| 摘要 |
PURPOSE:To prevent erroneous detection due to slant incident light to a filter and to make it possible to perform accurate flame detection for a broad range, by providing an optical system, which guides only a light beam that has passed a band pass filter comprising an interference filter vertically to a sensor. CONSTITUTION:A bandpass filter 2 comprises a flat shaped interference filter, which passes only light having a wavelength of 4.3mum. A positive lens L1 is arranged between the bandpass filter 2 and an incident port 4a of a light shielding plate 4 so that a front focal point is positioned on the incident port 4a. A cubic angle omega0 at the position of the incident port 4a is expressed by omega0=2<tan-1>(D1/ 2<f>1), where f1 is the focal length of the positive lens L1 and d1 is an effective diameter. The light rays within the cubic angle omega0 all become approximately parallel with a light axis X by the positive lens L1. The light rays become the rays, which are approximately perpendicular to the handpass filter 2. An infrared ray sensor 6 is arranged at the position of a rear focal point of a positive lens L2. The output is fed into a processing circuit, and the presence or absence of a flame is detected. |
