Unified Optical Scanning Technology

Chapter 4.3 - Scanner Devices and Techniques: Rotating Polygons

4.3 ROTATING POLYGONS

As introduced in Section 3.4.1, rotating polygons are classified in two major configurations: the pyramidal polygon of Figure 3.6 and the more utilized prismatic polygon of Figure 3.7. Although the dominant characteristic of both polygon forms is the regularity of the facet-to-facet (polar) angle and of the facet-to-axis angle, some are designed to be nonuniform in either or both parameters [She]. Nonuniform polar angle separation between facets yields nonuniform temporal and spatial scanned beam lengths (in the "along-scan" direction), whereas nonuniform angular orientation of the facets with respect to the rotating axis yields variations in scan line positioning (in the "cross-scan" direction). These options are rarely implemented intentionally. Most are designed for uniform facet angles in both directions. The prospect of encountering residual (nonintentional) angular deviations is, however, real—resulting from the application of practical machining accuracies and fabrication economies. The consequences of such "manufacturing tolerance" imperfections are the small (but sometimes critical) nonuniformities in scan line timing, length, and placement. Corrective action that may be taken to abate these errors is discussed in Chapter 5. A rarely used design option is the inverted polygon [She] having the facets formed on the inside surface of a ring-shaped rotating substrate rather than on the outside circumference of a continuous substrate. These may also be constructed with regular or irregular facet angles.

Significant distinctions appear in the operation of pyramidal and prismatic polygons, discussed below. Either form may have any practical number of facets. When the pyramidal type reduces to one facet, such as illustrated in Figures 1.9 and 1.10, it is commonly called a monogon, whereas in remote sensing, it may be called an oblique or single ax-blade scanner. When comprised of two oblique facets sym-metric about the axis, it is identified as a wedge, double ax-blade, or knife-edge scanner [B&J,Wol].

 

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