| 摘要 |
PURPOSE:To prevent the decrease in accuracy of detection due to contamination, by constituting a light receiving device, wherein a ceramics sintered body forms a light source, and a light sensitive element, a protecting film that is provided in front of the light sensitive element, and a heater that heats th protective plate are provided. CONSTITUTION:A light source 4 is provided in a light emitting device 4 and emits light. A heating resistor 6 is conducted so that the temperature in the vicinity of the resistor becomes 900 deg.C or more. The light emitted from the light source 4 is projected from a window 9 and transmitted through exhaust air. Then the light is transmitted through a protective plate 25 of a light receiveing device and projected on a light sensitive element 17. The smoke density in the exhaust air is obtained by detecting the transmittance of the light, which is projected on the light sensitive element 17. A ceramic sintered body having the heating resistor is used as the light source. Therefore, the light source itself has a high temperature. Even though smoke is attached, it is immediately oxidized and disappears. Since the protective plate 25 is provided in front of the light sensitive element 17, the light sensitive element 17 is not contaminated. |
