Language / Country ▼
Home > Media > News
March 23, 2017
LED Fire Notification Devices: The Case for the 20 Millisecond Strobe

The familiar xenon flash tube has been and continues to be an effective light source for the Signaling for the Hearing Impaired category as described in the National Fire Alarm and Signaling Code NFPA 72 and ANSI/UL 1971 Signaling for the Hearing Impaired. While its effectiveness remains unquestioned, its high current consumption triggered a quest for a better, more efficient light source. Advances in LED technology as demonstrated in today’s lighting products have served as a catalyst for LED devices to be considered by many manufacturers as a replacement to the xenon flash tubes.


The development of ANSI UL 1971 was based on research conducted by UL in cooperation with the Illinois School for the Deaf, during the late 1980s. The results of this research were published in the definitive report Emergency Signaling for Use by the Hearing Impaired.1 The test instrumentation available to the UL researchers at the time was relatively rudimentary compared to today’s technology, therefore, certain specifications developed were a bit limited in scope. Specifically, the methodology to determine the light energy output from a strobe was not well defined. .

These specifications were carried over to ANSI/UL 1971 and subsequently the Notification Appliances Chapter of NFPA 72. The UL1971 standard and the Notification Appliances section of NFPA 72 were consequently referenced in the Americans with Disabilities Regulation (commonly referred to as the Americans with Disabilities Act (ADA).

Fortunately, xenon flash tubes became the technology of choice and because these devices produce a very short, extremely bright flash they demonstrated consistent effectiveness. Lack of greater definition related to light energy output from the device (as mentioned above) was never an issue.

Research Advancements:

Ongoing advancements in LED technology that led to:

1. Ever increasing brightness output and,
2. A very high conversion efficiency of electrical to light energy for LEDs,

guided some manufacturers to consider these devices as a replacement for the xenon flash tube. While newer LEDs are very bright they are not as extremely intense as the xenon flash tubes and the LED flash duration is longer. The less than ideal specification in the current UL and NFPA standards then had the unintended consequence of a tradeoff between pulse duration and pulse intensity and a number of devices designed to this trade off were listed and appeared on the market. .

The listing and availability of these long pulse duration LED strobes raised effectiveness concerns within the standards development community triggering several less than conclusive research projects. In particular, during the development of the 2016 edition of the NFPA 72 Notification Appliance chapter, the Technical Committee as an interim safety measure added a specification that limited the pulse duration for strobes to 20 milliseconds (0.02 seconds). .

The publication of the 2016 edition of NFPA 72 with the 20 millisecond specification triggered the UL Standards Technical Panel, STP for UL 1971 to form a Pulse Duration Task Group to conduct research to experimentally identify the ideal pulse duration for LED strobes. The research was to be conducted in a highly controlled environment with a large group of test subjects of various ages and both genders so that a statistically significant analysis could be conducted leading to a hard data based solution.


The UL Task Group research was conducted with over 200 test subjects at Light Engine’s one-of-a-kind test chamber where room lighting ambient could be very accurately and uniformly controlled. Test subjects were randomly exposed to various pulse durations and intensities while actively engaged in a task. An extensive body of data was compiled and statistically analyzed and interestingly enough the conclusion from this work indicated that the 20 millisecond specification made by the Notification Appliances Technical Committee for the 2016 edition of NFPA was indeed correct. .

Further it was determined that pulses of a duration longer than 20 milliseconds would need to produce much more light intensity. For example, a 60 millisecond pulse would need to emit approximately three times the intensity of a 20 millisecond pulse in all room ambient lighting conditions resulting in much higher power consumption. .

See Figures 1 and 2.

1 Underwriters Laboratories Subject 1971 US187 89NK20252 March 20, 1991

Figure 1

Figure 2

Light Engine was proud to donate the use of its testing facility to the UL Task Group. We believe that through comprehensive research and the generation of supporting in-depth data, the various safety standards of today can be updated to help address not only the technology of today, but also prepare the market for the technology of tomorrow.

For any questions or comments about the use of LED technology and the advancements in Fire Notification device efficiencies, please email: or visit our website at

Back to the top
Connect across
Light Engine
Follow Light Engine on FaceBook Follow Light Engine on LinkedIn Follow Light Engine on Google+ Follow Light Engine on Weibo Follow Light Engine on Vimeo Follow Light Engine on Youtube Please Visit YouTube or Vimeo for Light Engine's Corporate Video Home | Sitemap | Disclaimer | Email