Why do you need vacuum when coating the lens?

First, explain qualitatively why the evaporation space needs to be vacuumized during lens evaporation coating The reasons are as follows,

1) Prevent the deterioration of evaporation source due to the reaction between air molecules and evaporation source to form compounds at high temperature.

2) Prevent the evaporation molecules from hitting the surface of the lens substrate directly due to the collision between the molecules of the evaporation material and the air molecules in the vacuum chamber, and form compounds on the way, or condense before reaching the lens substrate due to the collision between the evaporation molecules.

3) Prevent air molecules from mixing into the film as impurities or forming compounds in the film during the formation of the film on the lens substrate.