Lighting lore & law

Many of the bulbs here claim to produce more light than standard bulbs. This is typically achieved by using a thinner wire which glows more brightly. The glass tube is made larger so that a greater volume of gas can be added, to allow the filament to regenerate and prolong its life.

The bulbs we have tested here are all halogen — the glass capsule is filled with an inert gas such as argon and a small amount of a halogen. Xenon can replace the argon and this results in a slightly whiter light, though adding blue to the glass will also help achieve this.

Various manufacturers also talk about producing whiter light — a higher light ‘temperature’. Normal, yellow headlights are around 2000 Kelvin and daylight is 6000K. Whiter bulbs tend to be around 3500-4000K, roughly halfway.

LED (Light Emitting Diode) bulbs are becoming more and more common, as they use less energy than filament bulbs and give more light. There are direct replacement LED bulbs available — and we have included one in the test — but they generally don’t work well with reflectors designed for halogen bulbs.

The reflector in the headlight is what actually creates the beam pattern and this is why the design of the bulb and the filament position is so important. Dipped beam is designed to light the road when there are other road users around, so the beam will cut off below the other drivers’ eye-line while illuminating the near-side edge of the road.