LED Emergency Exit Sign Lighting

Why use LEDs in an Emergency Exit Sign?

In recent years, LED-based exit signs have begun to be seen in both new build and replacement applications, but usually, they aren't the first to be considered. Why is this exciting new technology slow to be taken up in applications other than the mobile phone and automotive industries?

Various technologies have been used in the past to illuminate emergency luminaires and exit signage, the most basic being the GLS (filament) lamp. This simple device has the advantage that it will operate without complex control gear on either AC or DC supplies. Filament lamps are also inexpensive and available in a range of light outputs to suit the application; however, the efficiency of such systems is fairly low, being in the order of 10-20 lm/W.

Unfortunately, the major disadvantage of GLS lamps is their lack of longevity; typically they have a lifetime of only around 2000 hours. This is sufficient for use in non-maintained emergency luminaries, but generally exit signs are used in maintained mode. Using tungsten lamps would require their replacement very regularly to ensure continued safety cover. Obviously, redundancy is a must, and multiple lamps are needed to allow for the inevitable failures.

Tungsten lamps also generate substantial amounts of heat; this has substantial design implications for luminaires, especially when redundant lamps are required. For these reasons, fluorescent tubes are the standard light source for emergency luminaires and in particular, exit signage.

Fluorescent and cold cathode

Fluorescent tubes are not dissimilar in cost to filament lamps and additionally offer the advantages of high efficiency and long life. A typical 8W lamp will produce around 450 lumens and last for up to 10,000 hours. If required, they are available with outputs of several thousand lumens for general lighting applications. Because DC supplies cannot directly run a fluorescent lamp, an inverter is needed to produce the high voltages necessary for operation, and thus fluorescents are more complex to use than filament lamps.

The average lifetime equates to over a year under ideal conditions. However, to give continued safety, preventative maintenance is preferable for fluorescent fittings. The lamp is best replaced at least annually to give continued operation so there are obvious benefits if a longer lifetime light source is used in emergency luminaires.

Two technologies have projected long lifetimes when used as light sources in such products, LEDs and cold cathode tubes.

Cold cathode can produce appreciable amounts of light and tubes based on this principle are available as slender devices, ideal for edge illumination of acrylic panels. They do however need to be fed from very high voltages and are relatively fragile, both aspects being a major consideration during the design of luminaires. Although cold-cathode exit signs and bulkhead emergency fittings are available, enhancements in LED technology are showing increasing benefits, resulting in many more becoming available.

LED Technology

In the past, LEDs were low output, inefficient, monochromatic devices. With power levels in the milliwatt range, clusters of devices were needed to give any appreciable light output. Although the unit cost of the individual elements was low, it still meant their use as a light source was expensive.

Being rugged is a major advantage, but the main reason for using LEDs was, and still is, their relative longevity. Indicators and instrument lighting have used LEDs for many years, their low power being less of a disadvantage in these applications. Other aspects of LED technology have changed, making solid-state light sources more suited to general use in emergency luminaires.