US Patent 7248173 Smoke detector

A dark chamber is adapted to accommodate smoke particles. A photo emitter and a photo detector are disposed in the dark chamber. In a first region, a region where light emitted from the photo emitter passes and a region where the photo detector is capable of detecting light are overlapped, so that light emitted from the photo emitter and scattered by the smoke particles is detected by the photo detector. A photo converger converges light passed through the first region at a second region where is an outside of the first region.