Time-of-Flight Image Sensor Background Interference Separation
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Solution Overview
Problem
Existing imaging devices face challenges in simultaneously capturing depth information and color images due to background interference from intrinsic brightness, which is often hundreds or thousands of times more intense than the reflected light pulse, especially when operating in visible light spectra.
Innovation Solution
The implementation of time-of-flight image sensors with a dual-band filter that passes both visible and infrared light, combined with a specific circuit configuration for time-of-flight pixels, allows for the separation of reflected light pulses from background intensity, enabling the capture of depth and color information simultaneously.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If the imaging device operates in visible light spectrum to capture color images, then color information is obtained, but background interference from intrinsic brightness becomes hundreds or thousands of times more intense than reflected light pulse
Solution Approach 1:
The imaging device segments the detection process into multiple time windows: a first time window for detecting reflected light pulses and a second time window for detecting background light. This temporal segmentation allows the device to separately measure and subsequently subtract background interference from the reflected light signal, resolving the contradiction between capturing color information and dealing with background interference.
Solution Approach 2:
The imaging device employs periodic emission of light pulses and corresponds periodic detection time windows. By emitting light pulses at regular intervals and opening detection windows synchronously, the device systematically separates reflected light detection from background light detection across multiple periods, enabling effective background subtraction while maintaining color information capture.
2Measurement precision
If the imaging device uses time-of-flight measurement to capture depth information, then depth data is obtained, but the reflected light pulse intensity is much weaker than background intensity making separation difficult
Solution Approach 1:
The imaging device performs preliminary detection of background light in a second time window before processing the reflected light signal. By measuring background intensity in advance during a dedicated time window when no reflected light is expected, the device can pre-calculate subtraction values and apply them to enhance the reflected light signal, making the weak reflected light pulse separable from the strong background.
Solution Approach 2:
The imaging device introduces an intermediary processing step that calculates the difference between light detected in the first time window (reflected light + background) and light detected in the second time window (background only). This intermediary subtraction operation acts as a mediator that isolates the reflected light signal from the dominant background, enabling precise depth measurement.
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
This solution effectively separates background light from reflected light pulses, allowing for accurate depth measurement and color image capture, overcoming the limitations of traditional imaging devices that struggle with background interference.
Implementation Method 1
Distance information is then determined based on the time-of-flight of the light between the emission and detection of the light and the known speed of light
Implementation Method 2
Image pixels and time-of-flight image pixels in the image sensors may include photosensitive elements such as photodiodes that convert the incoming light into electric charges
Data Source
AI summary
Electronic devices may include time-of-flight (ToF) image pixels. Each ToF pixel may include a photodiode, a first capacitor coupled to the photodiode via a first transfer gate, a second capacitor coupled to the photodiode via a second transfer gate, and a third capacitor coupled to the photodiode via a third transfer gate. The first transfer gate may be turned on for a given duration to store a first charge in the first capacitor. The second transfer gate may be turned on for the given duration to store a second charge in the second capacitor. The third transfer gate may be turned on for a duration that is longer than the given duration to store a third charge in the third capacitor. Depth information may be computed based on the first, second, and third stored charges and a corresponding pixel constant.