When people are not in direct view of signal lights they often become aware of emergency situations by noticing their reflection on other surfaces. How visible are these lights when not in direct view?
In this video, John Bullough, Ph.D. of the Lighting Research Center at Rensselaer Polytechnic Institute (RPI) explores this issue and discusses the project, "Parameters for Indirect Viewing of Visual Signals Used in Emergency Notification" that the Lighting Research Center conducted for the Fire Protection Research Foundation.
One viewer of the NFPA video on YouTube posted the following question: "What's the reason of candela and how do you know what candela to use?"
We ran the question by NFPA staff expert Lee Richardson. Here's what he had to say:
"The question “What’s the reason of candela and how do you know what candela to use?” is answered in section 18.5 of NFPA 72, National Fire Alarm and Signaling Code. Candela (cd) is the effective intensity of a strobe. As indicated in A.188.8.131.52, it is “the conventional method of equating the brightness of a flashing light to that of a steady-burning light as seen by the human observer.” The code provides two methods of determining the candela to use for indirect signaling. Table 184.108.40.206.1(a) or Table 220.127.116.11.1(b) provide the required candela ratings for given room sizes. Subsection 18.104.22.168 provides a performance-based alternative to the use of the tables."