Why use LEDs in an exit sign?

Ian Harwood of Cooper Lighting & Security presents the case for using LEDs to illuminate emergency luminaires and exit signage.

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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?

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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

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Figure 1
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 (see figure 1). 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.

Early LEDs were available only in red, green and amber. In North America, red or green have found a use in exit signs, as the legislation allows the 'Exit' legend itself to be the illuminated colour element. With current European regulations, such products use a green and white pictogram, which needs at least some illumination from a white light source. The advent of white LEDs with a reasonable brightness completes the requirements for European signage.

LEDs are now available as single devices with a power rating of several watts and a luminous flux in excess of 100 lm. They are easy to implement in multiples or clusters, making them an attractive alternative to traditional fluorescent light sources. Now that LEDs have the potential to be successfully used in emergency luminaires, other aspects of their operation can be exploited.

Efficiency

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Figure 2a
When taken at face value, fluorescent light sources would seem to have much better efficiency than LEDs. An 8W fluorescent lamp, used in the majority of bulkhead emergency luminaires and exit signs, is not particularly efficient for this genre, with a level approaching 60 lm/W. Larger lamps such as high efficiency T5 are generally in the region of 90 lm/W. By contrast, current LED devices are around 20 to 30 lm/W, although technology is advancing and 35 lm/W and higher devices will become generally available.

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Figure 2b
What needs to be considered however is the efficiency of the overall package of light source and luminaire. Taking an edge-lit blade exit sign as an example, a traditional method would be to use an 8W fluorescent lamp in a light box, to top illuminate the blade (Figure 2a). Clearly, this is an inefficient way to transfer light into the sign itself.

Improvements can be made by using a 'reflex' type of tube (Figure 2b), here an aperture of 50 deg. and a reflective coating gives 4 to 5 times the downward light, but the reflex lamp is not as readily available and costs around 5 to 10 times more than the standard tube.

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Figure 3a
LED light sources are compact enough to be either embedded within, or in intimate contact with, the blade of the sign. The focused light from a standard 5mm LED is a clearly defined cone and this allows a series of light sources to evenly illuminate the sign (Figure 3a).

Using side emitting LEDs, the polar pattern can be exploited to inject light directly into the sign, resulting in a very efficient system (Figure 3b).

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Figure 3b
For these systems, a much better transfer from the light source to the medium is obtained; so overall efficiency is comparable to fluorescent.

Cost issues

Again, an 8W lamp, costing less than $1 would seem to have the edge over an LED, which provides a fraction of the power at over 5 times the price. Although the cost of high-powered LEDs, in common with most electronic components, is falling, they are still relatively expensive. The key aspect to consider in drawing a more accurate comparison is the projected lifetime of the light source.

A fluorescent lamp lasts in the order of 10,000 hours, whereas a white LED can last over 50,000 hours when correctly used. This increase in life makes the unit cost of the light source begin to look attractive, but again the wider picture must be taken into consideration (see Figure 4).

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Figure 4
Although the initial capital outlay required for an LED exit sign may be several times that of a comparable fluorescent alternative, they both cost a similar amount to install. Installation costs are obviously variable, but on average it is probably more expensive to install than to purchase a low-cost exit sign. Post-installation cost differentials are therefore much less pronounced between the two technologies.

Plant maintenance costs are becoming a larger factor to be considered. Fluorescent lamps should be replaced annually; batteries generally have a 4-year lifetime and would therefore be replaced about halfway through a typical 8-year life of a luminaire. The potential of 8 to 10 year operation for LEDs means that, even with current storage cell technology, the battery would be the only maintenance requirement of the luminaire.

Advances in battery design and materials are ongoing, giving the prospect of fit-and-forget 10-year lifetime luminaires in the near future.

Into the future

Now that there is a justifiable case for using LEDs, further benefits can be taken into account. Since LEDs are a low voltage source, the luminaire designer is freed from the need to protect from the high voltages needed for fluorescents. The control gear can be safely tucked away and minimal wiring used for the sign itself.

In installations where the brightness of the exit sign may become intrusive in some circumstances, LEDs are easy to dim using pulse-width modulation or current limiting. The compact nature of the devices also opens up new possibilities, the sign can be designed around the pictogram and the light source concealed unobtrusively, rather than the product dimensions being determined by those of the lamp.

As lumen outputs and efficiency increase, LEDs will become more prevalent in general lighting applications. Within a few years, LED efficiency should be comparable to fluorescent lamps and light output levels are expected to multiply. This, together with attendant cost improvements, should result in a large increase in market share for LED based exit signage and general emergency lighting.

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