I don't think optical dot gain is something new or no one bring it out in this forum before. There should be many members know about what caused the dot gain.
Attached a pdf, which was posted (by me) in March in one the thread, that was summary of the dot gain explanation published in one of the magazine in late '90.
Franky to said that the ppt content is nothing special. Nowadays density is using more in process control instead of as a printing standard.