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Chapter 5:
Interlace and Dither

Interlace and dither reduce sensor sample spacing without increasing detector count. A high-resolution image frame is comprised of two or more lowerresolution fields or subimages taken sequentially in time. Between each field or subimage, a nodding mirror or other mechanical means is used to move the locations where the scene is sampled. Interlace and dither achieve high resolution while minimizing FPA complexity.

The mechanical operation of a dither or microscan mirror is illustrated in Fig. 5.1. A mirror is inserted between the objective lens and the FPA. The angular motion of the mirror about the scan axis is very small. Image translation on the FPA is subpixel (that is, less than a sample spacing). The figure indicates a horizontal translation of the image, but the mirror can be made to scan about any axis. The subpixel image translation provided by the scan mirror allows changing the sample phase of the FPA relative to the scene.

Interlace generally implies that the subimages or field images are taken and displayed in time synchronism. That is, the pixels from subimages taken at different times are not combined and then displayed, but rather the time sequencing of the sensor field images is maintained at the display. The reducedresolution field images are combined into a high-resolution image by the human visual system. Dither, on the other hand, generally implies that the field images are combined to form a higher-resolution image prior to the display.

Interlace is used to improve sensor sampling without increasing pixel rate or electronic throughput of the system. It takes advantage of the eye's ability to integrate multiple fields of imagery, presented in time sequence, into a higherresolution scene.

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