Tuesday, January 20, 2015

Aptina Proposes Discontinuous Exposure Mode

Aptina's patent application US20150009375 "Imaging systems with dynamic shutter operation" by Gennadiy Agranov, Sergey Velichko, and John Ladd presents the artifacts issue:

"In conventional imaging systems, image artifacts may be caused by moving objects, moving or shaking camera, flickering lighting, and objects with changing illumination in an image frame. Such artifacts may include, for example, missing parts of an object, edge color artifacts, and object distortion. Examples of objects with changing illumination include light-emitting diode (LED) traffic signs (which can flicker several hundred times per second) and LED stop lights of modern cars.

While electronic rolling shutter and global shutter modes produce images with different artifacts, the root cause for such artifacts is common for both modes of operation. Typically, image sensors acquire light asynchronously relative to the scenery being captured. This means that portions of an image frame may not be exposed for part of the frame duration. This is especially true for bright scenery when integration times are much shorter than the frame time used. Zones in an image frame that are not fully exposed to dynamic scenery may result in object distortion, ghosting effects, and color artifacts when the scenery includes moving or fast-changing objects. Similar effects may be observed when the camera is moving or shaking during image capture operations.

The proposed solution is:

"Each image pixel in a pixel array may include a shutter element for controlling when the photosensitive element acquires charge. For example, when a pixel's shutter element is “open,” photocurrent may accumulate on the photosensitive element. When a pixel's shutter element is “closed,” the photocurrent may be drained out from the pixel and discarded.

The shutter elements may be operated dynamically by being opened and closed multiple times throughout the duration of an imaging frame. Each cycle of dynamic shutter operation may include a period of time when the shutter is open and a period of time when the shutter is closed. At the end of each cycle, the charge that has been acquired on the photosensitive element during the cycle may be transferred from the photosensitive element to a pixel memory element. By repeating this sequence multiple times, the charge accumulated on the pixel memory element may represent the entire scenery being captured without significantly unexposed “blind” time spots.

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