A smoke detector includes a dark chamber surrounded by a peripheral wall structure having a plurality of nested vanes. The second element of each vane is shorter than the first, and intersects the first element intermediate its ends at an acute angle. Adjacent vanes define a tortuous path that requires a minimum of three, and in most cases four, reflections for light to reach the inside of the chamber. Adjacent vanes define twisted channels leading from outside the chamber into the chamber for blocking light without substantially restricting the flow of air. The channels each include an outer section that extends in a direction toward the center of said chamber to define a channel entrance that admits airborne smoke with approximately equal resistance from opposite directions. The channels also define second and third sections that bend inwardly from the entrance toward said chamber, first in one direction and then sharply in another direction.
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