Light output: lux versus lumen

Wattage means nothing in terms of light output. A 100 Watt quartz halogen globe is not brighter than a 90 Watt HID globe. A 100 Watt quartz halogen globe is not twice as bright as a 50 Watt quartz halogen 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 quartz halogen 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 globe output, however the most important part of a driving light is not how bright the globe is, but how well illuminated the road is … up to 700 metres away from the globe.

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.

We’ve used a quartz halogen globe in the example above to demonstrate why wattage and lumen aren’t as important as actual lux on the road where you need it. This becomes even more evident when you are looking at the latest LED technology. LED is ever-evolving to produce more and more light output while consuming less and less power, so comparing wattage and lumen doesn’t dictate how the product will perform in the real world. The key factor is how the design of the LED product takes that output and projects it in a meaningful way onto the road ahead.