Black Level Correction Using Dynamic Dark Current Ratio Adjustment
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Solution Overview
Problem
Conventional black level correction methods for image sensors are inadequate when the dark current generation rates differ between dark calibration pixels and imaging pixels, leading to inaccurate corrections due to non-uniformity in dark current and mechanical stresses.
Innovation Solution
Calculating and adjusting the exposure time of dark calibration pixels based on the dark current ratio with imaging pixels to ensure accurate representation of imaging pixel dark currents, allowing for precise subtraction of dark current and circuitry offsets.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional BLC methods subtract dark calibration pixel readout from imaging pixel readout, then offset error from readout circuitry is corrected, but inaccurate correction occurs when dark current generation rates differ between dark calibration pixels and imaging pixels
Solution Approach 1:
The patent applies local quality by adjusting the exposure time of dark calibration pixels differently from imaging pixels. Specifically, when dark calibration pixels exhibit higher dark current generation rates, their exposure time is reduced to compensate for the non-uniformity. This localized adjustment to exposure time parameters allows each pixel type to be optimized according to its specific dark current characteristics, resolving the contradiction between maintaining simple subtraction methodology and achieving accurate correction under varying dark current conditions.
2Measurement precision
If dark calibration pixels are physically shielded from light, then they can measure dark current without optical interference, but mechanical stresses from the shielding layer cause non-uniform dark current generation compared to imaging pixels
Solution Approach 1:
The patent employs parameter changes by dynamically adjusting the exposure time of dark calibration pixels based on measured dark current ratios. When the shielding layer causes elevated dark current in dark calibration pixels, the system reduces their exposure time proportionally to match the effective dark current contribution to imaging pixels. This parameter adjustment compensates for the mechanical stress effects introduced by the shielding layer, maintaining measurement precision while accounting for the non-uniform dark current generation.
3Measurement precision
If exposure time for dark calibration pixels is adjusted based on dark current ratio, then accurate representation of imaging pixel dark currents is achieved, but additional calculation and control complexity is introduced
Solution Approach 1:
The patent implements feedback by measuring the dark current ratio between dark calibration pixels and imaging pixels, then using this measured ratio to adjust the exposure time of dark calibration pixels in subsequent measurements. This closed-loop feedback mechanism automatically compensates for non-uniform dark current generation without requiring complex manual calibration or intervention. The system continuously monitors dark current characteristics and adapts exposure time parameters accordingly, achieving high measurement precision while keeping the control system relatively simple through automated feedback-based adjustment.
Data Source
AI summary
A technique for obtaining a corrected pixel value is disclosed. The technique includes measuring a first dark current of a dark calibration pixel of a pixel array and measuring a second dark current of an imaging pixel of the pixel array. A dark current ratio is calculated based on the first dark current and the second dark current. An image charge is acquired with the imaging pixel where the image charge is accumulated over a first time period. A charge is acquired with the dark calibration pixel where the charge is accumulated over a second time period. The second time period is approximately equal to the first time period divided by the dark current ratio. A corrected imaging pixel value is calculated using a first readout from the imaging pixel and a second readout from the dark calibration pixel.