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Practical Guide to EPLLA Determination (SAE J2999-2017)
🛠️ Overview and Scope
SAE J2999-2017 provides a standardized method for determining the Effective Projected Luminous Lens Area (EPLLA) of a lamp function using design analysis. This recommended practice addresses both traditional and new lighting technologies, ensuring consistent photometric measurements across automotive lighting systems.
Definition: EPLLA is the area of the orthogonal projection of the effective light-emitting surface of a lamp onto a plane perpendicular to a defined Test Pattern direction relative to the axis of reference.
The Test Pattern is defined as the rectangle formed by horizontal and vertical planes passing through the bounding photometric test points. Accurate EPLLA determination is critical for compliance with SAE standards such as J586, J588, J2087, and related regulations.
🔍 Design Analysis Procedure
The procedure requires a scalable drawing or CAD representation of the lamp function, showing orthogonal projection of all applicable elements. The analysis follows three key steps:
- Lamp Function Representation: Provide a scalable drawing or CAD data showing the projection of light source, reflective, refractive, and diffusion elements onto a perpendicular plane, including obstructive features.
- Element Identification: Identify elements according to the standard’s classifications.
- Determination of EPLLA: Calculate the area of orthogonal projections, handling overlaps and shared elements.
Below is a summary of element classifications and inclusion criteria:
| Element Type | Description | Inclusion Criteria | Examples |
|---|---|---|---|
| Light Source Element | Any portion of a light source that generates light (including envelope) | Include all applicable for the lamp function | LED die, phosphor, optical dome, bulb filament, glass envelope, exit port of light guide |
| Reflective Element | Surface designed to specularly reflect incoming light toward the test pattern | Only if contributes to primary intended trajectory; include entire element with manufacturing transitions | Metallic surfaces, vacuum metalized coatings, argent paint, white plastic, etc. |
| Refractive Element | Lens surface with optical structures to specularly refract light toward the test pattern | Only if part of primary intended trajectory; include transition features | Pillow optics, flutes, prisms, fresnel lenses |
| Diffusion Element | Surface or material with light scattering features | Only if receives light from primary intended trajectory; exclude those from split-off rays | Reflective or refractive surfaces with stipple, knurling, or similar structures |
Engineering Design Insight: The standard places emphasis on the primary intended trajectory of light rays. Reflective, refractive, or diffusion elements that only receive split-off rays from the primary path must be excluded. This prevents inflation of EPLLA and ensures that only optically active areas contributing to the test pattern are counted.
⚠️ Common Pitfalls and Compliance
Designers and engineers should be aware of several key pitfalls:
- Overlapping Elements: If the orthogonal projection of multiple elements overlaps, the overlapping area shall be counted only once. Double counting is explicitly prohibited by the standard.
- Shared Elements: An element shared with another lamp function may only be included if it changes intensity or color when the lamp function being measured is activated.
- Light Guides: The entire light guide should not be included unless it has optical properties to direct light toward the test pattern. The exit port of a light guide may be included as a light source element.
Warning: Overlapping areas between elements in the orthogonal projection cannot be counted twice. Sum the distinct areas after union.
Frequently Asked Questions
1. What elements should be excluded from EPLLA calculation?
Reflective, refractive, and diffusion elements that only receive split-off rays from the primary intended trajectory, or those that direct light away from the test pattern, must be excluded. Overlapping areas are not double counted.
2. How is a shared element handled between lamp functions?
An element shared with another function is counted only if it changes intensity or color when the measured function is activated. For example, a dual-function stop/turn lamp shared element would be included when measuring stop function if it intensifies.
3. Can I include the entire light guide in the EPLLA?
No, unless the light guide has optical properties to direct light toward the test pattern. Typically only the exit port of a light guide qualifies as a light source element. The body of the guide is excluded unless it functions as a refractive element.
4. What defines the primary intended trajectory?
It is the designed path of light rays from the source through the optical system toward the test pattern. Elements that receive only unintended split-off rays (e.g., spurious reflections) are not part of the primary trajectory and must be excluded.
By following the design analysis procedure in SAE J2999-2017, engineers can ensure accurate and consistent EPLLA values for automotive lamp functions, facilitating regulatory compliance and design validation.
