- optic taper to a time-delay integration mode, charge-coupled device (C CD) image array. The signal propagation through such a detector depend s on the intensity and angular emission of light from the phosphor scr een, the angular acceptance and transmission of light through the opti cs, and the spectral sensitivity of the CCD to the fluorescent light. The production of light by the phosphor screen was considered in a pre vious paper. Here, the issues related to the optics are examined. For phosphor screens coupled by lenses with limiting acceptance angles of less than 30 degrees, it was calculated that the coupling efficiency w ould be 10% greater than would be estimated under the assumption of a Lambertian source. These increases occur because a phosphor screen typ ically produces light which is more forward directed than a Lambertian source. Similar increases in efficiency are observed when a phosphor screen is coupled to a fibre-optic faceplate or taper. For fibre optic s, exact estimation of the optical coupling efficiency requires knowle dge of the angular-dependent transmission efficiency of the fibres.