Unified Optical Scanning Technology

Chapter 4.3.3 - Scanner Devices and Techniques: Over- and Underillumination (Over- and Underfilling) of the Facet

4.3.3 Over- and Underillumination (Over- and Underfilling) of the Facet

Overilluminated (overfilled) facets were introduced in Section 3.2.1 for discussion of the resolution of uniformly illuminated apertures. In Section 3.2.1.1 (illustrated in Fig. 3.4) appear four examples; the first two (triangular and keystone) represent aperture shapes of pyramidal polygons, the third (round or elliptical) is typical of flat vibrational mirrors, and the fourth (square or rectangular) typical apertures of vibrational devices or of prismatic polygons. Overillumination of polygon facets imparts unique characteristics to the scan process. Along with gleaning the highest possible resolution and speed from a given polygon by utilizing its widest possible aperture, it also allows the largest possible duty cycle of 1.0 (100%) as a limit. This is accomplished (Fig. 3.6) by overilluminating two adjacent facets simultaneously, such that the scan by the second facet is initiated at the instant it is terminated by the first facet. The trade-off is the great loss of light flux due to apodization of a wide beam, utilizing only the central portion of a Gaussian-like beam to approach uniform illumination. Further loss is due to selection of a small fraction (at best, one-half) of that residue at any one time, as the facet traverses the (at least twofold) wider and already apodized beam. The round or elliptical beam is a typical candidate for the underfilled case.

 

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