Fog lamp
Also called fog light, fog lights, front fog lamp, fog beam, foglamp
A fog lamp is a wide, low-mounted lamp with a sharp cutoff along the top of its beam, which lays light on the road surface and throws almost none above it, and it is that beam shape and its approval class that make it a fog lamp rather than the colour of the light.
What is a fog lamp?
UN Regulation 48 gives the purpose in one line: a front fog lamp is a lamp used to improve the illumination of the road ahead of the vehicle in case of fog or any similar condition of reduced visibility. Everything else about the lamp is engineering that follows from that, because the problem in fog, dust and falling snow is not a shortage of light. It is that light thrown into the air comes back at the driver, and the lamp has to put its output on the road without filling the air at eye level.
UN Regulation 19 is where a standards body writes that down in numbers, and it is free to read. A class F3 front fog lamp, set at its standard aim, must produce at least 2,700 candela along a line two and a half degrees below the horizontal, and no more than 485 candela on the horizontal itself, 360 one degree above it, 245 at two degrees, 150 at four and 130 at eight. It is capped as well as floored: no more than 12,000 candela in the zone between one and a half and three and a half degrees down. And it must be wide, with at least 450 candela still arriving 35 degrees off to each side.
2,700 candela required two and a half degrees down. 485 permitted on the horizontal. The shape is the whole specification.
The edge between those two figures is the cutoff, and Regulation 19 specifies it rather than describing it. The line must be horizontal for four degrees either side of straight ahead, aimed one degree below the horizontal, straight to within two tenths of a degree across the middle six degrees, and it must reach a sharpness factor of at least 0.08. The American standard, SAE J583, uses that same 0.08. Two standards bodies on two continents, one number for how hard the edge has to be.
Why a fog lamp sits low and switches on its own
Because a regulation says so, in the case of any vehicle approved to UN rules, and because the reasoning behind that rule holds everywhere. Regulation 48 allows front fog lamps as optional equipment, two of them, no closer than 250 mm to the ground and no higher than 800 mm on a car or light van, and no point of the lamp may sit higher than the highest point of the low beam headlamp. It also requires them within 400 mm of the outer edge of the vehicle, switchable on and off independently of the headlamps, and fitted with a tell-tale on the dash that tells the driver they are lit.
Low, outboard, separately switched, and with a light on the dash saying it is on. That is the whole discipline people work out the hard way when they wire auxiliary lamps, written as law for one lamp.
Why a fog lamp is not a colour
This is the most common confusion in off-road lighting vocabulary, and the regulation settles it in a sentence. Regulation 19, paragraph 7: the colour of the light emitted by the front fog lamp shall be either white or selective yellow, by choice of the applicant. Colour is the applicant's choice. The beam pattern is not. A yellow lamp with no cutoff is not a fog lamp, and a white lamp with the right pattern is one.
You can watch it happen in a product. KC's FLEX ERA 3 Dual Mode is listed as compliant with SAE J583 and sold with a clear SAE lens and a selective yellow SAE lens as alternatives on the same lamp. Whether the yellow buys you anything in dust is a separate question, and a genuinely contested one: the makers' claim is about glare and the folk claim is about penetration, and those are not the same claim. Our entry on amber light works through what the evidence supports.
What American law says about fog lamps
Less than most people assume. FMVSS 108, the federal lighting standard, neither requires nor regulates fog lamps on a private vehicle. The nearest federal statement is 49 CFR 393.24, which binds buses, trucks and truck tractors: they may carry front fog lamps for use in conjunction with, but not in lieu of the required headlamps, those lamps have to meet SAE J583, and they have to be mounted so the aim cannot be knocked out on the road and then actually aimed to the standard. Everything about a private pickup is left to state codes, which vary.
See also
Seen in
Sources
- UN Regulation No. 19, approval of power-driven vehicle front fog lamps, Official Journal of the European Union, L 250, 22 August 2014, CELEX 42014X0822(01) · read 2026-08-27Read in full. Paragraph 6.4.3 carries the class F3 photometric table quoted here, Annex 9 the cut-off line requirements and the 0.08 sharpness factor, paragraph 7 the choice of white or selective yellow. The UNECE's own site blocks automated retrieval, so this is the EU Official Journal reproduction, which carries the note that only the original UN texts have legal effect. Since 2019 the content has been consolidated into UN Regulation No. 149. It is a European approval regulation and it binds nothing in the United States, which is why we quote it for what a fog lamp is rather than for what you must fit.
- UN Regulation No. 48, installation of lighting and light-signalling devices, Official Journal of the European Union, L 14, 16 January 2019, CELEX 42019X0057 · read 2026-08-27Paragraph 2.7.19 defines a front fog lamp by its purpose; paragraph 6.3 sets how one may be installed, including the height limits, the independent switch and the mandatory tell-tale. The companion to Regulation 19: 19 approves the lamp, 48 governs putting it on a vehicle. This is the 06 series; later series exist and we have not checked them.
- 49 CFR 393.24, Requirements for head lamps, auxiliary driving lamps and front fog lamps, Electronic Code of Federal Regulations · read 2026-08-27The federal truck-safety rules, binding on commercial vehicles rather than private ones. Paragraph (b) incorporates SAE J581 by reference under its July 2004 title, Auxiliary Upper Beam Lamps; the current revision is retitled Auxiliary High Beam Lamps (https://saemobilus.sae.org/standards/j581_201102-auxiliary-high-beam-lamps) and is paywalled, and we have not read either revision. This section is where United States federal law comes closest to defining a driving lamp; FMVSS 108 sets no photometric requirement for one, reaching added lamps only through its rule that nothing installed may impair the lighting the standard requires.
- SAE J583 Test Results, May 2022, photometric testing by Calcoast ITL, Morimoto · read 2026-08-27This is where our J583 numbers come from: an independent laboratory report that reproduces the standard's photometric test-point table. We have the candela envelope and the cutoff gradient; we have not read J583's own text. It was published by a lamp maker whose two products passed and three of whose competitors failed, which is worth knowing when reading the framing, though the measurements are Calcoast ITL's. J583 is absent from the FMVSS incorporation-by-reference list at 49 CFR 571.5 (https://www.ecfr.gov/current/title-49/section-571.5), since fog lamps are optional equipment FMVSS 108 does not reach, which is why no free copy comes with the passenger-car rules. It is incorporated elsewhere: 49 CFR 393.7 approves it by reference for 393.24(b), the commercial-vehicle rule.
- 49 CFR 571.108, FMVSS 108, Lamps, reflective devices, and associated equipment, Electronic Code of Federal Regulations · read 2026-08-27Cited for an absence. It sets no photometric requirement for a front fog lamp and does not list one among required equipment, reaching added lamps only through its rule that nothing installed may impair the lighting the standard does require. J583 is likewise absent from the incorporation-by-reference list at 49 CFR 571.5.
- Specification tables: FLEX ERA 1, FLEX ERA 3 Dual Mode SAE, FLEX ERA 4 and the 30 inch FLEX Array combo bar, KC HiLiTES · read 2026-08-27Four tables, all read through the Internet Archive because kchilites.com blocks automated retrieval: the FLEX ERA 1 master kit at this URL, capture of 10 February 2025; FLEX ERA 3 Dual Mode SAE (https://www.kchilites.com/kc-flex-era-3-dual-mode-sae-master-pack-system-284.html), 13 February 2025; FLEX ERA 4 master kit (https://www.kchilites.com/kc-flex-era-4-light-master-kit-pair-pack-289.html), 9 August 2025; and the discontinued 30 inch FLEX Array combo bar (https://www.kchilites.com/30-flex-array-combo-beam-pattern.html), 9 September 2017. Every candela, lumen and metre in them is the manufacturer's own, published by the company selling the lights, and no independent laboratory has checked any of it. The arithmetic we do on them uses nothing but their own candela and distance columns.