Reference number: what is it and how does it work?

What exactly is the reference number?

The Reference number, often abbreviated to “Ref.”, is a dimensionless value derived from the maximum light output (in candelas) of a driving light. As it has no unit, you can objectively compare different lights, regardless of type or brand. On every approved driving light, you will find this number directly next to the ECE approval mark on the lens or housing, for example “Ref. 25” or “Ref. 50”.

The most common values for driving lights are 12.5, 17.5, 37.5 and 50. A higher value means a stronger beam. A driving light with Ref. 50 therefore delivers a significantly higher peak light output than one with Ref. 12.5.

Low or high reference number: what suits your situation?

The choice between a low or high reference number depends entirely on your driving environment. Both have their own strengths.

Low reference number (12.5 to 25)

These driving lights cast a wide light image closer to the vehicle. Ideal for winding roads, mountainous terrain or applications where the roadside and immediate surroundings are important. Think forestry, agricultural work or distribution in urban areas.

High reference number (37.5 to 50)

These lights bundle light narrower, but cast it further ahead. Perfect for long transports on highways and straight stretches where distance visibility is crucial.

How do you add up reference numbers?

The Reference numbers of all simultaneously operating high beams and driving lights are simply added together. Two driving lights with Ref. 17.5 together deliver a total value of 35.

Important to remember: the high beam of your truck itself also counts in this sum. Low beam, fog lights, position lights and work lights do not count. Only lighting that functions as a high beam or high beam supplement counts in the addition.

Legal limit: total value of 100

According to UN Regulation R48, a maximum combined reference value of 100 applies to high beam functions that can be switched on simultaneously. In older texts, this is described as a maximum combined light output of 430,000 candela, corresponding to reference value 100.

A calculation example for a typical tractor with high beam Ref. 40:

• Available space for additional lighting: 60 (100 minus 40)
• Two driving lights Ref. 25 plus one lightbar Ref. 12.5 = total 102.5, therefore not permitted
• Two driving lights Ref. 17.5 plus one lightbar Ref. 25 = total 100, therefore permitted

So always check the Ref. value of your existing high beam before ordering additional lighting. Too high a total value leads to rejection and to less safe lighting, because overexposure disturbs the perception of depth and contrast.

Maximum number of driving lights

In practice, heavy trucks often operate with two standard high beams and up to two additional high beam functions. For category N3, R48 allows additional main-beam headlamps, but the circuit and joint reference value remain decisive. Therefore, always check which high-beam functions can operate simultaneously.

Reference number is completely technology-independent. Whether you choose a classic incandescent driving light or a modern LED bar, Ref. 37.5 delivers exactly the same measured peak light output in both cases.

Low reference number driving lights

The driving lights and LED bars below are examples of TRALERT® driving lights. For example, the Atlas 220+ and Nova series only have a Reference number of just 25. You can filter the driving lights in the shop by reference value.

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Tralert Atlas 220+ driving light warning lights + dual position lights 12-24v

LED driving lights

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SKU: WD-5860A

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Nova Series LED driving lights white 9-36v - 150w - 13600lm | WD-15013W

LED driving lights

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– Nova Series LED driving lights white 9-36v - 150w - 13600lm | WD-15013W quantity +

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SKU: WD-15013W

Example: four driving lights on trucks

Four driving lights on a truck are common in practice, but this does not automatically mean that all four lights can be used as high beams at the same time. The decisive factor is the sum of the reference numbers of the main-beam functions that can operate simultaneously.

Four driving lights with Ref. 25 together already reach reference value 100. As soon as the vehicle's original high beam remains active at the same time, the total value exceeds that. With four driving lights with Ref. 37.5 or Ref. 50, the 100 limit is already exceeded by the additional lights alone.

In such situations, custom switching, use in pairs or lighting with a lower reference number are often used in practice. Certain lights may also be used only as position lights, show lights or for non-public areas. Therefore, always check the reference value, the circuit and the national application requirements before fitting additional driving lights.

Frequently asked questions

Where can I find the reference number on my driving lights?

The reference number is always on the lens or housing, right next to the ECE approval mark (a circled E with country code). You recognise it by the notation "Ref." followed by a number, e.g. "Ref. 25". Without this number, a driving light may not be used as a high beam supplement on public roads.

How many additional driving lights can I connect to my truck?

According to UN Regulation R48, a maximum of two additional driving lights may be fitted on top of the standard high beam. Furthermore, the sum of all Reference numbers for lights burning simultaneously must not exceed 100. If your truck high beam has a Reference number of 40, that leaves 60 for your additional lighting.

Does my LED bar count towards the total of 100?

Yes, any LED lightbar or driving light that can be switched on at the same time as the high beam is included in the total. Low beam headlights and fog lights are not included, as they are not classified as long-range lighting. Some lightbars combine multiple functions. In that case, only the part that operates alongside the high beam is included in the total.

Need help making the right choice?

Are you unsure which driving lights are suitable for your truck and existing main lighting system? Our technical advisors will be happy to calculate with you so that you stay within the 100 standard and choose the right light image for your work. View our complete range of LED driving lights with specified Ref. value or contact us directly for personal advice.

Source: UN regulation R48 - UNECE