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Establishing Consistent HUD Optical Performance Measurement: SAE J1757-2-2018
Automotive Head Up Displays (HUDs) have become critical for driver information and safety. To ensure consistent quality across different vehicle models and suppliers, SAE J1757-2-2018 provides a uniform terminology and metrology for evaluating HUD optical performance. This standard focuses on indoor measurements with simulated outdoor lighting, covering both windshield HUD (w-HUD) and combiner HUD (c-HUD), and references Augmented Reality (AR) HUD applications. While it does not mandate pass/fail thresholds, it offers recommended values and a repeatable framework for design validation and compliance testing.
Understanding the Scope of SAE J1757-2-2018
The standard defines measurement methods for HUD virtual images in typical automotive ambient conditions. Key elements include:
- Simulated ambient lighting – representing daylight, overcast, and night scenarios.
- Standard test images – such as even squares (n=m=5) or dots (n=m=1) for luminance and resolution checks.
- Reference to SAE J941, SAE J1757-1, and ISO 15008 for driver eye locations and display legibility.
By adhering to these guidelines, engineers ensure that HUD performance evaluation is both repeatable and representative of real-world driving conditions.
🛠️ Design Insight: Always reference driver eye location distributions (Eyellipse) to define Eyebox boundaries. The standard measurement points CE5, UE2, LE8, LFT4, R6 are derived from the centroid and extremes of the Eyellipse, enabling reproducible optical testing across different vehicles and HUD designs.
Key Definitions and Measurement Points
The following table summarizes the essential terms and their practical significance as defined in SAE J1757-2-2018:
| Term | Definition | Measurement Importance |
|---|---|---|
| Eyellipse | Statistical distribution of driver’s eye locations relative to the SAE SgRP. | Basis for all observer-related measurements; defines the range of possible eye positions. |
| Eyebox | Vertical plane through the centroid of the Eyellipse, bounded by Y and Z limits. | Defines the volume where the HUD virtual image must be visible; used for positioning instrumentation. |
| Paravan | Vertical screen with orthogonal graduations for evaluating virtual image qualities. | Enables measurement of luminance, size, color, distance, and field of view. |
| Measuring Points (CE5, UE2, LE8, LFT4, R6) |
Standardized Eyebox locations on the observer/instrument surface. | Ensure repeatable sensor placement for optical testing; CE5 is the centroid. |
| Field of View (FOV) | Angular extent of the virtual image active area measured from the CE5 point. | FOV determines how much information is visible; measured in degrees (horizontal and vertical). |
| Virtual Image Distance (dOV) | Distance from observer (center of Eyebox) to the virtual image on the paravan. | Incorporates path through windshield; affects image scale and accommodation. |
⚠️ Common Mistake: Incorrect calculation of dOV by ignoring the windshield position (dOW and dWV). The standard explicitly states that virtual image distance must account for the observer-to-windshield and windshield-to-virtual image segments.
Engineering Design Insights and Compliance
Implementing SAE J1757-2-2018 early in the HUD development cycle helps avoid costly rework. Key takeaways include:
- Repeatability – Use the defined Eyebox measuring points for all luminance, chromaticity, and contrast measurements.
- Ambient simulation – Test under dark, diffuse daylight, and direct sunlight conditions to ensure legibility across all scenarios.
- Contrast and contrast ratio – Understand the relationship: Contrast = (L_info – L_background) / L_background, and Contrast Ratio = L_info / L_background. Both are critical for display legibility and should be measured with the standard dark ambient (ambient light off) for baseline performance.
- Resolution – HUD resolution depends on the original optical engine and magnification; measure in pixels per degree to verify image clarity.
Frequently Asked Questions (FAQs)
How is the HUD field of view (FOV) measured per SAE J1757-2-2018?
The FOV is defined as the angular size of the virtual image active area, measured from the centroid CE5 point. Horizontal and vertical angles are determined using points V2, V8, V4, and V6 on the target image (standard image 3b). The solid cone originates at the Eyellipse centroid.
What are the standard measurement points and why are they important?
The five official points—CE5 (Center), UE2 (Upper), LE8 (Lower), LFT4 (Left), and R6 (Right)—represent extremes and center of the Eyebox. They ensure that optical instrument placement is consistent and that HUD performance is evaluated across the full range of expected driver eye positions.
How does this standard relate to other HUD regulations?
SAE J1757-2-2018 works in conjunction with SAE J941 (driver eye locations), SAE J1757-1 (general display metrology), and ISO 15008 (display legibility). It provides the missing link for HUD-specific optical measurement under automotive ambient conditions.
🔍 For a deeper dive into measurement procedures, refer to the full SAE J1757-2-2018 document.
