If you've watched 35mm feature films presented you have perhaps seen "Leader Ladies" or "China Girls." These are the colloquial terms for the test frame attached to each reel processed in a color lab. Color and light density tend to be perceived subjectively by the mind, so a uniformly colored photograph is used to gauge the quality and density of color information of processed motion picture stock. It has been traditional to use photographs of young women for this, sometimes young Asian women, hence the appellation "China Girl."
Even though you have seen them, you have likely never noticed it because they would have only been shown for 1/24th of a second, and only then at the beginning or end of a reel. Some of the photographs used are quite beautiful and the "Leader Lady" has become kind of a talisman of projection booths. They can often be seen taped to the walls with splicing tape.
Julie Buck and Karin Segal have given these anonymous models starring roles in their short film GIRLS ON FILM (2008). It's great to see so many beautiful frames in one place.