## 2 Fundamentals and Terminology
### Automotive Lighting System Development Stages  
Gantt charts are a popular tool for visualizing temporal discrete event sequences with dependencies, commonly used in manufacturing and computing [26]. Figure 2 illustrates a structured integration of DT technology into the development lifecycle of automotive headlamp systems. Each gate represents a critical validation point, ensuring continuous alignment between the DT framework and system engineering processes. The key gate activities are outlined as follows:  
Figure 1: V-Cycle development process for digital twin in automotives. G10--Initial DT Requirements: The system requirements--both functional and non-functional--are defined and analyzed from the perspective of the DT, ensuring that the DT-related needs are identified and aligned with the overall system objectives;
* G20--System DT Architecture: General system aspects are defined through the DT view, establishing a high-level structure that guides DT integration within the overall system architecture;
* G30--Concept DT Validation: This gate evaluates the feasibility of the conceptual DT representation. Verification activities include preliminary simulations and use-case alignment to confirm that the DT can represent essential system behaviors accurately at an abstract level;
* G40--SiL Integration with DT: The DT logic is integrated into the simulation environment to enable virtual testing and validation of system behavior during early development stages;
* G50--HiL Integration with DT: The DT components are connected to physical subsystems, enabling real-time validation of the system interactions;
* G60--Requirements Traceability via DT: This stage focuses on the behavior of test cases against the requirements, which are integrated with the DT concept to ensure complete traceability;
* G70--DT Calibration: This process aligns the DT with real-world data through calibration;
* G80--Scenario-Based Testing: This gate allows the DT to be tested across a wide range of real-world and edge-case scenarios, helping to assess system robustness, safety performance, and behavior under uncommon or critical conditions;
* G90--Analytics for DT-Based Diagnostics: Diagnostic analytics are integrated with the DT to enable fault detection, root cause analysis, and prognostics;
* G100--Standards Compliance Validation (via DT Simulations): DT-based simulations are used to verify compliance with industry standards and certification requirements;
* G110--Continuous Integration and Testing (CI/CT) with DT: The DT is incorporated into automated CI/CT workflows, supporting regression testing and continuous verification;
* G120--Final Deployment Readiness and Field Monitoring with DT: The final gate assesses deployment readiness, with the DT prepared for post-deployment monitoring. Figure 2: Development gates for digital twin in automotives. (1) Initial DT requirements; (2) system DT architecture; (3) concept DT validation; (4) SiL–DT; (5) HiL–DT; (6) REQ traceability–DT; (7) DT calibration; (8) scenario-based testing; (9) analytics for DT-based diagnostics; (10) standards compliance validation (via DT simulations); (11) continuous integration/testing (CI/CT) with DT; (12) final deployment readiness + DT for field monitoring. To further demonstrate the systematic role of the DT in the development and validation of automotive lighting systems, we analyzed the implementation of functional and non-functional technical requirements across all 12 validation gates (G10-G120), as shown in Table 1. This approach highlights not only the relevance of the DT at each phase but also the coherence and traceability it enables throughout the entire lifecycle from early design to field operation. The analysis shows that a well-defined DT can be aligned with each development milestone, acting as a central validation mechanism. In the early phases (G10-G30), the DT helps capture functional requirements and simulate key lighting behaviors in conceptual