What are the two types of photoelectric smoke detectors?

The correct answer identifies the two types of photoelectric smoke detectors as light scattering and light obscuration.

Light scattering detectors operate based on the principle that smoke particles entering the detector disrupt the light path from a light source to a sensor. When smoke is present, the particles scatter the light, which triggers the alarm. This type of detection is responsive to smoldering fires, which produce larger particles that are effective at scattering light.

Light obscuration detectors, on the other hand, function by measuring the decrease in light intensity due to smoke obstructing the light beam between a light source and a detector. In this case, as smoke fills the detector chamber, it physically blocks some of the light, and the increase of smoke results in a greater degree of light obstruction, signaling an alarm.

Both types utilize light in some form to detect smoke but operate under different mechanisms on how they respond to the presence of smoke in the environment.