Patent ID: 8152390

Claim:
A barrier unit comprising: a base, which defines a first window; a cap, which defines a second window and which is arranged so that the second window faces the first window; and at least one blade, which is arranged between the base and the cap so as to open or close with respect to the first and second windows, wherein a surface of the cap that is opposed to the at least one blade has a notched portion around the second window, wherein the surface of the cap that is opposed to the at least one blade is a reference plane, and the cap has, around the second window, a first sloped portion, which defines a first angle of less than 90 degrees with respect to the reference plane, and a second sloped portion, which is arranged so as to run from the first sloped portion toward the second window and to define, with respect to the reference plane, a second angle that is greater than the first angle and that is also less than 90 degrees, and wherein the height of the notched portion as measured perpendicularly to the reference plane is (a), the depth of the notched portion as measured parallel to the reference plane is (b), and the barrier unit satisfies 0.1<(a/b)<5.