PHOTO DETECTOR WITH FILM REFLECTOR

PROBLEM TO BE SOLVED: To improve the S/N ratio and the performance by using, as reflector, a reflection area, which is formed out of a cylindrical chromium layer on the internal surface of a housing and covered with a reflection material. SOLUTION: Radiation energy from a laser light source is admitted into a housing 12 via a radiation energy input part 14. All the incident non-scattered energy 26 is absorbed by a light trap 22 having an internal light absorption area 24 in an internal capacity. In this process, smoke particles flowing into the internal capacity 16 together with air passing through openings 20a -20n provided in the wall of the housing 12 cause a forward scattering as scattered light 32a , 32b and 32c . The forward scattered light is reflected by reflection areas 12c and 12d , which are formed out of a cylindrical chromium layer on the internal surface of the housing 12 and covered with a reflection material to be collected by a photosensor 29 at an end of a conical area 12b . The output of the photosensor 29 provides an index for incident scattered light. Thus, the use of the reflector formed in the reflection area inside can improve the S/N ratio thereby increasing the level of a signal with respect to the phtosensor 29.