Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1999-06-18
2000-11-28
Phan, James
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359223, 359226, 359900, 250236, G02B 2608
Patent
active
061543047
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for monitoring distortion in a reflective surface of a spinning mirror.
FIG. 1 illustrates a conventional imagesetter. A drum 1 carries a film to be exposed (not shown) on its inner surface. A laser 2 generates a collimated laser beam 3 which is directed along the axis 4 of the drum 1. The laser beam 3 is reflected by a spinning mirror 6 onto the film. The beam 3 is focused into a spot 9 by a focusing lens 5. The rotating mirror 6 is typically a beryllium mirror which is mounted in a mirror mounting 7 and spun at approximately 30,000 rpm by a drive 8. As the film is exposed, items 5-8 are driven along the axis 4 of the drum 1. Thus the spot 9 follows a series of azimuthal scan lines across the film. The geometry of the system illustrated in FIG. 1 defines an axis parallel to the axis 4 of the drum 1 and perpendicular to the scan lines, which is referred to hereinafter as the X-axis. Similarly the geometry of the system illustrated in FIG. 1 defines an azimuthal direction, substantially parallel with the scan lines, which is referred to hereinafter as the Y-axis.
Due to the high speed of rotation of the spinning mirror 6, centrifugal force tends to distort the reflecting surface 10 of the mirror. This creates an undesirable effect known as dynamic astigmatism which will now be explained with reference to FIGS. 2-4.
In an ideal case, where the spot 9 is precisely focused on the interior of the drum 1, and no dynamic astigmatism is present, the spot 9 will be perfectly circular (on the assumption that the incident beam 3 is perfectly circular and perfectly collimated). The presence of dynamic astigmatism in the spinning mirror 6 causes the spot 9 to deviate from its ideal circular form. This deviation is illustrated in FIG. 2. FIG. 2 illustrates the profile of the scanning dot 9 on the interior of the drum 1 with the lens 5 at two different focal positions and when dynamic astigmatism is present. The X and Y axes are as previously defined. Contour 20 is the contour of 50% beam intensity at the interior surface of the drum with the focusing lens 5 in a first axial position f.sub.x (referred to hereinafter as the horizontal focus position). When the lens 5 is in the horizontal focus position, the horizontal dimension (ie. in the X direction) of the spot 9 at the interior surface of the drum is at a minimum. The contour 20 is an ellipse having a maximum dimension L.sub.x and a minimum dimension S.sub.x. As the lens 5 is moved to a second axial position f.sub.y (referred to hereinafter as the vertical focus position) the profile of the spot 9 at the interior surface of the drum changes to give the 50% contour indicated at 21. When the lens 5 is in the vertical focus position, the vertical dimension (ie. in the Y direction) of the spot 9 is at a minimum. The contour 21 is an ellipse having a maximum dimension L.sub.y and a minimum dimension S.sub.y.
At an intermediate focal position between f.sub.x and f.sub.y, the 50% contour will be circular with a diameter greater than S.sub.y and greater than S.sub.x. Ideally the focal position of the lens 5 is set in the imagesetter at this intermediate position, and ideally the diameter of the spot in this focal position is as small as possible. However if the mirror 6 suffers from a significant amount of dynamic astigmatism, the diameter of the spot 9 with the lens in the intermediate focus position may be unacceptably large. Therefore if the dynamic astigmatism is too high the mirror should not be used. Dynamic astigmatism can vary widely between different mirrors 6.
It is clear that in the manufacture of the apparatus illustrated in FIG. 1 (or other imaging systems which use a spinning mirror, such as input scanners), it is desirable to measure the degree of dynamic astigmatism created by the spinning mirror 6. It is not sufficient to measure the astigmatic properties of a static (i.e. non-spinning) mirror since centrifugal forces can dramatically distort the mirror in use. A conventional method of measuring dynamic
REFERENCES:
patent: 3743427 (1973-07-01), Weiser
patent: 5535042 (1996-07-01), Takada
Fujifilm Electronic Imaging Ltd.
Phan James
LandOfFree
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