Unified Optical Scanning Technology

Chapter 4.3.2 - Scanner Devices and Techniques: Duty Cycle

4.3.2 Duty Cycle

The active portions of scan cycles are almost always buffered by blanking or retrace intervals, allowing time to reestablish the scanning aperture at the start of the new scan. This interval can include short overscan portions straddling the main format that may be used for synchronization and/or radiometric calibration. The ratio of the active portion to the full scan period is termed the duty cycle η, expressed as

in which τ is the blanking time and T is the full scan period. For the underfilled polygon scanners, a practical interpretation of the time loss factor in Equation 4-1 is τ/T = Dm/Da, yielding,

in which (per Fig. 3.7) Dm is the projected beam width on the facet (Section and Da is the full facet width. Thus this depletion in duty cycle is effectively the fraction of the full scan period that the traversing corner of the facet truncates and distorts the beam, preventing its full utility during that time. This limits the useful scan angle Θ to

where Θmax is the scan angle that could be available over the full period T. A reduced duty cycle imposes increased bandwidth for a given average data rate. It also increases the power required to accommodate a given exposure or detection sensitivity. With Equation 4-3 not limited to a particular coordinate (x or y), evaluation of the required exposure energy (or signal gain for detection) may entail determination of the duty cycle in the quadrature direction as well. Such evaluation is necessary, for example, in the case of repetitive two-dimensional (raster) generation for video recording or projection display.



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