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ELECTRICS

HOW LIGHTS WORK

The black art of automotive lighting explained.

Lights that lit the way for 4WDs used to be variations on the same incandescent-globe theme, but now there are HID and LED lights. Here’s how they all work.

First up, if you’ve been to a trivia night you’ll probably know already that Thomas Edison didn’t invent the incandescent light globe: what he and his ‘muckers’ did in 1879 was make reliable ones.

However, until Osram in 1906 came up with a globe using a tungsten filament that could withstand vibration, the motor vehicle relied on acetylene-gas headlamps.

Once electric globes were available, the ignition magneto supplied current to them and you could see for miles – well, metres, anyway.

Incandescent – from the Latin verb incandescere: to glow white - globes rely on the simple principle that virtually all substances glow when they get hot. When an electric current is run through a thin wire the resistance to electron flow causes the wire to get hot: white-hot, actually and emit light.

From the early days, incandescent globe development concentrated on increasing the brightness and extending the life of the filament.

It was realised at the outset that it was necessary to exclude oxygen from inside the globe, because the white-hot filament oxidised and literally burnt-out very quickly. Even in a so-called vacuum-globe the filament gradually vaporised, coating the inside of the globe with a black layer.

The impossibility of creating a perfect vacuum led to experiments with inert gas filling that was found to decrease blackening, while limiting oxidising.

That’s pretty much where things stayed until the 1960s, when the halogen-quartz globe was released.

Halogen gas inside the globe performed a neat trick: as the filament surface vaporised, tungsten atoms combined with the halogen gas and then re-deposited on the hot filament. The globe needed a higher operating temperature for the ‘halide’ cycle to work, but the result was far less loss of filament and a brighter light.

To tolerate the higher temperature a quartz globe material was used and, because that material reacts adversely with many common substances, required gloved hands to install a halogen-quartz globe.

Incidentally, halogen-light development occurred in Europe, because the USA had legislated in 1940 for one type of standard, seven-inch sealed-beam headlamp that was the only permitted fitment on all US-registered vehicles. The Yanks didn’t get a halogen sealed-beam until 1978, but it was still restricted to the same size and shape until 1984, when the US joined the modern automotive world and allowed variable-shape headlights with replaceable globes.

HIDs

In the 1990s came the next leap in lighting: high intensity discharge (HID) globes.

In the HID globe the halogen globe’s ‘halide’ effect is employed, but the tungsten filament has been replaced by two electrodes and the gas charge is highly-pressurised xenon. The globe contains a small amount of metal salts – usually compounds of sodium and scandium.

An electrical arc is struck between the two electrodes and the initial glow comes from ionised xenon gas. In a matter of seconds the heat from this discharge causes the metal salts to form a white-hot plasma around the electrodes and the result is brilliant light.

Because high-voltage is required to strike the initial arc HID lights come with internal or external ‘ballast’ units that convert 12-volt vehicle battery potential to around 20,000 volts. Once the arc has been struck the ballast units discharge.

 

LEDs

Light Emitting Diode lights work completely differently from incandescent and HID lights, generating photons of light at the atomic level.

A diode is an electrical device that allows current flow in only one direction - well, almost. In practice a diode isn’t a perfect current-blocker and some current ‘leaks’ past, but it’s a very small percentage of the diode’s capacity.

A diode is a semiconductor, which means it has a variable ability to conduct electricity.

The basis of most semiconductors is silicon, one of the most common elements on the planet, being the principal element in sand and quartz. Crystalline silicon is most commonly seen on solar panels and has a silvery, metallic appearance.

Pure silicon is not a good electricity conductor at all and is almost an insulator. However, if minute amounts of the right impurities are incorporated into silicon its crystalline structure changes and it becomes a reasonable conductor.

In California’s famous ‘Silicon Valley’, where the whole semiconductor business began in 1947, the process of modifying silicon crystal behaviour is known, appropriately enough, as ‘doping’.

Silicon ‘doped’ with phosphorus or arsenic has surplus electrons that give this material the name N-type silicon - N for negatively charged - and these free electrons can conduct an electric current. Silicon doped with boron or gallium has a depleted electron count and is therefore known as P-type silicon, because the absence of electrons creates a positive charge effect that seeks electron flow.

In a diode, P-type and N-type silicon layers are matched face to face, with electrical terminals connected to each. Without any current flow the electrons from the N-type silicon neutralise the positive charge on the P-type silicon at the interface and no current flows through the terminals.

However, connect a positive end of a circuit to the P-type terminal and a negative end to the N-type and repellent action in both materials causes current to flow through the diode.

Reverse the circuit polarity and a very interesting event takes place: no current flows through the diode, but the central ‘neutralised’ zone increases in size and the incoming electrical energy is converted into light energy.

Early glass-valve diodes emitted energy in the form of heat, but in the case of a solid-state LED the energy is emitted mainly as photons of light.

Low-energy LEDs emit in the infra-red spectrum and are used in appliance remote controls. Higher-energy designs emit light in the visible spectrum.

LEDs emit light directionally, so they can be arrayed with small, individual reflectors, as in LED light bars and driving lights, or with a large reflector, as in Hella’s Luminator LED and Narva's Enhanced Optic LED lights.


Lumens and lux 

When evaluating incandescent headlights and driving lights, including halogens, the common comparison method is ‘wattage’. Most driving light makers settled on the 100-watt halogen globe as the best compromise between light output and service life.

However, our laboratory and field testing over many years showed that wattage isn’t a reliable guide to performance, because the optics in the reflector and/or lens are critical to beam shape and avoiding wasted, ‘scattered’ light. Beam distance is another function of reflector shape and some manufacturer-quoted distance figures we’ve found to be misleading.

Wattage is of even less importance when evaluating HID lights, because they’re mostly in the 35-45W range. So, their makers are now quoting lumens, lux and colour temperature.

Like using wattage to evaluate the useful light spread and penetration of a pair of driving lights, lumens and lux figures need qualification. Both measurement units indicate brightness, but there’s a catch. Sure, a light putting out 10,000 lumens has ample power and, if spread over an area of 50 square metres, a value of 200 lux, but if spread over 100 square metres the lux figure drops to 100.

Beware the light chart that displays beam shape and distance in intensity of only 0.25 lux. A much more accurate measurement of beam intensity is the Isolux system that measures beam distance at one lux intensity.

 

Colour temperature

This is by far the most confusing of all light statistics, because it compares the Kelvin absolute temperature scale with the visible light spectrum.

William Kelvin was a physicist in the late 1800s and developed a temperature scale, using Absolute Zero (-273C) as its starting point, hence eliminating the minus numbers you get if the freezing temperature of water is used as the starting point. Scientists welcomed the Kelvin Scale, because calculations are much easier to do without minus numbers.

The same bloke heated a piece of carbon, noting that it changed colour as its temperature rose: dim red, bright red, dim yellow, bright yellow, yellow-white, bright white and blue-white. Kelvin temperature points were later added to colour-temperature charts.

Back in the days when light brightness was a function of applied heat – making a filament glow bright yellow to white hot, but stopping short of melting it – the scale had relevance. Back then, the higher the temperature, the whiter the light, but now there are cooler light sources – gas discharge (HID) and light emitting diodes (LEDs) – it’s plain confusing.

Checking out a colour temperature chart, it’s obvious that if you evaluate an HID or LED light purely on its colour temperature, more isn’t necessarily better. The ‘sweet spot’ is in the 4000-5500K region, because higher numbers give too much ‘blue’ cast and head for eventual darkness at the end of the visible spectrum.

 

What’s best

Our testing over many years has shown that HIDs outperform halogens and LEDs for spot beam distance, while using less than half the electrical power. Globe life is around 10 times that of halogens, but shorter than LEDs.

LEDs are brilliant in the mid-distance area – out to around 750 metres for the best performers – but use much more alternator current in the process. Large LED light bars have 300+Watts of power and that means more than 20 amps of current in a 12V system and 10+amps in a 24V system.

LEDs should last the life of the housing.

Halogens are now often the cheapest lights, but some very high powered ones rival HIDs for distance and LEDs for brightness. However, these lights have very high current requirements.



 


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