FMVSS 108 Turn Signal, Brake and Clearance Lamp Rules: Colors, Candela and Visibility Angles
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A rear turn signal under FMVSS 108 must reach at least 25 candela at 20 degrees outboard and 5 degrees up from its reference axis, and must never exceed 900 candela in any direction. Both numbers live in Table VII rather than in the running text of the regulation, which is why arguments about signal lamp compliance so often stall. The regulation fixes colors, counts and mounting heights in one table, lens area in another, candela across half a dozen more, and visibility in yet another.
This article covers the signal side of the rulebook only: turn signals, stop lamps, taillamps, side marker and clearance lamps, identification lamps, backup lamps and the reflex reflectors that sit in the same housings. Approval markings are a separate subject, handled in DOT vs E-Mark marking; headlamp beam testing belongs to FMVSS 108 headlamp photometry; aiming and glare belong to auxiliary light aiming and glare. Nothing here repeats those three.
Quick answer: where each requirement actually lives
FMVSS 108 separates a signal lamp into four questions and answers each in a different table. Which lamps a vehicle needs, in what color, at what height and when they must light: Table I-a for passenger cars, multipurpose passenger vehicles, trucks and buses, Table I-b for trailers, Table I-c for motorcycles. How much lens area a function needs: Tables IV-a to IV-c. How much light it must emit at each point of a test pattern, and how much it may not exceed: Tables VI-a and VI-b for front turn signals, Table VII for rear turn signals, Table VIII for taillamps, Table IX for stop lamps, Table X for side marker lamps, Table XII for backup lamps. How far around the lamp that light must still be visible: Tables V-a to V-d.
UN Regulation No. 148 answers the same questions in a different shape. It publishes one symbol per lamp function in its Table 1, one intensity table per function family in paragraphs 5.1 to 5.11, and one geometric field per function family in Annex 2. The table below is the map; the rest of this article fills in the numbers.
| Question | FMVSS 108 (49 CFR 571.108) | UN Regulation No. 148 |
|---|---|---|
| How many lamps, which color, what mounting height | Table I-a, I-b, I-c | Not in R148; R48 paragraph 6 (not retrieved for this article) |
| Effective projected lens area | Tables IV-a, IV-b, IV-c | No equivalent lens-area floor |
| Minimum intensity at each test point | Tables VI-a, VI-b, VII, VIII, IX, X, XII | Tables 3 to 11, by category |
| Maximum intensity in any direction | Rows inside the same tables | Third and fourth columns of Tables 3 to 11 |
| Field of visibility | Tables V-a to V-d, plus the 1,250 square millimeter option | Annex 2, Tables A2-1 to A2-3 |
| Color | Table I plus CIE 1931 boundaries | One color sentence per function plus shared chromaticity limits |
| What happens when a light source dies | S9.3.6, turn signal pilot indicator | Paragraph 4.6.1.2 and paragraph 5.6.3 |
| Production conformity | Not addressed in 571.108 | Annex 4, 20 percent unfavourable deviation |
The practical consequence is that a lamp can satisfy one table and fail another, and no single number describes a compliant signal lamp. A rear indicator that peaks at 300 candela is inside the Table VII maximum of 900 candela but says nothing about whether it reaches the 25 candela floor at the outboard corner of the pattern.
Which lamps count as signal lamps, and which do not
FMVSS 108 defines turn signal lamps as the signaling element of a turn signal system that indicates an intention to turn by giving a flashing light on the side toward which the turn will be made, and stop lamps as lamps giving a steady light to the rear to indicate that a vehicle is stopping or diminishing speed by braking. Taillamps are steady burning low intensity lamps used to designate the rear of a vehicle. Clearance lamps show to the front or rear, mounted as near as practicable to the upper left and right extreme edges, to indicate overall width and height. Side marker lamps show to the side, near the front and rear edges, to indicate overall length. Backup lamps illuminate the road to the rear and warn pedestrians and other drivers when the vehicle is backing up.
Four functions sit outside this article even though they share housings and wiring. Headlamps have their own photometry tables and their own aiming rules. Daytime running lamps have their own paragraph, S7.10, and are permitted rather than required on most vehicles. Rear fog lamps and maneuvering lamps are ECE functions with no FMVSS 108 equivalent in Table I. And warning lamps, the amber and blue beacons covered by SAE J845 and ECE R65, are emergency equipment rather than vehicle signaling devices; the buying logic for those sits in the R65 and J845 beacon guide, the hardware in the LED warning lights category and a single example in the Hornet TL beacon. Work lamps are a different case again: a pod from the off-road lights range is an auxiliary lamp, and fitting one does not create a signal lamp obligation.
The regulation is a standard for new vehicles and replacement equipment, not an inspection manual; what a state inspector accepts on a road is a separate question, covered in the DOT and E-Mark article.
FMVSS 108 Table I-a: number, color, height and activation
Table I-a is where the FMVSS 108 turn signal requirements begin, because it answers the questions a buyer asks first: how many, what color, how high, and when must it light. The version reproduced below comes from a state department of transportation exhibit that reprints Tables I-a, I-b and I-c and labels them as FMVSS 108 as of 29 August 2019. It is a faithful reprint of the table structure but it is not the current eCFR text, and the current text should be checked before certifying a vehicle.
| Lamp function | Number and color | Mounting height | Activation |
|---|---|---|---|
| Front turn signal | 2, amber | 15 to 83 inches | Flashes when the flasher is actuated by the turn signal operating unit |
| Rear turn signal | 2, amber or red | 15 to 83 inches | Same flashing control as the front |
| Taillamp | 2, red | 15 to 72 inches | Steady; on with headlamps, or with parking lamps on vehicles under 80 inches wide |
| Stop lamp | 2, red | 15 to 72 inches | Steady on service brake application; circuit must prevent the stop signal when a combined turn signal is flashing |
| Side marker, front side | 2, amber | At least 15 inches | Steady, may flash for signaling |
| Side marker, rear side | 2, red, not required on a truck tractor | At least 15 inches | Steady, may flash for signaling |
| Clearance lamps, vehicles 2,032 mm or more wide | 2 amber front and 2 red rear, rear not required on a truck tractor | As near the top as practicable | Steady |
| Identification lamps, vehicles 2,032 mm or more wide | 3 amber front, 3 red rear, centers 6 to 12 inches apart | As near the top as practicable | Steady |
| High mounted stop lamp | 1 red, or 2 red where the body splits the centerline | At least 34 inches except on passenger cars | Steady, service brake only |
| Parking lamp | 2, amber or white | 15 to 72 inches | Steady, on with headlamps |
| Backup lamp | 1, white, additional lamps permitted | No requirement | Steady when ignition is on and reverse is engaged; must not light in forward motion |
| License plate lamp | 1, white | No requirement | Steady with headlamps or parking lamps |
Three details in that table catch people who only read the color column. First, the front and rear turn signals share the same 15 to 83 inch height window, which is wider at the top than the 72 inch ceiling on taillamps and stop lamps; a lifted truck can therefore keep its indicators inside the height limit while pushing its stop lamps outside it if the lamps are not in the same housing. Second, the rear turn signal may be amber or red, so a red rear indicator is a design choice the federal standard allows, not a defect. Third, the stop lamp row carries a circuit obligation rather than a photometric one: when a stop lamp and a turn signal share a lens and a filament, the wiring has to prevent the stop signal from lighting while that side’s turn signal is flashing.
The color question has one more layer. S6.1.2 states that the color in all lamps and reflective devices must be as specified in Table I, and that the color identified as amber is identical to the color identified as yellow. FMVSS 108 therefore does not maintain two colors called amber and yellow; the words are synonyms in this regulation, which matters when a specification sheet offers a choice between them.
Table I-b and Table I-c: trailers and motorcycles differ
Trailers and motorcycles do not simply inherit the passenger car table. The trailer column of Table I-b drops the parking lamp and the high mounted stop lamp, allows a single taillamp and a single stop lamp on trailers less than 30 inches wide, and requires three red identification lamps on the rear for trailers 2,032 mm or more in overall width. Motorcycles under Table I-c are the most different: they need one red taillamp and one red stop lamp rather than two, and their turn signals carry separation distances that cars do not have.
| Requirement | Passenger car, MPV, truck, bus (I-a) | Trailer (I-b) | Motorcycle (I-c) |
|---|---|---|---|
| Rear turn signal | 2, amber or red | 2, red or amber | 2, amber or red, centers at least 9 inches apart |
| Front turn signal | 2, amber | Not required | 2, amber, centers at least 16 inches apart |
| Turn signal to headlamp or taillamp edge gap | Not specified in Table I-a | Not applicable | At least 4 inches |
| Taillamp | 2, red | 2 red, or 1 red under 30 inches wide | 1, red |
| Stop lamp | 2, red | 2 red, or 1 red under 30 inches wide | 1, red |
| Parking lamp | 2, amber or white | Not required | Not required |
| High mounted stop lamp | 1 red, or 2 red where the body splits the centerline | Not required | Not required |
| Identification lamps | 3 amber front, 3 red rear at 2,032 mm or more | 3 red rear at 2,032 mm or more | Not required |
Why the candela numbers are hard to quote, and how this article handles them
Here is the awkward part of writing about FMVSS 108 signal lamps. The FMVSS 108 turn signal requirements refer to Tables VI-a, VII, VIII and IX but do not restate them, and the official XML rendering of section 571.108 returns the paragraphs without the table bodies; the Cornell LII version reads the same way. The Canadian Technical Standards Document No. 108 contains them, but its text extraction stopped before the table pages.
The numbers in this article therefore come from a third party that reproduces the tables: a compliance test report issued by Zhejiang ATTC Automobile Technology Service on 29 June 2023, report number AT23XX1D66631, for a Morimoto rear group lamp. That report reprints the required minimum column of Tables IV-a, VII, VIII, IX, X and XII and the required maximum where a maximum applies, point by point, and then lists measured values against them. It is a test report, not the regulation, and every number below that is attributed to Tables VII, VIII, IX, X or XII should be read that way: as the table as reproduced by a laboratory, not as a quotation from the eCFR.
Two structural facts matter more than any single value. The per-point minima in Tables VII and IX depend on how many lighted sections the lamp has, because the group total carries the requirement for a three-section lamp. And every one of these tables contains a maximum as well as a minimum.
Rear turn signal photometry: 25 candela at the corner, 900 at the ceiling
For a rear turn signal certified as three lighted sections, Table VII sets the following minima. Test points are written as degrees from the reference axis: 5U and 5D are 5 degrees up and down, 20L and 20R are 20 degrees left and right, H is the horizontal line and V the vertical line through the axis.
| Test point | Minimum intensity | Group | Group minimum |
|---|---|---|---|
| 20L 5U, 20L 5D | 25 cd | 1 | 120 cd |
| 5L 10U, 5L 10D | 35 cd | 1 | 120 cd |
| 10L 5U, 10L H, 10L 5D | 65 cd at 5U and 5D, 90 cd at H | 2 | 220 cd |
| V 5U, V 5D | 150 cd | 3 | 825 cd |
| 5L H, V H, 5R H | 175 cd | 3 | 825 cd |
| 10R 5U, 10R H, 10R 5D | 65 cd at 5U and 5D, 90 cd at H | 4 | 220 cd |
| 5R 10U, 5R 10D | 35 cd | 5 | 120 cd |
| 20R 5U, 20R 5D | 25 cd | 5 | 120 cd |
| Any direction | Maximum 900 cd | — | — |
Read the first and last rows together. The outboard corner of the pattern needs 25 candela, the center needs 175, and nothing anywhere may exceed 900. The ratio between the center minimum and the ceiling is 5.1 to 1, and the ratio between the corner minimum and the ceiling is 36 to 1. A lamp that produces a very bright center and a weak corner is the classic failure mode: it passes at H, fails at 20 degrees outboard, and to a driver behind it looks impressively bright while being less visible from the angle where a following vehicle in an adjacent lane actually sits.
The group minimum column is the second thing to read. Groups 1 and 5 cover the outboard flanks and require 120 candela across four points each; group 3 covers the vertical center column and the horizontal center row and requires 825 candela across five points. A lamp measured point by point can meet every individual minimum and still fail its group minimum, because the group requirement is a sum over the group’s points rather than a check on each point.
For the record, the measured values in that same report sat far above the minima: 213.5 and 225.4 candela at V H for the two samples after one minute, against a 175 candela minimum, with the maximum recorded value 234.1 candela against the 900 candela ceiling. That is what a compliant rear indicator looks like in a laboratory: comfortably above the floor, far below the ceiling, and stable between the one-minute and thirty-minute readings.
Stop lamp photometry: 15 candela at the corner, 420 at the ceiling
Table IX uses the same nine-point pattern with lower numbers, because a stop lamp is read at close range and the design intent is contrast against the taillamp rather than reach.
| Test point | Minimum intensity | Group | Group minimum |
|---|---|---|---|
| 20L 5U, 20L 5D | 15 cd | 1 | 70 cd |
| 5L 10U, 5L 10D | 22 cd | 1 | 70 cd |
| 10L 5U, 10L H, 10L 5D | 40 cd at 5U and 5D, 55 cd at H | 2 | 135 cd |
| V 5U, V 5D | 95 cd | 3 | 520 cd |
| 5L H, V H, 5R H | 110 cd | 3 | 520 cd |
| 10R 5U, 10R H, 10R 5D | 40 cd at 5U and 5D, 55 cd at H | 4 | 135 cd |
| 5R 10U, 5R 10D | 22 cd | 5 | 70 cd |
| 20R 5U, 20R 5D | 15 cd | 5 | 70 cd |
| Any direction | Maximum 420 cd | — | — |
A retrofit lamp that triples the center value does not make braking more visible. It moves the lamp toward the ceiling, and if any single direction exceeds 420 candela the lamp fails Table IX no matter how good it looks. This is the arithmetic behind the rule that a stop lamp is not a place to chase lumens.
Taillamp, side marker and backup lamp: the low numbers
The remaining rear functions use smaller numbers still, and their maxima are as important as their minima.
| Function and table | Point minima | Group minima | Maximum |
|---|---|---|---|
| Taillamp, Table VIII, one section | 0.3 cd at 20L and 20R 5U and 5D; 0.4 cd at 5L and 5R 10U and 10D; 0.8 cd at 10L and 10R 5U, H and 5D; 1.8 cd at V 5U and 5D; 2 cd at 5L H, V H and 5R H | 1.4, 2.4, 9.6, 2.4, 1.4 cd by group | 18 cd at any point |
| Side marker, Table X, red | 0.25 cd at all nine points of the 10U, 10D and H rows | Not grouped | Not listed in the reproduced table |
| Backup lamp, Table XII | Group minima 45, 50, 100, 360, 50, 45 cd by group; individual points from 10 to 80 cd | As listed | 300 cd at most points |
Two observations follow from that table. A taillamp’s ceiling of 18 candela at any point is barely above its own center group minimum of 9.6 candela, so the taillamp has one of the tightest windows in the whole standard. A red side marker needs only 0.25 candela, which makes it a conspicuity device rather than an illumination device.
Lens area: 2,200 and 5,000 square millimeters
Before a lamp can be measured for candela, it has to have enough luminous area to be measured at all. Table IV-a sets effective projected luminous lens area floors by function and by vehicle width, and the numbers are much larger than most aftermarket lamps assume.
| Function | Single compartment, under 2,032 mm wide | Each compartment of a multiple lamp | Each lamp, 2,032 mm or more wide | Motorcycle |
|---|---|---|---|---|
| Front turn signal | 2,200 sq mm | 2,200 sq mm | 7,500 sq mm | 2,258 sq mm |
| Rear turn signal | 5,000 sq mm | 2,200 sq mm | 7,500 sq mm | 2,258 sq mm |
| Stop lamp | 5,000 sq mm | 2,200 sq mm | 7,500 sq mm | 5,000 sq mm |
The report we are using illustrates the wide-vehicle column. Its subject lamp was certified for a vehicle wider than 2,032 mm, so the requirement was 7,500 square millimeters for both the rear turn signal and the stop lamp; the lamp measured 8,940 square millimeters of effective projected luminous lens area for each function.
The geometry rules that change which table applies
Three numbers decide how a multi-lamp rear cluster is tested, and none of them is a photometric value.
The first is 560 millimeters. S7.1.2.11.2 and S7.2.11.2 say that when a multiple compartment lamp or multiple lamps are used on a passenger car or on a multipurpose passenger vehicle, truck, bus or trailer of less than 2,032 mm overall width, and the distance between adjacent light sources does not exceed 560 millimeters for a two-compartment arrangement, the combination must be used to meet the photometric requirements for the corresponding number of lighted sections in Table VII or Table VIII.
The second is 410 millimeters. The same paragraphs give 410 millimeters for a three-compartment or three-lamp arrangement. Beyond those distances each compartment must meet the single-section requirement on its own, which is a much harder test.
The third is 2,032 millimeters of overall width. At or above it, the rules change rather than tighten: S7.1.2.11.4 requires that multiple compartment rear turn signal lamps on multipurpose passenger vehicles, trucks and buses 2,032 mm or more in overall width be measured as the entire lamp and not per compartment, and Table IV-a raises the lens area floor to 7,500 square millimeters per lamp.
S6.4.3 adds a fourth rule that is not a distance at all: a 1,250 square millimeter lens area option, described in the visibility section below.
Spacing multipliers and ratio rules
Two FMVSS 108 mechanisms make the required intensity depend on where a lamp sits rather than only on what it is.
The first applies to front turn signals. S7.1.1.10.4 sets a photometric multiplier based on the measured distance between the turn signal and the lighted edge of the lower beam headlamp or a supplementary low beam or fog lamp. Under 100 millimeters the multiplier is 2.5; from 75 to under 100 millimeters it is 1.5; from 60 to under 75 millimeters it is 2.0; under 60 millimeters it is 2.5. Where more than one spacing relationship exists, S7.1.1.10.1 requires the highest applicable multiplier to be used. The logic is that a turn signal close to a bright headlamp has to compete with it, so the same lamp must produce more light.
| Spacing to the lower beam lamp | Photometric multiplier | What it means in practice |
|---|---|---|
| 100 mm or more | 1.0, the base table | No increase |
| 75 mm to under 100 mm | 1.5 | 50 percent more than the Table VI-a base |
| 60 mm to under 75 mm | 2.0 | Double the Table VI-a base |
| Under 60 mm | 2.5 | Two and a half times the Table VI-a base |
The second mechanism applies at the rear. S7.1.2.12.1 requires that when a taillamp, or a clearance lamp on a vehicle 2,032 mm or more wide, is combined with a rear turn signal, the turn signal’s intensity at each identified test point must be at least the taillamp’s intensity at that same point multiplied by the factor given in Table VII for that point. The rear indicator therefore cannot simply be a brighter version of the same lens; it must maintain a specified ratio to the taillamp across the whole pattern, which is why a shared-lens design has to be engineered as a pair rather than as two independent functions.
S7.1.3.1 adds a mechanical requirement to the same shared-lens idea: where a turn signal is optically combined with a taillamp or parking lamp and a two-filament bulb is used, the bulb must have an indexing base and the socket must be designed so that non-indexing bulbs cannot be fitted. That paragraph exists because a bulb rotated into the wrong position puts the high-intensity filament where the low-intensity one should be, and the result is a taillamp that blinds and a stop lamp that is dim.
ECE R148: one regulation, one symbol per function
UN Regulation No. 148 replaced nine separate regulations. It was adopted as an annex to the 1958 Agreement with an entry into force of 15 November 2019, and it consolidates the provisions of UN Regulations Nos. 4, 6, 7, 23, 38, 50, 77, 87 and 91 into a single text. Its scope lists eleven lamp types: rear-registration plate illuminating lamps, direction indicator lamps, position lamps, stop lamps, end-outline marker lamps, reversing lamps, maneuvering lamps, rear fog lamps, parking lamps, daytime running lamps and side marker lamps.
R148 is a lamp-level regulation, not a vehicle-level one. Paragraph 4.1 states that the requirements contained in sections 5 and 6 of UN Regulations Nos. 48, 53, 74 or 86 apply to R148 as well. Those sections are where the vehicle installation rules live, so the number of lamps a vehicle must carry and where they may be mounted are in R48, which this article did not retrieve. What R148 itself fixes is what a lamp of a given category must do on a test bench.
The categories are what makes R148 readable once you know the code. Table 1 of the regulation assigns a symbol to each function, and the front direction indicator family alone has four symbols because the required intensity depends on how close the indicator is allowed to sit to the passing beam headlamp.
| Symbol | Function | Distinguishing condition |
|---|---|---|
| 1, 1a, 1b | Front direction indicator | Distance to passing beam or front fog lamp: any distance, at least 20 mm, at least 40 mm |
| 11, 11a, 11b, 11c | Front direction indicator, category L vehicles | Distance to passing beam: at least 75 mm, at least 40 mm, at least 20 mm, any distance |
| 2a, 2b | Rear direction indicator | Steady, or variable intensity |
| 12 | Rear direction indicator, category L vehicles | Single category |
| 5, 6 | Side direction indicator | Vehicles up to 6,000 mm long, and longer vehicles |
| S1, S2, S3, S4, MS | Stop lamp | Steady or variable; S3 and S4 are central high mounted; MS is for category L |
| SM1, SM2 | Side marker lamp | All vehicle categories, and M1 specifically |
| A, MA / R1, R2, MR | Front and rear position lamps | General category, and category L |
| AM, RM1, RM2 | End-outline marker lamps | Front, and rear steady or variable |
| AR | Reversing lamp | Single category |
| F1, F2 | Rear fog lamp | Steady or variable |
| RL | Daytime running lamp | Single category |
| 77R | Parking lamp | Forward and rearward facing |
| L, LM1 | Rear-registration plate illuminating lamp | General and category L |
| ML | Maneuvering lamp | Single category |
The symbol is not decorative: paragraph 3.3.2 requires the approval marking to carry the function symbol, the regulation number, the series of amendments and an outward arrow where the lamp cannot be mounted on either side, so a marked lens states its own test criteria. Decoding those marks is the subject of the DOT and E-Mark article.
ECE direction indicator intensities by category
Table 8 of R148 gives three numbers for each direction indicator category: the minimum intensity in the reference axis, the maximum in any direction for a single lamp, and the maximum for a lamp marked D. For categories 5 and 6 the minimum applies in direction A rather than on the reference axis.
| Category | Minimum intensity | Maximum, single lamp | Maximum, lamp marked D |
|---|---|---|---|
| 1 | 175 cd | 1,000 cd | 500 cd |
| 1a | 250 cd | 1,200 cd | 600 cd |
| 1b | 400 cd | 1,200 cd | 600 cd |
| 2a | 50 cd | 500 cd | 250 cd |
| 2b | 50 cd | 1,000 cd | 500 cd |
| 5 | 0.6 cd | 280 cd | 140 cd |
| 6 | 50 cd | 280 cd | 140 cd |
| 11 | 90 cd | 1,000 cd | Not applicable |
| 11a | 175 cd | 1,000 cd | Not applicable |
| 11b | 250 cd | 1,200 cd | Not applicable |
| 11c | 400 cd | 1,200 cd | Not applicable |
| 12 | 50 cd | 500 cd | Not applicable |
The comparison with the American table is the useful part. A rear indicator in Europe is category 2a with a 50 candela minimum and a 500 candela maximum, while the FMVSS 108 three-section rear indicator has a 25 candela floor at its outboard corner and a 900 candela ceiling. The European category is brighter in the axis but capped at about half the American maximum.
R148 also fixes how a flashing indicator is measured, and this is a detail that aftermarket testing often gets wrong. Paragraph 5.6.4 permits measurement in flashing mode at a frequency of 1.5 plus or minus 0.5 hertz with a pulse width greater than 0.3 seconds, measured at 95 percent of peak intensity, which is 90 plus or minus 30 cycles per minute. Paragraph 5.6.10 requires that for any direction indicator not equipped with filament light sources, the intensities measured after one minute and after 30 minutes of operation in flashing mode at 1.5 hertz and 50 percent duty factor must both comply with the minimum and maximum requirements. An LED indicator that is compliant when cold and out of specification after half an hour of hazard flashers is a real failure mode, and R148 tests for it explicitly.
The final direction indicator paragraph worth knowing is 5.6.11, which allows sequential activation of light sources only under five conditions: each light source must remain lit after activation until the end of the on cycle; the sequence must proceed uniformly from inboard toward the outboard edge; there must be one signal with no interruption and no vertical oscillation, with adjacent distinct parts of the light emitting surface no more than 50 millimeters apart; the variation must finish no more than 200 milliseconds after the start of the on cycle; and the orthogonal projection of the light emitting surfaces must be circumscribed by a rectangle whose horizontal to vertical side ratio is at least 1.7. A sweeping indicator that runs the wrong way, or takes half a second to complete, is not an approved category 2a or 2b lamp however good it looks.
ECE stop lamp intensities: S1, S2, S3, S4 and MS
Table 7 of R148 sets the stop lamp numbers, and the split between steady and variable categories is where most of the complexity sits.
| Category | Minimum intensity in H-V | Maximum, single lamp | Maximum, lamp marked D |
|---|---|---|---|
| S1, steady | 60 cd | 260 cd | 130 cd |
| S2, variable | 60 cd | 730 cd | 365 cd |
| S3, steady, central high mounted | 25 cd | 110 cd | 55 cd |
| S4, variable, central high mounted | 25 cd | 160 cd | 80 cd |
| MS, steady, category L | 40 cd | 260 cd | Not applicable |
Paragraph 5.5.4 fixes the color as red, and adds that for a category S3 or S4 stop lamp intended to be mounted inside the vehicle, the colorimetric characteristics must be verified with the worst case combination of lamp and rear window or sample plate. That sentence is the reason a high mounted stop lamp behind tinted glass is a colorimetric problem and not only a photometric one.
Paragraph 5.5.3 sets a floor across the whole field rather than at the test points: the intensity must be at least 0.3 candela for S1, S3 and MS and for S2 and S4 by day, and at least 0.07 candela for S2 and S4 by night. A variable-intensity stop lamp therefore has two floors to satisfy, one in the axis and one everywhere in the field, and the night floor is the lower of the two.
ECE position, side marker, reversing and rear fog lamps
The remaining R148 functions fill in the numbers that FMVSS 108 spreads across Tables VIII, X and XII.
| Function and category | Minimum intensity | Maximum | Color |
|---|---|---|---|
| Front position, A or AM | 4 cd in H-V | 140 cd single, 70 cd marked D | White; MA may be amber |
| Rear position, R1 or RM1 | 4 cd in H-V | 17 cd single, 8.5 cd marked D | Red |
| Rear position, R2 or RM2, variable | 4 cd in H-V | 42 cd single, 21 cd marked D | Red |
| Parking, forward facing | 2 cd in H-V | 60 cd | White |
| Parking, rearward facing | 2 cd in H-V | 30 cd | Red |
| Side marker, SM1 | 4.0 cd in the axis, 0.6 cd elsewhere | 125.0 cd | Amber, or red if the rearmost lamp is grouped with a rear position lamp |
| Side marker, SM2 | 0.6 cd in the axis, 0.6 cd elsewhere | 25.0 cd | Amber, or red on the same condition |
| Reversing, AR | 80 cd in H-V | 300 cd in or above H, 600 cd below H to 5D, 8,000 cd below 5D | White |
| Rear fog, F1 | 150 cd | 300 cd | Red |
| Rear fog, F2, variable | 150 cd | 840 cd | Red |
| Daytime running, RL | 400 cd in H-V | 1,200 cd | White |
The reversing lamp’s maximum is not one number but three, depending on direction: 300 candela in or above the horizontal plane, 600 candela below it down to 5 degrees down, and 8,000 candela below 5 degrees down. The regulation is permissive downward because a reversing lamp is expected to light the ground.
Paragraph 5.4.6 adds a size constraint that has no FMVSS 108 counterpart: the apparent surface of a daytime running lamp in the direction of the reference axis must be not less than 25 square centimetres and not more than 200 square centimetres. The upper bound is the interesting one, because it caps how much lens area a DRL may use rather than how little.
Geometric visibility angles: ECE prints the field, FMVSS defines a pattern
The clearest structural difference between the two regimes appears in the field of visibility. R148 publishes the horizontal and vertical angles for each function family in Table A2-1 of Annex 2, as inboard and outboard angles on each side of the reference axis.
| Function | Horizontal, inboard and outboard | Vertical, above and below |
|---|---|---|
| Front direction indicator, categories 1, 1a, 1b | 45 and 80 degrees, or 20 and 80 degrees | 15 and 15 degrees, or 15 and 5 degrees |
| Rear direction indicator, categories 2a, 2b | 45 and 80 degrees, or 20 and 80 degrees | 15 and 15 degrees, or 15 and 5 degrees, plus 5 and 15 degrees |
| Stop lamp, S1 and S2 | 45 and 45 degrees, or 20 and 45 degrees | 15 and 15, 15 and 5, or 5 and 15 degrees |
| High mounted stop lamp, S3 and S4 | 10 and 10 degrees | 10 and 5 degrees |
| Front or rear position lamp, A and R | 45 and 80 degrees, or 20 and 80 degrees | 15 and 15 degrees, or 15 and 5 degrees |
| Side marker, SM1 | 45 and 45 degrees | 10 and 10 degrees, or 10 and 5 degrees |
| Side marker, SM2 | 30 and 30 degrees | 10 and 10 degrees, or 10 and 5 degrees |
Two footnotes in that table do real work. Reduced angles are used only below the horizontal plane for lamps mounted with the horizontal plane below 750 mm, and Annex 3 paragraph 1.3 adds that for direction indicators, position lamps, parking lamps, S1, S2 and MS stop lamps and side marker lamps installed at 750 mm or less, intensity is verified only up to 5 degrees downward. A lamp low on a bumper is therefore tested over a smaller field than the same lamp high on a tailgate.
FMVSS 108 reaches visibility through S6.4 instead, and the difference matters: a manufacturer certifies each lamp function either to at least 1,250 square millimeters of unobstructed effective projected lens area in every direction through a pattern of corner points, or to at least the luminous intensity in the corresponding table in every direction through that pattern. The FMVSS 108 turn signal requirements therefore name directions and values, not angles.
Color and mounting height: the conditions that change the result
Color is the requirement most often assumed to be universal. FMVSS 108 states that the color of every lamp and reflective device must be as specified in Table I, and that amber is identical to yellow. R148 states the color function by function: white for front position lamps, with MA permitted to be amber; red for rear position, stop and rear fog lamps; amber for all direction indicators including the rear; amber for side marker lamps, with red permitted where the rearmost side marker is grouped with a rear position, end-outline, rear fog or stop lamp or shares its surface with the rear reflex reflector; white for reversing lamps.
| Color | FMVSS 108 boundary as reproduced in the test report | Functions in FMVSS 108 Table I |
|---|---|---|
| Red | y equals 0.335 on the yellow boundary, and y equals 0.980 minus x on the purple boundary | Taillamp, stop lamp, rear side marker, rear clearance, rear identification, rear reflex reflector |
| Amber | y equals 0.390 on the red boundary, y equals 0.790 minus 0.670x on the white boundary, and y equals x minus 0.120 on the green boundary | Front turn signal, front side marker, front clearance, front identification |
| White | x equals 0.310 and x equals 0.500, y equals 0.150 plus 0.640x and y equals 0.050 plus 0.750x, y equals 0.440 and y equals 0.380 | Backup lamp, license plate lamp, headlamp |
The rear indicator is the one place where the two systems genuinely diverge. Table I-a allows a rear turn signal to be amber or red, and the compliance report we are using tested a rear indicator against the amber boundaries, confirming that amber at the rear is a valid American configuration. R148 paragraph 5.6.9 is not permissive in the same way: the color of the light emitted by a direction indicator shall be amber, and that requirement applies across the whole variable-intensity range of a category 2b rear indicator.
Paragraph 3.9 of R148 is the caveat that keeps the European side honest. Contracting Parties are not precluded from prohibiting, for lamps installed on vehicles registered by them, certain colors for which the regulation makes provision, or from prohibiting stop lamps that have only steady luminous intensity. An approval to R148 therefore proves that a lamp meets the regulation; it does not prove that every country applying the regulation permits that color on its roads.
What must happen when a light source dies
Both regimes treat a dead light source as a signaling event rather than only a maintenance item.
FMVSS 108 S9.3.6 requires that failure of one or more turn signal lamps, such that the minimum photometric performance in Tables VI or VII is not met, be indicated by the turn signal pilot indicator through a steady on, a steady off, or a significant change in the flashing rate. The regulation names exceptions for variable-load flashers on large multipurpose passenger vehicles, trucks, buses, trucks able to take a slide-in camper, and any vehicle equipped to tow trailers. That single paragraph is the reason the FMVSS 108 turn signal requirements reach into the dashboard: the lamp and its failure indicator are one system.
Paragraph 5.6.3 is stricter for indicators. A tell-tale must be activated for categories 1, 1a, 1b, 2a and 2b if any one light source has failed, or if a lamp designed for only two light sources drops below 50 percent of the minimum in the axis, or if the intensity in any of three named directions falls below the required minimum: H equals 0 and V equals 0, H equals 20 degrees outboard with V at plus 5 degrees, or H equals 10 degrees inboard with V at 0. A rear indicator that loses one segment but still passes at its center will fail this test if the outboard 20 degree direction drops below the minimum.
Why an LED retrofit can break a lamp that was compliant
The FMVSS 108 turn signal requirements apply to a lamp, not to a light source, and that distinction is where retrofits go wrong. S6.7.1.1 requires each replacement lamp to meet all requirements for that type of lamp and to include all the functions of the lamp it replaces. S6.7.1.2 requires a replacement lamp designed or recommended for particular vehicle models not to take the vehicle out of compliance when installed, judged without regard to other devices that might also be installed. Neither paragraph permits a replacement that only works when accompanied by a resistor, a decoder or a different flasher.
The physical reasons are specific. A filament bulb radiates in all directions, so the reflector and lens of the original lamp were designed around a small, roughly spherical source at the focal point. An LED replacement puts several emitting surfaces at different positions, with a different angular distribution, so the intensity at the 20 degree outboard corner of Table VII can fall while the center value rises. The candela test is directional; a retrofit that looks brighter on a wall can be dimmer in the one direction the table measures.
R148 makes the same point from the other side. Paragraph 4.3.1.1 restricts replaceable light sources to types approved under UN Regulation No. 37 or No. 128, paragraph 4.3.1.2 requires the applicant to declare a lamp’s restriction to vehicles originally equipped with that light source category, and paragraph 4.3.1.3 requires the lamp design to prevent the light source from being fixed in any position but the correct one. A lamp approved with a specific LED light source category is approved as a system, and substituting a different source is outside the approval.
A retrofit that meets the numbers when cold can drift out of the window once the emitter heats, which is why a lamp that passes a five-minute bench check can still fail the sustained test in paragraph 5.6.10.
Hyperflash and load resistors: what the rules actually require
Fast flashing after an LED conversion is usually described as a fault, but in many vehicles it is the failure indication working as designed. S9.3.6 lists a significant change in the flashing rate as one of the three permitted ways to signal a failed turn signal lamp, and a vehicle that monitors current will see a lower load and treat it as a failure. The flashing rate is not the problem; the vehicle’s assumption about the load is.
R148 paragraph 4.5.4 shows the same design pattern from the lamp side: a rear position lamp reciprocally incorporated with a stop lamp must either use a multiple light source arrangement or be intended for a vehicle with a failure monitoring system for that function. Both regimes assume that a shared or monitored circuit will detect a dead source, and both leave the detection method to the manufacturer.
The practical conclusion is narrower than the internet version. A load resistor restores the current the monitor expects, which may silence the indication without restoring the photometric performance the lamp needs. If the vehicle’s monitor was satisfied but the lamp now fails the outboard test point, the resistor has hidden the symptom and left the defect. The order that matters is: verify the lamp against the table first, then address the monitor.
Common misreadings
Four misreadings account for most of the confusion in this subject.
Brighter is better. Every FMVSS 108 signal table has a maximum as well as a minimum: 900 candela for a rear turn signal, 420 for a stop lamp, 18 for a taillamp, 300 for a backup lamp at most points. A lamp that exceeds a maximum fails, so the objective is the window, not the peak.
Amber and red are interchangeable. Table I-a allows amber or red at the rear for FMVSS 108, but R148 requires amber for every direction indicator, and paragraph 3.9 lets contracting parties prohibit colors the regulation permits. A color that is approved in one market is not thereby approved in another.
The European category number is a quality grade. Category 1b requires 400 candela because it may sit at least 40 millimeters from the passing beam lamp, while category 1 requires 175 candela because it may sit anywhere. The higher number is a compensation for a harder installation position, not a better lamp.
A DOT mark on a signal lamp means the government tested it. S6.5.1.2 says a lamp specified in Table I, other than a headlamp, may be marked with the DOT symbol, and that the symbol constitutes a certification by the manufacturer. Reading the FMVSS 108 turn signal requirements as an approval system rather than a self-certification system is the root of that error.
A verification checklist before you buy or install
The checklist below is the practical form of everything above. It assumes the lamp is being sold into the United States; the European column in each row is the equivalent check under R148.
| Check | FMVSS 108 reference | R148 equivalent |
|---|---|---|
| Function, color and count match the vehicle table | Table I-a, I-b or I-c | Category symbol in Table 1 |
| Lens area at or above the floor for the vehicle width | Table IV-a | No lens area floor |
| Minimum intensity met at the outboard and vertical points, not only the center | Tables VII, IX, X, XII | Tables 7, 8, 9, 10 |
| Maximum intensity not exceeded in any direction | Same tables | Maximum columns of the same tables |
| Multi-lamp cluster spaced within 560 mm or 410 mm, or each lamp certified alone | S7.1.2.11.2, S7.2.11.2 | Not applicable |
| Turn signal kept at least 100 mm from the low beam lamp, or the multiplier applied | S7.1.1.10.4 | Category chosen by distance: 1, 1a or 1b |
| Failure indication still works after any lamp change | S9.3.6 | Paragraph 4.6.1.2, 5.6.3 |
| Lamp marked with its category and approval where the market requires it | S6.5.1.2, optional DOT mark | Paragraph 3.3.2 approval marking |
Two rows deserve a comment. The spacing row is the one an installer can still influence after purchase, because moving a front indicator further from the headlamp can drop the required multiplier from 2.5 to 1.0. The failure row is the one an installer most often breaks, because a resistor that satisfies the monitor does not satisfy the photometry.
What this article does not establish
Four gaps should be stated plainly. The front turn signal minima in Tables VI-a and VI-b are not given here, because the table bodies could not be retrieved in text form from the eCFR rendering, the Canadian reproduction or any compliance report we could open; only the spacing multipliers that scale those tables are quoted. The corner-point patterns of Tables V-b and V-c are likewise not reproduced, so the 1,250 square millimeter option is described in structure but not in geometry.
The FMVSS 108 color and lamp tables in this article come from a state transportation department exhibit labeled as the standard on 29 August 2019, and the photometric tables come from a laboratory report that reproduced them. Neither is the current eCFR text. Before certifying a vehicle or a product, the current text should be checked.
The European installation rules are not covered. R148 paragraph 4.1 imports sections 5 and 6 of UN Regulation No. 48, and those sections hold the requirements on how many lamps a vehicle carries and where they are mounted. UN Regulation No. 48 was not retrieved for this article, and UN Regulation No. 150, which governs reflex reflectors, was not retrieved either.
Frequently asked questions
How many candela must a brake light have?
Under FMVSS 108 Table IX a stop lamp certified as three lighted sections needs 15 candela at 20 degrees outboard and 5 degrees up or down, 110 candela at the center, and no more than 420 candela in any direction. Under ECE R148 a category S1 stop lamp needs 60 candela in the axis and may not exceed 260.
Are red rear turn signals legal in the United States?
Yes. Table I-a of FMVSS 108 allows a rear turn signal to be amber or red. ECE R148 does not: paragraph 5.6.9 requires every direction indicator, including a rear category 2a or 2b, to emit amber light.
What separates an S1 stop lamp from an S3?
Mounting position and intensity. S1 is a normal stop lamp with a 60 candela axis minimum and a 260 candela ceiling. S3 is a central high mounted stop lamp with a 25 candela minimum and a 110 candela ceiling, because it is read through the windows of intervening vehicles rather than directly.
Why does my indicator flash fast after an LED conversion?
FMVSS 108 S9.3.6 lists a significant change in flashing rate as one of the accepted ways to indicate a failed turn signal lamp, so a vehicle that monitors current reads the lower LED load as a failure. A load resistor can silence the indication without restoring the photometric performance the lamp still needs.
Can I fit an LED bulb into a lamp approved for a filament bulb?
Not and stay compliant. S6.7.1.1 requires a replacement lamp to meet every requirement for that lamp type, and S6.7.1.2 requires it not to take the vehicle out of compliance. R148 paragraph 4.3.1.1 restricts replaceable light sources to types approved under UN Regulation No. 37 or No. 128.
What is the 1,250 square millimeter rule?
It is one of the two visibility options in S6.4.3. A manufacturer may certify a lamp by providing at least 1,250 square millimeters of unobstructed effective projected luminous lens area in every direction through a pattern of corner points, instead of certifying a minimum intensity through that pattern.
Do side marker lamps have a maximum as well as a minimum?
Yes, in both regimes. Table X requires 0.25 candela at nine points for a red side marker. R148 caps SM1 at 125 candela and SM2 at 25 candela, and limits a red side marker to 0.25 candela in the field toward the front of the vehicle.
Editorial note
Written by SAEC Lighting Engineering Team, Foshan. Last updated: 2026-10-20. This article quotes regulation text and table values as reproduced by the sources below, and states where a table body could not be retrieved rather than filling the gap. It is engineering reference material for lamp selection and installation review, not a certification, a test report or a legal opinion.
Sources
- eCFR, 49 CFR 571.108 Standard No. 108, Lamps, reflective devices, and associated equipment, full text via the official versioner API: https://www.ecfr.gov/api/versioner/v1/full/2025-01-01/title-49.xml?part=571§ion=571.108
- Cornell Law School Legal Information Institute, 49 CFR 571.108: https://www.law.cornell.edu/cfr/text/49/571.108
- Hawaii Department of Transportation, Exhibit A, Table I-a, I-b and I-c reproduced as FMVSS 108 as of 29 August 2019: https://hidot.hawaii.gov/highways/files/2021/03/Exhibit-A.pdf
- Zhejiang ATTC Automobile Technology Service, compliance test report AT23XX1D66631 according to U.S. FMVSS 108, 29 June 2023, reproducing Tables IV-a, VII, VIII, IX, X, XII and the CIE 1931 color boundaries: https://www.morimotohid.com/FMVSS108%20Documents/LF738%20LF739%20FMVSS108%20Compliance.pdf
- United Nations, UN Regulation No. 148, Uniform provisions concerning the approval of light-signaling devices for power-driven vehicles and their trailers, E/ECE/TRANS/505/Rev.3/Add.147, 9 January 2020, PDF copy: https://www.dun-bri.com/files//862baa9b-85f4-4aac-a630-b21a00c70863/UNECE%20Regulation%20No.%20148.pdf
- Transport Canada, Technical Standards Document No. 108, Revision 8, effective 1 April 2025: https://tc.canada.ca/sites/default/files/2025-05/108_tsd_rev_r8_en-web.pdf
- Note on access: the UNECE official pages at unece.org returned HTTP 403 to our requests, and the eCFR HTML rendering of section 571.108 redirected to an unblocking service, so the UN Regulation No. 148 text was read from the PDF copy above and the regulation paragraphs from the eCFR API response.
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