System Limitations and Boundary Conditions#

Understanding the constraints and limitations of the HSB Native Latency Measurement Tool is crucial for proper system design and accurate measurement interpretation.

Camera Constraints#

Not all camera sensors are interchangeable in this setup. The tool depends on HSB-level integration for hardware-synchronized frame events, and the shutter type determines how the LED trigger must be timed.

Sensor Compatibility#

  • Limited to HSB Supported Sensors: The tool works exclusively with cameras supported by the Holoscan Sensor Bridge

  • Tested Configurations: VB1940 (global shutter) and IMX274 (rolling shutter) are fully validated

  • Custom Sensors: Additional sensors may require specific driver development and validation

Shutter Type Dependencies#

  • Software Setup Variations: Rolling and global shutter cameras require different sequencer configurations

  • Timing Strategy Differences: LED trigger delay must be adjusted based on shutter type and sensor readout time

  • Frame Synchronization: Current frame (rolling) vs. next frame (global) LED capture strategies

Optical System Constraints#

The latency measurement relies on a physical light signal: an LED is illuminated at a precise moment and a photodetector mounted on the display detects when that light appears. The quality of this optical path — LED brightness, photodetector sensitivity, and ambient lighting — directly affects measurement accuracy.

LED Brightness Requirements#

  • Minimum Detectable Light Levels: LED must provide sufficient brightness to overcome ambient lighting

    • Recommended: High-brightness white LED (>10,000 mcd luminous intensity)

    • Tested Configuration: White LED - 20mA, 3.0-3.2V, 12,000-14,000 mcd

    • Ambient Light Handling: LED should be 3-5x brighter than ambient display brightness

  • Dynamic Range Considerations: Camera exposure settings affect LED visibility in captured frames

    • Recommendation: Use auto-exposure or fixed exposure settings that don’t clip LED brightness

Photodetector Sensitivity#

  • Environmental Light Interference: Ambient lighting can affect photodetector response accuracy

    • Mitigation Strategy: Use optical shielding or enclosure around photodetector

    • Tested Environment: Controlled indoor lighting (300-500 lux ambient)

    • Recommendation: Minimize direct overhead lighting on photodetector surface

  • Spectral Response: Photodetector sensitivity must match LED emission spectrum

    • Tested Configuration: Silicon photodiode with broad spectrum response (400-1000nm)

    • LED Compatibility: White LEDs (broad spectrum) work well with silicon photodetectors

  • Signal-to-Noise Ratio: Sufficient LED signal strength required above background noise floor

    • Minimum SNR: 10:1 ratio between LED signal and ambient noise

    • Verification: Monitor photodetector output with oscilloscope during setup

Physical Constraints#

The physical placement of the LED trigger area and the photodetector on the display affects whether the measurement signal is captured correctly. This section describes where components must be positioned and how they should be mounted.

Optical Alignment Requirements#

  • LED Position for Rolling Shutter: LED must be positioned in the lower right area of the display for rolling shutter cameras

  • Scan Line Timing: Rolling shutter row-by-row readout requires LED placement in later-scanned regions

  • Global Shutter Flexibility: LED position less critical for global shutter cameras due to simultaneous pixel capture

Distance Limitations#

  • Photodetector Positioning: Photodetector can be mounted directly on the display for optimal signal detection

    • Recommended Setup: Mount photodetector directly on display surface using tape

    • Tested Configuration: Photodetector taped directly to display screen - no separation distance

    • Alternative Setup: Small separation (1-5cm) acceptable if direct mounting not feasible

    • Advantage: Direct mounting eliminates ambient light interference and maximizes signal strength

  • Mounting Stability: Secure attachment prevents measurement inconsistencies

    • Recommendation: Use strong tape to ensure photodetector stays in position

    • Tested Setup: Photodetector taped directly to display in the LED trigger area

Performance and Latency Constraints#

The hardware components in the measurement chain each introduce a small but fixed overhead. Understanding these limits helps set realistic expectations for measurement resolution and minimum achievable timing intervals.

Hardware Timing Limitations#

Event Detection at GPIO:

  • Detection Latency: ~3 clock cycles (6.4ns × 3) = 19.2ns before event latches to PTP timestamp

  • Hardware Precision: Sub-microsecond event detection accuracy

Photodetector Response Time:

  • Typical Response: ~100us from light detection to electrical signal

Minimum Timing Intervals:

  • FSR to LED_Active: Minimum 1.2us between frame start reception and LED activation

  • Sequencer Processing: Hardware sequencer execution time for GPIO control

  • Event Acknowledgment: Minimum time between measurement cycles

System Performance Boundaries#

Measurement Frequency Limits:

  • Maximum Frame Rate: Limited by camera sensor and processing pipeline capabilities

  • Continuous Operation: Event acknowledgment required for repeated measurements