OPTICAL DETECTION APPARATUS

PURPOSE:To always keep proper detecting operation without being affected by dust, by differentiating the open areas of the small area opening end parts of a pair of funnel shaped members provided in the light path cylinders at both sides of a flue. CONSTITUTION:When a detection apparatus S is operated, the light from a light emitter 50 passes through the opening 33b of an inner cylinder 33, the opening 32c of an inner cylinder 32, openings parts 22a, 22b, the opening 42c of an inner cylinder 42 and the opening 43b of an inner cylinder 43 and is received by a light receiving element 60 to generate a light receiving signal. At this time, if dust is penetrated in bottomed cylinders 31, 41 through the opening parts 22a, 22b, because the open areas of the openings 32c, 42c are respectively smaller than that of the opening parts 22a, 22b and spaces are respectively formed in the bottomed cylinders 31, 41 outside the inner cylinders 32, 42, said dust diffused in the above-mentioned spaces within the bottomed cylinders 31, 41 and the concn. of the dust is lowered. By this method, the penetration of dust in the inner cylinders 32, 42 through respective openings 32c, 42c is prevented.