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NFPA 72 requirements for visible appliances are based on the ability of a person to "perceive" the flash of a light, in an open space. This is important to understand, as there will be many times that the direct viewing of an appliance is not possible. This could be due to partitions, or obstructions due to machinery. This is known as indirect viewing.
In order to alert a person, the effective intensity of the light must measure at lease 0.0375 lumens per square foot. This will provide enough "illumination" to alert occupants.
In a corridor, however, it is expected that a person will see the flash directly, as they will be in a "tunnel", so to speak. Therefore, in corridors, NFPA 72 bases design of notification appliances on "direct viewing".
© Hughes Associates,Inc. 2012