This project was conducted by the Lighting Research Center at Rensselaer Polytechnic Institute (www.lrc.rpi.edu) under FAA contract 10‐G‐013.  This

summary was prepared by the Lighting Research Center on behalf of the FAA.

Further details: Don Gallagher, donald.gallagher@faa.gov;  http://www.lrc.rpi.edu/programs/solidstate/aviation.asp.

Federal Aviation Administration 

Research Highlights 

Study Goals

Flashing lights, such as runway end identifier lights

(shown), are often used at airports to help avoid

collisions. The FAA uses a concept known as effective

intensity as the metric for quantifying the intensity of

such flashing light. It is defined as the luminous

intensity of a steady‐burning light that has the same

visual range as the flashing light. It is a measure of

effectiveness at the visual threshold, although many

airfield lights are designed to be

seen well above threshold

conditions. Further, the equation

used by FAA to define effective

intensity for signals using

multiple pulses of light that may

appear like a single flash of light,

differs from that used by the

Illuminating Engineering Society

(IES). The goals of the Lighting Research Centerʹs studies

were to validate the suitability of effective intensity as a

metric for above‐threshold visibility and to test the

suitability of the FAA’s effective intensity equation for

multiple‐pulse flashing lights.

Research Activities

In the first study, experimental subjects viewed flashes

of light varying in duration and in intensity (but always

well above the visual threshold), and were asked which

flash of light appeared to be brighter. Brightness

judgments were consistently related to the relative

effective intensities of the flashes, indicating that

effective intensity can be a useful metric for

characterizing the visual effectiveness of a flashing light.

In the second study, four different sequences of light

pulses separated by different dark intervals were

displayed to experimental subjects. The FAA effective

intensity equation for multiple‐pulse flashes of light

predicted all of the sequences to have the same effective

intensity; the IES equation predicted that the pulses

separated by shorter dark intervals would be more

effective. When asked to judge which one of a pair of

multiple‐pulse flashes was more visible, subjects

consistently judged the pulse sequences with the shorter

dark intervals as most visible.

For More Information

 http://dx.doi.org/10.1080/09500340.2013.831497  

 http://dx.doi.org/10.1177/1477153512444494  

Effective Intensity 

for Flashing Lights

The recommended equation for the effective intensity (Ie, in

candelas) of multiple‐pulse flashing lights in FAA guidance

documents is:

Ie = ∫t1t2 I dt / (0.2 + t2 – t1)

Where t1 is the start of the first pulse of light and t2 is the end

of the last pulse of light (in seconds), and I is the

instantaneous luminous intensity (in candelas) between times

t1 and t2.

Judgments of flashing signal light brightness were

correlated with the relative effective intensities of the lights.

Multiple‐pulse flashes of light were most visible when the

dark intervals between pulses were shortest.

Runway end

identifier light