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What is The optical density of gross fog is: and explain in detail?
An unexposed film, when processed, shows some density. This appearance is caused by the inherent density of the base and added tint and the development of a few unexposed silver halide crystals. This minimal density is called base plus fog and typically is 0.2 to 0.3. 
 
White and Pharoah's Oral  Radiology Principles and Interpretation 8th edition