Why the coating looks dimming?

1.The reason that coating looks dimming and rough is lots of macro particles are deposited on the coated products and diffuse reflection occurs due to the bombardment of arc ions.

2.The surface or some parts of the coating look dull and seem like a haze is because residential gas produced during coating react each other and form a dimming coating.

3.Another reason is that the crystal structure of coating is not compact. It becomes dimmer with the increased thickness of coating.