What would you call an "almost-closed" roller shutter?

Solution 1:

For captions and concise usage, I suggest nearly closed, tightly closed, time to nearly close, time to tightly close or, if shorter is needed, nearly closed time and fully closed time. Shut IMO works in nearly/tightly shut, but I would use close the shutter rather than shut the shutter.

You could explain at the first usage that nearly closed means almost closed, "with sunlight peeking through the holes."

With regard to questions: What is the time required/needed to tightly close the shutter? or What is the time it takes to nearly close the shutter?

I lived in Italy, Land of the Roller Shutter, for many years, and tapparella socchiusa is the term used there for a shutter that's not completely closed, as the illustrations show.

Tapparella in Alluminio Coibentato con Asola Larga da 18cm per una maggiore illuminazione e ricircolo dell'aria a tapparella socchiusa

The adjective socchiuso itself is often translated using ajar or slightly open. However, ajar in this case gives me the sense of open space under the bottom of the shutter, as might slightly/partially open. Other short descriptive phrases I can think of don't work when used before the noun time.