X-ray Beam Alignment
ZnS Phosphor (P-31) is used to allow us to know where the X-ray beam is, how big it is, and is it the right shape.
If your optical system allows you to see the fine crystals that are in a ZnS phosphor, then you can have better focus and alignment.
Checking Beam Position matches Correct Z-Height
Electron Count-Rate vs X-ray Beam Focus vs Optical Beam Focus
Optical focus can be wrong. Optical cameras on XPS systems do not always provide high quality images with useful depth-of-field capability.
When Optical focus is wrong, then count-rate can be wrong and beam alignment can be wrong, and you might analyze the wrong position!