Lighting the Way Ahead

Jan. 1, 2020
When Cadillac introduced the world's first factory-installed electric headlights in 1912, driving at night was still rare and risky. Steady improvements in power and brightness have reduced the risk, and today's headlights can virtually turn night in

New technology creates a mixed bag of lights.

When Cadillac introduced the world's first factory-installed electric headlights in 1912, driving at night was still rare and risky. Steady improvements in power and brightness have reduced the risk, and today's headlights can virtually turn night into day out to a distance of 200 feet. And focused beams can reach 600 feet or more. But driving relies almost completely on visual cues, and the world looks very different under artificial light.

Technology always has driven regulations that define things such as headlight design, bulb type and aiming point. Recent advances prompted the National Highway Traffic Safety Administration (NHTSA) to initiate a study that will help define the next generation of vehicle lighting. The study begins with the following statement:

"The goal of Advanced Forward lighting System (AFS) is to actively control headlamp beam patterns to meet the dynamic requirements of changing roadway geometries and visibility conditions...Currently, advanced front lighting systems are categorized by 'beam type.' These are: bending beam, town beam, motorway beam and adverse weather beam."

When the study is concluded and the regulations written, new vehicles will be required to have headlights or some additional light on the front of the vehicle that can change focus, aim, beam pattern and/or beam direction in response to commands from an electronic control unit.

These headlights already exist, developed by companies such as Hella, Delphi and Visteon, but implementing this technology is another matter. The major concern is how to avoid distracting or blinding other drivers with headlights that move. Technology and software are still evolving, and the government has yet to finalize the regulations defining AFS. While it's difficult to say what will finally reach production, we can at least explore the possibilities.

Regulations

The Federal Motor Vehicle Safety Standard (FMVSS) 108 that specifies headlight type and placement was last updated in 1995. It calls for at least two headlamps as far apart as possible at the same height. They must be between 22 inches and 54 inches (at the centerline) above the road. This wide range of allowable heights is part of the reason for the NHTSA study.

The inside rearview mirror on some cars is at the same height as the headlights of many SUVs. High Intensity Discharge (HID) headlights produce full-spectrum white light, and on some cars, the beam moves slightly side-to-side as the driver turns the steering wheel. The brightness and movement of that beam have generated concern (and drivers' complaints) about glare and distraction. Many SAE standards for lighting performance have been rewritten in just the past three years to address this problem.

AFS technology

Headlights have a single light source that is focused into a beam using reflectors and/or lenses. Most headlights on the road today use halogen light bulbs and a reflector that shapes and aims the beam, while HID systems use reflectors and lenses.

In a reflector-type headlight, the beam's aim can be adjusted "on the fly" by moving the whole lamp/reflector assembly or by adjusting the lamp's position inside the reflector. The lamp or reflector can be tilted up for high beams and down for low beams using a simple two-position actuator. More sophisticated HID systems use a stepper motor to move the reflector or the lens. So in addition to switching between low and high beams, the control unit reads suspension position sensors and operates the motor to keep the headlight beam level with the road.

This is the technology that will be used for "bending" the light beams. While there will probably be improvements, all that's needed now is software and government guidelines.

Distributed Lighting System

Distributed lighting refers to light generated at a source and transmitted through fiber optic "tubes" to one or more remote locations. One of the first cars to use this technology was the Fiat 131 in the late 1970s. The instrument panel gauges all were illuminated by a single light bulb transmitting light through fiber-optic tubes. Unfortunately the bulb socket was buried deep in the dashboard, and being a 1970s-vintage Fiat, electrical reliability was an issue.

Improvements have been made in both reliability and design. Although the light source itself is now likely to be a single LED, it can reasonably be expected to outlast the vehicle. Distributed lighting is typically used to add lights that might not otherwise be practical, such as locations exposed to harsh environments. Fiber-optic connections don't corrode.

More significantly, distributed lighting reduces traffic and noise interference on a vehicle's data communications bus. As the electronic content of new vehicles increases, distributed lighting will be used for more interior and exterior lighting.

High Intensity Discharge

High Intensity Discharge (HID) headlights are now relatively common, and they're even available as aftermarket conversion kits.

Compared to a standard 55-watt halogen H4 headlight bulb, HID headlights produce more than twice as much illumination using only 35 watts of power, though it takes up to 30,000 volts to strike the initial arc. The lamp itself operates on AC voltage, so each HID headlight assembly includes the lamp, a ballast unit and a starter, sometimes called the ignition unit. The ballast contains the DC/AC power converter and a control unit that regulates AC voltage, provides gradual warm-up of the lamp after cold-start and an instant restart if needed. The control unit also can sense a failed lamp and will turn off the power to prevent injury or unwanted ignition of other substances.

HID lights use a lens to focus the beam and are often called projector beam headlights. A Xenon bulb is mounted inside a reflector, and a lens is positioned a specific distance in front of it. Between the reflector and lens is a shutter that moves up and down to cut off part of the light pattern reaching the lens. As in the reflector beam system, the shutter can be moved with a simple two-position actuator or to infinite positions with a stepper motor.

However, since this reflector doesn't move, the beam's hot spot remains in the same position relative to the vehicle. This means as the vehicle moves over bumps in the road, the headlight beam's hot spot can momentarily flash at oncoming traffic. The same thing happens with conventional headlights, but it's more distracting because HID lights are so bright. This might make the moving-reflector auto-leveling system more likely (in the regulations), even with its increased cost and complexity.

Light-Emitting Diode

The latest advance in vehicle lighting is LED headlights. But what is a light-emitting diode?

A diode is often described as an electronic check valve: resistance to current flow is very low in one direction but very high in the other. A solid-state diode is made by joining together two different semiconductor chips (silicone wafers doped with different materials), and the current flow "gate" is the joint between the chips. Performance properties of a diode are partially determined by the semiconductor materials, and in the early 1900s, it was discovered that certain combinations will produce light when current flows through them. Serious development of solid-state diodes began in the 1960s to replace vacuum tubes. By the 1990s, light-emitting diodes (LEDs) had been developed for sensors, instrument lighting and dozens of other low-intensity applications. Today LEDs are available in color wavelengths ranging from infrared to ultraviolet.

Producing LEDs that generate white light is a challenge, because white light is a combination of all colors. Two different techniques are used. One method uses three primary-colored LEDs (red, green, blue) combined into one assembly and focused through a lens into a single beam. At certain angles the light will be tinged with varying colors, so the lens shape is critical. The other method is to cover the LED with a coating that glows (phosphoresces) white when exposed to blue light. This reduces overall efficiency (light produced for energy consumed), but it's usually less expensive, and the light is the same color at all viewing angles.

Producing LEDs bright enough to be used for illumination is also a challenge. More than one LED is needed to equal the light output of a single light bulb. Also, even without a hot filament, LEDs make almost as much heat per unit of light as incandescent bulbs. Most of that heat is produced at the mounting base, so the lighting assembly needs a heat sink and some kind of ventilation. Also, when an LED gets hot, it produces less light, further increasing the need to manage heat.

The 2009 Cadillac Escalade will be the second production vehicle with all-LED headlights (the 2007 Audi R8 sports car was first, and the 2007 Lexus LS 600h uses them only for low beams). Made by Hella, each headlight assembly uses an array of multi-chip LEDs and glass projection lenses called optical units. Two lenses on each headlight are identical, but the others are all different shapes. Sufficient lighting could be accomplished with fewer optical units, but Cadillac chose this design for styling reasons.

Each low beam has five optical units stacked at the outer edge of the housing. The top unit has a round 60mm projection lens that generates the low beam's extended pattern on the right side of the road. The four cushion-shaped freeform lenses below it generate a symmetrical pattern at close range in front of the vehicle. Daytime running lights are created by electronically dimming the low beam stack. Two additional optical units at the inner edge of each housing are the high beam.

Two white standard LEDs (no lenses) are used as a guide rod placed vertically between the low beam and the side marker. On the outer edge of the housing, seven vertically stacked amber standard LEDs form the side marker lights. Turn signals and fog lamps are mounted in the bumper.

Though a cluster of LEDs is not as efficient as one HID light, Cadillac wanted the unique styling of LED headlights. The penalty is heat, so Hella developed a powered ventilation system especially for this application.

Software

As with so many other items on today's vehicles, exterior lights will be operated by software. This enables new (old?) features like automatic high beam dimming and automatic on-off. Cameras and other optical sensors will be added for "adaptive" lighting. With these systems, vehicle lighting trouble codes will show up on a scan tool. So in a few years, replacing light bulbs will give way to reprogramming the control unit.

That is the future of automotive lighting technology.