TOF Depth Sensing with SPAD Histogram Linearity Correction
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Solution Overview
Problem
Time-of-flight (TOF) imaging systems using single-photon avalanche diodes (SPADs) suffer from differential nonlinearity (DNL) due to variations in gating interval widths and start times, leading to distorted peak locations and errors in depth measurements, particularly in high ambient light conditions.
Innovation Solution
Implementing dithering of temporal offsets between optical pulses and gating intervals, and using continuous-wave (CW) radiation for calibration to measure and correct deviations in bin widths and timing offsets, thereby compensating for DNL in histogram bins.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If dithering of temporal offsets is implemented, then measurement precision is improved, but device complexity increases
Solution Approach 1:
The system performs preliminary calibration using continuous-wave radiation to measure and store correction values for bin widths and timing offsets before actual time-of-flight measurements. This preliminary action eliminates the need for complex real-time corrections during measurement, thereby improving depth measurement accuracy without proportionally increasing device complexity during operation.
Solution Approach 2:
The system implements feedback through the calibration process where measured deviations in bin widths and timing offsets are used to generate correction values that are applied to subsequent measurements. This feedback mechanism systematically compensates for differential nonlinearity, improving measurement precision while keeping the correction logic manageable through structured calibration sequences.
2Manufacturing precision
If calibration with continuous-wave radiation is performed, then manufacturing precision is improved, but loss of time increases
Solution Approach 1:
The calibration process using continuous-wave radiation is performed as a preliminary action during system initialization or setup phases. By completing the time-consuming calibration measurements before actual depth mapping operations, the system achieves improved histogram bin linearity without significantly impacting the time available for primary measurement functions.
Solution Approach 2:
The calibration can be performed periodically or on-demand rather than continuously, allowing the system to balance between maintaining manufacturing precision and minimizing time loss. The calibration values are stored and reused for multiple measurement cycles, reducing the frequent need for time-consuming recalibration.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Averaging out DNL variations and correcting histogram distortions, resulting in accurate time-of-flight measurements and improved depth mapping accuracy.
Implementation Method 1
Single-photon avalanche diodes (SPADs), also known as Geiger-mode avalanche photodiodes (GAPDs), are detectors capable of capturing individual photons with very high time-of-arrival resolution
Implementation Method 2
The depth value at each pixel in the depth map is derived from the difference between the emission time of the outgoing pulse and the arrival time of the reflected radiation from the corresponding point in the scene, which is referred to as the 'time of flight' of the optical pulses
Data Source
AI summary
Optical sensing apparatus includes a radiation source, which directs a series of optical pulses toward a target scene. A first array of single-photon detectors receives optical radiation that is reflected from the target scene and outputs electrical pulses in response to incident photons. A second array of counters aggregates and counts the electrical pulses output by the single-photon detectors over respective periods indicated by respective gating signals applied to the counters. Control logic applies the respective gating signals to the counters, in each of a sequence of image frames, so as to cause different ones of the counters to aggregate and count the electrical pulses output by one or more of the single-photon detectors over different, respective gating intervals relative to each of the optical pulses, and to sum the frame histograms generated with different temporal offsets so as to compute and output a cumulative histogram.