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6.1 Laser Scanning 6.1.1 Reflective scanning prism A rotating reflective prism in the shape of a polygon cylinder, or spinner, is a well-known scanning technique. Figure 6.1 shows a rotating polygon where a fixed direction single laser beam partially illuminates a face. The reflected beam scans continuously until a facet corner is encountered, where there is a discontinuity or retrace interval between the end of the facet scan and the succeeding scan from the next facet. The pyramidal faceted scanner in Fig. 6.2 reduces or eliminates the retrace interval.1 A convergent beam illuminates at least two facets to produce a continuous-scan interval between these facets having a circular focal trace with constant angular velocity. Thus, two illuminated facets will produce one continuous-scan interval, three illuminated facets will produce two continuous scan intervals, and illumination of an entire pyramid having n facets will produce n − 1 continuous-scan intervals. 6.1.2 Refractive prism-beam scanning and steering 6.1.2.1 Single-wedge prism A single-wedge prism with vertex angle α and deviation angle δ, when rotated about an axis normal to one of the surfaces, produces a circular scan on a screen or generates a cone with half-angle δ. The exact deviation angle depends on the vertex angle, prism refractive index, and the direction of the prism with respect to the incident beam. When the wedge prism is in "A" orientation as in Fig. 6.3, the deviation angle δ can be calculated from (6.1a) (6.1b) (6.1c) |
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