Image Sensor Black Level Estimation for High Temperature Noise
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Solution Overview
Problem
Conventional CMOS image sensors experience significant noise and image saturation due to increasing dark current at high temperatures, limiting their dynamic range and effectiveness.
Innovation Solution
An image processing system with a pixel array, ADC unit, CDS unit, BLE unit, and BLC unit that generates a negative offset voltage to counteract dark voltage, improving signal-chain dynamic range through black level estimation and compensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional black level compensation scheme is adopted, then dark current noise is reduced, but image saturation occurs at high temperature
Solution Approach 1:
The patent applies preliminary action by performing black level estimation using dedicated estimating OBPs before the main black level compensation process. The BLE unit calculates dark voltage from estimating OBPs and generates negative offset voltages that are subtracted from pixel signals before ADC conversion. This preliminary subtraction of dark voltage expands the dynamic range, allowing subsequent BLC to work effectively without causing saturation, thus resolving the contradiction between noise reduction and preventing saturation at high temperatures.
2Object-affected harmful factors
If dark current compensation is increased, then noise is reduced, but dynamic range is limited
Solution Approach 1:
The patent segments the black level compensation process into two distinct stages: black level estimation (BLE) and black level compensation (BLC). The BLE unit uses estimating OBPs to calculate and subtract dark voltage, generating negative offset voltages that expand dynamic range. The BLC unit then performs traditional compensation using compensating OBPs. This segmentation allows the system to simultaneously achieve noise reduction through enhanced compensation while preserving dynamic range through the preliminary dark voltage subtraction, resolving the contradiction between noise reduction and dynamic range maintenance.
Data Source
AI summary
An image processing system for an image sensor includes an analog-to-digital conversion (ADC) unit that performs ADC on pixel signals, thereby generating digital pixel signals; a correlated double sampling (CDS) unit that performs CDS on the digital pixel signals; a black level estimation (BLE) unit that generates a negative offset voltage according to dark voltage obtained from CDS performed on estimating optical black pixels (OBPs) of a pixel array, the negative offset voltage being subtracted from the pixel signals before feeding the pixel signals to the ADC unit; and a black level compensation (BLC) unit that performs BLC on active pixels sensors (APSs) and the compensating OBPs of the pixel array.