Interior Illumination

Interior Illumination Shedding some light on the subject By Joe Escobar April 2001 In today’s aircraft, lighting plays an essential role. We are aware of outside lighting requirements such as position and anti-collision...

Interior Illumination

Shedding some light on the subject

By Joe Escobar

April 2001


In today’s aircraft, lighting plays an essential role. We are aware of outside lighting requirements such as position and anti-collision lighting. However, in addition to the critical role of exterior lighting, interior lighting is also important. We will discuss some of the types of lighting available as well as some of the newer technologies that are starting to make their way into the market.

Incandescent bulbs
As we know, Thomas Edison started the whole lighting movement. He was responsible for moving us from the age of candle and oil lighting to electrical lighting. Edison designed a bulb that basically worked as a resistor. A thin filament made out of carbon was placed in a bulb containing a vacuum. A vacuum was used because of air’s oxidation effect on the filament at high temperatures. When electricity was passed through the filament, its resistance caused it to convert the energy to heat and radiate the energy as light.
Today’s incandescent bulbs work on the same basic principles as Edison’s bulb did. Tungsten is now used as the filament material instead of carbon. An advantage of using incandescent lighting is that compared to other types of lighting, the cost of replacement bulbs is fairly low. In addition, you can replace a bulb without having to install totally new mounting units or power supplies.
One of the disadvantages of incandescent lighting is the heat produced. Because of this heat, technicians need to be aware of bulb placement to ensure that no flammable material is lying close to the bulb. Another disadvantage of this light is the relatively short life. Because of the high filament temperatures, usually over 3,500 degrees, the tungsten tends to evaporate. This evaporation is evident as a dark deposit on the inside of older bulbs. The evaporation tends to be uneven and eventually causes thin spots in the filament. It is at these thin spots that the filament has a tendency to fail — causing the bulb to burn out.
Instead of the past practice of manufacturing incandescent bulbs with a vacuum, many bulbs today are filled with an inert gas such as argon or an argon-nitrogen mixture. This gas helps to slow down the process of filament evaporation. Because of the large size of these gas atoms, the evaporating tungsten tends to bounce back onto the filament. Although it does not completely eliminate evaporation, it does slow it down enough to provide a longer bulb life.
Because of the design of incandescent bulbs, they are somewhat fragile. Technicians should use care not to subject them to extreme shock. Excessive shock or vibration, especially on bulbs that have already been in operation, can lead to shorter life of the bulb or even immediate failure.

Fluorescent lighting
When the technology of fluorescent lighting was introduced, it didn’t take long for it to make its way into aircraft interiors. It is used in many larger aircraft as upwash / downwash lighting.
Fluorescent lights do not rely on a glowing filament for illumination. Instead, they use a process called gas discharge to create light. An electrode is located at each end of the tube. The tube contains a small amount of mercury as well as an inert gas such as argon. The inside of the tube is coated with a phosphor that gives off light when excited with UV radiation. When electricity is applied to the electrodes, the current passing through the gases causes them to give off UV light. The phosphor coating on the inside of the tube then converts this UV light to visible light.
Fluorescent lighting has several benefits over traditional incandescent lighting. They are more energy efficient, and operate at much cooler temperatures for the same light output. A disadvantage is their limited application due to their size. Also, curved mounting areas can pose a placement problem. Their large size and thin walled construction requires careful handling so as not to break them. If one is broken, care should be taken during cleanup due to the small amount of mercury present in the bulb assembly.

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