Illuminating Options: LEDs and halogen sources light aircraft interiors

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.

White Light from Device
White Light
Blue light from the LED material and yellow light generated by an excited phosphor combine to form the white light output of an LED disc. Electronic circuitry connects the LED device to the aircraft power source. The glowing filament
inside a bulb interacts with a gas fill containing trace amounts of a halogen material to produce the white light and extend filament life. The source connects directly to the aircraft power source.

Retrofit and replacement
Because halogen sources have traditionally been used for aircraft interior lighting, the necessary fixturing, electrical systems, and backup systems to support them are already in place within most aircraft. Replacing halogen sources with LED clusters requires additional engineering design.

Replacing halogen sources can be easy or difficult, depending on the application. Although some halogen lamps are easily replaced by twisting a front cover off the fixture, some personal service units (PSUs) are difficult to access and disassemble. Retrofitting LEDs requires replacing an entire lighting module; here again, the complexity of the task depends upon the application. It is important to note, however, that if an LED source does not last the lifetime of the aircraft, very expensive labor and material charges will occur with a repair or replacement.

Tighter focused beam with Halogen lamp

Cost of ownership
Evaluating the cost of ownership of each technology is a science in itself. The following list enumerates the factors that decision-makers need to consider before selecting a particular type of light source for a specific task:

  • Source initial purchase price.Original installation cost, including labor.Cost of energy consumed by the light source.
  • Cost of energy consumed by additional electronics required.
  • Number of light sources required to generate a specific amount of light. (How many halogen sources vs. how many LED modules, to produce an equivalent result for a particular task?)
  • Maintenance costs, both labor and material, for upkeep of source components throughout lamp lifetime. (Factors such as dust on the lenses, yellowing of lenses, and replacement of lamp electronics.)
  • Replacement cost of failed unit.
  • Inventory costs of holding repair parts.Labor costs associated with replacing a failed unit.

In selecting either a halogen or an LED source for a particular application, decision-makers must consider many factors. The emergence of LED technology provides additional options for interior aircraft lighting design, but maintenance issues over the lifetime of the aircraft require that the benefits and shortcomings of each technology be considered. Initial purchase price of a particular source is also an important consideration, but factors such as long-term cost of both labor and materials and complexity of maintenance must be factored in as well. Choosing a source for a particular aircraft interior lighting task should depend, ultimately, on a consideration of the illumination task in conjunction with a thorough cost and benefit analysis.

Doug Rutan is the Original Equipment Marketing Manager for the Lighting Products Group of Welch Allyn Inc., Skaneateles Falls, New York. He has more than 20 years of experience in electrical product engineering, design, and program management in the lighting industry, and has been granted multiple U.S. patents.

Additional ReSource
Doug Rutan