Time-of-Flight Camera Multipath Interference Correction
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Solution Overview
Problem
Conventional time-of-flight cameras fail to accurately measure scene depth at mixed pixels due to multipath interference, where a single pixel receives light from multiple depths in the scene, leading to inaccurate depth determination.
Innovation Solution
A time-of-flight camera system uses amplitude modulated light and broadband codes, such as m-sequences, to emit and receive light, allowing for accurate depth measurement by solving a linear system through deconvolution algorithms like sparse or Wiener deconvolution, even in the presence of multipath interference, using a single fundamental modulation frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional time-of-flight cameras use single frequency modulation to measure scene depth, then the device complexity is low and operation is simple, but measurement precision deteriorates at mixed pixels due to multipath interference
Solution Approach 1:
The patent changes the modulation signal from simple sinusoidal to broadband coded signals (m-sequences), transforming the correlation waveform from sinusoidal to non-sinusoidal with distinct peaks. This parameter change enables accurate depth measurement at mixed pixels by providing unique temporal signatures for different path lengths, resolving the multipath interference problem while maintaining practical device implementation
Solution Approach 2:
The patent applies deconvolution algorithms (sparse deconvolution or Wiener deconvolution) to pre-process the received signal and separate contributions from different path lengths before depth calculation. This preliminary signal processing action removes multipath interference effects, enabling accurate depth measurement at mixed pixels without requiring complex hardware modifications
2Measurement precision
If conventional time-of-flight cameras use multiple modulation frequencies to resolve multipath interference, then measurement precision improves at mixed pixels, but device complexity and measurement time increase
Solution Approach 1:
The patent uses a single fundamental modulation frequency with broadband coded modulation patterns (m-sequences) instead of multiple frequencies. The coded signals create distinct peak patterns in the correlation waveform that enable temporal separation of multipath components, achieving accurate mixed pixel measurement without increasing measurement time or requiring multiple frequency sweeps
3Measurement precision
If conventional time-of-flight cameras use multiple modulation frequencies to resolve multipath interference, then measurement precision improves at mixed pixels, but device complexity increases
Solution Approach 1:
The patent transforms the modulation approach from multi-frequency sinusoidal to single-frequency broadband coded modulation. This parameter change simplifies the hardware while enabling complex signal processing through deconvolution algorithms that efficiently separate multipath components using the unique autocorrelation properties of the coded signals
Solution Approach 2:
The patent replaces the mechanical/approach of using multiple modulation frequencies with a signal processing substitution using deconvolution algorithms. This substitution processes the single-frequency broadband signal to achieve multipath separation, reducing hardware complexity while maintaining measurement precision at mixed pixels
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
The system effectively measures scene depth at mixed pixels, enabling accurate foreground and background depth determination even when direct views are blocked by transparent objects or diffusers, and captures light sweep images with improved robustness to noise.
Implementation Method 1
a light source emits light that is amplitude modulated (e.g., by rapid strobing on and off)
Implementation Method 2
In a conventional time-of-flight (ToF) camera, scene depth is measured based on the difference between the phase of light emitted by the camera and the phase of light reflected from the scene
Data Source
AI summary
In illustrative implementations, a time-of-flight camera robustly measures scene depths, despite multipath interference. The camera emits amplitude modulated light. An FPGA sends at least two electrical signals, the first being to control modulation of radiant power of a light source and the second being a reference signal to control modulation of pixel gain in a light sensor. These signals are identical, except for time delays. These signals comprise binary codes that are m-sequences or other broadband codes. The correlation waveform is not sinusoidal. During measurements, only one fundamental modulation frequency is used. One or more computer processors solve a linear system by deconvolution, in order to recover an environmental function. Sparse deconvolution is used if the scene has only a few objects at a finite depth. Another algorithm, such as Wiener deconvolution, is used is the scene has global illumination or a scattering media.