Hi Yao,

With Mirror material definition, or with surface defined as Reflective, what OpticStudio does is assuming the surface is coated with a thick layer of aluminum, with an index of refraction 0.7 - 7.0i. The aluminum layer is assumed to be thick enough that no light propagates past the layer. This means that an uncoated mirror surface has a reflectivity of less than 1, though the exact value will depend on the polarization of the rays. If you define and apply coating to a Mirror surface, the last layer in the coating definition is then assumed to be a semi-infinite thickness of substrate material (the surface thickness defined for the last layer will be irrelevant.). The same thing happens when you apply taper coating. Actually the taper profile in a single layer coating dose not result in a significant reflectivity variation over the Mirror surface. To see the taper coating effect on the Mirror surface, you may need to add another coating layer (let’s say Aluminum) as the last layer in the coating definition to be considered as the substrate. 

I hope this helps. You can read more here : The Libraries Tab > Coatings Group > About Coatings > Specifying Coatings on Surfaces.

Best regards,

Moj