A 50 year old male patient complains of reduced mouth opening. Patient gives h/o RTA 1 week back. A panoramic X-ray was taken using an intensifying screen containing gadolinium and lanthanum. On interaction with X-rays, the contents of this intensifying screen emits:
Red light
Green light
Blue light
Yellow light
Contemporary intensifying screens used in extraoral radiography use the rare earth elements gadolinium and lanthanum. These rare earth phosphors emit green light on interaction with X-rays.  Compared with the older calcium tungstate screens, rare earth screens decrease patient exposure by 55% in panoramic and cephalometric radiography.
White and Pharoah, Oral radiology, ed 7th, pg-34