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SAE J1957-2011: Center High Mounted Stop Lamp (CHMSL) Standard – Requirements and Guidelines
1. Scope and Applicability
The SAE J1957-2011 standard establishes test procedures, performance requirements, and guidelines for center high mounted stop lamps (CHMSL) intended for use on vehicles with an overall width less than 2032 mm. The CHMSL is an additional stop lamp that provides a brake-actuated steady warning light to the rear, improving visibility through intervening vehicles. This revision incorporates an updated photometry format without changing photometric requirements, and clarifies exempted tests for internally mounted lamps.
2. Key Requirements and Testing
The standard references multiple SAE documents and includes specific tests from SAE J575 with modifications tailored to CHMSL applications. The following table summarises the applicable tests and their requirements:
| Test / Requirement | Applicability | Key Notes |
|---|---|---|
| Vibration Test (SAE J575) | Applies to all CHMSLs | Standard procedure |
| Moisture Test (SAE J575) | Exempt for inside-mounted CHMSLs | Per Section 6.1.2 |
| Dust Test (SAE J575) | Exempt for inside-mounted CHMSLs | Per Section 6.1.3 |
| Corrosion Test (SAE J575) | Exempt for inside-mounted CHMSLs | Per Section 6.1.4 |
| Photometry Test | Applies to all CHMSLs | Photometer distance ≥ 3 m; meet zone totals per Figure 1 |
| Warpage Test (plastic components) | Applies to all CHMSLs | Use stop lamp cycle times and temperatures from SAE J575 Table 1 |
| Color Test (SAE J578) | Applies to all CHMSLs | Emitted light must be red |
| Plastic Materials (SAE J576) | Applies to all CHMSLs | Optical parts must conform to SAE J576 |
🛠️ Engineering Design Insight: The lamp must have an effective projected luminous lens area of at least 29 cm² (4.5 in²). For inside-mounted designs, ensure means are provided to minimise reflections at the rear window glazing visible to the driver. Replacement bulb identification should be permanently marked on the housing, and the lamp should be serviceable with commonly available tools.
3. Design, Installation, and Maintenance Guidelines
CHMSL mounting and design must meet several location and visibility criteria to ensure effectiveness and compliance:
- Mounting location: The lamp must be centred on the vehicle vertical centreline. If mounted below the rear window, the lens must not be lower than 152 mm (6 in) below the window on convertibles, or 76 mm (3 in) on other passenger cars.
- Visibility: The signal must be visible from 45° left to 45° right of the vehicle longitudinal axis.
- Activation: The CHMSL shall be illuminated only when the service brakes are applied.
- Combination restrictions: The CHMSL must not be optically combined with any other signal lamp or reflective device except a cargo lamp.
- Heat testing: Heat test cycles may be increased to reflect more severe mounting environments.
- Serviceability: Minimise trim pieces for lamp servicing and rear window cleaning; bulb identification should be on the housing.
⚠️ Common Mistake: Applying moisture, dust, and corrosion tests to internally mounted CHMSLs. These tests are explicitly exempt per J1957 Sections 6.1.2, 6.1.3, and 6.1.4.
Designers should also note that CHMSL plastic optical parts must comply with SAE J576, and the lamp should be positioned higher than conventional stop lamps for optimum visibility through traffic.
Frequently Asked Questions (FAQs)
What is the minimum effective projected lens area for a CHMSL?
Per Section 6.4, the lens area measured perpendicular to the lamp axis must be at least 29 cm² (4.5 in²).
Are environmental tests such as moisture, dust, and corrosion required for inside-mounted CHMSLs?
No. Sections 6.1.2, 6.1.3, and 6.1.4 explicitly exempt inside-mounted CHMSLs from the moisture, dust, and corrosion tests specified in SAE J575.
What colour must the CHMSL emit?
The emitted light must be red, in accordance with SAE J578 (Section 6.2).
How should the CHMSL be activated?
It must be activated only upon application of the service brakes, not by any other brake mechanism (Section 6.5.5).
