Abstract:
A beam scanner has a beam deflector mounted within a hollow shield having a beam entrance aperture and a beam exit aperture. The beam exit aperture is sealed by a window through which the deflected beam exits the hollow shield. A spin motor rotates the hollow shield, beam deflector and the window about a spin axis. A hydraulic system maintains the periphery of the window in an undistorted condition.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to beam scanners in general, and, more particularly, to a high speed beam scanner for use in drum-type laser imagesetters. 
     High resolution drum-type laser imagesetters require low wobble, low noise and first surface spin mirror with minimal optical aberrations. As the electronic imaging market demands faster and faster scan speeds, a variety of problems arise with increased scan speed. First, the inherent nature of the spinners&#39; 45 degree surface increases windage and noise when the speed is increased. Windage is considered to be a forcing function for air-bearing wobble, and the audible noise becomes a work environment concern at high rotational speeds. Additionally, any increase of the pumping of ambient air across the spin mirror surface increases both erosion and chemical degradation. This is especially true for highly polluted areas. 
     It is a general object of the present invention to provide an improved high speed beam scanner. 
     It is a specific object of the invention to reduce acoustic noise associated with a high speed rotating beam deflector. 
     It is another object of the invention to reduce windage in the high speed rotation beam deflector. 
     It is still another object of the invention to prevent deleterous effects upon the rotating beam deflector caused by environmental conditions external to the rotating beam deflector. 
     It is a further object of the invention to enclose the rotating deflector in a hollow shield having beam entrance and exit apertures. 
     It is a still further object of the invention to environmentally seal the exit aperture with a flat window. 
     It is yet another object of the invention to prevent edge distortion of the flat window during rotation of the hollow shield. 
     SUMMARY OF THE INVENTION 
     The invention comprises a beam scanner that has a beam deflector mounted within a hollow shield having beam entrance and exit apertures. The beam exit aperture is sealed by a window through which the deflected beam exits the hollow shield. The beam deflector and hollow shield are rotated together about a spin axis by a spin motor. A hydraulic system maintains the periphery of the window in an undistorted condition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded view in perspective of a beam scanner having a beam deflector, a hollow shield with entrance and exit apertures, a window sealing the exit aperture and a spin motor; 
     FIG. 2 is a view in cross-section taken along line 2--2 in FIG. 1 showing the static or non-rotating condition of the sectional portion of the scanner; 
     FIG. 3 is same view as FIG. 2, but showing the dynamic or rotating condition of the sectional portion of the scanner with component displacements exaggerated for purposes of illustration; 
     FIG. 4 is a plan view of a hollow hydraulic liquid filled container; 
     FIG. 5 is a view in cross-section taken along line 5--5 in FIG. 4 showing the curvature of the hollow hydraulic liquid filled container with respect to the spin axis of the beam scanner; and, 
     FIG. 6 is a diagrammatic view of the beam deflector and window with the window tilted with respect to the spin axis of the beam scanner. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, and particularly to FIG. 1 thereof, there is shown a beam scanner constructed in accordance with the present invention and indicated generally by the reference numeral 10. The beam scanner 10 comprises a spin motor 12 that rotates a beam deflector, such as a mirror, 14 about a spin axis 16. The beam deflector 14 is enclosed within a hollow shield 18 that also is rotated by spin motor 12. 
     The hollow shield 18 has a beam entrance aperture 20 and a beam exit aperture 22. The exit aperture 22 is environmentally sealed by means of a window 24 that is secured to window mount 26 which in turn is secured to the inside of the hollow shield 18. A hollow hydraulic liquid filled container or &#34;O-ring&#34; 28 is positioned between a portion of the window mount 26 and the inner surface 30 of the hollow shield 18. 
     Referring now to FIGS. 2 and 3, FIG. 2 illustrates the static or non-rotating condition of the beam scanner 10. The exit aperture window 24 is secured to its window mount 26 by an optical adhesive 32. The window mount 26 is compliantly secured to the hollow shield 18 by an elastomer 34 such as RTV. The hydraulic liquid filled container or &#34;O-ring&#34; 28 is positioned in an annular groove 36 formed in window mount 26. 
     Referring now to FIG. 3, this figure illustrates, in exaggerated form, the dynamic or rotating condition of the beam scanner. It can be seen from FIG. 3 that the rotational forces have peeled away the peripheral edge 38 of the exit beam aperture 22. This &#34;fish mouthing&#34; of the exit beam aperture allows a shifting of the exit beam aperture window 24 from the static condition shown in dotted form to the dynamic or rotating condition shown in solid lines. In the static condition of FIG. 2 and the dotted condition of the window shown in FIG. 3, the flat window has no optical power. Under the rotating condition shown in FIG. 3, the window &#34;bulges&#34; and has an optical power, but this power can be compensated for by focus adjustment in the scanning system (not shown). 
     It will be appreciated that in order to avoid the introduction of optical aberrations of astigmatism, tricorn and coma, all points on the periphery of window 24 must be uniformly displaced. However, non-uniform forces are generated upon rotation of the hollow shield and window. The distribution of forces around the periphery are matched to provide equal displacement of all points on the periphery of window 24. This matching of the force distribution is achieved by varying the area of the hydraulic liquid filled container 28 as a function of its radial distance from rotation axis 16 such that the further the distance from rotation axis 16 the greater the area of the hydraulic liquid filled container 28. The shape of the liquid filled container 28 thereby varies according to the angle theta shown in FIG. 4. Thus, as depicted in FIG. 4, the top and bottom portions 40 have a greater area than the left and right portions 42 of the hydraulic liquid filled container 28. 
     Referring to FIG. 5, the container or &#34;O-ring&#34; 28 contains a hydraulic liquid 44 such as a hydraulic oil. The container 28 is curved to conform to the curvature of the inner surface of hollow shield 18 so that radius R1 equals radius R2. 
     Looking at FIG. 6, the exit aperture 24 is tilted with respect to the spin axis 16. The tilting of the beam deflector 14 from the 45 degree position prevents back reflections from the beam scanner. In this configuration, the window is tilted 5 degrees a shown in FIG. 6. 
     It will be appreciated from the preceding discussion of the beam scanner that the window 24 provides an environmental seal with respect to the environment external to the rotating hollow shield. Preferably, the entrance aperture 20 is also sealed with a window 46 illustrated diagrammatically in FIG. 6. 
     Numerous variations can be made in the components described above while still achieving the objectives of the present invention. For example, althoug the hollow shield has been shown as a hollow cylinder, a spherical shield with corresponding entrance and exit apertures can be employed. Furthermore, the geometric shape of the exit aperture can be circular or non-circular with the window having a corresponding configuration. Although the window 24 has been depicted as a planar window, and is preferable because of its relative inexpensive cost, non-planar windows can be employed with a corresponding adjustment of the configuration of the hydraulic liquid filled container 28 including a uniform area in contrast to the varying area depicted in FIG. 5. 
     Having described in detail a preferred embodiment of my invention, it will now be apparent to those with skill in the art that numerous modifications can be made therein without departing from the scope of the invention as defined in the following claims.