An apparatus for detecting fire including a light source for generating light, a light sensor for receiving light from the light source and providing an analog signal representing the intensity of the received light, and a clear plastic light guide for transmitting the light from the light source to the sensor. The light guide include notches for allowing smoke to enter the notches. Smoke entering the notches decreases the intensity of the light passing therethrough. A calibration sensor is included. The sensitivity of the fire detector is increased by a lens assembly which collimates the light generated by the light source. The color of smoke entering the light guide may be determined by generating colored light. Temperature, humidity and carbon monoxide levels are also detected and the information is integrated with the smoke detection data to provide a reliable fire detector.