Flash Exposure Control Using Preflash Statistics
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Solution Overview
Problem
In low-light conditions, electronic devices struggle to capture properly exposed images due to the unpredictable effect of strobe flash illumination, as the image capture circuitry cannot observe the flash's impact until it occurs, leading to potential overexposure or underexposure.
Innovation Solution
The device gathers image capture statistics during a 'strobe off' period and a 'preflash' period, normalizes these values, and uses them to determine 'main flash' image capture control statistics, adjusting exposure settings to ensure proper exposure when the strobe emits its main flash.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Illumination intensity
If a strobe flash is used to supplement ambient light in low-light conditions, then image illumination is improved, but exposure control becomes unpredictable leading to overexposure or underexposure
Solution Approach 1:
The system performs a preflash before the main image capture to preliminarily assess the scene's reflectivity and lighting characteristics. Based on this preliminary action, the system calculates appropriate exposure parameters (shutter speed, aperture, ISO) for the main flash, ensuring reliable exposure control while maintaining improved illumination.
Solution Approach 2:
The system uses the preflash image data as feedback to adjust and determine the optimal exposure settings for the main flash. The preflash statistics (luma values, exposure histograms) provide feedback about scene characteristics, enabling the system to calculate and apply appropriate exposure compensation, thereby resolving the unpredictability in exposure control.
2Productivity
If exposure settings are determined without preflash statistics, then capture speed is improved, but image exposure accuracy deteriorates
Solution Approach 1:
The system performs a quick preflash capture before the main exposure to preliminarily measure scene lighting conditions. This preliminary action provides accurate exposure statistics without significantly delaying the overall capture process, as the preflash and main flash occur in rapid succession within the same capture sequence.
Solution Approach 2:
The preflash and main flash are integrated into a continuous capture sequence rather than separate operations. The preflash serves dual purposes: it provides preliminary exposure data and also contributes to the final image illumination. This continuity minimizes time loss while maintaining exposure accuracy.
Data Source
AI summary
Systems, methods, and devices for obtaining a properly exposed strobe-illuminated image are provided. One method for doing so may include, for example, gathering image capture statistics during a first period when a strobe is not emitting light and during a second period when the strobe emits a preflash. These image capture statistics may include distinct image capture control statistics and luma values associated with the periods. Final image capture control statistics then may be determined based at least in part on the first luma value normalized to the first image capture control statistics and the second luma value normalized to the second image capture control statistics. Thereafter, the final image capture control statistics may be used to capture a properly exposed strobe-illuminated image when the strobe emits a main flash.