Each Doran Light Tight Darkroom Louver is designed to supply just the right amount of air to a particular model exhaust fan.
Exhaust fans work best in a darkroom equipped with fresh air supply louvers or some other kind of air supply. If there is no air supply to the darkroom the fan will be working against a vacuum and can do nothing.
Doran L-1212 Specs
13.75 x 13.75" (WxH) (outside grill) Cut out size is 12.25 x 12.25" (back of grill is 1.25" deep)