Method and apparatus for generating a scanned optical output signal

A system is disclosed for transforming a collimated beam of light, such as that generated by a laser, into a line scan or a raster image for projecting onto a screen or other object. The laser beam is directed at a rotating mirror having a plurality of facets disposed at equal circumferential intervals around the mirror axis of rotation. The light reflected from the rotating mirror generates a first scanned reflected output beam. A first reflecting surface is aligned to receive the first scanned reflected output beam and generates a second scanned reflected output beam. This second scanned reflected output beam passes through a first pivot vertex and is directed back onto the rotating mirror facets by a second reflecting surface to generate an angularly amplified output beam which scans a first plane. The angularly amplified output beam is directed through a second pivot vertex and is then deflected through a second plane which is oriented perpendicular to the first plane. The angularly amplified output beam which has been deflected through the first and second planes thereby projects a raster image on a screen.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to optical scanning methods and apparatus, and more 
particularly, to an optical scanning method and apparatus which develops a 
scanned output beam from a collimated beam of light. 
2. Description of the Prior Art 
Optical systems for transforming a collimated beam of light into a raster 
scan are known in the prior art. However, many of the prior art systems 
are generally characterized either by considerable complexity or by 
limited performance. Some comparatively high performance prior art systems 
utilize a plurality of rotating mirrors which have a large number of 
facets disposed circumferentially about the axis of rotation. In order to 
achieve the required high scan rate and scan efficiency, comparatively 
large diameter polygon mirrors having a multiplicity of facets must be 
provided and rotated at high rates. Because of bearing strength 
limitations and the inability to achieve suitable dynamic balance, it has 
been impractical to fabricate a scanning device utilizing a single polygon 
mirror which is capable of being rotated at a sufficiently high R.P.M. to 
produce a television raster scan. 
One solution to the problem experienced by prior art devices is disclosed 
in U.S. Pat. No. 3,782,803 (Buck). This patent discloses a flat mirror 
scanner including a multi-faceted pyramidal mirror which is rotated in a 
vertical plane, a multi-faceted polygon mirror which is rotated in a 
horizontal plane and a pair of spherical mirrors. The pyramidal mirror and 
the polygon mirror are driven by separate motors at separate rates and 
must be closely synchronized to generate the desired raster scan image. 
The pyramidal mirror rotates at a comparatively slow rate and provides 
vertical deflection while the polygon mirror rotates at a comparatively 
fast rate and provides horizontal deflection. 
SUMMARY OF THE INVENTION 
It is therefore a primary object of the present invention to provide an 
optical scanning method and apparatus for projecting a scanned beam on an 
object or screen which requires only a single, small diameter rotating 
polygon mirror which achieves a significantly increased scan efficiency by 
reflecting an input light beam from the polygon mirror facets two separate 
times. 
Another object of the present invention is to provide an optical scanning 
method and apparatus for projecting a raster image on a screen which uses 
a rotating polygon mirror to generate the horizontal deflection for the 
raster image and an oscillating galvanometer driven planar mirror to 
generate the vertical deflection for the raster image. 
Yet another object of the present invention is to provide an optical 
scanning method and apparatus for projecting a scanned beam on a screen 
which includes an optical modulator for varying the intensity of the light 
beam. 
Still another object of the present invention is to provide an optical 
scanning method and apparatus which is capable of projecting either a 
monochromatic or color television image on a large screen and which is 
compact, portable and less expensive than prior art systems. 
Briefy stated, and in accord with one embodiment of the invention, an 
optical scanner receives an input light beam and directs that light beam 
onto a rotating mirror at a first position to produce a first scanned 
reflected output beam. A first reflectting surface is aligned to receive 
the first scanned output beam and generates a second scanned reflected 
output beam. The second scanned reflected output beam is directed through 
a first pivot vertex back onto the rotating mirror to generate an 
angularly amplified output beam which scans a first plane. To generate a 
two dimensional raster scan for television or other uses, the angularly 
amplified output beam is deflected through a second plane oriented 
perpendicular to the first plane and is then projected onto a screen or 
other object to produce a raster image corresponding to the video input 
signal. The television image projection version of the present invention 
includes an optical modulator which modulates the intensity of the input 
light beam to correspond to the intensity of the video signal component of 
a composite video signal.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In order to better illustrate the advantages of the invention and its 
contributions to the art, a preferred hardware embodiment of the invention 
will now be described in detail. 
Referring now to FIGS. 1 and 2, a source of collimated light from a laser 
or equivalent device generates an input light beam indicated generally by 
reference number 10. Light beam 10 is properly aligned with the optical 
scanning apparatus of the present invention by directing it through a 
conventional bore sighting fixture 12. Input light beam 10 is directed 
vertically downward by mirror 14 to the input of an optical modulator 16 
which modulates the intensity of the light beam to correspond to the 
intensity of a video signal of the type provided by a closed circuit 
television system or by a commercial television receiver. In the preferred 
embodiment of the present invention, modulator 16 is available from 
Intra-Action Corp. of Bellwood, Ill. and is designated as acoustic optic 
modulator Model No. AOM-70. Acoustic optic modulator 16 generates two 
significant angularly displaced optical output signals. In the preferred 
embodiment of the invention, a mask 17 is positioned as shown in FIG. 1 to 
block the zero order output signal from modulator 16. The operation of 
this acousto optic modulator will be discussed in greater detail below in 
connection with FIG. 7. Other different types of optical modulators, such 
as an electrooptic modulator, can also be used to modulate light beam 10. 
The modulated optical output signal from modulator 16 is redirected into a 
horizontal plane by mirror 18. Mirrors 20 and 22 direct the modulated 
light beam to an input aperture of telescopic objective 24. Telescopic 
objective 24 and positive lens 26 function as a beam expander to expand 
the diameter of the modulated light beam. Positive lens 28 intercepts this 
expanded beam and converges it slightly. The converged, modulated light 
beam is directed onto a multi-faceted or polygon mirror 30. Polygon mirror 
30 includes 24 facets each having a face length of 0.266 inches. The 
inscribed circle within the facets of mirror 30 has a diameter of 2.02 
inches. The specific polygon mirror utilized in the preferred embodiment 
of the invention is available from Lincoln Laser Co. of Phoenix, Ariz. and 
is designated as Model No. PO-24-202-037. 
Polygon mirror 30 is coupled to the output shaft of a high speed 
synchronous electric motor which is supported by motor housing 32. The 
entire motor/motor housing/polygon mirror assembly is inclined 
approximately 5.degree. with respect to the vertical axis of the optical 
scanning apparatus as illustrated. 
In order to recreate the desired 15,750 Hertz horizontal deflection rate, 
the polygon mirror is rotated at 1/24th of that rate which equals 656.25 
revolutions per second or 39,375 revolutions per minute. The appropriate 
rotational rate and highly stable bearing support systems requird are 
provided by an electrical motor assembly manufactured by the Lincoln Laser 
Company of Phoenix, Ariz. and designated as Model No. 225 XLIM. 
Referring now also to FIGS. 3, 4 and 5, the modulated light beam incident 
on the facets of mirror 30 is reflected outward and downward as a result 
of the angle of inclination of motor housing 32 onto a first reflecting 
surface in the form of a spherical mirror section 34. The central section 
of the complete spherical mirror of which spherical mirror section 34 
forms an element would prevent the passage of the modulated light input 
beam from lens 28 to polygon mirror 30 necessitating the removal of 
approximately the upper 1/32 of an inch of the lower half of the spherical 
mirror. The vertical axis of spherical mirror 34 is oriented parallel to 
the vertical axis of the video display apparatus of the present invention. 
The modulated light input beam which is reflected from the facets of 
polygon mirror 30 and which scans the upper surface area of spherical 
mirror section 34 will be referred to as the first scanned reflected 
output beam and will be designated by reference number 36. 
Spherical mirror 34 receives the first scanned reflected output beam 36 and 
generates a second scanned reflected output beam designated by reference 
number 38. Second scanned reflected output beam 38 passes through a first 
pivot vertex designated by reference number 40 which is defined by the 
radius of curvature of first reflecting surface 34. 
Directing means in the form of a prism or a pair of inclined mirrors 42 is 
positioned to receive second scanned reflected output beam 38 after it has 
passed through first pivot vertex 40 and both elevates and redirects this 
light beam back onto the rotating mirror facets which are positioned on 
the rear surface of mirror 30. The reflection of second scanned reflected 
output beam 38 from a rear mirror facet produces an angularly amplified 
output beam 44 which is deflected within a single plane and corresponds to 
the horizontal raster scan. Angularly amplified output beam 44 is 
deflected at an angular rate substantially greater than the rate of 
deflection of either the first or second scanned reflected output beams. 
The phenomenon of angular amplification will now be described in greater 
detail by reference to FIGS. 4 and 5. Note first that the impact point 
designated by reference number 46 of the input light beam on the front 
mirror facet of mirror 30 moves along the length of the facet as the 
mirror rotates since the incident light beam remains fixed in space while 
the mirror rotates. In FIG. 4A, incident point 46 occurs at the leading 
edge of a front facet; in FIG. 4B, incident point 46 lies at the midpoint 
of the front facet; and in FIG. 4C incident point 46 lies at the end of 
the front facet. The position of the impact point designated by reference 
number 48 on the rear facet of mirror 30, on the other hand, maintains a 
relatively constant position somewhere in the vicinity of the midpoint of 
the rear facet. In other words, impact point 48 tracks the rotary motion 
of the rear facet and an angularly amplified output beam 44 is generated 
as a result of this unique relationship. In FIGS. 5A and 5C, reference 
number 50 designates the virtual pivot vertex of angularly amplified 
output beam 44. 
Angularly amplified output beam 44 is reflected from the rear facet of 
mirror 30 and is directed to focusing means which takes the form of a 
second reflecting surface or spherical mirror section 52 which is oriented 
parallel to first reflecting surface 34. The reflected output beam from 
second reflecting surface 52 is directed vertically upward by mirror 54 to 
a 1/4 inch diameter planar mirror 56 which is deflected back and forth as 
indicated by the arrows designated by reference number 58 by a 
galvanometer movement as will be described below. Mirror 56 is positioned 
at a second pivot vertex which is defined by the curvature of second 
reflecting surface 52. The location of mirror 56 at this pivot vertex 
permits the overall size of mirror 56 to be extremely small resulting in a 
low mass system which is readily reflected by a galvanometer movement. The 
deflection indicated by the arrows designated by reference number 58 
provides the vertical or Y-axis deflection for the raster scan. The focal 
point of the light beam reflected from mirror 56 is controlled to be 
located at or close to the surface of the screen on which the raster image 
is to be projected. 
Prior art optical scanniong systems that utilize rotating polygon mirrors 
have been unable to achieve both the high velocity rotational scanning 
rate and the ninety percent scan efficiency required to duplicate a 
television raster image due to mechanical limitations which limit the 
maximum rotational velocity of the polygon mirror. Polygon mirrors equal 
in size to the mirror utilized in the preferred embodiment of the present 
invention are available and can be rotated at the required 40,000 R.P.M. 
rate required to duplicate a television raster scan, but the prior art 
technique of reflecting a light beam from only a single facet of a small 
diameter rotating polygon mirror can achieve scanning efficiencies of only 
approximately seventy-five percent and is therefore unacceptable for 
television raster reproduction requirements. A seventy-five percent scan 
efficiency causes the television raster image to be compressed and 
distorted in the horizontal plane since the spots intended to reproduce 
the raster image overlap one another instead of having the required 
adjacent, non-overlapping relationship. 
The arc designated by reference number 80 in FIG. 5C indicates the 
deflection angle of the second scanned reflected output signal which is 
generated after having been reflected from only a single facet of rotating 
mirror 30. Reference number 82 indicates the deflection angle of angularly 
amplified output beam 44 which has been reflected a second time from the 
rear facet of mirror 30. The substantially increased deflection angle of 
the angularly amplified output beam causes the angularly amplified output 
beam to trace a significantly longer horizontal path on a screen and 
yields the ninety percent scan efficiency required to project an 
uncompressed, non-distorted image on the screen. This undistorted image is 
formed from a plurality of spots positioned adjacent to one another in a 
non-overlapping relationship. 
The unique method and structural configuration of the present invention 
generates an angularly amplified output beam which permits a small 
diameter polygon mirror capable of rotation at the required 40,000 R.P.M. 
rate to project a horizontal raster scan line having the required length. 
Thus a small diameter rotating polygon mirror is rendered equivalent to a 
significantly larger diameter polygon mirror having longer facets which is 
incapable of rotation at the required 40,000 R.P.M. rate due to mechanical 
limitations. 
In the preferred embodiment of the present invention, the first reflecting 
surface 34 is commercially available from the Melles Girot Company of 
Irvine, Calif. and is designated as Model No. 01 MCG 023. First reflecting 
surface 34 is positioned approximately 13/4 inches from the closet facet 
of mirror 30. Second reflecting surface 52 is commercially available from 
the Melles Girot Company and is designated by Model No. 01 MCG 027. Second 
reflecting surface 52 is positioned approximately 33/4 inches from the 
nearest mirror facet of mirror 30. The total optical path length between 
second reflecting surface 52 and mirror 56 is approximately 71/4 inches. 
Mirror 56 is driven by a galvanometric optical scanner manufactured by 
General Scanning, Inc. of Watertown, Mass. and designated by Model No. G 
100 PD. This device is driven by a servo controller designated by Model 
No. CCX-100 and is also available from General Scanning, Inc. FIGS. 1 and 
2 are drawn to scale and other geometric dimensions not specifically 
recited above can be ascertained directly from these figures. 
Referring now to FIG. 7, the manner in which vertical and horizontal 
synchronization is imparted to the moving mechanical elements of the 
present invention will now be described in some detail. A composite video 
signal including a video component and horizontal and vertical 
synchronization signals is coupled to a conventional sync separator 
designated in FIG. 7 by reference number 60. The video output portion of 
the signal from sync separator 60 is coupled to a commercially available 
acousto optic modulator 16. The vertical sync signal from sync separator 
60 is coupled to the commercially available galvanometer driver which 
drives the galvanometer which provides deflection from mirror 56. 
Sync separator 60 also provides a horizontal sync signal which is coupled 
to an input of motor sync circuit 64. FIG. 2 illustrates that an 
unmodulated portion of input light beam 10 is picked off from the input 
optics of the present invention and is directed against the rotating 
facets of mirror 30 as is indicated by reference number 66. An optical 
detector 68 is positioned behind a knife edge (not shown) to detect the 
passage of the light beam reflected from the facet edges of mirror 30. 
This facet synch detector output signal is coupled to another input of 
motor sync circuit 64 which generates a motor sync output signal to 
control the operating speed and phase angle of the motor driving polygon 
mirror 30. 
It has been assumed up to this point that the source of collimated light or 
laser comprises a source of monochromatic light and that the output light 
beam will project a raster image on a screen comprising various 
intensities of the monochromatic output signal from the laser. In order to 
project a full color raster image on a screen, a laser, such as a krypton 
laser, having a plurality of different color monochromatic outputs can be 
utilized in the manner illustrated in FIG. 6. A plurality of dichroic 
filters indicated generally by reference number 70 are provided to filter 
out different single color output beams from the laser output beam. A 
separately modulated acousto optic modulator 72 then modulates each 
filtered monochromatic signal with the appropriate color intensity 
information. The modulated single color outputs from each acousto optic 
modulator 72 are then directed onto dichroic reflectors and recombined to 
produce a color output signal 74 which is then directed in the manner 
described above to the input of telescopic objective 24. The operation of 
the full color version of the preferred embodiment of the invention from 
this point on is identical to that described above. 
Numerous different versions of the preferred embodiment of the present 
invention utilizing somewhat different components and different structural 
arrangements would be readily apparant to one of ordinary skill in the art 
and would not deviate from the scope of the present invention. For 
example, a comparatively wide mirror could be positioned at the point 
indicated by reference number 76 and deflected up and down by a 
galvanometer in order to provide the vertical deflection for the optical 
output beam from the optical scanning apparatus of the present invention. 
This galvanometer-deflected mirror would be wider than mirror 56 since it 
is not positioned at an optical vertex. In another embodiment of the 
present invention, a lens might be substituted for the spherical mirror 
sections 34 and 52 illustrated in FIGS. 1 and 2. 
In addition, it has been found that performance of the present invention is 
maximized by directing the second scanned reflected output beam onto a 
rear facet of mirror 30. However, the optical scanning apparatus of the 
present invention can be rendered fully operational by directing the 
second scanned reflecting output beam back onto the same mirror facet 
which produced the first scanned reflected output beam or to any other 
mirror facet positioned around the circumference of mirror 30. If the 
second scanned reflected output beam is directed back onto the same mirror 
facet that produced the first scanned reflected output beam, the optical 
path traversed by the second scanned reflected output beam must be 
extended by reflection from a plurality of plane mirrors or their optical 
equivalent to cause the first pivot vertex to be formed before that beam 
is reflected by the same mirror facet. The degree of angular amplification 
obtained is affected by the particular mirror facet relationship chosen 
and it has been found that the greatest angular amplification can be 
obtained when the second scanned reflected output beam is directed onto 
the rear facet opposite the front facet of mirror 30. 
Although the optical scanner of the present invention has been disclosed as 
being capable of projecting a television raster scan image on a screen, it 
may also be adapted to scan the surface of an object, such as a silicon 
chip, printed circuit board, or various other surfaces, for quality 
control inspection. In this application, an unmodulated light beam is 
deflected across the surface of the item to be inspected and an optical 
detector is positioned to receive the light beam reflected from that 
surface. 
The light beam generated by the optical scanner of the present invention is 
deflected in the horizontal plane as disclosed above. Vertical deflection 
can be accomplished either by a galvanometer movement in the manner 
described above, or in the alternative, by mounting the object to be 
inspected on a vertical stage which is either incrementally or 
continuously displaced with respect to the horizontally deflected output 
beam from the optical scanner. Numerous other uses for the optical scanner 
of the present invention will be readily apparent to one of ordinary skill 
in the art. 
Accordingly, it is intended by the appended claims to cover all such 
modifications of the invention which fall within the true spirit and scope 
of the invention.