Final answer:
A thin sheet of aluminum is placed onto the output phosphor in an image intensification tube to prevent light from scattering back towards the photo cathode layer.
Step-by-step explanation:
In an image intensification tube, to prevent light from scattering back towards the photo cathode layer, a thin sheet of aluminum is placed onto the output phosphor. The output phosphor is where the intensified image is created, and the aluminum prevents the reversed direction of light which could degrade the image quality by re-exposing the photo cathode.
An image intensification tube is used to convert light into electrical signals. To prevent light from scattering back towards the photo cathode layer, a thin sheet of aluminum is placed onto the output phosphor. This helps to improve the efficiency and quality of the image produced.
Therefore answer is 1. output phosphor.