CA1043011A - Flying spot scanner with plural lens correction - 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 the scanned medium.
A light source illuminates a portion of the mirrored sides of the polygon during each scanning cycle, to provide a desired sequence of spot scanning. In each scanning cycle, information is transmitted to the scanned medium by modu-lating the light from the light source in accordance with a video signal. An optical convolution of elements including at least two lenses having power in one optical plane and substantially no power in the other plane, is provided in combination with the polygon. One of these lenses is placed in the optical path between the polygon and the scanned medium with its power plane substantially parallel to the axis of rotation of the polygon for allowing a wide variance in runout tolerance of the scanning system.
The other lens is placed in the optical path between the light source and the polygon with its power plane substantially perpendicular to the axis of rotation of the polygon.

Description

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BACKGROI~D OF THE I~VE~TIO~
This invention relates to a flying spot scanning system for communicating video information to a scanned medium, and more particularly to a scanning system which utilizes a multifaceted rotating polygon for controlling the scanning cycles.
Much attention has been given to various optical approaches in flying spot scanning for the purpose 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 thereonO
Such arrangements 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 to provide for an optimum utili-zation of the scanning system.
One such effort is that described in United States Patent ~o. 3,675,016. The approach used was to 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 assuring the constancy of the size of the aperture of a rotating mirror scanning system. sy either illuminating several facets of the mirror or by directing light in a beam that is sufficie~tly narrow to assure that less than a full facet is the most that can ever be illuminated by the beam and limiting scanning to that J

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portion of the rotary travel of the facet when such facet is illuminated by all of such lic~ht beam. However, such system apertures are dimensional:Ly invariant because the dimensions of the rotating facets have no influence on such apertures.
While the system as described in U. S. Patent No.
3,675,016 may have advantages over the prior art, neverthe-less, various constraints must be imposed upon the spot size and other relationships of optical elements within the system which are not always desirable.
In United States Patent No. 4!040~096 issued August 2, 1977, and assigned to the assignee of the present invention, a flying spot scanning system is provided which does not have constraints lmposed upon the spot size and other relationships of optical elements within the syst~n which are not always desirable. As taught therein, a finite conjugate imaging system may be 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 arrangement. Additionally, a cylindrical lens is positioned in the optical path between the polygon and the scanned medium to ccmpensate for runout and polygon facet errors.
SUMMA~Y OF THE INVENTION
In accordance with one aspect of this invention there is provided apparatus for recording information from an electrical signal onto a scanned medium comprising: means for providing a beam of high intensity light; means for modu-lating the light beam in accordance with the information content of an electrical signal; optical means for imaging said modulated beam to a spot in a focal plane at the surface of a light sensitive medium at a predetermined distance from : -3-. .

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said optical means; a multifaceted polygon having reflective facets for reflecting the modulated beam incident to it onto said medium and means for rotati:ng said polygon such that the reflected light is scanned in successive traces across said medium; said optical means including a first lens, having power in one optical plane and substantially no power in the other plane, positioned in the optical path of said light :-beam between said light providing means and said polygon with its power plane substantially perpendicular to the axis of rotation of said polygon for providing a predetermined distri-bution of light incident upon said polygon; and a second lens, having power in one optical plane and substantially no power in the other plane, positioned in the optical path of the imaged beam between said optical means and said medium, with its powex plane substantially parallel to the axis of rotation of said polygon, the aperture of said second lens being in convolution with the aperture of said optical means such that ; runout errors are corrected.
In accordance with another aspect of this invention there is provided apparatus for recording information from an electrical signal onto a scanned medium comprising: means for providing a beam of high intensity light; means for modulating the light beam in accordance with the information content of an electrical signal, first optical means, having power in one optical plane and substantially no power in the other plane, for expanding said modulated beam; second optical means in convolution with said first optical means, said second optical means defining a finite conjugate imaging system for imaging said expanded beam to a spot in a focal plane having a large depth of focus coextensive with the surface of a light sensi-tive medium at a predetermined distance from said second optical means; a multifaceted polygon having reflective facets ~ 4 1043Cl ~ -positioned in the optical path o:E said imaged beam such that certain of said facets are illuminated to reflect said beam toward said medium; said first optical means being positioned with its power plane substantially perpenaicular to the axis of rotation of said polygon; 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, and third optical means, having power in one optical plane and substantially no power in the other plane, in the optical path of the imaged beam between said second optical means and said medium with its power plane substantially parallel to the axis of rotation of said polygon such that runout errors are corrected.
In accordance with another aspect of this invention there is provided a flying spot scanning system for recording information from a video signal onto a scanned medium compris-ing: a laser for emitting a beam of collimated light of sub-stantially uniform, high intensity; means for modulating the light beam in accordance with the information content of a video signal represented by a stream of binary digits; means - for focusing said beam to a spot upon the surface of a light sensitive medium; a multifaceted polygon having reflective facets positioned in the optical path of said focused beam such that certain of said facets are illuminated to reflect said beam toward said medium; said focusing means including a first cylindrical lens positioned with its power plane substantially perpendicular to the axis of rotation of said polygon; means for rotating said polygon such that said re-flected beam is scanned through a scan angle to provide successive spot scanning traces across said medium; and a second cylindrical lens positioned in the optical path of said focused beam between said polygon and said medium with ~ -4a-~ 943CI ~
its power plane substantially paxallel to the axis of rotation of said polygon such that runout errors are corrected.

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,~ A feature of an embodiment of the invention is the in-clusion of a lens, having power in one optical plane and sub-stantially no power in the other plane, in the optical path between the polygon and the scanned medium with its power plane substantially parallel to the axis of rotation of the polygon. The light beam reflected from the facets of the polygon impinge upon the convex surface of the lens to focus at a predetermined position on the surface of the scanned medium regardless of runout and facet errors.
Another feature of an embodiment of the invention is the inclusion of a second lens similarly characterized in -the optical path between the light source and the polygon with its power plane substantially perpendicular to the axis of rotation of the polygon for imaging the beam of light onto the facets of the polygon.
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 itself. The medium to ~e scanned is one which is responsive to the modulated beam and records its information content as contained within the scanning spot in a usa~le form on its surface - ~ across the scan width.
Yet another feature of an embodiment of the invention in-cludes an embodiment of the flying spot scanning system for utilization in high speed xerography. The scanned medium 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 1~430~1~
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 ilnage from the dru~ to the - copy paper. A fusing device then fixes the images to the copy paper as it passes to an output station.
This invention will best be understood in the following description when considered in conjunction with the accompanying drawings.

DESCRIPTIO~ OF THE DRAWI~GS
Figure 1 is an isometric illustration of a flying spot scanning system in accordance with the invention.
Figure 2(a) is a side perspective view of the utilization of the correction lenses which is an integral part of the flying spot scanning system shown in Figure 1.
Figure 2(b) is a top perspective view of the utilization of both correction lenses.

In Figure 1, an embodiment 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 system. The light source 1~3~D~L~
1 is preferably a laser wh.ich generates a collimated beam of monochromatic light which may easily be modulated by modulator 4 in conformance with the information con-tained in a video signal, although any source of directed light may be employed.
Modulator 4 may be any suitable modulator, such as those electro-optical or acousto-optical in nature, for recording the video information in the form of a modulated light beam 6 at the output of the modulator 4. The modu-lator 4 may be, for example, a Pockel's cell comprising a potassium dihydrogen phosphate crystal, whose index of refraction in periodically varied by the application of the varying 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 any event, by means of the modulator 4 the information within the video signal is represented by the modulated light beam 6.
The light beam 6 is reflected froml-~mirror 8 in convolution with a cylindrical lens 10. The lens 10 is positioned in the optical path between the mirror 8 and the polygon 16 with its power plane substantially per-pendicular to the axis of rotation of the polygon 16.
The lens 10, in combination with an imaging lens 18, images the beam 6 to a horizontal fan of energy which defines a beam distribution on the order of 40 millimeters in the power plane and one millimeter in the direction parallel to the axis of rotation of the polygon 16 at the polygon 16 to illuminate at least two facets of the polygon 16. The lens 10 is required to image either a virtual or real axial ~430~
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 impinges upon at least two contiguous facets of a scanning polygon 16. The lens 10 may even be spherical or torric provided that the power plane is similarly oriented to the 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 convergent beam 20 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 preferred embodiment, imaging lens 18 is a 1-n element lens. The focal length f of lens 18 is related to Sl, S2 and d by the following thin lens equation: 1 + 1 = 1 Sl S2+d f An optimum relationship between the lens 10, the lens 18, and the polygon 16 is established as described in the following mathematical expressions:

The focal length of the lens 18, just described, produces a magnification or minification M in the distance along the beam path from lens 10 to the recording surface.
This distance, DT, is Sl + S2 + d. The magnification of the lens 18 is then M = S2 + d Sl The focal length fO of the lens 10 is such that the horizontal extent of the spot produced by it is governed in the following manner. Since a laser beam is 1~43~
essentially collimated, then the horizontal spot size WH is WH = 2.44 A fo/h.
where A is the w,~velength of light and h is the diameter of the beam directed by laser or light source 1 at the focal point of the lens 10. In this case, the spot size WH is said to be diffraction limited since the beam extent and focal length of the lens 10 determines the spot size and not source extent. Should the light source 1 have appreciable size, then the spot size WH
is determined by the following equation:
WH ~ fo tan e where e is the angular subtend of the emitting area of the source 1 as seen from the position of the lens 10.
If the desired spot size at the recording surface is Q, then the required magnification of the lens 18 would be M = Q
WH
for the diffraction limited case. ~!' For the case of appreciable source size, then M = Q
W~
Once M is known, then the focal length fl of lens 18 can be approximately determined by the following equation:
fl = Sl + S2 + d = DT
(M + l/M + 2) M + 17M + 2) In the preferred embodiment, 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 reflection of any illuminatin~
light impinging upon them. With the rotation of the polygon 16, a pair of light beams are reflected from the 3~
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 p:iezoelectric crystals or planar reflecting mirrors which axe driven in an oscillatory fashion.
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 reflection of any illuminating light impinging upon them. With the rotation of the polygon 16,assuming two configuous 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 piezoelectric crystals or 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 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 re~pective focal spots of the convergent light beams throughout a scan width x.
A substantially uniform spot size is assured throughout the scan width x even though a curved focal plane 24 is defined throughout the scanning cycle. The lens 10 in convolution with the imaging lens 18 provides a finite : -10-~04301~
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. In an alternate embodiment, the imaging lens 18 could be positioned in the optical path between the polygon 16 and the medium 25, as describe~ in aforementioned ~ U.S. Patent 4,040,096.
Medium 25 may be a zerographic drum (not shown) which rotates consecutively through a charging station depicted by corona discharge device, an exposure station where the beam from the rDtating polygon 16 would traverse a scan width x on the drum, through a developing station depicted by a cascade development enclosure, a transfer station where a web of copy paper is passed in contact with the drum and receives an electrostatic discharge to enduce a transfer of the developed image from the drum to the copy paper. A fusing de~ice fixes the images to the copy paper.
Usable 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 position within the scan width x. As the spot traverses a charged surface through a given scan angle c~, the spot dissipates the electrostatic charge in accordance with its light intensity. The electrostatic charge pattern thus produced would be developed inthe developing station and then transferred to the final copy paper. The xerographic drum would be cleaned by some cleaning device such as a rotating -1~3~

brush before being recharged by the charging device. In this manner, the information content of the scanned spot is recorded on a more permanent and useful medium. of course, alternative prior art techniques may be employed to cooperate with a scanned spot in order to utilize the information contained therein.
Since runout errors and polygon facet errors may cause poor results in terms of the quality of ima~e transfer to the scanned medium, a cylindrical lens 36 is positioned in the optical path between the pol~gon and the scanned medium with its power plane substantially parallel to the axis of rotation of the polygon 16 and with its aperture aligned with the aperture of the polygon 16. The lens 36 may also be spherical or torric provided that its power plane is similarly oriented. The lenses 10 and 36 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 pre-determined position in an ordinate perpendicular to the direction of scan on the surface 26 of the medium 25.
As shown in Figure 2, the polygon 16 is continuously driven by a motor 40 and may be synchronized in rotation to a synchronization signal representative of the scan rate used to obtain the original video signal.
In the case of the utilization of a xerographic d~um, 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 1~43~
of ~ from the desired axi~; of rotation for the polygon 16, a runout angle of ~ defines the deflection from the intended direction of scanO Other misalignments ~f optical elements within the system, such as facet mis-alignment, also may cause the same r~nout effects.
The disposition of the cylindrical lens 36 in the optical path, though, compensates for such effects.
The lens 36 is located at a distance b from the origin of the angular deflection ~ D The compensation is effected in that the off-axis beam passes through the convex surfaces of lens 36. Then, the lens 36 focuses the beam 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 provided approximately equal to or greater than the scan width x.
In this preferred embodiment, in order to determine the optimum relationship between the imaging elements 18 and 36, the aperture D of the imaging lens 18 shall be equal to approximately 2ax/fl, with an (f/number)l equal to approximately x/2a tan2 ~ . Furthermore, the focal length of lens 36 is f2, which is defined as l/b + l/b' = 1/f2 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 pra~tice the invention, the following relationship is preferred:
D2 - 2b tan~
Having defined f2 and D2, it is helpful to determine the necessary (f/number)~ for the lens 36:

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(f/number)2 = f2/D2 = b'/2 tan~ (b' + b) - The number of facets in this preferred embodiment has been found to be optimum if at least 20 to 30 facets are employed. The scan angle ~ traversed would be equal to the number of facets chosen in relation to one complete revolution of the polygon 16. An extremely useful arrangement would have the polygon 16 with 24 facets and a se~an angle ~ of 15 degrees. Since the depth of focus requirement d~of the converging beam 22 is related to the scan angle ~ in that as the scan angle ~ increases the radius of curvature of the focal plane 24 increases, it is important to define a scan angle ~ in realtion to the desired scan width x. For a scan width x of approximately 11 inches it has been found that the scan angle ~ 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 optical system shown in Figure 2(a).
Another o~timum relationship is that the cylindrical lens 36 be located at a distance from the surface 26 of the medium 25 approximately equal to the focal length f2 of the lens 36.
- 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 illumination of two contiguous facets is preferred. With such illumination, another scanning spot is provided at a distance equal to the scan width x behind the leading scanning spot with virtually no wait between successive scans. With the continuous rotation of the polygon 16 additional contiguous facets ~04~0~9.
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.
Another benefit of the particular convolution of optical elements herein defined is that the focal length f2 of the lens 36 may be sufficiently large to yield an increased depth of focus df and to enable distancing lens 36 from the recording medium 25 to reduce otherwise stringent surface quality requirement for the lens 36.

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Claims (17)

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

1. Apparatus for recording information from an elec-trical signal onto a scanned medium comprising: means for providing a beam of high intensity light; means for modulat-ing the light beam in accordance with the information content of an electrical signal; optical means for imaging said modulated beam to a spot in a focal plane at the surface of a light sensitive medium at a predetermined distance from said optical means; a multifaceted polygon having reflective facets for reflecting the modulated beam incident to it onto said medium and means for rotating said polygon such that the reflected light is scanned in successive traces across said medium; said optical means including a first lens, having power in one optical plane and substantially no power in the other plane, positioned in the optical path of said light beam between said light providing means and said polygon with its power plane substantially perpendicular to the axis of rotation of said polygon for providing a pre-determined distribution of light incident upon said polygon;
and a second lens, having power in one optical plane and substantially no power in the other plane, positioned in the optical path of the imaged beam between said optical means and said medium, with its power plane substantially parallel to the axis of rotation of said polygon, the aperture of said second lens being in convolution with the aperture of said optical means such that runout errors are corrected.

2. The apparatus as defined in claim 1 wherein said first and second lenses are cylindrical lenses.

3. The apparatus as defined in claim 2 wherein said light providing means is a finite source and the focal length of said first cylindrical lens is approximately equal to Wh/tan .THETA., wherein Wh is the horizontal size of the spot at said medium and .THETA. is the angular subtend of the emitting area of said light providing means.

4. The apparatus as defined in claim 2 wherein said light source is a laser which emits beam of collimated light of substantially uniform intensity.

5. The apparatus as defined in claim 4 wherein the focal length fo of said first cylindrical lens is approximately equal to h Wh/2,44.lambda., where h is the diameter of the laser light beam directed at the focal point of said first lens, Wh is the horizontal size of the spot at said medium, and .lambda. is the wave-length of the laser light beam.

6. The apparatus as defined in claim 4 wherein the focal length of said first cylindrical lens is approximately equal to h Wh/2Ølambda., where h is the diameter of the laser beam directed at the focal point of said first lens, Wh is the horizontal spot size at said medium, and .lambda. is the wavelength of the laser light beam.

7. The apparatus as defined in claim 6 wherein said cylindrical lens has a focal number (f/number)2 ? b'/[2 tan .beta. (b' + b)], where b is the distance of said lens from the origin of runout error, b' is the distance of said lens from said focal plane, and .beta. is the angular measure of runout error.

8. Apparatus for recording information from an elec-trical signal onto a scanned medium comprising: means for providing a beam of high intensity light; means for modulating the light beam in accordance with the information content of an electrical signal; first optical means, having power in one optical plane and substantially no power in the other plane, for expanding said modulated beam; second optical means in convo-lution with said first optical means, said second optical means defining a finite conjugate imaging system for imaging said ex-panded beam to a spot in a focal plane having a large depth of focus coextensive with the surface of a light sensitive medium at a predetermined distance from said second optical means; a multifaceted polygon having reflective facets positioned in the optical path of said imaged beam such that certain of said facets are illuminated to reflect said beam toward said medium;
said first optical means being positioned with its power plane substantially perpendicular to the axis of rotation of said polygon; means for rotating said polygon such that said reflect-ed beam is scanned through a scan angle to provide successive spot scanning traces across said medium, and third optical means, having power in one optical plane and substantially no power in the other plane, in the optical path of the imaged beam between said second optical means and said medium with its power plane substantially parallel to the axis of rotation of said polygon such that runout errors are corrected.

9. The apparatus as defined in claim 8 wherein said light source is a laser which emits a beam of collimated light of substantially uniform intensity.

10. The apparatus as defined in claim 9 wherein said first optical means is a first cylindrical lens and said third optical means is a second cylindrical lens.

11. The apparatus as defined in claim 10 wherein the focal length fo of said first cylindrical lens is approximate-ly equal to h Wh/2.44 .lambda. , where h is the diameter of the laser beam directed at the focal point of said first lens, Wh is the horizontal size of the spot at said medium, and .lambda. is the wave-length of the laser light beam.

12. The apparatus as defined in claim 10 where the focal length fo of said first cylindrical lens is approximately equal to h Wh/2.0 .lambda. , where h is the diameter of the laser light beam directed at the focal point of said first lens, Wh is the horizontal spot size of the spot at said medium and .lambda. is the wavelength of the laser light beam.

13. The apparatus as defined in claim 12 wherein said second optical means has a focal length f1 = (DT)/M+1/M+2 where DT is the distance of said first cylindrical lens from said medium and M is the desired spot size at the medium divided by Wh.

14. A flying spot scanning system for recording informa-tion from a video signal onto a scanned medium comprising: a laser for emitting a beam of collimated light of substantially uniform, high intensity; means for modulating the light beam in accordance with the information content of a video signal represented by a stream of binary digits; means for focusing said beam to a spot upon the surface of a light sensitive medium; a multifaceted polygon having reflective facets posi-tioned in the optical path of said focused beam such that certain of said facets are illuminated to reflect said beam toward said medium; said focusing means including a first cylindrical lens positioned with its power plane substantially perpendicular to the axis of rotation of said polygon; means for rotating said polygon such that said reflected beam is scanned through a scan angle to provide successive spot scan-ning traces across said medium; and a second cylindrical lens positioned in the optical path of said focused beam between said polygon and said medium with its power plane substantially parallel to the axis of rotation of said polygon such that run-out errors are corrected.

15. The apparatus as defined in claim 14 wherein the focal length fo of said first cylindrical lens is approximate-ly equal to h Wh/2.44.lambda., where h is the diameter of the laser light beam directed at the focal point of said first lens, Wh is the horizontal size of the spot at said medium, and .lambda. is the wavelength of the laser light beam.

16. The apparatus as defined in claim 14 wherein the focal length fo of said first cylindrical lens is approximately equal to h Wh/2Ølambda., where h is the diameter of the laser light beam directed at the focal point of said first lens, Wh is the horizontal size of the spot at said medium, and .lambda. is the wave-length of the laser light beam.

17. The apparatus as defined in claim 16 wherein said cylindrical lens is positioned at a distance from said medium approximately equal to the focal length f2 of said lens.