Need to know: Driving lights

The black art of lighting up Aussie country roads has produced a dazzling array of add-on equipment, which makes selecting a pair of night-vision aids quite a business.

The reason for buying after-market driving lights should be to enhance standard headlight penetration, which is often inadequate for driving on Australian country roads. However, as happens also with the purchase of many aftermarket accessories, appearance often influences the buyer, rather than performance.

Our laboratory and real-world light tests over the years have shown that price isn’t a reliable guide to light performance, because many of the most expensive lights on the market have performed ordinarily, while some of the cheaper ones blazed brilliantly.

Looking at a light will tell you if it seems to be well made and if its mounting bracketry is substantial, but it won't tell you much about its performance.

The most confident light makers have supporting literature that shows approximate light patterns on the road surface and pattern distances in metres. However, even such diagrams can be ‘fiddled’ because light intensity is very difficult to quantify.

Round spotties the go but do they fit?

The traditional auxiliary lighting choice is a pair of round spotties, but there’s not much point settling on a pair and then discovering they won’t fit your bar. Vehicle registration authorities in some States are policing light fitments that project beyond the front profile of the vehicle or the ’roo bar.

Most buyers settle on a pair of round lights with diameters from 150mm up to 230mm.

Light makers usually recommend a pair that combines a pencil or ‘spot’ beam light for maximum distance illumination and a spread beam, to light up the road edges.

Another arrangement preferred by some is a pair of pencil beams and yet another combination is a pair of spreads.

LED (light emitting diode) round lights and light bars are increasingly popular, but our testing shows that LEDs can't produce the one-kilometre-plus-distance illumination that big HID lights can.

However, LEDs have good mid-distance illumination and are virtually maintenance-free.

What is a ‘halogen’ light?

In any incandescent bulb, electricity passing through a thin filament causes it to glow: the higher the temperature, the brighter the bulb. However, during the process, atoms of the wire fly off, so that the wire decays to the point where it burns through or breaks.

In a conventional bulb, as used in most parking and taillight globes, the atoms deposit on the inside of the glass, eventually clouding the bulb and reducing its output.

A halogen bulb is pressure-filled with an inert halogen gas that greatly reduces the amount of ‘evaporation’ from the filament, but a halogen bulb will wear out, through the effects of filament degradation, vibration and the extreme heat of the bulb: around 400 degrees C.

Rough roads, poorly installed lights and clear plastic light covers used at low speeds, where there’s insufficient cooling airflow, all contribute to shortening of bulb life. In harsh conditions, a halogen 55W bulb may last for only 200 working hours, but a 100W bulb for as little as 50 hours.

Directional lights

If halogen and HID globes were similar to household lights, wattage would be the main indicator of brightness, but these lights differ in a very significant way from house lights. When you buy a new globe for your house, you don't expect it to be ‘directional’, unless you’re buying outside spotlights or indoor down lights.

Vehicle lights need to be directional, in a precise way, and that's where the science comes in. This directional component comes from the reflector finish and shape, or from the lens fitted to the front of the light.

A pencil beam usually relies solely on reflector shape and finish for beam direction, and has a plain lens.

Spread-beam lights used to rely totally on shaping of the lens – fluting or bars moulded into the glass – to bend the light from the reflector into a shorter, but wider shape. However, there’s an increasing trend to use free-form reflectors to shape the beam spread, through a clear lens.

From our experience with measuring laboratory light patterns and relating those patterns to real-world conditions, we know that the best pencil beam lights have a large ‘hot spot’ in the centre of the combined-light beam, with little diffusion into the bands of lesser intensity.

The best spread-beams show flattish concentrations of light around a shallow-oval centre, fanning out to at least 10 degrees either side of centre.

Less effective pencil beams have a wider diffusion of light, with a smaller central hot spot, while some wide-spreads spray light over a very wide angle, losing intensity in the process.

HID Lights

High Intensity Discharge lights are the brightest stars in the lighting galaxy. We know that LED driving lights have excellent spread, but the best long-distance-beam choice is the gas discharge type.

HID lights are automotive equivalents of the household fluorescent tube, which is 20 per cent efficient and almost everlasting. In contrast, the best halogen globes are around five per cent efficient; with most of the electrical input being dissipated as heat.

HID lighting technology replaces the filament of the light bulb with a capsule of gas. The light emanates from an arc discharge between two closely spaced electrodes that are hermetically sealed inside a small quartz glass capsule.

To operate, HID lights require high-voltage ballast units that supply and maintain high voltage and control the current.

The amount of light produced is greater than from a standard halogen bulb, while the HID globe consumes less power and more closely approximates the ‘colour temperature’ of natural daylight.

Light engineers talk of colour temperature in Kelvin units. It’s not important to understand the basis of the rating, but the comparative levels indicate why HID and LED lights are brighter than halogen lights.

The typical colour temperature of a standard incandescent globe is less than 3000 Kelvin; a halogen globe falls into the 3000-4000 Kelvin range and HID and LED lights exceed 4000 Kelvin.

HID lights have globes with wattages that are much lower than halogen lights. A typical HID globe is only 35-70W, compared with the average 100W for halogen driving light globes or typical LED lights. Because the HID globe has no wire filament it’s much more durable than a conventional ‘hot wire’ globe and should last at least 2000 working hours.

In summary, HID lighting has three key advantages over halogen lights: more light output, whiter light and longer service life.

LED Driving Lights

Since Hella’s Luminator LED Series’ Australian release in early 2012 the LED light market has boomed.

LEDs deliver bright, white, even beams and turn the road to virtual daylight out to around 550-750 metres.

Brightness is on a par with HIDs and there is no sign of blotchiness, or unwanted reflector flare in the beams.

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

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 in numbers with small, individual reflectors, as in light bars and most LED driving lights, or with a large reflector, as in Hella’s Luminator LED and Narva Enhanced Optic lights.

In most lights LEDs range from 5W to 15W output, so they’re arranged in multiples. Round lights and light bars typically have 20 or more in each housing and the Hella and Narva reflector types have two or three LEDs in each housing.

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.

One lux is sufficient light for reading small printed text.

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.

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.

LED globes 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.