Flash Synchronization for Rolling Shutter Imagers

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Solution Overview

Problem

Existing electronic rolling shutter (ERS) systems in CMOS imagers face challenges in ensuring proper exposure of objects in bright scenes, leading to underexposure due to non-synchronous flash illumination, especially when integration time is reduced to prevent overexposure.

Innovation Solution

Implementing middle curtain synchronization, where the flash is synchronized to illuminate the central portion of the frame, ensuring that the object of interest is properly exposed by both flash and ambient light, while underexposed portions can be digitally zoomed and cropped for optimal image capture.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Illumination intensity

If integration time is reduced to prevent overexposure in bright scenes, then overexposure is avoided, but underexposure occurs due to non-synchronous flash illumination

Engineering Contradiction:
Improveexposure adequacyVSAvoidexposure consistency
Core Design Contradiction:
Illumination intensityVSReliability

Solution Approach 1:

The flash is triggered in advance to illuminate the scene before the imager begins its rolling readout sequence. The flash controller receives the trigger signal and activates the flash at a calculated time based on the imager's frame rate and shutter speed, ensuring that the flash duration coincides with the integration period of the sensor.

Inventive Principle:
Principle #10Preliminary action

Solution Approach 2:

The system uses feedback from the imager's controller to the flash controller to synchronize flash timing with the rolling shutter sequence. The controller calculates the optimal flash trigger time based on the current frame rate and integration time, creating a closed-loop system that adapts to changing imaging conditions.

Inventive Principle:
Principle #23Feedback

2Ease of operation

If leading curtain synchronization is used, then flash timing is simplified, but underexposure occurs in portions of the frame

Engineering Contradiction:
Improveflash timing controlVSAvoidframe coverage
Core Design Contradiction:
Ease of operationVSIllumination intensity

Solution Approach 1:

The system dynamically adjusts the flash trigger timing based on the rolling shutter progression. Instead of using fixed leading or trailing curtain synchronization, the flash is triggered at a calculated moment during the exposure sequence that optimizes illumination coverage across the entire frame, adapting to the continuous movement of the shutter curtains.

Inventive Principle:
Principle #15Dynamics

3Ease of operation

If trailing curtain synchronization is used, then flash timing is simplified, but underexposure occurs in portions of the frame

Engineering Contradiction:
Improveflash timing controlVSAvoidframe coverage
Core Design Contradiction:
Ease of operationVSIllumination intensity

Solution Approach 1:

The system dynamically adjusts the flash trigger timing based on the rolling shutter progression. Instead of using fixed leading or trailing curtain synchronization, the flash is triggered at a calculated moment during the exposure sequence that optimizes illumination coverage across the entire frame, adapting to the continuous movement of the shutter curtains.

Inventive Principle:
Principle #15Dynamics

4Device complexity

If flash is synchronized to leading or trailing curtain, then synchronization complexity is reduced, but image quality deteriorates due to underexposed areas

Engineering Contradiction:
Improvesynchronization controlVSAvoidexposure uniformity
Core Design Contradiction:
Device complexityVSManufacturing precision

Solution Approach 1:

The flash is triggered in advance to illuminate the scene before the imager begins its rolling readout sequence. The flash controller receives the trigger signal and activates the flash at a calculated time based on the imager's frame rate and shutter speed, ensuring that the flash duration coincides with the integration period of the sensor.

Inventive Principle:
Principle #10Preliminary action

Solution Approach 2:

The system uses feedback from the imager's controller to the flash controller to synchronize flash timing with the rolling shutter sequence. The controller calculates the optimal flash trigger time based on the current frame rate and integration time, creating a closed-loop system that adapts to changing imaging conditions.

Inventive Principle:
Principle #23Feedback

Data Source

PatentUS8994877B2Method and system for synchronizing a flash to an imager
Publication Date: 2015.03.31 SEMICON COMPONENTS IND LLC
  • US8994877B2 patent drawing
  • US8994877B2 patent drawing
  • US8994877B2 patent drawing

AI summary

A digital camera system synchronizes a flash to an array of image sensing elements. The array of image sensing elements senses an image focused on the array by a lens. The system includes an array controller that implements an electronic rolling shutter to capture the image as a frame. The electronic rolling shutter includes read and reset pointers that the array controller scans through the array to capture integrated photo charges on the image sensing elements to generate the frame. The number of lines between the reset and read pointers defines the integration time of the imager. The system also includes a flash controller that synchronizes the flash to a portion of the frame. When the flash is activated, the camera system reduces the integration time and synchronizes the flash to the scanning of a selected portion of the array.