Blinded by the light – car headlights explained
Halogen, HID, LED, laser and adaptive headlights. These are just some of the car headlight choices available to car buyers… Here’s what you need to know about it.
HOW I PINE for simpler times. When you could have your Model T in any colour you liked, as long as it was black. These days, the plethora of options available for even economy models just about needs a PhD to decipher. In this article, we look at light, what it is, how it’s measured and the variety of lighting available for your vehicle.
What is Light? As part of the electromagnetic spectrum, visible light is electromagnetic radiation that can be seen with the naked eye. That’s enough techno babble, but it’s important to understand that there is light that isn’t visible to the naked eye. Grab your TV remote control and have a look at the front of it while pressing a button, using the camera in your smart phone. These cameras are sensitive to IR (infrared light) and you’ll be able to see it on the display. This light is above the colour that the human eye can see. Just below visible light is Ultra Violet light. This light is responsible for the sunburn I receive on my pasty white Anglo-Saxon skin on all but the most overcast days!
Illuminance of a Light is measured in Lumens. Don’t make the mistake of thinking you can compare the illuminance of two different lights using the wattage figure, for example 100W Halogen vs 75W HID. It’s like trying to compare apples and oranges. But more on that later. One thing to note is the illuminance of the light source follows the “inverse square law”. That is, at twice the distance to the original illuminance you’ll only get one fourth of the illuminance. Conversely, if you want to have twice the illuminance, you’ll need to increase the lights power by four!
Colour Temperature. Ever noticed when shopping for house hold globes, they’ll usually give you two choices, either “warm white” or “cool white”? The warm white globes produce a light that is yellower and better mimics a setting Sun’s light output that the bluer light produced by the cool white, that is akin to noon day light colours. Automotive globes are similar, in that they are available in different colour temperature configurations, which produce different shades of white light.
As anyone who has seen a rainbow can attest, white light comprises of all of the colours of the rainbow! Just like mixing a cake, mixing different proportions of different coloured light will produce different coloured white light. Halogens for example, usually produce a yellow light of around 3000K (Kelvin), Metal Halide HID lights on the other hand produce a whiter light that’s around 5000-6000K. For comparison purposes, sunlight when directly overhead at noon has a colour temperature of around 5600K.
The human eye has a varying sensitivity to different colour lights. It stands to reason that the human eye has evolved to be most sensitive to day light colours. That is, you’ll need less light to perceive an object lit with to the same brightness, when you mimic daylight colour temperatures.
Acetylene Gas Lamps, were the first widely adopted automotive headlamps. They featured on the first of the Ford Model T’s. Otherwise known as carbide lamps, the headlight worked by dripping water onto calcium carbide pellets producing acetylene gas. This gas was then lit, producing a bright broad light. Once all of the Calcium carbide had been expended, it produced a slurry of slaked lime (calcium hydroxide) this could then be removed from the reaction chamber, replenished with fresh calcium carbide pellets, filled up with water and the whole process was ready to start again. Up until recent times, recreational cavers still preferred the light produced by a carbide lamp than more modern alternatives.
Tungsten Lights otherwise known as incandescent, made inroads very early on in the piece. In fact the later Model T Fords featured tungsten headlights. A filament of resistive tungsten is electrified in a vacuum or inert gas atmosphere. As part of normal operation, some of the tungsten metal filament boiled off of the surface of the filament leaving black deposits on the inside of the globe. They are all but obsolete today.
Tungsten-Halogen Lights, more commonly known as halogens, were the next emerging headlight technology. Similar to tungsten lights, though instead of operating in a vacuum or strictly inert atmosphere, they have small amounts of halogens, such as bromide or iodine. With the addition of the halogen, this created the “halogen cycle”, in which, instead of the boiled tungsten from the filament depositing on the inside of the globe, it was redeposited back onto the filament. This major development lead to higher filament temperatures (and therefore more light) and much longer life. Many new cars are still using this technology today.
Halogen Infrared Reflective lights, also known as simply HIRs, are the latest iteration of incandescent globe technology. Working similarly to the tungsten-halogens with the exemption that during the manufacturing process, a dichromic coating is added. This coating allows only the visible light produced to pass and reflects the infrared light. The effect of which is the reflected infrared light provides additional heating to the tungsten filament. This produces more light without extra power consumption.
HID or High Intensity Discharge lights have come to the fore of the last few years, unlike tungsten globes, HIDs don’t use a resistive tungsten heated element to produce light. Instead they create an electric arc that is generated in the presence of metallic salt in an “arc chamber”. There are several different types of HIDs available. The HPS or High Pressure Sodium HID is the reddish light used in most street lamps. Though for automotive applications MH or metal halide is the HID of choice. They may also be referred to as Xenon as the globe contains xenon gas. HID lights produce much more light that any derivative of the tungsten for the same amount of power used.
One peculiar characteristic of HID are once lit, they take a little while to achieve full brightness. Also you can’t flick HID on and off rapidly, as you would if warning another driver of an upcoming traffic situation. They can take a little while to restrike.
LED or Light Emitting Diode, without going into too much theory, it is a semi-conductor lighting source. When you provide an appropriate voltage to an LED, the electrons from one side hop across to the vacant electron holes on the other. This in turn releases photons (light).
LEDs while being more energy efficient than HIDs, up until recent years weren’t able to produce the light output required for a headlight applications. White light LEDs were also one of the last to the market. LEDs are more energy efficient than HIDs, and they reach near full light output within milliseconds of turn on, unlike HID lights.
LASER is an acronym for “light amplification by stimulated emission of radiation”, is the bleeding edge of headlight technology. That’s right just like “sharks with fricken laser beams!” So while a laser light pointer may provide hours of amusement tormenting the cat on the lounge floor, it can also illuminate your way on a dark road, well not exactly, but read on.
In the laser headlight featured in BMW’s i8, three blue lasers are focused into a single stream of laser light through a lens, this is diverted through a mirror that is then shone into a yellow phosphorus panel. This panel then in turn produces a brilliant white light. The Audi version works in a similar fashion. Why go to all this additional trouble and expensive? Efficiency primarily, laser headlights are up to thirty percent more power efficient than a comparable LED headlight. Presently laser is only used for high beam applications, though into the future they will be used in low beam application as well.
DESIGN OF A HEADLIGHT
Reflectors need to be used to reflect the light produced in the intended direction efficiently. If you imagine a satellite dish with the receiving antenna elevated somewhere over the centre of the dish. The dish shape can be described mathematically as a parabola, with the receiving antenna situated it the focal point of the parabolic dish. When a radio wave strikes anywhere on the surface of the dish, it is reflected directly to the dish’s focal point.
The inverse is also true. If you project light from a parabolic reflector’s focal point it will be reflected in the intended direction, it doesn’t matter what part of the reflector the light is reflected from. With subtle manipulation of the reflectors surface the designer is able to tailor the headlight’s beam pattern to suit the design.
Predominantly parabolic type reflectors are used with halogen type lights.
Projectors Type Reflectors are becoming the new mainstay, with the widespread adoption of HID and LED headlamps. They utilise a much smaller parabolic reflector behind the globe, with a glass lens in front used to control and shape the headlight’s beam pattern. The projector unit also contains an electro-mechanical shield that raises up from the bottom of the projector to cast a shadow and create a low beam to avoid blinding oncoming drivers.
The astute reader will probably be asking why the shield protrudes from the bottom. As you need to cut off the top of the light pattern to avoid burning the retinas of oncoming drivers.
When the human eye see and image, it enters the front of the eye, passed through the lens, is turned upside down by the lens and is projected inverted on the retina. The brain then is able to interpret the image right side up. New born babies actually see everything upside down for the first few days until their brain is able to sort out which way is up!
Projector type reflectors work in the same way, so in order to shadow the top of the head light beam before the lens, you must place an obstruction from the bottom.
Projector type reflectors also produce the cleanest cut off. That is the line when the low beam projected shadow finishes. Projector cut offs are also able to be tailored to left and right hand drive countries by simply changing the left and right side height of the low beam cut off shield.
Lighting Driver Aids
Adaptive Headlight Technology has been heralded as the bleeding edge in headlight driver aids. Imagine driving down a windy mountain range, where the speed advisory signs are constantly advising you of recommended speed of less than forty kilometres per hour to successfully navigate the corner without becoming one with the armco barrier. It’s night, there are no streetlights and your headlights are doing a great job of disturbing the cows in the adjacent paddocks, though unfortunately not illuminating the road ahead.
This is the scenario that adaptive headlight technology was invented for. The light beam now follows the road, instead of disturbing the bovine’s peaceful slumber.
Just as flared jeans fall in and out of fashion, so do some automotive technologies. While the current implementation of adaptive headlight system might be different, a variation was seen back as far as the 1948 Tucker ‘48. Their system used a centrally mounted “Cyclops eye” that turned with the steering once the front wheel angle reached past ten degrees. Interestingly Tucker also supplied a cover for the Cyclops eye so the car could be driven in US states that only allowed vehicles with no more than two headlights. Something akin to legislators now playing catch up to driverless car technologies of today.
Adaptive Front Headlights of the modern iteration, utilise various electronic sensor such as speed, steering wheel angle and yaw. (That is how the car is turning.) These factors are used to determine where the headlights should be pointing. Projector headlight such as used in HID and LED headlights predominantly use small electronic motors to move the projector in the computed direction.
Most systems allow the headlight projector to turn fifteen degrees from the straight ahead position, making for a total movement of thirty degrees. They are also “smart” enough to not to turn while idling around looking for a spot in a car park.
Cornering Lights are a complementary technology to adaptive headlights. Though instead of pivoting the cars headlight, they use an additional light that is pointed in the direction of travel. Some systems can pivot a fog lamp while others utilise a sideways pointed light contained within the headlight.
Again this isn’t new technology. Some of the later model of the Nissan GQ Patrol (1994+) had a turning light integrated in the front mud guard, which was activated but the indicators. I’m considering buying a pair of flare jeans to stay ahead of the pack!
Automatic or Dusk-Sensing Headlights are a common feature found in cars today. They utilise a photoresistor or LDR (light Dependant Resistor) to determine how dark it is outside. When the electronics determine it has reached a pre-determined level of light, the headlights switch on. Usually on the switch you’ll have a park setting, headlight and “auto”. You can just leave most cars with automatic lights in the “auto” setting and let the electronics do the work, day to night. You’ll be able to tell the cars behind you with the headlight set on auto as you pass under long overpasses in the daytime. Their headlight will switch on as they head under, and off after they exit.
Auto Dipping High Beams is another complementary autonomous automotive headlight technology. Anyone who has driven sparsely driven country roads at night will be able to attest how much of a pain it is to continually be turning on and off the high beams as you come across oncoming traffic or are passed by another vehicle. You’ll also have to turn them off when passing though small, street lit towns. Automatic high beams automate this for you, using a forward facing camera usually mounted in the rear vision mirror, it is able to determine, oncoming traffic, tail lights of vehicles that you have either caught up to or have passed you and street lights. It’ll turn off the high beam lights when not required or could be a hazard to other road users and turn them back on when able.
Selective high beam filtering is another cutting edge headlight technology, though will no doubt work its way down to economy models in the years to come. It’s utilises a forward facing radar and camera to determine exactly where other vehicles are, and how far away they are in real time. Once the computer has worked out where the cars are, it can shade the vehicles from the driver’s high beam headlights individually.
Most implementations use a multi segmented LED headlight. They utilise at least twenty five individually controllable LEDs in each head light, and by referencing the radar and camera system are able to individually shade certain portions of the road. All the while tracking multiple vehicles and updating the data in excess of one hundred time per second. In time, systems will be smart enough to integrate data from the vehicle’s GPS navigation system and be able to augment the lights when you’re approaching a roundabout.
Automotive night vision is available in two flavours, both passive and active.
Passive systems utilise a thermographic camera that sees heat as opposed to colours (anyone recall the vision in the Predator movies?) to see through bad weather and the darkness that is then able to be displayed to the driver via a screen or HUD (Heads Up Display). Passive systems have a greater range and higher contrast than active systems. Though are lower resolution, larger sensor and don’t work as well in warmer weather condition.
Active systems on the other hand while also using a thermographic camera, are lit with infrared lights from an external source usually located in the headlight. These systems have a higher resolution, utilise a smaller sensor and work better in hotter conditions, however are shorter range, and don’t work as well in rain or fog.
Various systems not only are able to see over three hundred meters, but also have the ability to high beam flash a detected pedestrian in the intended path of travel without dazzling oncoming vehicles. Also detect animals to the side of the road out of the vehicles head light beam and light the animal using special purpose lights while setting the brakes to maximum standby and using audible alerts such as BMW’s night vision as featured in the 2014+ 5 series.
Into the future this technology will be utilised in an augmented reality system, where not only will you be able to see through the screen, but pertinent information will be relayed directly onto the screen for the driver, somewhat like a fighter pilots helmet.
DRL or Daytime Running Lights have become almost the norm in modern day cars, just as activated head lights on motorcycles have been used to attract the attention of other motorist many years. Of recent years, manufactures have begun to integrate a dedicated white lights in the headlight assemble. Many installations use dual coloured LEDs as the indicator and DRL, then turn off the DRL when the indicator is being used.
CAN I IMPROVE MY EXISTING HEADLIGHTS?
Retrofits of HID/LED into Halogen Reflectors is one of the better ways to win friends and influence people! If you cast your mind back to the parabolic reflector used by Halogen lights, the light is produced right at the reflector’s focal point, just the way the designers intended.
Putting an aftermarket HID/LED in the reflector specifically designed for a halogen will move the point at which the light originates away from the focal point. The effect is to blind most of the oncoming traffic with glare, not only are you pushing out considerably more light, it’s no longer casting the low beam shadow needed in order to not blind oncoming drivers.
You can, however, install HID lights to your vehicle, but you’ll need to retrofit proper HID projectors to the original light fitting, install headlight washers and some means of levelling the light when you’ve got a load in the back. Most standard lights use a motorised mount within the headlight.
In conclusion your lighting need will vary with the type of driving you’re doing, if you mainly commute to town and back to the suburbs, the standard headlight may be all you need. If your heading to the outback, aftermarket halogen, HID or LED accessory lighting could be considered mandatory. The options (if any) available for your new car depend on you driving requirements and your eyesight. Choose wisely.