CMOS Image Sensor Dynamic Range via Capacitance Switching
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Solution Overview
Problem
Current CMOS image sensors face challenges in capturing images with a wide dynamic range without saturation, as existing solutions either require complex processing, reduce overall resolution, or introduce noise due to technological dispersions and fixed read noise in column mode.
Innovation Solution
A method involving a CMOS image sensor with a largely-pinned photo-diode and additional capacitors, where the sensor is read out in both low and high full-well modes to store and retrieve charge carriers, allowing for increased dynamic range while maintaining a low noise floor by switching between modes and using capacitors to store charges.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If successive images with different integration times are captured to achieve wide dynamic range, then the sensor can detect signals in both low and high lighting conditions, but the overall acquisition time increases and complex processing is required
Solution Approach 1:
The pixel circuit dynamically switches between two capacitance values (C1 and C2) to achieve different integration modes. By controlling the switch connected to the second capacitor, the pixel can adapt its charge storage capacity in real-time, enabling wide dynamic range capture within a single integration period without requiring multiple successive images.
Solution Approach 2:
The pixel's charge storage capability is segmented into two distinct modes using two separate capacitors. The first capacitor (C1) provides baseline storage for normal dynamic range, while the second capacitor (C2) provides additional storage capacity for high dynamic range requirements. This segmentation allows the pixel to select the appropriate storage mode based on lighting conditions.
2Adaptability or versatility
If a mixed matrix with small and large pixels is used to achieve wide dynamic range, then the sensor can handle both low and high light conditions, but the overall resolution of the matrix is reduced
Solution Approach 1:
All pixels in the matrix are designed with the same physical dimensions and can perform multiple functions by switching between the two capacitance modes. Each pixel can operate in low-capacitance mode for high-resolution imaging or high-capacitance mode for wide dynamic range capture, eliminating the need for a mixed matrix of different pixel sizes and maintaining uniform resolution across the entire sensor.
3Adaptability or versatility
If pixels with logarithmic or linear-logarithmic function are used to achieve wide dynamic range, then the sensor can capture both low and high light conditions, but the solutions are sensitive to technological dispersions such as threshold voltage variations
Solution Approach 1:
The patent changes the capacitance parameter of the pixel by switching between two discrete capacitance values (C1 and C2) rather than relying on complex logarithmic transfer functions. This approach uses linear charge storage with different capacities, which is less sensitive to transistor threshold voltage variations and other technological dispersions, thereby improving reliability while maintaining wide dynamic range capability.
4Adaptability or versatility
If three sampling in-pixel capacitors are used as proposed in WO99/34592 to achieve wide dynamic range, then the sensor can capture both low and high light conditions, but the capacitors occupy very large surface area and fixed read noise is amplified
Solution Approach 1:
Instead of using three capacitors as in the prior art, this patent uses two capacitors (C1 and C2) where C2 is specifically sized to provide the necessary additional dynamic range capacity. The second capacitor is dimensioned to be just sufficient for high dynamic range requirements rather than oversized, optimizing the balance between dynamic range capability and surface area consumption.
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 approach significantly increases the dynamic range of the image sensor while maintaining a low noise floor, allowing for effective capture of both low and high light conditions without saturation, and can be further enhanced by adding additional operational modes.
Implementation Method 1
an array of pixels for sensing incident light, each pixel comprising a largely-pinned photo-diode
Implementation Method 2
a first capacitance arranged parallel to the largely-pinned photo-diode, a second capacitance that can be switched in parallel with the first capacitance
Data Source
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AI summary
The invention relates to a method of reading out a CMOS image sensor. The respective pixels havinga first mode and a second mode. The method comprises: i) setting a respective pixel (Pxl) in the first mode (SS);ii) resetting the respective pixel (Pxl) such that the predefined voltage (V_ref) is set over the photo-diode (Dde) and the first capacitance (C_low); iii) collecting charge carriers generated by the incident light on the respective photo-diode (Dde), wherein the collected charge carriers reduce the pixel potential (Vp) on the respective photo-diode (Dde)iv) reading out the respective pixel (Pxl) while set in the first mode (SS) and storing the pixel potential (Vp);v) reading out the respective pixel (Pxl) while set in the second mode (LS) and storing the pixel potential (Vp); vi) resetting the respective pixel (Pxl) such that the predefined voltage (V_ref) is set over the photo-diode (Dde), the first capacitance (C_low), and the second capacitance (C_high);vii) reading out the respective pixel (Pxl) while set in the second mode (LS) and storing the pixel potential (Vp), and viii) reading out the respective pixel (Pxl) while set in the first mode (SS) and storing the pixel potential (Vp).The method results in an increased dynamic range of the CMOS image sensor. The invention also relates to the CMOS image sensor as such.