Light Obscuring Smoke Detectors In the Light obscuring smoke detector, smoke interferes with a light beam between a light source and photocell. The variation in photocell output is being used to initiate an alarm. This type of detection can be used to protect large areas with the light source and photocell positioned some distance apart. Watch the video and answer the question. A photocell and light source are used in Light Obscuring Smoke Detectors. How do the 2 work together to detect smoke? T