Scanner system having a dual trace spinner

The present invention encompasses a scanning optical system such as internal drum photoplotters that has a raster scanner for advancing a circularly polarized optical beam across a substrate surface in a first direction to form a scan line and for advancing the optical beam in a second direction substantially perpendicular to the first direction. There is a curved platen for receiving a substrate and an apparatus for switching the optical beam polarization between first and second directions in response to polarization control signals;. An encoder generates signals indicative of the position of the optical beam along a current scan line. There is a controller that receives the encoder signals and generates the advancement signals and the modulator control signals. The controller further provides the optical beam polarization switching signals in dependence on the encoder signals such that the optical beam polarization is switched after the completion of the current scan line. The system is characterized by a spinner which uses polarization switching to generate two scan lines per spinner rotation.

TECHNICAL FIELD 
The present invention relates to scanners and imagers in general and, more 
particularly, to scanners in having a dual scan spinner for an enhanced 
operational efficiency. 
CROSS REFERENCE TO RELATED APPLICATIONS 
Some of the subject matter herein is disclosed and claimed in the following 
U.S. patents, all of which are incorporated herein by reference. 
U.S. Pat. No. 5,291,392 entitled "Method And Apparatus For Enhancing The 
Accuracy Of Scanner Systems"; 
U.S. Pat. No. 3,555,254, entitled "Error Correcting System And Method For 
Use With Plotters, Machine Tools And The Like"; 
U.S. Pat. No. 4,851,656 entitled "Method And Apparatus For Enhancing 
Optical Photoplotter Accuracy". 
BACKGROUND OF THE INVENTION 
Raster scan photoplotters or imagers having both planar and internal drum 
design are known in the art. These devices are used in the fabrication of 
printed circuit boards. Conversely, scanners which read data from a 
substrate have similar geometries. Planar photoplotters such as disclosed 
and claimed in U.S. Pat. No. 4,851,656 have a planar surface for receiving 
a substrate. An optical exposure head is located on a movable gantry 
apparatus and is rastered above the substrate during exposure. Internal 
drum photoplotters are characterized by a substantially cylindrical 
surface portion which receives the substrate. The exposure beam emanates 
from an optical exposure head and is scanned across the substrate by a 
rotating spinner. The optical exposure head is indexed along the 
longitudinal axis of the cylinder to complete the substrate exposure. 
Internal drum raster photoplotters of the type disclosed in U.S. Pat. No. 
5,291,392 have inherent advantages over planar type scanners, including 
simplicity of design and lower costs. 
An exemplary internal drum laser raster imager, the Crescent 42 
manufactured by Gerber Scientific, Inc. of South Windsor, Conn., has an 
internal drum that utilizes a 180.degree. curved surface to receive the 
substrate. It also has a spinner centered on a longitudinal drum axis. 
With this configuration, one rotation of the spinner with its nominal 
45.degree. scan mirror produces one scan line; yielding a duty cycle of 
about 50%. As the raster image processing or "RIPing" technology of 
transferring data and thereafter interpreting it progresses, so does the 
desire to image faster. However, there are difficulties in advancing the 
imaging speed of internal drum imagers. The spinner itself is limited to a 
speed in the range of 20,000 to 24,000 RPM by the air bearing/motor 
technology and mirror deformation considerations. Another avenue of 
inquiry involves the use of multiple beams, However, a multiple beam 
approach is highly difficult to implement due to the internal drum 
scanning geometry which produces an undesirable rotation in the image 
plane of multiple beams so that they no longer lie in a plane with respect 
to the motion axes. Solving this problem requires the addition of a costly 
and complicated rotating prism assembly which must be synchronized to the 
spinner. 
A further, related issue is the desire to increase the temporal efficiency 
of the scanner or imager. As noted, prior art systems are limited to a 
maximum 50% duty cycle. Internal drum imagers can be manufactured with 
higher angular utilization (i.e. 270.degree.) with higher duty cycle but 
they add complexity for material handling. A limited duty cycle is 
undesirable from two respects. First, the lower the duty cycle, the faster 
the video electronics must be for an equivalent scan rate. Secondly, for 
systems such as computer-to-plate and direct imaging of printed circuit 
boards, there can be an exposure limitation. A higher duty cycle improves 
the system's ability to expose the substrate media. 
Earlier efforts to improve the overall throughput of imaging or scanner 
systems include the device disclosed in U.S. Pat. No. 5,187,606 to Kondo 
et al. The '606 device shows a scanning optical apparatus that has a light 
source for emitting a light beam and a deflector, such as a rotating 
polygonal mirror, with a plurality of mirror surfaces for deflecting the 
light beam. Each mirror surface of the polygonal mirror has a pair of 
reflecting surfaces inclined toward the center axis of rotation of the 
polygonal mirror and orthogonal to each other. There is a fixed reflecting 
mirror arranged in an opposed relationship with one of the pair of 
reflecting surfaces so that the light beam deflected by the reflector is 
reflected, to be returned to the deflector again. The '606 system is used 
to increase the scanning angle of the laser beam to twice the width as 
compared to that of conventional polygonal mirrors, thereby increasing the 
speed of scan without increasing the rotational speed of the polygonal 
mirror. U.S. Pat. 4,445,126 to Tsukada discloses an image forming 
apparatus in which recording medium is scanned with a plurality of light 
beams. The '126 apparatus includes a beam generator for generating a 
plurality of light beams and presenting them simultaneously to a facet of 
rotating polygonal mirror. The purpose of the '126 apparatus is to 
generate a plurality of scan lines at a given time during operation. 
An image recording device which relies on multiple beams is disclosed in 
U.S. Pat. Nos. 4,506,275 and 4,517,608 to Maeda et al. The Maeda et al 
device includes a recording unit for duplicating and recording halftone 
images on photosensitive material. The recording unit comprises an 
acousto-optic light modulating element including a plurality of ultrasonic 
wave exciting portions disposed side by side on a single acousto-optic 
medium. The ultrasonic wave exciting portions independently modulate an 
incident light beam into a plurality of modulated light beams in response 
to image signals from a photoelectrical scanning means. There is a scaled 
down optical system which then reduces the diameter of the plurality of 
modulated light beams at a plurality of light transfer elements to 
transfer the light beams from the scaled down optical system to a focusing 
lens to be projected onto a film in a recording cylinder. The system as 
set forth in the Maeda et al patents relies on a fixed scanning head. The 
substrate is located on external surface of the rotating drum. 
A multiple beam optical modulation system is disclosed in U.S. Pat. No. 
5,251,057. The '057 system is used in a raster output scanner that employs 
one original beam and a facet of a rotating polygon to generate to 
consecutive scan lines. The original beam is first separated into two 
beams in a beam splitter. The resultant beams are polarized ninety degrees 
apart, and directed to a modulator. The beams are a sufficient distant 
apart so that the acousto-optic (a/o) modulator can modulate each beam 
with a minimum of crosstalk. The output beams are put brought together to 
within one scan line separation by a beam recombination device, which is a 
reversed beam splitter. The beams can be brought together to close 
proximity without optical interference because the beams are polarized 
ninety degrees apart. 
None of the systems disclosed by the prior art offer a doubling of scan 
system efficiency nor is there found a system which achieves any 
improvement in throughput without extensive and cumbersome modifications 
to system optics and electronics. It would be advantageous to have a 
system for use with internal drum type scanners or photoplotters which 
provides two scans for each rotation of the system spinner. The present 
invention is drawn toward such a system. 
SUMMARY OF INVENTION 
An object of the present invention is to provide an optical spinner for use 
with a photoplotter or scanner that provides two scan lines for each 
rotation. 
Another object of the invention is to provide a spinner of the forgoing 
type that allows for approximately one hundred per cent duty cycle 
operation. 
Still another object of the present invention is to provide a system of the 
foregoing type in which the system throughput approximately doubles for a 
given spinner rotation speed. 
According to one aspect of the present invention, a scanning optical system 
includes an optical source for generating a circularly polarized optical 
beam. There is a curved platen to receive a substrate and a modulator for 
providing optical modulation to the circularly polarized optical beam in 
response to received modulator control signals. A raster scanner is 
responsive to advancement control signals and advances, relative to the 
substrate, the circularly polarized optical beam across the substrate in a 
first direction forming a scan line. The raster scanner also advances the 
circularly polarized optical beam relative to the substrate in a second 
direction substantially perpendicular to the first direction displacing 
one scan line from another. There is also an apparatus for switching the 
optical beam polarization between first and second directions in response 
to polarization control signals. An encoder generates signals indicative 
of the position of the circularly polarized optical beam along a current 
scan line. A controller receives the encoder signals and generates the 
advancement signals and the modulator control signals. The controller 
further provides the optical beam polarization switching signals in 
dependence on the encoder signals such that the optical beam circular 
polarization is switched after the completion of the current scan line. 
The scanning optical system also includes a spinner that receives the 
optical beam from the polarization switching apparatus. The spinner has an 
quarter wave plate that receives the circularly polarized optical beam and 
provides a linearly polarized scan beam. A polarization sensitive 
beamsplitter reflects the linearly polarized scan beam at an internal 
beamsplitter surface if the linearly polarized scan beam is polarized in a 
first linear direction. A quarter wave plate receives the linearly 
polarized scan beam from the polarization sensitive beamsplitter if the 
linearly polarized scan beam is polarized in a second linear direction 
orthogonal to the first linear direction. The quarter wave plate rotates 
the second direction polarized scan beam by ninety degrees as it transits 
the same. There is a retroreflector for returning the ninety degree 
rotated second direction polarized scan beam through the quarter wave 
plate to the polarization sensitive beamsplitter. 
According to another aspect of the present invention, a spinner for use in 
a scanning optical system that has an apparatus for generating a 
circularly polarized optical beam, an apparatus for switching the optical 
beam polarization between first and second directions, a curved platen for 
receiving a substrate and a raster scanner responsive to control signals 
for advancing, relative to the substrate, the circularly polarized optical 
beam across the substrate in a first direction forming a scan line and an 
encoder for generating signals indicative of the position of the 
circularly polarized optical beam along the current scan line. The spinner 
includes an quarter wave plate for receiving the circularly polarized 
optical beam and provides therefrom a linearly polarized scan beam. There 
is a polarization sensitive beamspliter reflecting the linearly polarized 
scan beam at an internal beamsplitter surface if the linearly polarized 
scan beam is polarized in a first linear direction. A quarter wave plate 
receives the linearly polarized scan beam from the polarization sensitive 
beamsplitter if the linearly polarized scan beam is polarized in a second 
linear direction orthogonal to the first linear direction. The quarter 
wave plate rotates the second direction polarized scan beam by ninety 
degrees. A retroreflector receives and returns the ninety degree rotated 
second direction polarized scan beam through the quarter wave plate to the 
polarization sensitive beamsplitter.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to both FIGS. 1 and 2, there is shown in simplified schematic 
form a portion of an internal drum raster photoplotter 10 having an 
internal drum 12 with a surface 14 that comprises a portion of a cylinder. 
The internal drum is carefully fabricated and must maintain the 
cylindricity of the drum surface with great accuracy regardless of 
variations in environmental parameters such as temperature. To that end 
the internal drum is a substantial structure preferably of cast aluminum 
with a series of reinforcing ribs (not shown) spaced along an outside 
perimeter. 
The drum surface is adapted to receive a substrate and includes a plurality 
of holes 16 which communicate with a plurality of internal channels 18 
through which a vacuum is generated by conventional apparatus not shown in 
the drawing. The vacuum is used to hold a substrate 21 in place during the 
exposure process. Alternative methods can be equivalently used to hold the 
substrate in place, including electrostatic and mechanical retention 
techniques. 
The photoplotter also includes a rail 20 that has a carriage mounted raster 
scanner 22 for scanning an optical beam 24 about the substrate surface in 
response to command signals received from controller 26 in a manner 
detailed hereinafter. The raster scanner includes a linear encoder 28 for 
generating signals indicative of the position of the raster scanner as it 
moves along the rail. Also included is a fast scan apparatus 30, 
preferably comprised of a motor 32 and a spinner 34, for receiving the 
optical beam at a mirror surface 35 from an optical beam source, such as 
laser 36, and for exposing a series of scan lines 38 on the substrate by 
rotating the spinner about a spin axis 40, typically at 12,000 rpm. A 
rotary encoder 42 is included for generating signals indicative of the 
angular position of the mirror surface during a scan. The optical beam is 
provided along the spin axis to be received at a central point on the 
mirror surface. 
FIG. 3 is a simplified schematic illustration of a portion of a prior art 
photoplotter 44. Shown in FIG. 3 is a first portion of a rotary drum 
substrate surface 46 which receives a beam of light 48 reflected from a 
mirror surface of spinner 50. The spinner 50 is rotated about a rotational 
axis 52 and advances the beam from right to left in the figure. The 
spinner mirror surface is oriented at 45 degrees along the central axis of 
the internal drum which also corresponds to the optical axis along which 
the exposure beam traverses before presentation to the substrate surface. 
The mirror surface is oriented to the optical axis of a laser beam and 
presents the beam directly to the surface. A full rotation of the spinner 
will yield a laser beam presented to the entire internal drum surface; 
both the section containing the substrate and the remainder thereof. 
There is an initial spinner position 54 before which the beam would 
otherwise be presented above the internal raster drum substrate surface 46 
and therefore not to the substrate. FIG. 4 shows a second spinner position 
56 subsequent to the initial position shown in FIG. 3 in which the beam is 
almost completely advanced across substrate. The rotational extent of 
these two positions is displayed diagrammaticaly with respect to FIG. 5 by 
curve 58. Beyond spinner position 56, the spinner must rotate around to 
its initial position shown in FIG. 3 before the controller can again 
present the modulated exposure beam for creating a scan line. In many 
scanners, the internal drum surface which receives the substrate extends 
only 165 degrees, much less than the practical upper bound of 180 degrees. 
As a result, the duty cycle of prior art systems is even less than 50%. 
FIGS. 6 and 7 are simplified schematic drawings showing a spinner 60 
provided according to the present invention. The spinner allows for two 
scan lines for each rotation. In the present invention, the speed of the 
spinner is substantially the same as in known systems. Fundamental to the 
present design is the concept of polarization switching of the incident 
laser beam. In FIG. 6, a collimated beam of light 62 which feeds this 
scanner is circularly polarized in a clockwise rotation. A first quarter 
wave plate 64 positioned on a first spinner surface 66 to receive the 
beam. A linearly polarized, first scan beam 68 is created through the 
interaction of the 1/8th wave plate and transits a polarization sensitive 
beamsplitter (PSBS) 70 with an "S"" orientation. This first scan beam is 
received and reflected by a internal surface 72 of the PSBS such that the 
reflected beam exits the spinner to follow the direction of scanner 
rotation. The internal surface is polarization sensitive such that 
incident light of select polarizations will be transmitted while other 
polarizations (e.g., "S" orientation) will be reflected. The PSBS surface 
reflects nearly 100% of the linearly polarized light. There is also a lens 
74 which focuses the first scan beam before presentation to the substrate. 
The first scan beam, therefore, is generated in a manner similar to that 
done in prior art systems and constitutes the initial beam generated by 
the present system. For the second scan, an input (feed) beam to the 
scanner is polarization switched by 180.degree. to 
circular/counterclockwise, as represented by beam 76 in FIG. 7. The first 
quarter-wave plate now creates a linearly polarized second scan beam 77 in 
a "P" orientation perpendicular to the S-beam. The light of the second 
scan beam propagates through the polarization sensitive beam splitter past 
the internal surface with nearly 100% efficiency. Following the PSBS, the 
polarization of the light is further rotated by 90 degrees by quarter wave 
plate 78 and reflected back by retroreflector 79. On return, the 
retroreflected beam is again polarization rotated by an additional 90 
degrees by the quarter wave plate to be polarized in the "S" orientation, 
as was the first scan beam. The PSBS internal surface reflects the now S 
polarized second scan beam which transits a focusing lens 80 and presents 
the same to the substrate. 
The present system takes advantage of the above spinner by including an 
accousto-optical device 81 which receives switching signals from the 
controller to change the polarization of the input beam between clockwise 
and counterclockwise polarizations. Since the preferred encoder generates 
a once per revolution signal, the controller now enables presentation of 
the modulated beam at two predetermined times during a each revolution of 
the spinner, as opposed to once per revolution. Similar changes to the 
other components and system parameters are accomplished as well, including 
a doubling of the advancement speed in the slow scan direction. 
Other examples of an optical return which can be substituted for the flat 
mirror of FIGS. 6 and 7 include a retroreflector, a roof prism or a roof 
mirror. Systems built in accordance with the present invention and which 
incorporate either a simple mirror or a retroreflector are burdened by the 
need for almost perfect alignment of the optical components which comprise 
the optic train. 
Without such ideal alignment, the spinner will present the first and second 
beams to necessarily different positions on the substrate when they 
should, in fact, be superimposed (as determined without slow scan 
indexing). As shown in FIG. 8, there is seen two scan lines written on a 
substrate. Lines 116, 118 are respectively generated by first and second 
scan beams by a system with a simple mirror as an optical return and with 
some error introduced. The lines clearly deviate from the ideal positions 
of nominal scanlines 120, 122 which would be produced by a perfect system. 
The optic train which generates and guides the first and second scan beams 
in the present system can (and typically does) have some degree of 
misalignment among the several optical components or inaccuracies in the 
components themselves. Accordingly, the optic train, and the spinner in 
particular, must be tolerant of first scan beam deviation from the input 
optic axis and the subsequently induced deviation from the preferred 
optical path of the second scan beam, or simple variation of the second 
scan beam from its preferred path. In certain situations the second scan 
beam will be provided to the substrate at a location that is different 
from the substrate position which receives the first scan beam even when a 
roof prism or mirror is employed as an optic return. As a result, the 
"written" scan lines will differ in position according to whether written 
by the first or second scan beams. The tolerance for this misalignment is 
extremely small; errors less than or equal to 20 microinches are 
problematic. 
The embodiment of the present invention as set forth with respect to FIG. 9 
incorporates a roof prism or roof reflector as an optical return of the 
second scan beam, and as such, can tolerate a wider range of input angles 
and displacements and yet still superimpose the first and second scan 
beams. In FIG. 9 there is schematically shown a simplified illustration of 
an alternative spinner 82 provided according to the present invention. The 
alternative spinner is substantially similar to the spinner described 
above. A circularly polarized beam 84 is presented along input axis 86 to 
a quarter wave plate 88 and thereafter to a polarization sensitive beam 
splitter 90. The light either is reflected from internal surface 92 or 
passes therethrough in dependence on the beam's polarization. 
A first scan beam 94 exits the polarization sensitive beam splitter 90 
along output axis 96 through lens 98. A second scan beam 100 passes 
through quarter wave plate 102 and enters an optical return 104 which is a 
roof prism in the Figure. The light is turned by reflection within the 
prism or roof reflector and presented again to quarter wave plate 102 
which rotates the polarization of the beam, resulting in the second scan 
beam being reflected by the internal surface 92 and presented to lens 106 
along axis 108. Deficiencies in the optic train corresponding to an 
angular deviation 110 in the input beam from ideal coincidence with input 
axis 86. As a result, scan lines vary from their respective preferred 
positions as written on a substrate surface. 
In the embodiment of the present invention shown in FIG. 9, there is also 
included apparatus 126, 127 for allowing lateral adjustment of the lenses 
98, 106 relative to the output optic axes 96, 108. The apparatus are of a 
type known in the art and can be manually adjustable, as either or both of 
the lens positions can be adjusted during an initial alignment to remove 
any mispositioning of the scan lines relative to one another as written by 
the respective first and second beams, and thereby compensate for any 
errors from the optic train. In the alternative, the spinner may include 
an optical wedge or wedges 112 operated by control apparatus 114 to remove 
error from the input beam. 
It is further understood that only an adjustment mechanism cause scan line 
superposition in the slow or cross scan dimension is required. Adjustment 
during a scan can be provided effectively by means of modification to the 
pixel clocking electronics. 
Spinners which have only simple reflectors (e.g. planar, reflective 
surfaces) are burdened by the need for dynamic compensation of scan line 
position errors. These systems must dynamically compensate for errors "on 
the fly" (as the scan line is written), since the optics of these spinners 
do not allow for a single correction to be made which is valid for every 
pixel in the scan line. Accordingly, compensation apparatus must be 
programmed with the appropriate magnitude of compensation for each pixel 
position in a scan line written by each of the first and second scan 
beams. 
FIGS. 13-15 graphically show the computed effect of error in the scan lines 
produced by the first and second scan beams with the embodiment of the 
present invention shown in FIG. 1 as compared with that of FIG. 9. The 
error can be the result of input beam misalignment, a mispositioning of 
one or more optic train elements, defects therein or combinations thereof. 
In FIG. 13, error is deliberately introduced into the first or primary scan 
in a system as provided by the present invention and is manifested as bow 
error (curve 170). The bow error is a function of spinner rotation angle 
from 0 to 180 deg. and has a magnitude of plus or minus approximately 0.22 
mm. Curve 172 is shown in FIG. 14 and results in a system in which a 
simple mirror is used as the optic return, while a system which uses a 
roof mirror or prism yields curve 174 in FIG. 15. A comparison of curves 
172 and 174 reveals that only the system with the roof mirror produces in 
the second scan line the same error, both in magnitude and sign, as was 
introduced into the first. 
Accordingly, a simple adjustment to remove the error from the first scan 
beam will remove the error from the second scan beam. Apparatus to remove 
the error is selected in dependence on the application. In a system that 
employs a cube/beamsplitter and roof mirror, a wedge prism may be inserted 
about the spin axis corotating with the assembly and rotated to remove the 
error, as noted above. The compensation apparatus described above is also 
preferred in Wollaston systems described hereinafter. 
Referring now to FIG. 10, there is shown a simplified schematic 
illustration of a alternative embodiment 128 of the present invention 
characterized by a Wollaston prism 130. The alternative Wollaston system 
also employs polarization switching and a polarization sensitive optical 
component. As with other embodiments, an incident beam 132 of light having 
either a left or right circular polarization is presented through a 
quarter wave plate 134 producing a beam of one linear polarization, (S) as 
an example. As the light transits the Wollaston prism, a first scan beam 
136 is generated which is displaced from axis 138 by a deviation angle 
141. Light in a second scan beam 140 with the complimentary (P) 
polarization proceeds down a second path with an equal and opposite 
displacement angle from the optic axis. As seen in the schematic, 
sectioned illustration of the prism in FIG. 11, the beams 142, 144 of 
different polarization transiting a Wollaston prism 130 are deflected an 
equal amount from the optic axis 145. 
Following the Wollaston prism, is a lens 146 centered about the axis and 
common to both polarization paths. The lens focuses the collimated light 
to two separate foci 148, 150. In the embodiment of FIG. 10, the lens is 
of moderate complexity because of the off axis performance requirements 
but is otherwise of conventional design. A sectioned schematic 
illustration is found in FIG. 12 showing elements 152-156 that comprise 
the lens providing a scan beam 158 to a substrate 160. 
A double mirror 162 is also shown in FIG. 10 that has an "ax blade" 
geometry and which is positioned to receive the focused scan beams from 
the lens. Each beam is presented to a respective surface 164, 166 of the 
mirror to fold out the beam towards the cylindrical imaging surface that 
holds the substrate. In other embodiments, the lens structure may be 
deleted, assuming that the "ax mirror" or equivalent optics contains some 
optical power to bring the beams into focus at the cylinder's surface. 
The scanning system provided by this embodiment presents several advances 
over some of the other embodiments of the present invention described 
hereinabove. The Wollaston system is inherently symmetrical. In the 
presence of alignment errors to the scan assembly, the scan lines which 
are produced are exactly equal. The two complimentary beam paths are 
collinear and are superimposed on the surface of the drum, assuming that 
the drum is not moved in the slow scan direction. In other words, the 
input beam is now parallel to the spin axis. In the event of a 
misalignment within the rotating scanner, for example ax blade mirrors 
which are located at slightly different angles relative to the spin axis, 
compensation is straightforward and is provided by shifting the scan lens 
laterally in its position with respect to the spin axis or by alignment of 
a co-rotating wedge prism. 
In addition, the Wollaston system of FIG. 10 avoids spinning of the 
focusing lenses off the input axis, and thereby avoids all of the 
challenges of mechanical stress and stress induced optical birefringence 
which would otherwise occur. As a result, the system can be created to 
spin at higher speeds than would otherwise be possible in a system with 
optical components located off axis. Moreover, the Wollaston system 
presents fewer parts than does other embodiments of the present invention, 
and the tolerance requirements on these parts are generally lower than 
that of the above described embodiments. 
Similar systems may be built using a Rochon prism in place of the Wollaston 
prism as shown in FIG. 16. As is known, a Rochon prism 176 which receives 
a beam 178 will deflect light of a selected polarization. A scan beam 180 
of a first polarization will exit the prism along optic axis 182 directly, 
with the second scan beam 184 presented at an angle thereto. A Glan or 
Glan-Thompson prism can also be used to construct a system of the present 
type without substantial modification to the systems described 
hereinabove. Those skilled in the art will note that a Glan-Thompson prism 
is similar to a Nicol prism which produces plain polarized light, but has 
its internal faces normal to the optic axis. 
Referring now to FIG. 17 there is shown in diagrammatic form an 
illustration of an alternative embodiment 186 of the present invention 
characterized by the simultaneous presentation of dual scan beams. In 
comparison to the embodiments described hereinabove, the dual scan system 
of FIG. 17 has two light sources 188 and 190 which are both circularly 
polarized but whose polarization have the opposite sense (e.g., right hand 
vs. left hand circular polarization). Both beams are combined at beam 
combiner 192 for presentation along scan axis 194. Both beams are then 
presented simultaneously to scanner 196 which is substantially as 
described hereinabove with respect to FIG. 9. An adjustment mechanism to 
198 may also be included to perform the same functions as noted above. 
With the embodiment of FIG. 17, each beam 200 and 202 maps directly back to 
its laser modulator and data stream. Consequently, the number of scans per 
revolutions has doubled. The embodiments described previously generate two 
scans per revolution whenever the active scanning is less than .pi. 
radians (180.degree.). The embodiment of FIG. 17 no longer has the limit 
of 180.degree.. However, two modulators as well as two optical sources 
(lasers) and beam combining optics are required with this embodiment of 
the present invention. 
The Wollaston system set forth above suffers from several drawbacks which 
can affect system performance. These include an asymmetry in deviation 
angles of the beams output from the Wollaston prism. An input beam which 
traverses a path at a slight angle to optic axis will exit a Wollaston 
prism at an angle whose magnitude is dependent on beam polarization. In 
practice, typical deviation angles are -9.7.degree. and +10.9.degree.. 
These may be balanced to a mean separation of 10.3.degree. with the 
addition of an optical wedge. However, this configuration does not solve 
other problems which afflict the Wollaston system. 
The Wollaston system also possess an asymmetry in the angular 
magnification. The input beam is received by the prism at a small angle to 
the optic axis. This small input angle is typically due to a residual 
alignment error between the optical chassis and the scanner (e.g. 0-5 arc 
minutes). Beam magnification is different for each polarization. In order 
for the system as a whole tolerate alignment errors, the Wollaston system 
needs to be highly symmetrical. For an input angular error of S arc 
minutes, the Wollaston prism will generate a constant tilt error in the 
output beams within 0.2 arc seconds, in addition to the input tilt. The 
standard Wollaston prism has a tilt response of about 1 arc minute to a 5 
arc minute input (or 300 times the tilt error). Moreover, the Wollaston 
prism has an exit pupil, representing the axial tilt position, which is at 
unequal planes for the two cases. 
All of these problems are effectively addressed in a modified Wollaston 
prism system 204 of FIG. 18, also referred to as a Straayer prism 
assembly. The system 204 is substantially the same as shown with respect 
to FIG. 10, but includes a prism assembly 206 comprised of first and 
second prisms 208, 210 substantially identical, but with crystal axes 212, 
214 oriented orthogonal to one another. The crystallographic axis of the 
first prism extends out from the page, while that of the second prism is 
lengthwise in the Figure. The prisms each have base angles of 67 deg, 20' 
plus/or minus 10', with an apex angle of 48 deg, 50' plus/minus 10'. In 
the embodiment of FIG. 18, the prisms are cemented together so the 
assembly has a base of 12 mm plus/minus 0.2 mm and a height of 13.2 mm. 
The specifications set forth above are exemplary of a selected aperture 
and wavelength (488 mm). 
There is also a glass (SF57) window 216 having a known anti-reflection 
coating. The window is 6 mm plus/minus 0.25 mm in thickness and 20 mm 
plus/minus 0.25 mm in diameter and is placed in the system to receive an 
input optical beam prior to presentation of the same to the prism 
assembly. The index, thickness, and tilt angle of this window are chosen 
to force the beam deviations to occur from a common exit pupil plane. 
The design of this modified Wollaston system has several distinguishing 
characteristics. The beam has an angle of incidence on first prism surface 
which is equal to the mean angle of exit or average angle between the two 
deviation beams. The angle of incidence in the system shown in FIG. 18 is 
approximately 22.5.degree.. The prism angle of the deviation prisms being 
nearly equal (approximately 48.degree.). The addition of a plane parallel 
deviation window brings the exit pupils of the two beam deviation cases 
into coincidence. 
FIGS. 19 and 20 illustrate the performance of the modified Wollaston system 
of FIG. 18 when receiving first and second beams 218 and 220, 
respectively, of different polarization. In FIGS. 19 and 20, as in FIG. 12 
the beams traverse from right to left in contrast to the several other 
Figures. 
In FIG. 19, input beam 218 is presented first through window 216 before 
transiting prism assembly 206. The beam is then deflected downward in the 
Figure to be received by a first surface of the double mirror as in the 
system of FIG. 12. The double mirror is not shown in FIGS. 19 or 20, but 
is substantially as indicated with respect to the Wollaston prism 
embodiment of FIG. 10. In FIG. 20, the second beam 220 which has the 
opposite circular polarization as compared with beam 218 is presented 
through the same window 216 and prism assembly but now is deflected 
upwards in the Figure for presentation to a second surface of the double 
mirror. 
Similarly, although the invention has been shown and described with respect 
to a preferred embodiment thereof, it would be understood by those skilled 
in the art that other various changes omissions and additions thereto may 
be made without departing from the spirit and scope of the present 
invention.