Final answer:
The active layer of the CR imaging plate is made of barium fluorobromide, a phosphor used to store and release energy as light for computed radiography.
Step-by-step explanation:
The active layer of the CR imaging plate is made of barium fluorobromide. This substance is a phosphor that is used due to its capability to store energy from radiation and then release it as visible light when stimulated, a process essential for computed radiography (CR). It is different from other substances like amorphous selenium, which is used in direct radiation capture for digital radiography, where the exposure to radiation directly creates an electric charge without the need for a separate stimulating light source.