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Chapter 6:
Automatic High Dynamic Range Control
In most linear response sensors, there are mechanisms for selecting the correct exposure and correct gain to provide the best exposure and noise tradeoff. These regulators act on exposure and gain settings to target a certain average image brightness for a complete image or for a region of an image (or a weighted average of several regions, sometimes called tiles). If the image is too dark, exposure time or gain can be increased. If the image is too bright, exposure time or gain can be decreased. The decision to work on gain or exposure depends on the situation and on the sensor. High gains should be avoided because of the resulting noise amplification and reduction of precision in exposure time settings due to enlarged steps. Exposure time variation is preferable when possible, i.e., until exposure reaches a maximum value, defined by the type of scene and frame rate requirements. Still scenes allow for long exposures, while moving objects require short exposures and a higher gain. Compared to photography, exposure time is the shutter time, and gain is the ISO speed.
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