Imaging Apparatus Exposure Control Circuit for S/N Ratio and Frame Rate
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Solution Overview
Problem
Conventional exposure control in imaging apparatuses often deteriorate the signal-to-noise ratio (S/N ratio) when dealing with varying subject brightness, particularly in low-light conditions, and can reduce frame rates when trying to expand exposure time, which is problematic for capturing moving images.
Innovation Solution
The proposed solution involves an imaging apparatus with a feedback-controlled exposure unit that adjusts exposure time and gain settings through multiple control modes, including standard and enlarged frame periods, and front-stage and rear-stage gain control circuits to maintain a stable frame rate and S/N ratio, utilizing hysteresis for stable state transitions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the exposure time is expanded to increase signal level in dark conditions, then the S/N ratio is improved, but the frame rate is reduced due to the upper limit determined by the vertical scanning period
Solution Approach 1:
The patent segments the gain control into two distinct stages: exposure gain (affecting signal level during exposure) and processing gain (affecting signal level after exposure). This segmentation allows the system to first maximize exposure time within the frame period constraint, then apply processing gain only when necessary to achieve the target signal level, thereby preserving frame rate while improving S/N ratio
Solution Approach 2:
The patent implements dynamic control of both exposure time and gain settings based on real-time signal level feedback. The exposure control circuit dynamically adjusts the exposure time within the frame period constraint, and the processing gain is dynamically adjusted based on the integrated signal level, enabling adaptive optimization of S/N ratio while maintaining frame rate
2Illumination intensity
If the vertical scanning period is enlarged to allow longer exposure time, then the signal level can be increased, but the frame rate is further reduced
Solution Approach 1:
The patent applies partial action by using processing gain to achieve the remaining signal level enhancement needed after maximum exposure time within the standard frame period has been utilized. This avoids the excessive action of enlarging the vertical scanning period, thereby maintaining frame rate while still achieving adequate signal level through the combination of maximum exposure and controlled processing gain
3Illumination intensity
If the gains of AGC circuit and DGC circuit are increased to raise signal level, then the signal level is improved, but the noise component is also amplified deteriorating the S/N ratio
Solution Approach 1:
The patent applies preliminary action by first maximizing the exposure time within the frame period constraint before applying any processing gain. This ensures that the signal level is maximized through the most S/N-friendly method (exposure time extension) before resorting to gain amplification, thereby minimizing noise amplification while achieving the target signal level
Solution Approach 2:
The patent changes the control parameter sequence: first optimizing exposure time (a parameter that does not amplify noise), then applying processing gain (a parameter that can amplify noise) only when necessary. This parameter change strategy ensures that gain is applied minimally, preserving S/N ratio while achieving adequate signal level
Data Source
AI summary
In exposure control of an imaging apparatus, if a vertical scanning period is enlarged to increase an exposure time when the exposure time reaches an upper limit value, there occurs a problem in that a frame rate is reduced. Accordingly, when a subject is dark, an auto exposure control circuit sets the vertical scanning period V to a standard value Vst, increases the exposure time E to the upper limit value Emax (P60) and then increases a gain α of an AGC circuit (P62). If the image is still dark even when the gain α reaches the upper limit value α max, the auto exposure control circuit switches the vertical scanning period V to an enlarged value Vex and thus more increases the exposure time E (P64).