Automotive Lighting: HID vs LED Headlights and Beyond

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The lights on and in your vehicle can be as important to the safety of you and your passengers as seatbelts, and as aesthetically distinct as great rims or an unusual paint color. As the automotive industry grows, consumers are looking beyond incandescent lights to high intensity discharge headlights and other 12v LED lighting solutions, but even more options and modifications are available to consumers. While there are amazing new technologies available, they are not all true improvements or even street-legal. Let’s shed some light on the matter.

Automotive Lighting Technologies

Automobiles have been around for over a decade – almost as long as the first mass-marketable light bulbs.  These first light bulbs were incandescent bulbs that simply run any current, AC or DC, through a high-impedance filament that gets hot, glows, and emits light.  Their efficiency is very poor (as low as 10%) but they emit light in all directions and are inexpensive.  The filament material (typically tungsten) evaporates under the heat required to generate light and tends to collect on the inside of the glass bulb, darkening the bulb over time. 

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Halogen Lights for Cars

Halogen bulbs solve this problem by burning the filament in a halogen gas atmosphere inside a quartz or aluminoscilate glass bulb.  This setup takes advantage of chemical reactions that let evaporated tungsten return to the filament rather than collect on the glass surface and enables a brighter light with a longer lifetime.  The brighter light emits more heat and the specialized glass dictates a smaller bulb size, so these lights get extremely hot and require special handling even when cool to avoid introducing grease or chemicals to the surface that could cause the bulb to heat unevenly and explode.  Halogen bulbs are slightly more efficient than incandescent bulbs and the increased light output makes them the most common light source for headlights even today. 

HID Lights for Cars

HID (high intensity discharge) bulbs are a type of arc lamp and generate light by heating metal salts to the point of forming plasma.  Most drivers could identify older HID headlights by their distinct blue or green tint, but newer technologies have normalized the color temperature.  These fixtures are more efficient than filament bulbs and can produce more light but contain dangerous chemicals like mercury and emit proportionally more UV light that can damage surrounding parts. 

LED Light Bulbs for Cars

LED (light emitting diode) retrofits often require bulky power converters to operate off the AC voltage in a building on the grid, but are perfectly suited to work with the 12VDC in an automotive system.  LEDs are special diodes that emit photons (and therefore light) when an electrical current allows the combination of electrons and holes within the device material.  Each material emits a specific wavelength of light rather than a full spectrum like incandescent bulbs, so most white LED light sources are actually a blue LED with a phosphor conversion layer that convert the light into a combination of wavelengths that appear white. 

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OLED Lighting

OLED (organic light emitting diodes) stretch out the light emitting surface into a thin film of organic compound that uniformly glows in response to an electronic current. OLED displays do not require backlighting like traditional displays and thus consume much less power while achieving richer blacks and higher contrast ratios.  These displays can also be flexible, allowing for diffuse, curved light emitting surfaces. 

Automotive Lighting Regulations & Standards

Design rules and guidelines vary widely by country and region.  In the United States, we look to the FMVSS for guidance.  The Federal Motor Vehicle Safety Standards (FMVSS) are federal regulations specifying design, construction, performance, and durability requirements for motor vehicles and regulated automobile safety-related components, systems, and design features.  As much of a mouthful as that is, the standards say surprisingly little about headlights, and most regulations are actually decided at a state level.   Federal standards require headlights to have a maximum of two settings (like low beams and high beams) that can be actuated from inside the vehicle.  State standards vary, but most require the low beams to illuminate a human 100’ away, the high beams to illuminate a human at least 200’ away, and mandate that high beams not be used when within about 300’ of another vehicle.  Actual laws and enforcement policies vary by state, but those standards are a fair representation of expected headlight behavior.   Tail lights have a similar requirement – there must be two light levels available from inside the car, with a markedly brighter level activated automatically by the brake pedal. 

The color of light is more clearly defined than intensity.  Only white and amber light may be visible from the front of a vehicle, and only red and amber may be visible from the back. There are no specific color temperatures or wavelengths called out in federal or state regulations, but most of the standards contain a clause that basically asks the driver or manufacturer to use common sense.  If the majority of humans would say a certain color of light is red, that is acceptable for your tail lights.  If the majority of drivers would say your headlights are too bright, they are too bright. 

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The FMVSS are still used to regulate the production of products available for off-road use, but most laws only pertain to vehicles when they are being used on a public road.  A light bar used for off-road driving where there are no other drivers to affect can be as bright and multi-directional as your vehicle’s electrical system can support.  If you want to put rainbow headlights in a tractor, that is between you and the cows. 

Types of Headlights: HID vs. LED Headlights

Incandescent and halogen filament bulbs are still used in most vehicles because electrical efficiency is typically not a major concern, and they emit a warm white light that renders colors well and gives our eyes more information than harsher bulbs.  Although their lifetimes are shorter than those of LED light sources, they are relatively inexpensive and simple to replace.  HID lights have been criticized as being too blue and too bright for road use, but they have found favor with drivers looking for a bright light with a unique appearance.  The brightness of a headlamp as experienced by an oncoming driver has much more to do with the angle of the beam than actual light output, and much of the brightness criticism is likely due to the HIDs being installed after-market and improperly adjusted.  Though the technologies are different, incandescent, halogen, and HID light sources all result in a completely uniform radial emission that maximizes the use of reflective panels within the headlight fixtures.  LEDs each emit from a flat surface, so even though must bulbs use three surfaces in the shape of a triangle to mimic 360o emission, the beam pattern tends to narrower and longer than other bulbs. 

Higher efficiency is almost universally considered desirable, but LED headlights have run into an interesting issue.  They are significantly more efficient than traditional bulbs which means they emit much less heat while producing the same amount of light, but it turns out that the heat was actually great for keeping snow and ice from collecting on the headlight lenses.  LED traffic lights face the same issue, and some manufacturers have introduced separate heating elements to melt accumulating snow. 

LED Running Lights

Although headlights are only required in the dark and under low visibility conditions, automotive manufacturers recognized the importance of front running lights for improving how well other drivers can see your vehicle.  Some cars just automatically have the low beam headlights on at all times, but high end cars tend to have a full or partial ring of LED lights around the outside of the normal headlight.  These rings are a common aftermarket modification and are colloquially called “angel eyes.”    LEDs are perfect for this application because the cooler color temperature of highly efficient LEDs shows up even under full sun, and they are able to operate with relatively little power.   Full headlights are typically in use for a low enough percentage of the time that efficiency is not a core concern, but running lights are literally designed to be on whenever the vehicle is running.  Running 10W LEDs instead of 60W incandescents is not going to double your mileage, but the reduced battery load helps prolong battery life and lets you not worry about charging your phone through the cigarette lighter port for the entire drive. 

LED Light Bar for Cars

On the other end of the spectrum, LEDs are also excellent for extremely bright off-road bar lights.  Their high efficiency allows them to consume significantly less power to produce the same amount of light as other sources or consume the same amount of power and produce huge amounts of light.  A 100W incandescent flood light producing 1500lm could be replaced with a 13W LED bulb, or an LED bulb could consume 100W to provide over 10,000lm.  Even though 100W is a reasonable amount of power to draw from a car battery, the battery voltage is only 12V.  This means the PCB supporting the LEDs in a bar light needs to be able to handle moving over 8A of current without incurring voltage drops that would cause LEDs at the extremities to dim.  The numbers definitely say you can get a lot of light from the energy available in an automotive system, but managing power on a PCB does not get any less tricky. 

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Matrix LED Headlights and Laser Powered Headlights

There are several technologies trying to break into the automotive headlamp market, but regulations have to be updated to accommodate them before they will be street legal in the United States.  Two that seem to have popular support are adaptive LED matrix lights and laser powered headlights.  The former uses a matrix of individual LEDs with narrow beam angles to intelligently illuminate or deemphasize parts of the environment. 

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Image from motorauthority.com

Manufacturers from Audi to Mazda have been working on this technology that can do anything from emphasize a pedestrian by flashing the lights on them alone, to preventing glare to oncoming traffic by dimming the LEDs that would shine directly on the approaching vehicle.  While the advantages are obvious, the adaptive concept defies the two-beam rule put in place in the United States.  Until legislation catches up, this technology is not street legal. 

Laser powered headlights sound like they would also fail the “street legal” test but are technically designed for use when you are the only car on the road.  As mentioned earlier, LEDs are typically emitting a blue wavelength of light that gets converted to white through a phosphor shield because it is easy to generate highly efficient, highly intense blue light.  These headlights take that concept to a whole new level by generating a high energy blue laser that reflects within the headlight fixture before striking a phosphor disc at the front of the lamp.  The excited phosphor casts a very bright white light farther down the road than most high beam lights, illuminating objects quite a long distance away without just using a brighter light.

LED Brake and Tail Light Regulations

There are fewer rules around the lighting required and/or allowed on the rear a vehicle because there is a well-defined set of information that must be communicated and lighting has to have a lower impact to the visibility of other drivers.  Rear lights must be only red and amber, and there must be a notable difference in brightness between running tail lights and brake lights. Though state rules vary, brake lights that flash multiple times when the vehicle decelerates quickly are controversial in spite of proof that they can help prevent the fender benders that occur when a vehicle is forced to slam on its brakes for one reason or another.  Flashing lights are good for conveying information, but their use is discouraged on civilian vehicles because they are so strongly associated with emergency vehicles on the roads. 

LED Brake and Tail Lights

LED tail and brake lights are popular because they illuminate much more quickly than filament or arc based bulbs that have to heat up before producing light.  This is the closest thing we have to fully street legal rear flashers, and LED lights have been shown to produce a faster reaction time in drivers than filament based bulbs.  Even if a car primarily uses filament bulbs, adding a red LED light bar at the top of the rear window is a common aftermarket modification because drivers acknowledge the safety benefits of having a bright, instant-on braking indicator.

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The crisp on/off contrast of LEDs is also desirable in turn signal indicators, for the same reason  Most indicators based on filament bulbs use a single light source and complicated reflectors and diffusion grids to funnel light both out the back and sides of the fixture.  These systems tend to capture and reflect incoming light (like that from the sun or the headlights of a vehicle behind them) and may appear to glow and send false signals to surrounding drivers.  Individual LEDs can be placed in just about any configuration, allowing multifaceted surface of light to wrap around the curves of a vehicle and give high-contrast visual information from any angle.  Turn signals in particular could also be a candidate for OLED surfaces because they are flexible and could literally wrap around curves without a complete light fixture behind them, but the “cool” effect is unlikely to convince manufacturers to pursue the more expensive technology. 

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LED License Plate Light Strip

One often overlooked but important light on the back of a vehicle is the license plate illuminator.  The small bulbs are traditionally filament bulbs that are tied into the rest of the electrical system, but they are tricky to replace and often go unchanged even after burning out.  The enforcement rules vary by state in the US, but not having a working license plate illuminator can definitely get you pulled over anywhere.  LEDs have lifetimes that can outlast the drivable life of cars, so a properly driven LED strip above the license plate can nearly guarantee the light will not go out for the lifetime of the vehicle. 

Automotive Interior LED Lighting 

While interior and accent lighting is much more fun to customize than your license plate light, the white/red/amber color limitations apply to any lights seen while the car is in motion.  The idea of DIY lights that make your car look like something out of Tron might be immensely appealing but would sadly just be an immensely quick way to get ticketed if you use them on public roads.   This even applies to map lights, though many lights now use color tunable LEDs or colored diffusers to tint the light red or blue.  Exposure to full spectrum white light makes it more difficult for your eyes to readjust to darkness than tinted light with some wavelengths removed. 

However, lights that are only on when the car is stationary can be customized in any way you desire.  Interior under-dash lighting is a quick project that can be done with a strand of neopixels and a power converter to add ambiance in just about any color.  The more ambitious modifier might use EL wire as piping on upholstery. 

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OLED Dashboard Displays

Dashboard displays commonly use high contrast blues or greens and face no issues because the display is not visible from outside of the vehicle.  These displays are another common application for OLED screens because of their high contrast ratio even under direct sunlight.  This contrast is traditionally forced by having die cut plastic with backlighting for dash information tools like speedometer and gas gauge, requiring custom plastics and mechanical arms that are fragile compared to a flat display. 

The Future of Automotive Lighting

The future of automotive lighting is bright, but the immediate future is not quite as “smart” as that of other verticals because the regulations are simply not prepared to handle the newest technologies.   Adaptive headlights are already rising in popularity in Europe, so hopefully regulatory agencies in the states agree that there is a need to bring that technology into the fold.  In the meantime, LEDs have made braking more obvious to surrounding drivers and lowered the power consumption of internal and external lights throughout the automotive industry.  This trend will likely continue as LEDs become more affordable and get designed into vehicles from the original manufacturer, rather than only being available as an aftermarket modification.  You will likely never be cleared to have electric blue under lighting while on public roads, but you can install custom interior lighting in any color you desire. For more insights into the latest LED technology, check out our article State of LEDs in 2019

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