Protect sensor from X-Ray Damages.
Even the best scintillators can’t prevent some X-rays from slipping past the barrier, high-end sensors feature a fiber optic plate to absorb stray X-ray photons. The plate is composed of some Xray absorption material. Which could make the output xray noise as low to 1%.
Some sensor manufacturers exclude fiber optic plates to keep the sensor head thin (and production cost low). However, without the added protection from direct X-rays, it’s hard to guarantee the quality of the final radiography captured with such a sensor.