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| LED Technology | Vehicles |
LED’s for Automotive Applications
Benefits
Light Emitting Diodes (LED) use a solid state component to replace the traditional bulb as a light source. They can be used in the signal lamps such as rear combination lamps (RCL) and the headlamps (HL).
LEDs provide automotive manufacturers many benefits over traditional light sources. These benefits include:
- Energy Efficiency
- Enhanced Styling / Design Capability
- Long System Life
- Lower Light Source Temperatures
- High Performance
- Faster “On” Time
For technical information on LEDs follow the link below:
In general, today's light emitting diodes are more energy efficient than incandescent bulbs and halogen light sources. A typical full function LED automotive rear combination lamp requires around 10 watts of power with all functions activated. A comparable incandescent bulb lamp requires nearly 50 watts under the same conditions.
LED manufacturers are steadily increasing their "lumens per watt" rating. LEDs with 50 lumens per watt are now available and industry analysts project this growth to continue. As the output of each LED increases, the number of LEDs required per lamp can be reduced resulting in additional power savings.
Enhanced Styling / Design Capability
LEDs are much smaller than conventional light sources allowing for alternative approaches to design, styling and personalization. The smaller light sources create an opportunity to use several light sources and distribute them to create an appearance of multiple points of light over a surface. Examples of different methods to direct the light are as shown below:
Lens optics over LEDs for Center High Mounted Stop Lamp
Another design approach is to direct the light from the LEDs into a light guide giving a smooth appearance and unique shapes.
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| Tall, narrow rear lamp with lens optics | Tall, narrow rear lamp lighted |
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| Round lamps with white and red LEDs using indirect reflector optics | |
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| Lighted contour shapes can also be designed with the LEDs mounted in complex three-dimensional positions. Another alternative is to place the LED so that it is hidden from direct view using a reflector to collect and direct the light into the appropriate lighting pattern. These two solutions create a uniform jeweled appearance over the surface of a complex shaped lamp. | |
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LEDs provide stylist the opportunity to design the tail lamp with more color options. Halogen incandescent light sources typically require the lens or the bulb to provide the color of the lamp function. The tail / stop function would require a red lens and the turn function would require a red lens or an amber lens or light source. This requirement would introduce color to the lamp in the unlit condition that may detract from the styling intent of the vehicle. With LEDs, the lamp can have a clear or white look in the unlit condition, but light the appropriate color in the lit condition as shown below. |
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 Halogen Unlit |
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 LED Unlit |
 LED Lit |
Packaging is directly related to styling shape and appearance requirements and will be the primary driver for lamp size. LEDs offer the opportunity to reduce the depth of the lamp due to the differences in the light radiation pattern and size of the LED sources compared to the incandescent or halogen bulb.
Optical efficiency of a LED lamp can be greater than a traditional bulb lamp. The specific wavelength produced by LEDs gives a higher percentage of useable light radiation for a given color. The design nature of the LED is much like a point light source and the optics can be designed to collect a higher percentage of the available light to use for the required beam pattern.
LED lamps can be 60 – 75% of the depth of a comparable incandescent bulb lamp. Some signal lamps can be reduced to as little as 8 to12 mm thickness in certain applications.
Thin LED lamp design (< 12 mm)
This design capability is accomplished by the flexibility of the circuit board as shown in the below samples.
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Since LEDs are solid state devices they have no moving parts, glass or filament, making them very rugged, durable and a good choice for use in automotive applications. The LED does not burn out but will lose intensity over time. LEDs are insensitive to shock and vibration and have the potential to last the life of the vehicle, eliminating a major cause of maintenance and warranty cost. Typical LEDs are projected to last as long as 10,000 hours with minimal degradation compared to the life span of 1,000 to 2,000 hours for a typical automotive filament based bulb.
LED long system life provides a potential safety advantage over conventional light sources for both forward and signal lighting reducing the probability of the light source burning out during vehicle use.
Lower Light Source Temperatures
In an automotive lamp, the temperature of the light source and heat radiated outward can affect the space needed between the light source and the outer lens. Since the LED creates no radiated heat toward the lens, typical lamp thickness can be significantly reduced.
Thermal image of incandescent lamp
Thermal image of same lamp with LED
source
Due to the scalable nature of LED lighting (the number of actual LEDs can be increased or decreased depending upon the application and performance requirements), LED based lamps have the potential for light output equal to Xenon (high intensity discharge) lamps.
The LED product provides a potential safety advantage for signal lighting by decreasing the reaction time for a sudden stop. LEDs turn on instantly compared to a filament bulb. A driver behind an LED-equipped vehicle will see the stop lamp up to 200 milliseconds sooner compared to a similarly equipped incandescent lamp vehicle. This provides the opportunity of approximately 20 feet of additional stopping distance when traveling at 65 mph.
Industry Definition: LED is an indivisible discrete light source unit, containing a semiconductor junction(s), in which visible light is non-thermally produced when forward current flows as a result of applied voltage.
Light: Light output from the LED is a true color due to the narrow wavelength of light energy produced. The light is also directional and can be collected and directed into a beam pattern in a very effective manner. This allows for a more efficient use of light.
Color: LEDs are available in all colors required for automotive lighting applications worldwide.
Optical Systems: The four main optical styles are:
1. Direct: The LED itself is visible inside the lamp either in a small reflector or behind a collecting lens. The light is emitted in the same direction as the projected beam. This design style is currently the most commonly employed and tends to have a discrete appearance with each LED individually visible. There are three main methods of direct LED lighting:
2. Indirect: The LED is hidden from observation behind a bezel or housing component. The light pattern is developed by a system of lenses or reflectors and redirected in the required direction so that when unlit the lamp has no apparent light sources. The light from the LEDs tends to be more mixed than typical in the direct style so the lit appearance is more uniform. The packaging size of an indirect system also tends to be larger.
3. Light Guide: In a light guide the LED is not only hidden from view but often is used to illuminate several discrete elements within the lamp. The light is collected into an optically clear material (usually glass or acrylic) and is propagated along the guide through the process of Total Internal Reflection (TIR). TIR is a phenomenon where light rays are reflected perfectly from the interface between the guide and the surrounding environment. By carefully manipulating the angles at which rays hit the surface of the guide the light can be propagated around corners and even split up into many different elements. This type of system takes full advantage of all the benefits LEDs offer over bulbs and allows lamps to be designed which would otherwise be impossible. Many different styling opportunities are available from long uniformly illuminated tubes to systems which appear to have many LEDs but which in fact have only one.
4. Flat Mirror:
Difference between LED, Halogen and Xenon bulb
Typical incandescent lamp efficiencies are 30-40%, where the LED lighting solutions can approach 70% efficiency. Various LED viewing angles and radiation patterns in conjunction with unique optical concepts provide optimum light collection, thereby minimizing the number of LEDs (and wattage) required to perform a given function.
Click on image to enlarge
Click on image to enlarge
Typical LED signal lamps are lower in complexity than incandescent lamps.
Three components
Six components
LED types for automotive applications
LEDs are available in various sizes, color and power ratings
Emission Patterns
One of the advantages LEDs have over bulbs is that their emission pattern is more compact allowing for better optical efficiency. This means that a designer does not need to use as much volume within the lamp to collect the light. An incandescent source radiates over a large solid angle so much of the light cannot be collected and is wasted. An LED emits light within a much narrower solid angle so collection efficiencies are much higher.
Color Temperature
Color Temperature is a description of how a light source appears to an observer in reference to the temperature of a blackbody emitter with a similar spectrum. As a blackbody increases in temperature, the wavelengths of the light emitted shifts from red end towards the blue end. This means that light with higher color temperature appears bluer and light of lower color temperature appears more yellow. It is important to note that color temperature is not a measure of brightness or intensity only of what temperature of blackbody corresponds to the spectrum under consideration.
A typical tungsten halogen headlamp bulb has a color temperature of around 3,400 Kelvin (the Kelvin temperature scale has the same sized units as Celsius but starts at absolute zero which is -273C). Since the light from tungsten halogen bulb is being generated by hot metal this is about the temperature of the filament.
The color of an HID bulb is not determined by the temperature of a filament but by the emission spectra of the gasses in the bulb. Since the gas mixture can be adjusted by the bulb manufacturer the color temperature of HID bulbs can vary greatly. A typical OEM bulb will have a color temperature of about 4,200 degrees but bulbs with color temperature as great as 7,500 degrees can be manufactured. Daylight has a color temperature of about 5,500 degrees so a 7,500 degree bulb would appear very blue and not render color as well as a bulb with lower temperature. White LED light is created by using a white phosphor pumped by a blue LED.
Like an HID source the color temperature of an LED is not related to its actual temperature but to the spectrum it is designed to emit. This value can be adjusted by changing the composition of the LED coatings to match the color of an associated HID lamp or to emit at a daylight value of 5,500 degrees.
Spectral Power Distribution
The Spectral Power Distribution (SPD) of a light source is a plot of the energy of the source at each wavelength of the visible spectrum. This characterization allows different types of light source to be compared objectively.
Tungsten Halogen Bulb – Light from a tungsten halogen bulb is generated thermally. An electric current heats a thin tungsten wire filament which emits as a blackbody radiator according to its temperature. The resulting spectrum is smooth and continuous but substantially more yellow than daylight. At the daylight temperature of 5,500 degrees, the filament disintegrates very quickly making it useless for automotive applications. Most of the energy is released as infrared light so the entire system is exposed to considerable heat loads.
HID Bulb – Light from an HID bulb is generated by quantum transitions in an excited gas. Unlike the tungsten halogen bulb, which has a filament, HID lighting uses arc of extremely high voltage placed across an exotic mixture of gasses each of which, upon stimulation, emits a characteristic spectrum. The combination of the spectra of all the components approximates white light. The resulting spectrum is not smooth and some wavelengths are missing, reducing the ability of the light to render color as well as daylight. Because there is no infrared component the system as a whole is much cooler than halogen.
LEDs – Light from an LED is generated by quantum transitions in a doped semi-conductor junction. Electric current energizes electrons across a forbidden zone whose energy depends on the composition of the semiconductor. Since this band gap determines the wavelength of the light and is fixed, the resulting emission is monochromatic. In the case of white LEDs, high energy blue light is used to pump a white phosphor which then re-emits across a broader spectrum. The tunable nature of the band gap and the phosphor composition allow the resulting light to be tuned to provide the desired color temperature. When used in isolation the light can be tuned to match daylight giving the most pleasing and comfortable beam. When used in combination with other light sources the output can be matched to harmonize.
