Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
Understanding SAE J1889: LED Signal and Marking Lighting Devices – 2019 Update
The SAE J1889 standard provides test methods, requirements, and guidelines for motor vehicle signaling and marking lighting devices that use light-emitting diodes (LEDs) as light sources. The 2019 revision introduces critical updates to reflect modern measurement technologies and streamline production testing while maintaining photometric accuracy. This article outlines the key changes—including a new definition of photometric stability, support for camera-based photometry (SAE J3100), and a pre-determined ratio method—and offers practical advice for engineers implementing this standard.
Key Updates in the 2019 Revision
The 2019 revision of SAE J1889 addresses two primary goals: integrating new measurement technologies and allowing shorter warm-up times through a pre-determined ratio method. Below are the major changes.
1. Redefined Photometric Stability
Earlier versions of J1889 did not provide a precise definition of photometric stability. The 2019 revision establishes a clear, quantifiable criterion: photometric stability is the point at which the device under test (DUT) photometric value is stable to within 1% over the photometric test duration. This definition is independent of light source type (bulb, LED, laser, etc.) and measurement method, ensuring that faster instruments are not penalized. It also bounds drift error, making it a known and reasonable limit for photometric testing.
🔍 Key Definition
Photometric Stability: The point when the DUT photometric value is stable to within 1% over the photometric test duration.
Photometric Stability: The point when the DUT photometric value is stable to within 1% over the photometric test duration.
2. Camera-Based Photometry (SAE J3100)
Camera-based photometry records all test points simultaneously, eliminating drift error during measurement. The 2019 revision explicitly recognizes this method, aligning with SAE J3100. This is a significant step forward, as previous versions primarily assumed goniometer measurements with sequential point acquisition.
3. Pre-Determined Ratio Method
To enable shorter warm-up times in production testing, the standard introduces the concept of a pre-determined minimum ratio. By establishing a known ratio between early photometric values and stabilized minimum values, manufacturers can verify compliance without waiting for full stabilization. This expedites quality checks without sacrificing accuracy.
| Aspect | Previous Versions (pre-2019) | 2019 Revision |
|---|---|---|
| Photometric stability definition | Not explicitly defined (assumed ±5% over arbitrary time) | Quantified: 1% over test duration |
| Measurement methods | Primarily goniometer; sequential points | Explicitly includes camera-based (J3100): simultaneous recording |
| Warm-up time for production | Fixed, often 30 minutes | Flexible via pre-determined ratio method |
| Drift error management | Not explicitly bounded | Bounded to 1% by stability definition |
Practical Implementation and Testing Strategies
Applying the updated standard requires engineers to re-evaluate their testing protocols. The following guidance can help ensure compliance and efficiency.
Verifying Photometric Stability
To confirm stability, monitor photometric output over time until the variation remains within 1% for the intended test duration. If using a goniometer, ensure that the entire measurement sequence (including all required positions) is completed within this stable window. For camera-based systems, stability is less of a concern because all points are captured simultaneously, but the device should still be at thermal equilibrium to represent realistic operating conditions.
Implementing the Ratio Method
For production testing, pre-determine the ratio R = (minimum stabilized value) / (value at shorter time, e.g., 5 minutes). Once validated, this ratio allows you to calculate minimum values from early measurements, significantly reducing test time. The standard requires that the ratio be established with representative samples and periodically verified.
Inter-Laboratory Correlation
Consistency across laboratories is a major goal of J1889. The new stability definition provides a clear metric that, when combined with the pre-determined ratio, minimizes variability due to warm-up conditions. Engineers should document their verification methods and share ratio values to improve correlation.
⚠️ Common Mistake
Assuming LED luminous flux is stable after a fixed warm-up time without verifying per the new stability criteria. Always measure and confirm that drift is within 1% over the test duration, or use the pre-determined ratio method to account for initial drift.
Assuming LED luminous flux is stable after a fixed warm-up time without verifying per the new stability criteria. Always measure and confirm that drift is within 1% over the test duration, or use the pre-determined ratio method to account for initial drift.
Engineering Design Insights
Designers should consider the photometric output versus time characteristic of their LED devices to determine appropriate test conditions. The pre-determined ratio method is particularly valuable for production testing: by knowing the early thermal behavior, manufacturers can shorten quality checks while maintaining accuracy. Additionally, because camera-based photometry eliminates drift error, it offers an advantage for both internal testing and compliance verification.
Frequently Asked Questions
1. What is photometric stability according to SAE J1889 (2019)?
Photometric stability is defined as the point when the device under test’s photometric value is stable to within 1% over the photometric test duration. This definition applies to any light source and any measurement method.
2. How does the pre-determined ratio method reduce warm-up time?
The method uses a validated ratio R = (minimum stabilized value) / (value at a shorter time, e.g., 5 minutes). Once R is known, the minimum value can be calculated from the early measurement, eliminating the need to wait for full stabilization during routine tests.
3. Why was camera-based photometry added to the standard?
Camera-based photometry records all test points simultaneously, which eliminates drift error during measurement. This aligns with the goal of not penalizing faster instruments and provides an alternative to sequential goniometer measurements.
4. Does the 2019 revision affect how I validate my LED lighting devices?
Yes. You must now verify photometric stability per the new 1% criterion. If you previously relied on a fixed 30-minute warm-up, you may need to confirm that drift over the measurement duration meets the new requirement. The pre-determined ratio method offers a practical path for production testing to shorten warm-up times while remaining compliant.
