In-cabin Driver Monitoring Systems (DMS) and Occupant Monitoring Systems (OMS) are becoming regulatory requirements under Euro NCAP and GSR mandates. While industry focus centers on algorithmic advances, real-world performance is dominated by environmental factors—particularly lighting conditions and cabin geometry. These systems rely on near-infrared illumination paired with specialized cameras. However, the vehicle cabin presents extreme optical challenges. Key failure modes include under-exposure (creating noise that degrades pupil and eyelid detection), over-exposure from sunlight (saturating pixels and eliminating contrast), and non-uniform illumination from shadows cast by hats, visors, or A-pillars. Success depends on maintaining optimal NIR energy levels at the driver’s face—balanced between eye safety regulations and image quality needs. Cabin materials further complicate this: glossy surfaces create bright artifacts, matte materials absorb light reducing available illumination, and windshield geometry redirects NIR energy unpredictably. Seat recline, child seats, and passenger orientation alter reflective patterns. Even minor design choices like dashboard trim finish or visor angles significantly impact monitoring system reliability and tracking consistency.