Abstract: A process for the conversion of X-rays into blue and/or green light with a phosphor which on exposure to X-rays emits light the spectrum of which contains main intensity lines below 460 nm and/or main intensity lines between 530 and 570 nm, wherein said phosphor comprises a gadolinium gallium garnet compound having following empirical formula:Gd.sub.8-x-y Ln.sub.y Ga.sub.x O.sub.12wherein: 5.14.gtoreq.x.gtoreq.4.90 and 0.45.gtoreq.y.gtoreq.0.001, and Ln is scandium (Sc) and/or one or more rare earth elements other than gadolinium.The invention is directed particularly to the use of said phosphor in an X-ray conversion screen applied in medical radiography.