The emergency lights, also known as emergency heads or lamps, represent a key performance factor of the emergency lighting equipment: during a power failure they must supply enough light on the floor to help people evacuate the building easily and in a timely manner. How much light is required for a safe evacuation? The minimum illumination level on the path of egress is specified in the Life Safety Code issued by the National Fire Protection Association: NFPA 101 (2006 Edition), art. 7.9.2.1. The Code also sets limits for the minimum level of illumination, at the beginning and at the end of the back-up time, as well as for the illumination uniformity:

7.9.2.1 Emergency illumination shall be provided for not less than 1½ hours in the event of failure of normal lighting. Emergency lighting facilities shall be arranged to provide initial illumination that is not less than an average of 1 footcandle (10.8 Iux) and, at any point, not less than 0.1 ft-candle (1.1 Iux) measured along the path of egress at floor level. Illumination levels shall be permitted to decline to not less than an average of 0.6 ft-candle (6.5 Iux) and, at any point, not less than 0.06 ft-candle (0.65 Iux) at the end of the 1½ hours. A maximum-to-minimum illumination uniformity ratio of 40 to 1 shall not be exceeded.

As the lamps draw the power from batteries, which have a direct impact on the equipment cost, both the equipment manufacturer and end-user have a legitimate interest to utilize energy-efficient lamps, with performing light output and distribution. So, what lamps are available for the emergency lights and which ones are the best?

 

 

 

Miniature lamps: incandescent and quartz halogen

The most popular emergency heads are lighting fixtures including a plastic housing, a miniature incandescent bulb plugged in a socket, a metal reflector, and a plastic lens with diffusing ribs. Being battery-operated, the bulbs have low-voltage ratings (6, 12, 24 Volts) and use a tungsten filament in gas mix of Argon and Nitrogen. Usually, these lamps are referred to as “tungsten incandescent.”

better performance is obtained by using quartz halogen bulbs. The halogen gas in the bulb (iodide, fluoride) allows the tungsten filament to run at higher temperatures so the light output is about 25 - 30% higher than for incandescent bulbs with the same power consumption and life expectancy. Table 1 shows a comparison between an incandescent and a quartz halogen miniature lamp. The light output is rated in initial Mean Spherical Candlepower, MSCP.

You may already have noticed the unusually short life of these lamps: why would anyone use a lamp lasting only 50 hours? Simply because it offers a higher light output (25-30%) than a long-life (say 1000 hours) lamp for the same power consumption and technology. By design, the lamp filament is overdriven at higher temperatures, which increases the light output but also accelerates the vaporization of the filament atoms and consequently shortens its life.

However in emergency lighting the power failures don’t occur too often (maybe 4 - 6 times a year). With a back-up lighting duration of 90 minutes the emergency lights are actually used for not more than 6 - 10 hours a year; hence, a 50-hour life lamp should be operational for an average of five years or more.

Lamp manufacturers like GE or Sylvania offer both “long life” and “high-output” miniature lamps of the same construction, e.g. incandescent, quartz halogen, etc. Table 2 shows a few examples of trade-off between lamp life and light output.

Reflector lamps: sealed-beam

Aside from the bulb, the performance of the emergency head depends on the main optical components: the reflector and the lens. This may become a challenge especially in damp and wet locations where water vapours or leaks can deteriorate the electrical contact in the socket or the reflector performance. A solution for these applications are the so-called reflector lamps or directional lights. The lamps have a built-in glass reflector and lens which are factory-designed for a particular type of beam angle: narrow, medium, large, etc. The best candidates for weatherproof performance are the sealed-beam lamps. Usually of a 4.5” diameter (PAR36) and with screw connections for the electrical wires, these lamps are quite popular in the emergency lighting and are available from manufacturers like GE and Sylvania in both incandescent and halogen versions. Same as for miniature bulbs, there are dedicated sealed beam lamps for long-life applications (4000 hours, 7 – 8 Lumen/Watt) and for emergency lighting (50 – 300 hours, 12 - 20 Lumen/Watt). The same trade-off applies between the lamp life and its efficacy.

While the lamp life was initially considered a non-issue, it has become an increasing concern in the last 10 - 15 years with the introduction of more complex emergency lighting equipment, featuring periodical self-testing and self-diagnostic circuitry. During the monthly self-test, a micro-controller circuit forces the equipment in emergency mode for a period of minimum 30 seconds and checks the battery capacity and the lamps integrity. Although the duration of self-testing is negligible compared to the lamp life, the repeated power turn-on and -off increases the risk of premature failure, due to the in-rush current at the lamp start-up.

The new generation: MR16

Fortunately, the lamp life dilemma has found an answer with the MR16 lamp technology. With increasing popularity in the last decade the MR16 lamps include, in a one-piece design: a low voltage (12V) miniature halogen lamp, a metal-coated glass reflector (MR-16 stands for Multi-facet Reflector of 16/8-inch diameter) and - for most popular models - a clear glass lens cemented on the reflector. Easy to install, the MR16 lamps are commonly used in the commercial and residential applications and are increasingly specified in the emergency lighting. Why? Apart from their superior colour temperature (bright, rich white) and precise directional beam, these lamps offer a good light efficacy (11 - 18 lumen/W) and long life (2000 - 6000 hours). How is it possible? One of the tricks is the use of a thin, clear glass lens (sometimes no lens at all) with low light absorption, compared to the classical diffusing lens. Instead, it is the multi-faceted reflector that plays the role of “beam diffuser.”

Photometry and light distribution simulation

Evaluating the lamp performance based on candlepower or the Lumen/Watt ratio is good, but not good enough. What matters in the end is the goal of application, in our case the illumination on the path of egress. In the commercial spaces for example, where the equipment cost is of main concern, the goal translates to something like: how many emergency heads and battery power are required to illuminate a path of egress - say - 6 ft wide and 100 ft long?

For exemplification we have run a few lighting simulations using a standard software package (AGI) and lamp photometric data (IES files) available on the web sites of lamp manufacturers and equipment OEMs. To keep things simple we have chosen a generic set up, close however to real-life applications: the path of egress is a 6-foot wide surface on the floor; the lamp is installed at a given height (8 feet, 15 feet) above the centre of the floor and has a vertical aim; the walls and the ceiling have no reflectance (in real life the indirect lighting slightly adds to the floor illumination). For each lamp we will analyze the average illumination on a path of egress of a given length.

Sealed-beam lamps: halogen vs. incandescent

The first comparison (Figure #1) is between two sealed beam PAR36 lamps, a halogen (GE # H7557, left) and an incandescent one (GE # 4044, right) installed at 8 feet above the floor. The picture shows a floor view with iso-foot-candle lines. The illumination values (average, maximum, minimum) are calculated on a path of egress 6ft x 18ft. The two lamps project similar iso-lines on the floor, yet the average illumination of the halogen lamp (left) is superior: 1.27 foot-candles (fc) compared to only .97 fc for the incandescent lamp (right), a value at the edge of the Code requirements.

Emergency heads vs. sealed beam lamps

With a well-designed reflector and lens, a small-size emergency head can be as good as a large PAR36 lamp. Figure #2 shows the performance of a 3-inch diameter head of 12V 12W (model # EF9(HD) from EMEGI-LITE / Thomas& Betts) using the GE quartz halogen lamp #783, on the same path of egress as above. The average illumination (1.05 fc) is close to the value of the GE sealed-beam lamp, while the EF9 head delivers a better light uniformity (max. / min. ratio).

The MR16 revelation

Figure #3 illustrates, in colour, rendering the performance of a PAR36 halogen lamp (GE # 19880, left) and a standard MR16 lamp (GE # Q50MR16/C/CG40, right) of similar power and life: 12V 50W, beam angle 30deg. and lamp life of 4000 and 6000 hours respectively. Given the levels of light we used a mounting height of 15 feet and a means of egress of 6 ft x 28 ft. The MR16 lamp delivers 40% more illumination: 2.1 fc compared to the 1.5 fc of the PAR36. Consequently, for the same illumination average of 1.5 fc, the MR16 lamp covers a longer path of egress: 40 ft.

The Infra-Red shield

Due to the market success of the MR16 technology, manufacturers like GE, Sylvania and Philips have developed product series tailored to different market segments. Among them, of a particular interest is the infrared lamp design: the MR16-IR. The lamp reflector has a special coating able to recycle a part of the dissipated heat (basically infra-red “IR” waves) by sending the heat back to the filament and thus keeping it white-hot with less electrical power consumption. The MR16-IR lamps are available in power ranges up to 50W and generate 35 - 40% more light output than standard MR16 lamps. The Philips catalogue for example presents IR lamps with lumen ratings leading to efficacies between 19 and 26 Lumen/Watt.

Figure #4 shows a grey-tone rendering of the floor illumination by two GE lamps: a 37-Watt IR (left) and a 50-Watt standard MR16 (right) mounted 15 feet above the floor. On a 6 ft x 40 ft path of egress the 37-Watt challenger delivers an average of 1.38 fc, close enough to the 1.5 fc performance of the 50-Watt lamp.

The last stop: LED emergency heads

The technology power LED (developed by manufacturers like: Cree, Lumileds, Nichia, Osram) is also evaluated by the emergency lighting industry. As of today, white LED products like the Luxeon Series are advertised with appealing light efficacies of 22 – 27 Lumen/Watt and an estimated operating life of 50,000 hours (until 50% of initial output). However, there are still limitations, like:

 

• Low power range: one to five Watts;
• Need for a power adaptor (constant current driver) which adds cost and power consumption;
• Need for heat sink and temperature protection; the LEDs - like most semiconductors – witness rapid degradation and failure when their junction (internal) temperature reaches or exceeds 125 - 150’C; and
• Cost, compared to incandescent lamps with equivalent light output.

Nevertheless, as the LED manufacturers forecast to improve the LED efficacy (50 –100 Lumen/Watt by the year 2010) while continuously reducing the manufacturing costs, some emergency lighting OEMs (Gilbert Industries, Chloride, EMERGI-LITE / Thomas&Betts) have already started to include power LEDs in their emergency lights.