A modular transceiver head including cooperative cover and base members operable as a transmitter or as a receiver of infrared energy. A controller including a processor is coupled to a transceiver head pair respectively operative as a transmitter and as a receiver of infrared energy to controllably project a beam of infrared energy therebetween through a protected region. The controller is periodically operative in a self-test mode to reduce the intensity of the projected-beam to simulate a smoke condition. Plural alarm thresholds are operator selectable to provide enhanced confidence detection, and particularized signal indications are provided to readily and quickly identify possible alarm and trouble conditions.