| CPC G03F 7/7025 (2013.01) [G02B 5/005 (2013.01); G02B 17/0848 (2013.01); G02B 17/0892 (2013.01)] | 20 Claims |

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1. A stop, comprising:
a light-transmissive aperture;
a first stop element; and
a second stop element,
wherein:
the aperture is provided in the first and second stop elements or at the first and second stop elements;
outside the aperture, each of the first and second stop elements comprises an opaque region and a fluid-permeable region;
the fluid-permeable region of the first stop element is offset from the fluid-permeable region of the second stop element so that:
i) a fluid passing through the fluid-permeable region of the first stop element is deflected by the opaque region of the second stop element before passing through the fluid-permeable region of the second stop element; and
ii) light passing through the fluid permeable region of the first stop element is unable to pass through the fluid permeable region of the second stop element.
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