Ventilation

[Flack]

I have a spare bathroom extractor fan so I was thinking of fitting it in my darkroom, but what is puzzling me how do I stop the light from coming through the fan. It will be going through a double brick wall with a self closing flaps panel on the outside.
What have others used to stop the light from coming through the opening.

 

[ian barber]

You can use inverse square to your advantage if you put a black duct on both ends