What is the difference between silver & vesicular microfilm?

 
What is the difference between silver & vesicular microfilm?
 
Silver - is a 4 mil polyester base film treated with silver gelatin to conform to ANSI/NAPM standard IT 9.1 and ANSI/AIIM MS14.

Vesicular - is on 3.6mil polyester base film and conforms to ANSI/NAPM standard IT 9.12