Scope and starting point
Electrical input becomes optical radiation and heat. A junction-temperature estimate needs the heat portion and all relevant thermal paths. Use the manufacturer’s definition of thermal resistance: some LED thermal data use electrical power conventions rather than a corrected optical heat balance. Do not mix conventions.
Selection notes
Use heat dissipation, not optical output, in the thermal budget. Include junction-to-case, interface and sink-to-ambient paths. Heat-sink resistance depends on orientation, airflow and test conditions; do not transfer one catalog number to an enclosed assembly without validation.
Keep a reproducible record
Save the complete part number, source revision, operating point and the reason for each decision. Keep calculated examples separate from observed results. A measurement report needs its sample identifier, instrument and calibration details, stabilization, ambient conditions, raw record and uncertainty statement. Do not replace a missing value with zero or a typical value with a guaranteed limit.