CA1041209A - Flying spot scanner - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A flying spot scanning system is provided by utilizing reflected light from a multifaceted rotating polygon which is then directed to t?? scanned medium.
A light source illuminates at least two contiguous facets of the polygon during each scanning cycle, to provide the desired sequence of spot scanning. To assure a uniform spot size at the scanned medium, an optical convolution of elements is selected in combination with the light source such that an adequate depth of focus at the medium is assured. In each scanning cycle, information is transmitted to the scanned medium by modulating the light from the light source in accordance with a video signal.

Description

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BACKGROUND OF THE INVENTION
This invention relates to a flying spot scanning ~ystem for communicating video information to a scanned medium, and m~re particularly to a scanning system which u~ilizes a multi~aceted rotating polygon for controlling the sca~n~g cycles.
~ uc~ attention has been given to a various optical approaches in flying spot scanning for t~e purposa of imparting the information content of a modulated light beam to a scanned medium. GaLvanometer arrangements have been used to scan the light across a document for recording its information content thereon.
Suc~` a~rrangements have included planar reflecting mirrors which are driven in an oscillatory fashion. Other approaches have made use of multifaceted mirrors which are driven continuously. Various efforts have been made to define the spot size in order t:o provide for an optimum utilization of the scanning system.
One such effort is tbat described in United States Patent NoO 3,675,016. The approach used was to ~ . .
~i make the spot size invariant and as small as possible by defining the dimensions of the focused beam so that only part, preferably half,~of a mirror facet is illuminated during scanning. This teaching alludes to generalized techniques for assurin~ the constancy of the size of the ~perture of a rotating mirror scanning system. By either illuminating several facets of the mirror or by directing light in a beam that is sufficiently narrow to assure that less than a full facet is the most that can evèr be illuminated by the beam and limiting scanning to that ~rtion of the rotary travel of ~he facet when such facet i8 illumu~ted ~y all of such light beam. ~owever, such system apertures are dimensionally invariant because the dimensions of the rotating facets have no influence on such apertures. 2 ., .

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While the system as described in U.S. Patent No.
3,675,016 may have advantages over the prior art, nevertheless, various constraints must be imposed upon the spot size and other relationships of optical elements within the system which are not always desirable.

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SUMMARY OF THE INVENTION
In accordance with an aspect of this invention there is provided a flying spot scanning system comprising:
means for generating a beam of high intensity light~ a medium, optical means for imaging said ~eam to a spot at the surface of said medium at a predetermined distance from said optical means, a multifaceted polygon located in the path of said imaged beam between said beam generating means and said medium and having reflective facets for reflecting the beam incident to it onto said medium, means for rotating said polygon such that the re~lected light is scanned in successive traces across said medium, and said optical means including a positive cylindrical lens positioned in the optical path of the beam between said polygon and said medium at a location such that runout errors are substantially corrected at said medium, the plane of no power of said cylindrical lens being oriented in the direction of the scan.
In accordance with another aspect of this inven-tion there is provided a flying spot scanning system compris-ing: means for generating a beam of high intensity light, amedium, first optical means for expanding said beam, second optical means in convolution with said first optical mean~
said first and second optical means defining a finite conjugate imaging system for imaging said expanded beam to a spot on the surface of said medium at a predetermined distance from said second optical means, a multifaceted polygon having reflec~
tive fac~ts positioned in the optical path of said expanded beam to reflect said beam toward said medium, means for rotating said polygon such that said reflected beam is scanned through a scan angle to provide successive spot scanning traces across said medium, said second optical means including "...~

-12~9 a positive cylindrical lens in the optical path of the beam so positioned between said polygon and said medium that runout errors are substantially corrected at said medium, the plane of no power of said cylindrical lens being oriented in the direction of scan.
A feature of an embodiment of the invention is that the beam of light incident upon`the multifaceted polygon illuminates at least two contiguous facets of the polygon during each scanning cycle to provide the desired sequence of spot scanning. Such feature provides a flying spot scanning system which has an extremely high duty cycle.
Another feature of the embodiment of the invention is that a very large depth of focus is provided for the spot at the contact loci at the surface of the scanned medium. This feature is provided by utilizing a finite conjugate imaging system in convolution with the light beam and the rotating polygon. A doublet lens, in series with a convex imaging lens between the light source and the medium provides such an arrangement. The doublet lens enables the original light beam 2Q to be sufficiently expanded for illuminating multiple contigous facets of the polygon, ~4a~

whereas the imaging lens converges the expanded beam to focus at the contact loci on the surface of the scanned medium. Employing such an optical system assures a uniform spot size at the scanned medium even though a substantial scan width is traversed by the spot.
Still another feature of an embodiment of the invention is the modulation of the original light beam by means of a video signal. The information content within the video signal is thereby imparted to the light beam ~10 itself. The medium to be scanned is one which is re~ponsive to the modulated beam and records its information content as contained within the scanning spot in a usable form on its surface across the scan width.
- Also, another feature of an embodiment of the in~ention is that a ~tart/stop of scan detection apparatus is in combination with and substantially matches the con-~olution of imaging elements which focus the flying spot ~;~ at ~he surface of the scanned medium, although such detection apparatus is spacially distant from the scanned medium.
Yet another feature of an embodiment o~ the invention includes an embodiment of the flying spot scanning sy~tem ~or utilization in high speed xerography. The scanned m ium in such an embodiment would consist of a xerographic drum which rotates consecutively through a charging station, an exposure station where the spot traverses the scan width~
of the drum, through a developing station, and a transfer station where a web of copy paper is passed in contact with the drum and receives an electrostatic discharge to induce the transfer of the developed image from the drum to the copy paper~ A fusing device then fixes the images to the copy paper as it passes to an output station.

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~` 10~12(~9 DESCRIPTION OF THE DRAWINGS

Figure 1 is an isometric illustration of a flying spot scanning system in accordance with the invention.
Figure 2 is a perspective view of the utilization of the scanning beam and embodies additional features of the invention.
Figure 3 is a~ isometric illustration of a first alternate t~ the flying spot scanner of Figure 1 and with the same numbers indicating the same or similarly operating parts.
Figure 4 is a perspective view of the utiliæation of the scanning beam according to the first alternate embodiment.
Figure 5 is an isometric illus-tration of a second alternative to the flying spot scanning system of Figure 1 and wherein the same number indicates same or similar parts.
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- 5a -ZC~9 Figure 6(a) is a side perspective view of the utilization of the cylindrical correction lens of Figure 6.
Figure 6(b) is a top perspective view of the utilization of the cylindrical lens.
Figure 7 is an isometric illustration third alternative to that of Figure 1 of a flying spot scanning system with the same numerals indicating the same or similar parts.
. DESCRIPTION OF THE PREFERRED EMBODIMENT
In Figure 1, an em~odiment of a flying spot scanning system in accordance with the invention is shown. A light source 1 provides the original light beam for utilization by the scanning s~stem. The light source 1 is preferably a laser which generates a collimated beam o monochromatic liyht which may easily be modulated by modulator 4 in confonnance with the inormation contained in a video signal.
Modulator 4 may be any suitable electro-optical modulator for recording the video information in the form of a modulated light beam 6 at the output of the modulator 4.
The modulator 4 may be, for example, a Pockel's cell comprising a potassium dihydrogen phospkate crystal, whose index of re-fraction is periodically varied by the application of the vary-ing voltage which represents the video signal. The video signal may contain information either by means of binary pulse code modulation or wide-band frequency code modulation. In "i any event, by means o~ the modulator 4 the information within the video signal is represented by the modulated light beam 6.

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~ he light beam 6 is reflected from mirror 8 in convolution with a doublet lens 10. The lens 10 may be any len~, preferably of two elements, which elements are in spaced relation to each other such that the external curved surfaces are provided in symmetry with the internal surfaces. Preferably the internal surfaces of lens 10 are cemented toge~her to form a common contact zone. Of course, .~. ~, as is often the case in the embodiment of such a lens as a microscope objective, the elements may be fluid spaced.
The lens 10 is required to image the aess~ point of beam 6 through a focal point on the opposite side of lens 10.
At the focal point, beam 6 diverges or expands to form beam 12 which impinges upon at least two contiguous facets of a scanning polygon 16.
In this emblodment, the rotational axis of polygon 16 is orthogonal to the plane in which light beams 6 travels.
The facets of the polygon 16 are mirrored surfaces for the refle~tion of any illuminating light impinging upon them.
With ~he rotation of the polygon 16, a pair of light beams are reflected from the respective illuminated facets and turned through a scan angle for flying spot scanning.
Alternatively, flying spot scanning could be provided by any other suitable device, such as mirrored piezoelectric crystal~ or planar reflecting mirrors which are driven in an oscillatory fashion.
~ In all of these arrangements, however, the re~lec~ing surfaces would be at a distance S from the originating focal point of light beam 12 and in orthogonal relatio~ to the plane bounded by the beam 6 such ~hat ~he reflected beams would be in substantially the same plane a-~ beam 5.

ll~gtlZ(~9 At a distance ~ from the leading illuminated facet of polygon 16 is positioned an imaging lens 20. Lens 20 is of a diameter D to cooperate with the respective reflected light beams throughout an angle of 2 ~ to render convergent beams 22 which define a focal plane 24 at a distance f from the imag-ing lens 20. In this preferred embodiment, imaging lens 20 is a five element compound lens as disclosed in United States - Patent 3,741,521 issued January 10, 1973 and assigned to the assignee of the present invention. The focal p~ane 24 is proximate a recording medium 25 whose surface 26 is brought in contact with the respective focal spots of the convergent light beams throughout a scan width x.
A uniform spot size is assured throughout the scan width x even though a curved focal plane 24 is defined through-out the scanning cycle. The lens 10 in convolution with the ; imaging lens 20 provides a finite conjugate imaging system which allows a large depth of focus d which is coextensive with the contact loci of a spot throughout the scan width x on the surface 26 of the medium 25.
The beam of light from the light source 1, through ~`~; the optical elements and reflected from the scanning polygon to the scanning medium 25 defines a light path.
; ~ As shown in figure 2~a), medium 25 may be a xero-graphic drum which rotates consecutively through a charging ; - station deplcted by corona discharge device 27, exposure surface 26 where the beam from the rotating polygon 16 traverses the ~can width x on the drum 25, through developing station 28 depicted by a cascade development enclosure, transfer station 30 where a web of copy paper is passed in contact with the drum 25 and receives 4~LZ()~l an electrostatic discharge to incluce a transfer of the developed image from the drum 25 to the copy paper.
The copy paper is supplied from the supply reel 31, passes around guide rollers 32 and through drive rollers 33 into receivi~g b n 35. A fusing device 34 fixes the images to the ~y paper as it passes to bin 35.
~ sable images are provided in that the information content of the scanning spot is represented by the modulated or variant intensity of light respective to its postion wit~in the scan width x. As the spot traverses the charged suxface 26 through a given scan angle c7~, the spot dissipates the electrostatic charge in accordance with its light intensity. The electrostatic charge pattern thus pr~duced is developed in the developing station 28 and then transferred to the final copy paper. The xerographic drum 25 is cleaned by some cleaning device such as a rotating brush 36 before being recharged by charging device 27. In this manner, the information content of the scanned spot is recorded on a more permanent and useful medium. Of co~rse, alternative p~ior art techniques may be employed to cooperate with a scanned spot in order to utilize the information contained therein.
The polygon 16 is continuously driven by a motor 40 an~ synchronized in rotation to a synchronization signal representative of the scan rate used to obtain the original video signal. The rotation rate of the xerographic drum 25 determines the spacing of the scan lines. It also may be peferable to synchronize the drum 25 in some manner to the signal source to maintain image linearity. The source image is reproduced in accordance with the signal and is transf~e~red to printout paper for use or storage.

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,3LZQ9 The number of facets has been found to be optimum if at least 20 to 30 facets are employed. The scan angle o~
~raversed would be equal to the number of facets chosen in relation to one complete revolution of the polygon 16. An extremely useful axrangement would have the polygon 16 with 24 facets and a scan anglec~ of 15 degrees. Since the depth of focus d of the converging beam 22 is related to the scan angle in ~hat as the s~an angle c~ increases the radius of curva~ure of the focal plane 24 increases, it is important , to define a scan angle in relation o the desired scan width ~` x. For a scan width x of approximately ll lnches it has ~een found that the scan angle o~of 12 to 18 degrees, with ~` 20 to 30 facets on the polygon 16, is optimum. Figure 2(b) is a top perspective view of the op1:ical system shown in Figure 2(a).
The optical system of the present invention provides a virtually 100% duty cycle scan for the entire scan angle o~ by virtue of the illumination of at least two contiguous facets. The illumin~tion of two contiguous facets is ~ preferred. ~ith such illumination, another scanning spot is provided at a distance equal to the scan width x ~ehind - the leading scanning spot with virtually no wait between successive scans. With the continuous rotation of the polygon ~; ~ 16 additional contiguous facets are subsequently illuminated~
; thereby providing successive convergent beams following the leading convergent beam 22 by no more than the scan angle, if so desired. Thus, a flying spot scanning system which has an extremely high duty cycle is provided.
The optical system of the present invention projects the highest energy portion of the laser beam onto the rotating - 9a -polygon and to the photoreceptor surface. The energy output of the laser beam has a Gaussian distribution with the highest intensity distribution heing concentrated near the axis of projection. In ~he Gaussian distribution the highest inten-sity energy is located at the centre of the beam. By spreading the beam out over a plurality of facets, the beam centre portion having the highest intensity radiation energy fills all the polygon facets reflecting the beam onto the photoconductive surface. The fringes of the beam having the lowest Gaussian energy intensities are spread out beyond those polygon facets and are excluded from the information signal scanned across the photoreceptor surface. Shaping the `; beam to spread it across a plurality of facets thereby ~ilters the lower energy radiation within the beam and excludes it, permitting the utilization of the maximum energy content of the beam.
In the first alternate e~bodiment as shown in Figure 3, the light beam 6 is reflected from mirror 8 in convolution with a doublet lens 10. The lens 10 may be any lens,~preferably of two elements, which elements are in ; spaced relation to each other such that external curved suraces are provided in symmetry with the internal surfaces.
Preferably the internal surfaces of lens 10 are cemented together to form a common contact plane. Of course, as is often the case in the embodiment of such a lens as a micro-scope objective, the elements may be fluid spaced The lens 10 is required to image either a virtual or real axial point of beam 6 through a focal point, for example, on the opposite ~ side of lens 10 for a real image. At the focal point, beam 6 diverges or expands to form beam 12 which would be more than sufficient to impinge upon a given facet of a scanning polygon 16.

Z~9 At a distance S2 from the leading illuminated facet of polygon 16 is positioned an imaging lens 18. Lens 18 is of a diameter D to cooperate with the expanded light beam 12 to render a convergent beam 2 which illuminates the desired facets to reflect respective light beams 22 to focus to focal plane 24 at a distance d from the polygon 16. In this first alternate embodiment, imaging lens 18 is a l-n element lens.
The focal length f of lens 18 is related to Sl, S2 and d by the following thin lens equation: 1 +
, Sl - :
The rotational axis of polygon 16 is orthogonal to the plane in which light beam 6 travels. The facets of the polygon 16 are mirrored surfaces for the reflection of any illuminating light implnging upon them. With the rotation of the polygon 16, assuming two contiguous facets are illuminated at a given time, a pair of light beams 22 are reflected from the respective illuminated facets and turned through a scan angle ~ for flying spot scanning. Alternatively, flying spot scanning could be provided by any other suitable device, such as mirrored piezo- -electric crystals or planar reflecting mirrors which are driven in oscillatory fashion.
In all of these arrangements, however, the reflecting ` surfaces would be at a distance Sl from the originating focal point of light beam 12 and in orthogonal relation to the plane ` ~ bounded by the beam 6 such that the reflected beams would be `~ in substantially the same plane as beam 6.
The focal plane 24 is proximate a recording medium :~ .
25 whose surface 26 is brought in contact with the respective ::
` focal spots of the convergent light beams throughout a scan width x.

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z~9 A uni~orm spot size is assured throughout the scan width x even though a curved focal plane 24 is defined through-out the scanning cycle. The lens 10 in convolution with the imaging lens 18 provides a finite conjugate imaging system which allows a large depth of focus of which is coextensive with the contact loci of a spot throughout the scan width x on the surface 26 of the medium 25.
As shown in Figure 4, medium 25 may be a xerographic drum and in all respects is similar to the arrangement of Figure 2 except for the position of the imaging lens as shown in Figure 3.
The second alternative embodiment is as shown in Figure 5. At a distance a from the leading illuminated facet of polygon 16 is positioned an imaging lens 20. As shown, the l~ns 20 is located between the polygon 16 and the medium 25.
Alternatively, the lens 20 may be located between the polygon 16 and the lens 10 as shown in Figure 3. In this embodiment, lens 20 is of a diameter Dl to cooperate with the respective , reflected light beams throughout each scan of 2 d~ to render convergent beams 22 which define a focal plane 24 at a distance f, from the imaging lens 20. In the first embodiment, imaging lens ~0 is a five element compound lens as disclosed in United States Patent 3,741,621 to G.L. McCrobie issued 3une 26, 1973 and assigned to the assignee of the present invention. The focal plane 24 is proximate a recording medium 25 whose ~urface - 26 is brought in contact with the respective focal spots of the convergent light beams throughout a scan width x.
Since runout errors and polygon facet errors may cause poor results in terms of the quality of image transfer to the scanned medium, a cylindrical lens 36 is positioned in the optical path between the polygon and the scanned medium with its ~g,t12~1i9 aperture aligned with the aperture of the polygon 16. The lens 23 may be either bi-convex, plano convex, or meniscus.
The light beam 22 impinges on the convex surface of the lens 36 to focus at plane 24 at a predetermined position in an ordinate perpendicular to the direction of scan on the surface 26 of the medium 25.
As shown in Figure 6, the polygon 16 is continuously drlven by a motor 40 and may be synchronized in rotation to a synchronization signal representative of the scan rate used to obtain th~ original video signal. In the case of the utilization of a xerographic drum, the rotation rate of the drum determines the spacing of the scan lines. The rotation of the polygon 16 off-axis from that desired causes runout errors or, in this case, a deflection of the beam 22 in the vertical direction away from the desired scan line. Assuming an angular deviation of ~ from the desired axis of rotation `for the polygon 16, a runout angle of ~ defines the deflection from the intended direction of scan. Other misalignments of optical elements wlthin the system, such as facet misalignment, also may cause the same runout effects.
The disposition of the cylindrical lens 36 in Figures 6(a), 6(b), the optical path, though, compensates for such~
effects. The lens 36 is located at a distance b from ths origin of the angular deflection ~ and a distance u from the imaging . ,~
lens 20. The compensation is ef-fected in that the off-axis beam passes through the convex surfaces of lens 36. Then, the - lens 36 focuses the ~eam 22 to a spot at a predetermined line .
of scan in the focal plane 24 at a distance b' from the lens 36.

To insure that lens 36 is sufficiently wide a length L is pro-vided approximately equal to or greater than the scan width s. The runout dimensions at lenses 20 and 36 are Dl' and D2', ZQ~
respectively.
In this embodiment, in order to determine the optimum relationship between the imaging elements 20 and 36, the aperture Dl of the imaging lens 20 shall be equal to approximately 2ax/f1, with an (f/number)l equal to approximately s/2a tan2 c~.
Furthermore, the focal length of lens 36 is f2, which is defined as l/b + l/b = l/fz. From this we may define the optimum focal length f2 for determining the distance of the focused spot from the lens 36.
Since it is also necessary to define the aperture D2 of the lens 36 to practice the invention, the following relationship is preferred:
D2 ~ 2b tan ~
Having defined f2 and D2, it is helpful to determine the necessary (f/number)2 for the lens 36:
(f/number)2 = f2/D2 = b'/2 tan ~ (b' ~ b~
An optimum relationship is that the cylindrical lens 36 be located at a distance from the surface 26 of the medlum 25 approximately equal to the focal length f2 of the lens 36.
A third alternative embodiment shown in Figure 7, wherein the same numerals correspond to the same or similar parts as in Figure 3.
~ he light beam 6 is reflected from mirror 8 in con-volution with a doublet lens 10. The lens lO may be any lens, preferably of two elements, which elements are in spaced ; relation to each other such that the external curved surfaces are provided in-symmetry with~the planar internal surfaces.
Preferably the internal surfaces of lens 10 are cemented together to form a common contact zone. Of course, as is often the case in the embodiment of such a lens as a microscope 2Q~
objective, ~he elements may be fluid spaced. The lens 10 is required to image either a virtual or real axial point of beam 6 through a focal poin~, for example, on the opposite side of lens 10 for a real image. At the focal point, beam 6 diverges or ~xpands to form beam 12 which would be more than sufficient to impinge upon a given facet of a scanning polygon 16.
At a distance S2 from the leading illuminated facet of polygon 16 is positioned an imaging lens 18. Lens 18 is of a diameter D to cooperate with the expanded light beam 12 to render a co~vergent beam 20 which illuminates the desired facets to reflect respective light beams 22 through a positive cylindrical lens 23 to focus to the focal plane 24 at a distance S3 from the polygon 16. In this preferred embodiment, imaging lens 18 is a five element compound lens as disclosed in afore-mentioned U.S. Patent 3,741,521.
The rotational axis of polygon 16 is orthogonal to the plane in which light beam 6 travels. The facets of the polygon 16 are mirrored surfaces parallel to the axis of rotation for the reflection of any illuminating light imping-ing upon them. With the rotation of the polygon 16, assumingtwo contiguous facets are illuminated at a given time, a pair of light beams 22 are reflected from the respective illuminated facets and turned through an angle 2 ~L for flying spot scanning.
Alternatively, flying spot scanning could be provided by any ; other suitable device, such as mirrored piezoelectric crystals of planar reflecting mirrors which are driven in an oscillatory fashion.
In all of these arrangements, however, the reflecting surfaces would be at a distance Sl from the originating focal point of light beam I2 and in orthogonal relation to the plane bounded by the beam 6 such that the reflected beams would be in .Z~9 substantially the same plane as beam 6.
The cyllndrical lens 23 is positioned in the optical path between the polygon 16 and the desired line of scan in the focal plane 24 with its aperture aligned with the aperture of the polygon 16.
The function of the lens 23 is to compensate for runout errors in the scanning system. The lens 23 may be either bi-convex, plano-convex or meniscus and further relates to the scanning system as described with respect to Pigure 6.
The focal plane 24 is proximate a recording medium 25 whose surface 26 is brought in contact with the respective focal spots of the convergent light beams throughout a scan width x~
A su~stantially uniform spot size is assured through-out the scan width x even though a curved focal plane 24 is ; defined throughout the scanning cycle. The lens 10 in con-volution with the imaging lens 18 provides a finite conjugate : imaging system which allows a large depth of focus d which is coextensive with the contact loci of a spot throughout the ~` 20 scan width x on the surface 26 of the medium 25.
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- 15a -As is further shown in figure 7, a mirror 42 is positioned proximate the start of scan location to deflect at least a portion of the beam 22 to direct a beam 4 th~augh a positive cylindrical lens 46 to focus at a ~tector 48. The detector 48 includes a photodiode (not shown), or other optically sensitive element, which produces an eledtrical pulse to indicate the start of scan upon Lllumination by the beam 44. The detector 48 further includes a timing element (not shown) in combination with the optically sensitive element which is responsive to ~he start of scan pulse. The timing element through well known techniques tl~es out the predetermined duration of a scanning cycle to produce a stop of scan pulse. An example of such a te~hnique would be a capacitive element charged at the star~ ~f scan pulse which charge decays in relation to a predetermined time con~tant to trigger a one-shot multivibrator at the stop of scan. The start~stop of scan signals are then used to slave the video signal to the scan rate of the ~canning system.
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The detection elements 46~and 48 are substantially matched wlth the convolution of imaging elements which focus the flying spot at the surface of the scanned medium. The cylindrical lens 46 is distanced along its respective optical path from the lens 18 precisely at the same length as the cylindrical lens 23 is distanced aIong its respective optical path from the lens 18. Furthermore, the aperture, focal length, and focal number of the lens 46 is substantially identical to that required for lens 23. Therefore, the focused spot of the beam 44 is in a focal plane at a distance ~~ S3 along the optical path from the polygon 16, where the detector 48 is located. Thereby, an effective detection system i5 provided which contributes to a high degree of ~ynchroni~ation accuracy and constancy, with no interference with th~ ~pot scanning elements wi~hin the system.

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Claims (20)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A flying spot scanning system comprising: means for generating a beam of high intensity light, a medium, optical means for imaging said beam to a spot at the surface of said medium at a predetermined distance from said optical means, a multifaceted polygon located in the path of said imaged beam between said beam generating means and said medium and having reflective facets for reflecting the beam incident to it onto said medium, means for rotating said polygon such that the reflected light is scanned in successive traces across said medium, and said optical means including a positive cylindrical lens positioned in the optical path of the beam between said polygon and said medium at a location such that runout errors are substantially corrected at said medium, the plane of no power of said cylindrical lens being oriented in the direction of the scan.

2. The structure as recited in claim 1: said beam generating means being a laser, means located between said laser and poly-gon for modulating the light beam in accordance with informa-tion content of electrical signals, and said medium being light sensitive.

3. The structure as recited in claim 1, wherein said facets are focused in the sagittal plane onto the surface of said medium.

4. The structure as recited in claim 3, wherein an inter-mediate focal point of the beam in the sagittal plane is at a location other than said facets.

5. The structure as recited in claim 4, wherein optimum spot size is provided at the surface of said medium.

6. The structure as recited in claim 1, wherein said cylindrical lens has a minimum focal number (f/number2? b'/
[2 tan.beta. (b' + b)] where b is the object distance of the origin of runout errors as seen by said lens, b' is the distance of said lens from said medium surface, and b' is the angular measure of runout error.

7. The structure as recited in claim 6: said beam generating means being a laser, means located between said laser and polygon for modulating the light beam in accordance with information content of electrical signals, and said medium being light sensitive.

8. The structure as recited in claim 6, wherein said facets are focused in the sagittal plane onto the surface of said medium.

9. The structure as recited in claim 8, wherein an inter-mediate focal point of the beam in the sagittal plane is at a location other than said facets.

10. The structure as recited in claim 9, wherein optimum spot size is provided at the surface of said medium.

11. A flying spot scanning system comprising: means for generating a beam of high intensity light, a medium, first optical means for expanding said beam, second optical means in convolution with said first optical means, said first and second optical means defining a finite conjugate imaging system for imaging said expanded beam to a spot on the surface of said medium at a predetermined distance from said second optical means, a multifaceted polygon having reflective facets positioned in the optical path of said expanded beam to reflect said beam toward said medium, means for rotating said polygon such that said reflective beam is scanned through a scan angle to provide successive spot scanning traces across said medium, said second optical means including a positive cylindrical lens in the optical path of the beam so positioned between said poly-gon and said medium that runout errors are substantially correct-ed at said medium, the plane of no power of said cylindrical lens being oriented in the direction of scan.

12. The structure as recited in claim 11: said beam generat-ing means being a laser, means located between said laser and polygon for modulating the light beam in accordance with infor-mation content of electrical signals, and said medium being light sensitive.

13. The structure as recited in claim 11, wherein said facets are focused in the sagittal plane onto the surface of said medium.

14. The structure as recited in claim 13, wherein an inter-mediate focal point of the beam in the sagittal plane is at a location other than said facets.

15. The structure as recited in claim 14, wherein optimum spot size is provided at the surface of said medium.

16. The structure as defined in claim 11, wherein said cylindrical lens has a minimum focal number (f/numher)2 ? b'/
[2 tan .beta. (b' + b] , where b is the object distance of the origin of runout errors as seen by said lens, b' is the distance of said lens from said medium surface, and .beta. is the angular measure of runout error.

17. The structure as recited in claim 11: said beam generating means being a laser, means located between said laser and polygon modulating the light beam in accordance with infor-mation content of electrical signals, and said medium being light sensitive.

18. The structure as recited in claim 16, wherein said facets are focused in the sagittal plane onto the surface of said medium.

19. The structure as recited in claim 18, wherein an inter-mediate focal point of the beam in the sagittal plane is at a location other than said facets.

20. The structure as recited in claim 19, wherein optimum spot size is provided at the surface of said medium.