IR Illumination Design for Depth Sensing
The physics and engineering of infrared illumination systems for ToF and structured light depth cameras.
Depth sensors need light to see. Unlike passive cameras that use ambient light, active depth sensors emit their own illumination - typically in the near-infrared (NIR) spectrum around 850nm or 940nm.
Why NIR?
Invisible to users: 850nm+ is outside human visual range Efficient sources: VCSELs and LEDs are mature at these wavelengths Sensor sensitivity: Silicon sensors still respond well (though sensitivity drops above 900nm) Sunlight filtering: Narrow bandpass filters can reject most solar spectrum
Illumination Architectures
Flood Illumination (ToF)
Cover the entire field of view with uniform IR light. Simple conceptually, but challenges include:
- Uniformity: Edges of FOV receive less light (cosine falloff + lens vignetting)
- Power density: More range requires more power, but eye safety limits peak intensity
- Efficiency: Much light falls on areas we don't need (sky, distant objects)
Patterned Illumination (Structured Light)
Project a known pattern (dots, lines, speckle) and use deformation for triangulation.
- Diffractive Optical Elements (DOE): Split single beam into pattern - very power efficient
- Pattern design: Must be unique enough for unambiguous matching
- Density trade-off: More points = more resolution but harder matching
Spot Illumination (Scanning)
Single beam scanned across the scene (like LiDAR).
- Power efficient: All energy concentrated in measurement point
- Slow: Mechanical scanning limits frame rate
- Eye safety: Easier to manage with low duty cycle per point
Design Constraints
For our headset:
- Eye safety: IEC 62471 Class 1 limits power density at the eye
- Power budget: under 200mW for illumination
- Range: 0.3m to 5m indoor, 0.3m to 3m outdoor
- Ambient rejection: Must work in 10,000 lux sunlight
The math is harsh. Sunlight at 940nm contributes ~0.2 mW/cm²/nm. Our narrow filter (10nm bandwidth) still passes 2 mW/cm². We need to exceed this with our illumination at 5m range while staying within power and safety budgets.
Multi-Frequency Approaches
One promising direction: modulate illumination at frequencies where ambient light doesn't contribute. Time-domain or frequency-domain filtering can reject DC (sunlight) while preserving our modulated signal.
Next month: diving deeper into the optics design.