Flash Pattern (Warning Lights)
A flash pattern is the sequence in which a warning lamp switches on and off: how many flashes make up each group, how long each flash lasts and how often the group repeats. The repeat rate is quoted in flashes per minute or hertz and is regulated: SAE J845 allows 1 to 4 Hz, while UN Regulation No. 65 requires 2.0 to 4.0 Hz.
A warning lamp’s pattern decides how it reads to other road users, and the limits are not the same in every market. A pattern built for North America can fall outside the European rule, which is why the flash pattern belongs in the lamp specification rather than being left to whatever the lamp ships with.
How flash patterns are counted
Most patterns repeat a group of short flashes: single, double, triple or quad. The rate is counted per group, not per flash, so a double flash at 75 flashes per minute means 75 double flashes each minute. UN Regulation No. 65 counts the same way: its frequency is the number of flashes or groups of flashes within one second. When two or more lamps are fitted to one vehicle, they can flash in sync, lighting together, or alternate, lighting one after the other.
What the standards require
SAE J845, the standard for 360-degree warning lamps that 49 CFR 393.25(e) calls up for amber warning lamps on US commercial vehicles, requires a flash rate of at least 1 Hz and not more than 4 Hz, which is 60 to 240 per minute. UN Regulation No. 65 on special warning lamps requires 2.0 to 4.0 Hz, which is 120 to 240 per minute, limits the on time to 0.4 divided by the frequency and requires an off time of at least 0.1 second. It also states that a lamp approved under the regulation must not let the user switch to a pattern outside these limits.
Local rules can add their own limits. Pennsylvania, for example, requires 60 to 260 flashes per minute observed from a fixed position (67 Pa. Code 173.3). Which colours a vehicle may use is a separate question, covered in beacon light colors.
Why the market decides the pattern
The two sets of limits overlap less than they appear to. A pattern that repeats 75 times a minute, 1.25 Hz, meets SAE J845 but falls below the 2.0 Hz minimum of UN Regulation No. 65. A lamp sold into both markets therefore needs a pattern set that meets both, or a version for each market. Write the pattern into the order: the flash type, the rate, and whether a pair of lamps runs in sync or alternates.
We set the flash pattern and rate of our warning lamps to the market the vehicle ships to, and build pairs that flash in sync or alternately. Alternating headlamps are a different device with their own rules, covered in wig wag lights. For the range, see LED beacon and warning lights.
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Related terms
ECE regulations (shown by an E-Mark) govern vehicle lighting in Europe and most of the world; DOT/FMVSS 108 and SAE standards govern the United States. The right certification depends on the destination market and whether the lamp is road-facing.
Multi-Volt Input (9–32V DC)A multi-volt lamp accepts any supply from roughly 9 to 32 volts DC, so the same part number runs on 12 V systems (pickups, utility tractors) and 24 V systems (trucks, excavators, mining equipment) without a converter.
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