Definition (40–60 words):
Stray light refers to unwanted light reaching a detector through reflection, scattering, or leakage paths outside the intended optical design. In optical modules, stray light reduces contrast, introduces noise, and creates artifacts. Effective blackening focuses on controlling specular and diffuse reflectance at specific internal locations—not on making every surface uniformly black.
Not all internal surfaces contribute equally to stray light. The most critical areas are those that intersect high-angle rays, secondary reflections, or partially collimated beams.
Lens barrel inner walls near aperture edges
Sensor cavity sidewalls adjacent to active pixels
Baffle edges and aperture stops
Mechanical joints and housing seams
Interfaces between matte plastics and metal frames
In ray-trace diagrams, these hot spots correspond to regions where off-axis rays undergo one or more specular reflections before reaching the detector.
| Application | Primary Stray Light Risk | Critical Blackening Zones |
|---|---|---|
| Camera Modules | Flare, ghosting, contrast loss | Lens barrels, sensor cavity walls |
| Optical Sensors | Signal drift, background noise | Housing interiors, aperture stops |
| LiDAR | False returns, reduced ranging accuracy | Emitter shrouds, detector surroundings |
| AR / VR Optics | Veiling glare, reduced immersion | Waveguide edges, frame interiors |
| Microscopes | Background haze, contrast washout | Tube interiors, baffles, stage openings |
Applying the same black coating everywhere often fails because:
High-gloss regions dominate stray light even if visually dark
Thermal or mechanical stress causes local cracking
Critical specular paths remain untreated
Effective stray-light control prioritizes low specular reflectance at hot spots, not maximum absorbance everywhere.
Failure Modes: Why Optical Blackening Peels, Cracks, or Turns Gray
Reflectance vs Absorbance vs Gloss in Optical Black Design
No. It is a system problem involving geometry, surface finish, and material stability.
Not necessarily. Many matte plastics still exhibit high specular reflectance at grazing angles.
Yes. Near-IR reflectance and thermal durability are far more critical in LiDAR systems.
Bidirectional reflectance (BRDF) for angular behavior
Specular reflectance at grazing incidence
Module-level stray light testing under off-axis illumination
Optical System Design, Robert E. Fischer
Stray Light Analysis and Control, SPIE Press
ISO 9358 – Optical radiation measurements