The lights go out the minute the Silver Nitrate is being added to the kettle of salts and gelatin. They stay out until the film is packed in the paper backing and sealed.
What you saw was probably a film of a test run or pilot run at full scale of the equipment. We do this even today, sometimes with dummy chemistry and sometimes with just water. I ran a lot of pilot "water runs" to test new equipment or software.
Sometimes I ran real stuff in the light just to see if things went the way I wanted them to and the result was an analysis of the grain and things like that.