The optical density of gross fog typically is:

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. Radiographic density is influenced by exposure and the thickness and density of the subject. Image receptors in Oral and Maxillofacial Radiology, pg -16, White and Pharoah, 7th ed, pg- 75