Image Sensor Voltage Sampling Circuit Eclipse Phenomenon
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Solution Overview
Problem
Image sensors face the 'eclipse phenomenon' where strong lighting causes the electric signal to decrease to zero, leading to very lit areas being interpreted as black, especially in high-intensity conditions like incandescent lamp filaments, due to the discharge of junction capacitance during acquisition phases.
Innovation Solution
A method involving double sampling of voltage levels, where a first sample is taken during photodiode reset, and a second sample is taken after a settling time with a slope check, replacing the first if below a threshold, and correcting the first sample with a coupling voltage, implemented in an image sensor circuit with transistors and capacitors to maintain accurate lighting measurements.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the photodiode junction capacitance is discharged by photocurrent during the acquisition period to measure lighting level, then the measurement capability is improved, but under strong light intensities the electric signal decreases to zero causing the eclipse phenomenon where strongly lit areas are interpreted as black
Solution Approach 1:
The patent applies preliminary action by performing a first sampling of the voltage across the photodiode during the reset phase, before the acquisition period begins. This preliminary sample captures the initial voltage state, and if the discharge slope during acquisition is insufficient (indicating strong lighting), this pre-acquisition sample is used instead of post-acquisition samples, thereby preventing the eclipse phenomenon where strong light would cause signal collapse to zero
2Measurement precision
If a first sample is taken during reset and a second sample is taken after acquisition, then the lighting measurement capability is improved, but under strong lighting conditions the first sample becomes unreliable due to the eclipse phenomenon
Solution Approach 1:
The patent implements feedback by monitoring the discharge slope of the photodiode voltage during the acquisition period. When the slope is found to be insufficient (indicating strong lighting that would cause eclipse), the system feedbacks this information and switches to using the first sample taken during reset, rather than relying on post-acquisition samples that would be corrupted by the eclipse phenomenon
Solution Approach 2:
The patent applies dynamics by making the sampling strategy adaptive rather than fixed. The system dynamically selects which sample to use (first sample during reset or second sample after acquisition) based on the measured discharge slope, allowing the measurement process to adjust its behavior according to lighting conditions to maintain reliability across varying illumination levels
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 effectively avoids the eclipse phenomenon by ensuring accurate voltage measurements across varying light intensities, preventing false black readings and maintaining image quality in high dynamic range conditions.
Implementation Method 1
An image sensor pixel comprises a photodiode used in reverse and having its junction capacitance discharged by a photocurrent according to a received light intensity
Data Source
AI summary
An image sensor and a method for acquiring images by sampling of the level of a voltage representative of the charge of a photodiode, in which a first sample is taken during a reset of the charge level of the photodiode; and a second sample is taken provided that the decrease slope of the voltage after the reset is lower than a threshold, said second sample replacing the first one.