Abstract: A scattered light fire detector and a method for evaluating scattering signals of a fire detector are disclosed. The microprocessor-based scattered light fire detector measures the scattering signals at two scattering angles and determines an alarm threshold. An alarm value is determined as a function of the ratio of the scattering signals and compared with the determined alarm threshold. The fire detector can be used with mixed fires without prior calibration. Fraudulent measurement values arising, for example, from water vapor can be stored in a memory.