Interior Illumination

April 1, 2001

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.

Halogen lighting
There has been an increase in use of halogen lighting in aircraft interiors. Halogen bulbs are basically incandescent bulbs with a few differences. In a halogen bulb, a process known as the halogen cycle helps to extend the life of the tungsten filament. Halogen is added to the gases inside the bulb. As the tungsten evaporates and settles on the inner surface of the bulb, the halogen chemically reacts with it and forms tungsten halides. When these halides reach the filament, they break down, and the tungsten is redeposited on the filament.
One company that has adapted halogen bulbs for aircraft applications is LaVerne, California- based DPI Labs. They have recently developed a color-corrected halogen light. Instead of using a traditional aluminum reflector, they use a diachrillic reflector for their bulbs. This reflector allows infrared light to escape out the back of the light assembly while allowing the light to be reflected forward. This results in a cooler, brighter beam than standard incandescent lamps. These bulbs can be used for many applications from reading lights to directional lighting.
Because of the high temperature at which halogen lights operate, care needs to be taken when handling them, as handling a recently operated bulb can result in burns. Also, when handling unprotected halogen bulbs, technicians should avoid touching the glass of the bulb with their bare hands. Oils and salts from the hands can carbonize on the quartz glass of the bulb. This leaves a dark spot that absorbs radiation from the filament and becomes extremely hot, possibly causing the bulb to crack or shatter.

Fiber optic lighting
Another technology that is seeing increased use in interior illumination is fiber optics. Avtec, Inc. based in Cahokia, Illinois is a manufacturer of aircraft fiber optic lighting systems. Their system is used for upwash / downwash, galley, and accent lighting applications.
Fiber optic systems are made up of two major components — the illuminator and the fiber assembly. The illuminator is the part of the system that produces the light. Avtec’s product consists of a 60-watt lamp, ballast, cooling fan, end even color wheels to allow for change in lighting color. The fiber assembly of the system is the part that actually gives off the light. The fiber assembly starts with a lead-in section covered by heat shrink to prevent light loss. This section goes from the illuminator assembly to the area where lighting is required. From there, the illuminated section of fiber provides light along its entire length and is terminated with an end cap. One illuminator is able to illuminate 45 feet of fiber with up to 25 feet of lead-in cable.
Fiber optic lighting offers several benefits. First of all, there are less bulbs involved to change out. The lighted cable is cool to the touch and is durable. It is also flexible — allowing for a minimum bend radius of 4.0 inches. About the only required regular maintenance is cleaning the fiber with a mild soap and water solution. As with any plastic, subjecting the fiber to harsh solvents can damage it — causing decreased light output or even necessitating replacement.

LED lighting
Light Emitting Diode (LED) lighting technology is emerging as one of the most widely researched methods of illumination and has made its way into aircraft lighting. As the name implies, an LED is basically a diode that emits light when electricity flows through it. Although LEDs have been around for quite a while, white LEDs have not been available until recently.
Many different companies are developing LED lighting systems. Whelen, a Chester, Connecticut company, has developed an overhead LED light that gives off a bright light while at the same time offering a lower profile and cooler operating temperature than other types of lighting. They are also developing LEDs for other uses including post lights and exterior position lighting.
Muskego, Wisconsin-based EMTEQ also offers overhead LED light assemblies. In addition, they have a unique LED product in the form of flexible and semi-flexible tube lighting. As the name implies, their product is a series of LEDs encased in a plastic tubing that is flexible enough to bend around contours. This offers interior refurbishing companies as well as OEMs more "flexibility" when addressing lighting issues.
B/E Aerospace is another company that is developing LED lighting products. They offer LED reading lights as well as a Smart Dimmer Control Panel, which is capable of controlling up to four cabin lighting zones independently with memory.
LEDs offer many advantages including extended life (usually up to 100,000 hours of operating life) and solid state design (no glass to shatter or filaments to break). They also consume 10 percent the current draw of conventional lights at the same light output. Many new applications are being developed for these lights, and their use is becoming more widespread.
Rod Stoehr of B/E Aerospace points out that where LEDs are concerned, even though they have a long life, it can be significantly shortened if they are operated at temperatures higher than their design specification. Be careful to chose LED products from a reputable manufacturer that has taken this issue into consideration. Also, as with a regular diode, when installing an LED, it can only be installed in one direction to work properly. This needs to be kept in mind whenever replacing LEDs.

Photoluminescent lighting
Although not useful for direct lighting applications, photoluminescent lighting is being used for exit path lighting in more and more aircraft these days. Photoluminescent lighting uses the basic theory of phosphorescence — the ability of some substances to glow for a time after the energy source exciting the substance has been removed. In the case of photoluminescent lighting, the energy source is ambient light. UK-based STG Aerospace has developed SafTGlo, an emergency exit lighting system for aircraft, which it patented in 1995. It is a fairly simple track system requiring no electrical power. The system only requires a minimum charge of 45 minutes each day in order to provide emergency lighting for up to 12 hours in total darkness.
The photoluminescent track is virtually maintenance free — requiring only cleaning the surface every now and then with a mild soap solution. As with other plastic materials, contact with solvents can harm the material and should be avoided.

Electroluminescent lighting
As is the case with fluorescent and photoluminescent lighting, electroluminescent lighting is based on the principle of phosphorescence. An electroluminescent light strip is similar to a capacitor in that a thin dielectric material is placed between two conductive electrode layers. In the case of electroluminescent lighting, a phosphor is placed on the dielectric material. This phosphor illuminates when electricity is applied to the electrode layers. Electroluminescent lighting systems only work with AC. Some sort of inverting system needs to be installed if the system is to be used with DC power supplies.
Luminescent Systems Inc. (LSI), a Lebanon, New Hampshire company, is a leading manufacturer of electroluminescent lighting. They offer both floor-mounted and seat-mounted emergency egress lighting. The system is powered by a rechargeable battery that can operate independently of the main power source in the event of power failure.
An advantage of electroluminescent lighting is the fact that it provides an even glow throughout the lighting surface. This light produces little heat and is energy efficient.
The electroluminescent lighting system is durable, with no major maintenance necessary. The light panels are resilient, and have an extremely long life. In fact, LSI says that the lights will last longer than the interior will, with the only concern being damage due to negligence by passengers or maintenance personnel.

So many choices!
These are only a few of the lighting systems available to light up an aircraft interior. There are many more lighting options available today than there were just a few short years ago. If you are wishing to upgrade the lights in your aircraft, the choices are numerous. Do your homework — talk to the different manufacturers out there and see what product best fits your needs. A lighting option that suits one particular application is not necessarily the best option for all lighting concerns. With the abundance of choices available and the rapid pace that new products are being introduced, you should be able to find a product that can give your aircraft a bright future.

The Source

Additional resources....
AC 25-10 Guidance for Installation of Miscellaneous Nonrequired Electrical Equipment

AC 25.812-1A Floor Proximity Emergency Escape Path Marking

Avtec, Inc.
3939 Mississippi Ave.
Cahokia, IL 62206

B/E Aerospace
75 Beacon Drive
Holbrook, NY 11741
(631) 563-6400

DPI Labs Inc.
1350 Arrow Highway
LaVerne, CA 91750
(909) 392-5777

S84 W18693 Enterprise Drive
Muskego, WI, 53150
Toll Free (888) 679-6170

4 Lucent Drive
Lebanon NH 03766
(603) 643-7766

STG Aerospace
3261 SE Slater Street
Stuart, FL 34997
Toll-Free (888) GLO-4USA

Whelen Engineering Company, Inc.
Rt. 145
Chester, CT 06412
(860) 526-9504