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Understanding SAE J2040-2024: Tail Lamps for Wide Vehicles and Its Supersession by J585
SAE J2040 has long been the reference standard for tail lamps (rear position lamps) on vehicles 2032 mm or more in overall width—typically heavy trucks, buses, and trailers. As of February 2024, this standard has been cancelled and is now superseded by SAE J585. This article summarizes the key technical requirements of J2040, offers design insights, and explains what the change means for engineers and compliance professionals.
⚠️ Notice: SAE J2040 has been cancelled as of February 2024 and is superseded by SAE J585. All new designs and certifications should reference SAE J585 for tail lamp requirements on wide vehicles.
Scope and Cancellation
J2040 was originally issued in 1991 and last reaffirmed in 2016. It defined test procedures, performance requirements, and guidelines for tail lamps used on vehicles with an overall width of 2032 mm or more. The rational for cancellation is that SAE J585 now incorporates tail lamp requirements for wide vehicles, making J2040 redundant. Lamps previously designed to J2040 may continue to be used, but new designs must comply with J585.
The lighting identification code for lamps meeting J2040 was T2, per SAE J759. This code is still valid under J585, but engineers should verify any additional requirements.
Key Testing and Performance Requirements
J2040 required a series of environmental and photometric tests per SAE J2139: vibration, moisture, dust, corrosion, photometry, and warpage. If LED light sources were used, SAE J1889 testing also applied. The table below summarizes the critical requirements that engineers must consider.
| Requirement | Detail |
|---|---|
| Minimum effective projected lighted lens area (single lamp) | 75 cm² |
| Spacing for multiple lamp arrangements | Two lamps: ≤560 mm between adjacent light sources; more than two: ≤410 mm. If spacing exceeds these, each lamp must meet photometry individually. |
| Stop / turn to tail intensity ratio (combined lamps) | ≥3 times tail intensity at most test points; ≥5 times at H-V, H-5°L, H-5°R, and 5°U-V |
| Prohibited optical combination | Tail lamp cannot be optically combined with a clearance lamp |
| Replaceable light source indexing | Retention system must prevent random reinsertion unless lamp functions correctly in any orientation |
| Photometric test distance | Light source ≥3 m from photometer; H-V axis parallel to vehicle longitudinal axis |
| Color | Red per SAE J578 |
| Plastic materials | Must meet SAE J576 |
Photometric compliance was verified through zone total requirements (see Figure 1 of the standard). For multiple lamp arrangements, engineers could either photometer all lamps together (provided the angle between each light source and the photometer axis did not exceed 0.6°) or sum the individual lamp outputs at each test point.
Design Insights and Installation Guidelines
Several engineering lessons emerge from the requirements:
- 🛠️ Indexing is critical for combined lamps. When a tail lamp shares a housing with a stop or turn signal, the replaceable light source must be keyed to prevent incorrect installation. This avoids performance degradation or non-compliance.
- 🔍 Zone totals depend on spacing. To achieve the aggregate photometric values in Figure 1, designers must respect the 560 mm / 410 mm limits. Otherwise, each lamp must be treated individually, which may increase cost or packaging complexity.
- Combined lamp intensity ratios demand careful optical design. The higher ratio (5×) at the four specified test points often drives the choice of bulb or LED intensity and the reflector/lens design.
Installation requirements (Section 6.5) mandate that tail lamps be mounted on the vehicle’s permanent structure at the same height, spaced laterally as far apart as practicable, and oriented rearward. The lamp must provide either 13 cm² of unobstructed projected area or a minimum luminous intensity of 0.05 cd over a wide field of view (45° inboard/outboard, 15° up/down).
💡 Design Tip: When certifying a tail lamp for use on vehicles ≥2032 mm wide, always verify the spacing of multiple lamps early in the design phase. This determines whether you can sum photometric outputs or must meet per-lamp minimums—a decision that significantly affects cost and lamp placement.
Although J2040 has been superseded, its technical requirements remain directly applicable as they have been incorporated into J585. Engineers working on legacy designs may still reference J2040 for baseline performance, but new compliance should be based on the latest revision of J585.
Frequently Asked Questions
Why was SAE J2040 cancelled?
SAE J2040 was cancelled because its content has been absorbed into SAE J585, which now covers tail lamps for all vehicle widths, including those 2032 mm or more. This consolidation reduces redundancy and maintains a single reference for rear position lamps.
What are the key spacing limits for multiple lamp arrangements?
For two tail lamps, the distance between adjacent light sources must not exceed 560 mm. For three or more lamps, the limit is 410 mm. Exceeding these distances means each lamp must individually satisfy the photometric minimums, which can affect lamp design and cost.
How does the intensity ratio requirement affect combined lamps?
When a tail lamp is combined with a stop or turn signal, the stop/turn intensity must be at least three times the tail intensity at most test points, and at least five times at four specific points (H-V, H-5°L, H-5°R, and 5°U-V). This ensures the stop or turn signal is clearly distinguishable from the tail function, especially in critical visibility directions.
What are the photometry methods for multiple lamp arrangements?
There are two methods: (1) photometer all lamps together, provided the angle between each light source and the photometer axis is ≤0.6°, with the H-V axis at the midpoint between the extreme light sources; or (2) photometer each lamp individually and sum the outputs at corresponding test points. Both methods yield the total luminous intensity for compliance with the zone requirements.
