Illuminating Options: LEDs and halogen sources light aircraft interiors

Halogen sources create light using a filament, seen here in three different lamps, using a gas containing trace amounts of a halogen material. LEDs and halogen sources light aircraft interiors By Doug Rutan Aircraft interior lighting...

Illuminating Options Halogen sources create light using a filament, seen here in three different
lamps, using a gas containing trace amounts of a halogen material.

LEDs and halogen sources light aircraft interiors
By Doug Rutan

Aircraft interior lighting choices impact maintenance complexity and cost, from passenger reading lights and galley illumination to the cockpit and flight attendant service quarters. While halogen light sources have been the traditional standard in aircraft interiors, the emergence of white light emitting diodes (LEDs) into the marketplace provides interior designers additional options for getting a job done.

Whether through initial installation, routine maintenance, or retrofit, maintenance professionals will likely deal with both technologies. Understanding the differences between LEDs and halogen sources can help maintenance professionals, and will allow decision makers greater insight into the implications of selecting a technology for a particular task.

Lighting functions and fundamentals
Lighting within an aircraft performs two basic functions: lights are either indicators, or they provide illumination within an area. Indicator examples include "Exit," "No Smoking," and "Fasten Seat Belt" signs, as well as the many lights on dials and gauges within the aircraft's cockpit. These lights draw low power, and often employ LEDs. On the other hand, illumination tasks include lighting the overhead baggage compartments ("soffit" lighting along the cabin's length), the galley, restrooms, and the cockpit. Halogen sources are used in many of these areas.

Halogen lamps and white LEDs create light in fundamentally different ways. Halogen sources use a glowing filament inside a bulb with a special gas-fill that extends the lifetime of the lamp. LEDs are solid-state devices that provide both self-generated light and light emitted from excited phosphors, such as those found in fluorescent lamps. Special electronic circuitry is required to regulate the current in an LED unit.

Lamp brightness, power consumption, heat management, and lamp lifetime differ between the two technologies.

One of the key differences between the two sources is brightness. Halogen sources are typically brighter than LEDs because the 2-mm-long filament within a halogen lamp concentrates light output into a smaller area than does the 4- to 5-mm diameter of a typical white LED disc. This can be visualized by thinking of light output emerging in a "cone" from the source. A 2-mm filament will concentrate its light output within a narrower cone than will a larger diameter disc. Even if the two sources emit the same amount of light, the narrower cone of the halogen source will allow greater illumination within a particular area. This makes halogen a better choice for applications such as overhead reading lamps.

First- and business-class cabins, on the other hand, often have "goose-necked" reading lamps that passengers can adjust. These lamps often employ LED sources. Although the LED disc spreads light output over a wider area than a halogen source will, the passenger can easily change the distance between the lamp and reading material to increase or decrease illumination on the task surface. Small business jets in which reading lamps are positioned very close to the passenger are good choices for LED sources as well.

Power, heat management, and lamp life
Power, or energy delivered to the light source per unit time, is also an important consideration both in terms of overall lighting power budget, and for heat management, since energy that does not generate light will generate heat. While LEDs have typically required more power than halogen sources for an equivalent task, new, white LED sources are more efficient than their older counterparts and consume roughly the same amounts of power as halogens.

The two technologies exhibit different heat flow, so lighting engineers must employ different heat management strategies.

Standard incandescent and halogen lamps lose their heat through radiation. The lamp is combined with a parabolic-type reflector with a metal coating. Heat in the form of infrared (IR) radiation is projected out of the front of the fixture into the surrounding area.
In comparison, heat flows off the back of an LED assembly. So to retrofit an overhead lighting application that previously employed halogen sources, additional engineering and cooling may be required to accommodate LEDs.

Lamp lifetime is another important maintenance characteristic. Individual LED lifetimes can range from 1,000 to 100,000 hours, although because LEDs are packaged as lamp clusters, a cluster lifetime of approximately 10,000 hours is average. Though halogen source lifetimes vary, a high-quality halogen source also exhibits a 10,000-hour lifetime.

Maintenance intervals
When must maintenance personnel change out a light source? A 10,000-hour lamp lifetime equates to about four or five years. Halogen and LED sources do not fail in the same way. A halogen bulb that has burned out is easily noticed, but when individual diodes fail at different times within an LED cluster, they contribute incrementally to illumination decline. Maintenance personnel should be aware that LED cluster light output might drop significantly before the 10,000-hour lifetime is reached.

And while lifetime and maintenance interval predictions are based on average performance, strong mechanical shocks or electrical surges can cause source failure long before the expected life limit. LEDs are particularly durable to shock and vibration; high-quality halogen assemblies incorporate design features that conform to the durability requirements of airframe manufacturers.

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