Bright ideas! Substance over style – the law on after-market light bulbs


When it comes to the legality of blue halogen car bulbs, it is the Kelvin rating (K) that really matters.

Author: Howard Tilney
Reading time: 2 minutes

This article is 7 years old.

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A common question from customers looking for bulbs with styling benefits is ‘…are blue car bulbs legal?’

When it comes to the legality of blue halogen car bulbs, it is the Kelvin rating (K) that really matters. Halogen bulbs with a rating over 4200K are not legal for road use, and should only be used off-road.

Bulbs are classified as white if they fall under the rating of 4200K; bulbs can physically appear to be coloured blue but the light emitted must be white.

There is a significant difference in the appearance of bulbs that have a rating of around 5000K when compared to bulbs with a rating within 4200K; these will appear to have a distinctive blue light, with a slightly purple effect. Typically if someone was to look at the light emitted and says that it isn’t white, then the bulb should only be used off-road.

Bulbs with a rating of 5000K are illegal for road use but many people tend to buy them for styling purposes and off road use such as car shows and events.

However, many new vehicles including Audi, BMW and Mercedes use High-intensity discharge lighting (HID), which can give a very slight blue tint. These are not illegal as they are factory-fitted HID lights and not an aftermarket HID kit. Standard HID bulbs usually run at around 4600K and produce a very bright, crisp white light.

Howard Tilney

Legal Advisor

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