Wattage means nothing in terms of light output. A 100 Watt QH globe is not brighter than a 35 Watt HID globe. A 100 Watt QH globe is not twice as bright as a 50 Watt QH globe. A 2000 Watt vacuum cleaner produces no light output. So how do we measure light in a way that is meaningful?

There are many terms that relate to light output, but the two that relate best to automotive lighting are lux and lumen.

We’ll start with lumen. Lumen can be thought of casually as the total amount of visible light in a defined beam or angle emitted from a source. With this in mind, every 100 Watt QH driving light, regardless of pattern, lens or design, will (at the source) have a luminous flux of the same amount of lumens. This is fine for comparing bulb output, however the most important part of a driving light is not how bright the bulb is, but how well illuminated the road is … up to 1100 metres away from the bulb.

Colour Temperature is measured in Kelvins. The Kelvin scale is a thermodynamic temperature scale based on the principle that a black body radiator will begin to emit light when heated, the colour of which is dependent on the temperature of the radiator (think a blacksmith heating a piece of steel so it glows red). As the temperature increases, the colour changes from red, yellow, white through to blue hues (blue being the hottest). To give an indication, 3500 Kelvin is equivalent to 3,226.8 degrees Celsius and 6000 Kelvin is 5,726.8 degrees Celsius. To give you a reference point, orange flames in a fire are around 1200 degrees Celsius.
In lighting, innovative new technologies have created globes that run much cooler (such as HID and LED) than in older technology. We therefore talk about the light output as having a Correlated Colour Temperature or CCT; that is, the colour that human colour perception matches most closely to the true Kelvin scale. In some light sources (such as an incandescent light bulb) the light that is emitted is thermal in origin and therefore has about the same actual colour temperature and CCT.

This is where lux becomes the defining measure in automotive lighting. Lux takes into account the area that the luminous flux is spread. Have a look at our diagram. It shows two identical light sources, both have a flux of 1000 lumens. One is a focused pencil beam that shines onto an area of one square metre. The other’s beam is spread across an area of ten square metres. The one square metre is lit to an illuminance of 1000 lux, whereas the ten square metre area is lit to only 100 lux.