Output Of Light

Wattage Vs Lux Vs Lumen

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.

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.

 

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