Candela (cd) is the standard international (SI) unit for luminous intensity, which measures the power emitted by a light source in a particular direction. NFPA 72, National Fire Alarm and Signaling Code, specifies the required light output for visible notification appliances (strobes) in terms of effective candela. This rating ensures the appliance is sufficiently intense to alert occupants, including those with hearing impairments, by penetrating their field of view and drawing attention to the alarm condition. The standard provides tables for proper device placement based on the candela rating.

A. lux: Incorrect. Lux is a unit of illuminance, measuring the amount of light falling onto a surface (lumens per square meter), not the intensity of the source itself.

C. lumen: Incorrect. Lumen is the unit of luminous flux, which measures the total quantity of visible light emitted by a source in all directions, not its directional intensity.

D. foot candle: Incorrect. A foot candle is an imperial unit of illuminance, measuring light falling on a surface (lumens per square foot), not the source's intensity.

1. NFPA 72, National Fire Alarm and Signaling Code, 2022 Edition.

Chapter 3, Section 3.3.36 Candela (cd): Defines candela as "The SI unit of luminous intensity."

Chapter 18, Section 18.5.3.1: States, "The light source for visible notification appliances shall be xenon or equivalent." The subsequent sections and tables (e.g., Table 18.5.5.4.1(a)) base all spacing and application requirements on the appliance's effective intensity rating in candela (cd).

2. SFPE Handbook of Fire Protection Engineering, 5th Edition.

Chapter 55, Section 55.5.2, "Visible Notification": States, "The intensity of a visible signal is measured in candela (cd). The effective intensity of a flashing light is a function of the peak intensity and the flash duration." This directly confirms candela as the unit for source intensity. (DOI: 10.1007/978-1-4939-2562-455)

3. Jensen, D. (2017). Fire Protection Systems (University Courseware Material), Oklahoma State University.

Chapter 4, "Notification Appliances": Course materials typically explain that visible appliances are rated by their light output intensity, which is measured in candela, and that NFPA 72 requirements are based on these ratings for proper design and installation.