Which of the following is not a type of smoke detector?

• A. Ionization
• B. Photoelectric
• C. Heat detector
• D. Light obscuration

Answer: (C)

Smoke detectors alert based on particles or light interactions in the air, not on heat. The main types of smoke detectors are ionization, which uses a small radioactive source to create ionized air and triggers when smoke disrupts that current, and photoelectric (optical), which detects smoke by how it affects light—either by scattering light to a sensor or by obscuring a light beam. Light obscuration is a recognized method within optical detection, where smoke blocks a light path and the sensor responds. A heat detector, by contrast, responds to heat—either a fixed high temperature or a rapid temperature rise—and does not detect smoke particles. So the one not a smoke detector is the heat detector.