Photocomposing machine and method

A laser beam is widened in one dimension to cover an array of a substantial number of electro-optic gates. The beam is divided by the gates into a plurality of potential spot-forming beams. The transmission of each beam to a photosensitive surface is selectively inhibited in accordance with a pre-determined pattern or program, while the beams are swept relative to the photosensitive surface to form characters and other images. Preferably, the gates are formed by a wafer of PLZT electro-optic ceramic material, with a large number of closely-spaced electrodes on the surface, in cooperation with a crossed polarizer. Preferably, the beams are collimated and a traveling lens and reflector combination is moved parallel to the photosensitive surface in the collimated beams to space the spots on the film from one another. The collimated beams are made convergent so that a relatively small, light-weight lens can be used to compose relatively long lines of text without excessive loss of light or vignetting. Also provided are a relatively simple but stable lens-reflector carriage and rail structure on which the carriage travels in order to sweep the beam relative to the photosensitive surface.

This invention relates generally to photocomposition. More particularly, 
this invention relates to so-called "Fourth Generation" photocomposing 
machines and methods based on the use of laser light. 
Prior laser photocomposing machines and methods capable of producing 
commercially acceptable type composition tend to be expensive and complex 
in construction. 
Most prior "raster" type fourth-generation photocomposers utilize a 
rotating multi-faceted polygon-shaped mirror to scan a laser spot-forming 
beam across a photosensitive surface. Such machines require relatively 
large amounts of computer memory and relatively high computing speeds to 
store and retrieve the data necessary to produce acceptable composition. 
Such machines also require the maintenance of stringent mechanical and 
optical tolerances. For example, there is the mechanical problem 
associated with the wobbling of the rotating polygon, the accuracy of the 
polygon reflecting facets such as facet pitch angle, facet-to-facet errors 
and facet-to-spin axis errors and other mechanical problems. The optical 
problems to be dealt with are no less serious and costly, and they 
increase considerably as the desired length of lines of text increases. 
Various means have been proposed to solve or alleviate the latter problems, 
such as the use of lenses of special design to insure a flat field, and a 
linear relationship between the angular displacement of the 
polygon-reflected beam and the focused spot in the flat image field and 
also to prevent the size of the spot from increasing as it approaches the 
ends of the text line. Other means, such as the use of toroidal lenses, 
curved mirrors and the like have been proposed. All of these factors 
contribute to the cost and complexity of the equipment. 
Other proposed fourth generation photocomposers use a series of 
acousto-optic deflectors instead of a polygonal mirror. However, such 
photocomposers still do not solve the cost and complexity problems, and 
have had other significant problems. 
One object of the invention is to provide a fourth generation 
photocomposing machine and method which have high versatility, high speed 
and productivity, and very good to excellent composition quality to 
faithfully reproduce original type designs, and yet are relatively low in 
cost; a machine and method which solve or greatly alleviate the foregoing 
problems. 
It is another object of the invention to provide such a machine which is 
compact enough to fit into the top of an ordinary desk. 
A further object of the invention is to provide such a machine which can be 
an adjunct to existing microcomputers with appropriate interface and 
software for office environments. 
An additional object of the invention is to provide such a machine and 
method in which a high level of light intensity is available to produce 
photocomposed text and graphics on a variety of photosensitive recording 
media, such as silver halide film or paper, zinc oxide plates or paper, or 
even plain paper. The proposed machine and methods preferably should be 
able to supply enough energy to expose printing plates of various kinds, 
thus enabling one to produce printing plates directly. 
Yet another object of the invention is to provide a relatively simple 
mechanism and method capable of producing relatively long lines of textual 
or pictorial elements without excessive loss of light or vignetting. 
An additional object of the invention is to provide such a machine in which 
very few adjustments need be made manually in order to keep the quality of 
the photocomposition output at a relatively high level. 
A further object of the invention is to provide a compact photocomposing 
machine with very few moving parts yet capable of producing long lines of 
text of constant quality from one end to the other with simple optical 
means. 
In accordance with the present invention, the foregoing objectives are met 
by the provision of a photocomposing machine and method in which a 
relatively wide laser beam is formed and modulated so as to divide it into 
a series of small, thin spot-forming beams. The transmission of each of 
the small beams to a photosensitive surface is controlled while the beam 
is swept relative to the photosensitive surface so as to form images from 
the spots. 
Preferably, the small, thin beams are aligned to form one or more lines of 
contiguous or overlapping spots, the length of the lines varying with the 
position of the beams relative to the photosensitive recording surface. 
It also is preferred that the modulator be an electro-optic device such as 
a PLZT ceramic wafer with an plurality of interleaved electrodes, which, 
together with a crossed polarizer, forms an array of very small light 
gates. Preferably, the spots are made contiguous or overlapping by making 
different lateral portions of each electrode offset from the other. 
The beam preferably is swept relative to the recording surface by the use 
of a collimator and a traveling lens and reflector. Preferably, the 
collimated beams are made convergent to avoid excessive loss of light or 
vignetting when composing relatively long lines of text with a focusing 
lens of moderate size. Also, the beam transmitted preferably are 
transmitted on or very close to the optical axis of the device, in at 
least one plane. 
The lens and reflector are mounted for travel on a pair of rails. A simple 
but accurate rail support structure is formed by a simple beam with walls 
forming two angles, and a rail member fitted into each angle. A simple 
extruded aluminum channel beam can be used. 
The carriage is maintained accurately in position on the rails by the use 
of a bearing member engaging one rail, a tapered roller engaging the other 
rail, and resilient means urging the roller and bearing member against the 
rails. 
Each vertical line of spots representing a small slice of a pre-selected 
character is projected at a substantially reduced size onto the 
photosensitive media along the optical axis of the relatively simple lens 
assembly so that relatively long lines of high quality images can be 
produced at a relatively low cost. The machine also is relatively compact 
and simple, and fully meets the objects set forth above.

TECHNOLOGICAL BACKGROUND 
FIG. 1 of the drawings illustrates one of the principles used in the 
present invention. In the preferred embodiment of the invention, an 
elongated transparent wafer 2 of lanthanum-modified lead zirconate 
titanate (PLZT) ceramic material is utilized. This material has 
electro-optic properties and operates as follows. 
A beam of light 8 from an unpolarized light source (not shown) is polarized 
in a first plane 18 by a first polarizer 6. With the switch 12 open so 
that the voltage from a source 10 is not applied to the wafer 2, the 
polarized light from polarizer 6 is transmitted through the wafer 2 
without any change of polarization. A second polarizer 4 receives the 
light transmitted through the wafer 2. The plane 14 of polarization of the 
polarizer 4 is rotated 90.degree. with respect to the polarization plane 
18 of the first polarizer 6 so that no light passes through the polarizer 
4. The wafer 2 is provided with two opposite electrodes 3 and 5. When the 
switch 12 is closed a voltage is applied to the electrodes from the source 
10 in order to create an electric field through the wafer. The wafer 2 
then acts as an optical retarder, shifting the relative phases of light 
polarized parallel and perpendicular to the applied field. With the proper 
electric field, the wafer behaves as a half-wave retardation plate, 
rotating the plane of polarization by 90.degree., from plane 18 to 14, so 
that light is transmitted through the second polarizer 4. Thus, the device 
shown in FIG. 1 is an electro-optic shutter which either transmits or 
blocks the transmission of light. For further details see: "PLZT 
Electro-optic shutters"- Applied Optics, Vol. 14, page 1866, August 1975. 
OPTICAL SYSTEM-GENERAL DESCRIPTION 
The major optical opponents of the photocomposer are shown in FIG. 2, which 
is a schematic representation. A laser light source 20 such as a He-Ne 
laser produces a constant, polarized narrow beam 21 of light. The beam 21 
is deflected approximately 180 degrees by a retro-reflector prism 22. The 
beam emerging from the prism passes through a first cylindrical lens 24 
which, in association with a second cylindrical lens 26 of smaller 
convergence, spreads the beam 21 vertically, and shapes the beam into an 
elongated, wide ribbon-like beam of light parallel to the vertical plane. 
The beam is made slightly diverging in the horizontal plane by a 
cylindrical lens 33 for reasons to be explained hereinafter. The output 
from laser source 20 preferably is polarized. However, an additional 
polarizer 27 is located at the output of lens 33 in order to correct any 
change of polarization introduced by the passage of light through the 
preceding optical elements. 
From lens 33 and polarizer 37 the shaped beam of light is directed to a 
modulator assembly 28 including a mirror from which the modulated beam is 
reflected to a crossed polarizer 30 which blocks selected ones of the 
light rays corresponding to unactivated elementary areas of the modulator, 
as it will be explained in greater detail below. The beams of light 
emerging from the polarizer 30 are spread horizontally by a cylindrical 
lens 32 and are deflected by mirrors 36 and 40 toward a spherical 
collimator lens 38. Lens 32, being cylindrical, has no effect on the rays 
parallel to the vertical plane. 
The collimator lens 38 is located at an optical distance from the 
reflective surface of the modulator 28 substantially equal to its focal 
length and therefore produces collimated rays shown at 54 in FIG. 2. Those 
rays reach a spherical imaging lens 42 and mirror 44, both of which are 
mounted on a slidable carriage 46. The lens 42 and mirror 44 move along a 
path defined by rails 48 at constant distance from film 50. That distance 
is determined so that lens 42 always forms an image 52 of reduced size on 
the film regardless of the position of the lens and mirror along their 
path of movement. Preferably, one line of text is produced by movement of 
the lens and mirror combination in one direction, and another when it 
reverses its motion to move in the opposite direction. 
A mechanical shutter shown schematically at 87 is moved into active 
position to intercept the laser beam 21 during relatively long periods 
between successive operations of the phototypesetter. 
A character spacing mechanism utilizing collimated light and traveling 
focusing lens and reflector is known in the art, but the present invention 
makes it possible to reduce the mass of the optical components on the 
carriage, thus decreasing its size and weight and making it possible to 
move it at a relatively high speed, particularly since it moves in a 
continuous fashion, first in one direction, then in the opposite 
direction. Another notable feature of the invention is the fact that the 
production speed, expressed in lines per minute, can be higher for shorter 
lines than for long lines because the carriage displacement can be varied 
with the length of the lines of text, which is not the case in systems 
utilizing a continuously rotating multi-faceted polygon, such devices must 
scan the light beam across the full width of the character receiving area 
on the film, as if the maximum length line were being composed, even if 
the actual length of line is much shorter. 
MODULATOR 
The structure and mode of operation of the modulator 28 is illustrated in 
FIGS. 8 through 18. 
Referring first to FIGS. 8-10, the modulator 28 includes a spherical 
plano-convex lens 62 cut off at the sides to form an elongated rectangular 
outline, a PLZT wafer 84, and a front-surfaced mirror 85. The wafer 84 is 
secured to the front surface of the mirror 85 by means of a transparent 
gel 82 which is flexible to allow the wafer 84 to expand and contract as 
it tends to do. The lens 62 is mounted with its planar rear surface 
parallel to and close to the wafer 84, and the assembly is secured with 
the wafer 84 in a hole 19 in a printed circuit board 88 by appropriate 
mounting means (not shown). Also mounted on the printed circuit board 88 
are integrated circuit chips 92 forming the driving circuitry for the 
modulator. 
FIGS. 8 and 9 are vertical cross-sectional views, with the vertical plane 
being aligned or parallel to the line 89 in FIGS. 8-11. Thus, the vertical 
plane is aligned horizontally in FIGS. 8-10. However, the modulator 
assembly 28 is shown rotated 90.degree. in FIG. 11 so as to illustrate the 
entering light rays 64 and the leaving light rays 65 and the angle 81 
between them. Angle 81 is greatly exaggerated in FIG. 11 for the sake of 
clarity in the drawings. Actually, the angle 81 is small, so that the 
entering and leaving light rays pass through substantially the same area 
of the wafer 84. 
Referring to FIG. 9, both outside surfaces 63 of the lens 62 are covered 
with an anti-reflection coating of appropriate composition for the 
characteristics of the traversing light. A small air gap 83 is left 
between the flat surface of the plano-convex segment and the PLZT wafer 
shown at 84. The surface of the wafer 84 which faces the lens is also 
covered with the appropriate anti-reflection coating. Controlling 
electrodes, to be described below, are located on the opposite surface of 
the wafer 84--that is, on the surface attached to the mirror 85. 
The light rays 64 entering the assembly 28 pass through the PLZT wafer 84 
twice, entering the lens 62, then passing through the wafer 84 and the gel 
coat 82 a first time, and being reflected from the reflecting surface of 
mirror 85 along a path slightly angled relative to the entering path but 
passing through substantially the same elementary area of the wafer, then 
proceeding through the lens 62 to emerge as rays 65. 
The exiting rays 65, as explained above, go through the polarizer 30. The 
combination of the modulator 28 and the polarizer 30 comprise a light 
shutter unit--an array of small, closely-spaced light gates. 
The control electrodes on the PLZT wafer 84 are shown in FIGS. 12 and 18. 
The electrodes are very small and preferably are produced by 
photolithography. 
The manner of operation of a PLZT device with thin, parallel closely-spaced 
electrodes will be explained with references to FIGS. 14 and 15. Input 
electrodes 100, 142, 143, etc., are interleaved between adjacent arms such 
as arms 101 and 105 which are joined together by a common electrode 103 
which is connected to ground. When an electrical signal is applied to one 
of the input electrodes such as the upper electrode 100, this creates an 
electric field in the PLZT material in the zones between the input 
electrode 100 and the adjacent grounded electrodes 101 and 105 and rotates 
the plane of polarization of light transmitted through those zones. 
If the device of FIG. 14 were used as in the modulator in the 
photocomposing machine and method of the present invention, with light 
shining on the zone 97, and if, for example, electrodes 142 and 143 were 
energized with a voltage high enough to rotate the plane of polarization 
of the incident light sufficiently, each of the two zones on the sides of 
each input electrode would, in effect, become an "open" half-window, 
allowing light passing through those zones to pass through the polarizer 
30 and the other downstream optical elements and onto the film 50. 
FIG. 15 is a waveform diagram plotting the intensity of light transmitted 
through the polarizer 30 versus position along the length of the wafer 84. 
Thus, FIG. 15 shows two light peaks 146 or 147 for each electrode, with an 
obscured zone between. The final result on the film or other 
photosensitive medium will be a spot, shown schematically in FIG. 16, 
comprised of two "light" spots 139 with two "dark" spots 138. 
It is a feature of the invention to provide electrodes shaped and located 
as shown in FIG. 12. Each electrode, having a useful length represented by 
97, is composed of two horizontal sections 97-1 and 97-3 of equal length 
joined by a slanted section 97-2 approximately twice the length of 
sections 97-1 and 97-3 so that the light going through the "open" 
half-windows or gates adjacent to electrodes 140 and 141 has the waveform 
144 or 145 as shown in FIG. 13. Although the intensity 98 of light of the 
areas 144 and 145 is lower than the intensity 102 in the FIG. 14 device, 
the total amount of light, represented by areas 144 and 145, is equal to 
the total amount of light represented by areas 146 and 147 of FIG. 13. The 
difference is that the device of FIG. 12 produces a single spot with no 
dark areas. 
FIG. 17 represents (approximately) one spot or dot 107 of a light column as 
produced on the film by one energized electrode of FIG. 12, for example, 
electrode 141, which opens gates 148 and 149. The dot 107 has a height 
proportional to four times the width of the electrodes and spaces, 
assuming that those widths are equal (see FIG. 13). Ideally, the dot shown 
in FIG. 17 has a width 106 of approximately 20 microns and an equal height 
109. The fact that electrodes arranged as in FIG. 12 produce dots with 
slanted upper and lower edges is of no importance since the protruding 
corner shown on the right side of the dot 107 is only one electrode width 
lower than the left side corner, divided by the reduction ratio of the 
optical system, only about 5 microns on the film. 
It should be understood that the incident light beam 64 covers the entire 
effective zone 97 of the electrodes. Therefore, the light rays forming one 
dot are transmitted through a zone whose width is much larger than its 
height. However, the width of the resulting light area is reduced by the 
optical system to produce a dot whose dimensions are substantially 
symetrical. 
It also should be understood that the electrodes and wafer structure shown 
in FIGS. 12 and 18 are greatly enlarged, and that only a few of a much 
greater number of electrodes are shown in the drawings, for the sake of 
clarity of illustration. Actually, in a typical wafer, there are enough 
electrodes to form 256 windows or gates. This produces more than 1,000 
spots or dots per inch for each vertical dot array on the film and thus 
produces lines of excellent quality. 
FIG. 18 shows a preferred arrangement of the electrodes controlling the 
gates of the PLZT light modulator 24. In this arrangement, both the 
electrodes and the spaces between them are twice as wide as in the 
arrangement of FIG. 12. For example they are approximately 140 microns or 
0.14 mm wide. One of the advantages of this arrangement is that it 
increases the distance between consecutive electrodes. This makes the 
wafer less costly to manufacture. Also, it has been found that the large 
electrodes and larger windows or spaces provide a higher contrast ratio, 
that is, there is better contrast between "open" and "closed" gates or 
windows. This decreases cross-talk and also, because of the wider windows, 
the divergence of diffracted rays is reduced, which reduces the size of 
the imaging optics as well as giving higher image quality. 
The arrangement of FIG. 18 also differs from the arrangement of FIG. 12 in 
that the left electrodes, represented by odd reference numbers 253, 255 
and 257, are single elements electrically isolated from one another and 
having terminals which permit individual electrodes to be connected to 
different voltage sources or circuits. Thus, for example, the left 
electrodes can be selectively connected to voltage source A or B by the 
energization of switches 289, 291 and 293, and the right electrodes 254, 
256 and 258 can be selectively connected through switches 290, 292 and 294 
to voltage sources C and D. In the embodiment of FIG. 18, window 276 has 
been "opened" by the simultaneous actuation of switches 293 and 292 which 
creates an appropriate difference of potential between the electrodes 256 
and 293. 
It has been found experimentally that, by the proper selection of the 
control voltages, the voltage difference between electrodes of unselected 
gates is of negligible significance and does not create fogging of the 
usual photosensitive material. 
To explain the foregoing further, when a voltage V.sub.1 is applied to an 
input electrode such as the electrode 254 in FIG. 18, and electrode 253 is 
grounded, the voltage difference between electrodes 253 and 254 is 
V.sub.1, a voltage sufficient to give an adequate amount of rotation of 
the polarization plane of light transmitted through the gate 273. However, 
if electrode 255, which is located on the other side of electrode 254, 
also were grounded, then the same voltage potential V.sub.1 would be 
established between electrodes 353 and 255, and the gate 274 will be 
opened inadvertently. To prevent this from happening, there is applied to 
electrode 255 a voltage V.sub.2 which is substantially greater than zero 
but substantially less than V.sub.1, so that the voltage difference 
between electrodes 255 and 254 is insufficient to create a degree of 
rotation of the polarization plane adequate to open the gate 274. This 
same arrangement is used for all of the gates on the wafer. 
The way in which the switches of FIG. 18 operate to assure independent 
opening and closing of each gate is that the full voltage V.sub.1 is 
applied only to line D, and the lower voltage V.sub.2 (e.g., 1/2 of 
V.sub.1) is applied to each of lines A and C, while line B is grounded. 
The switch contacts normally are in the left-most position, where they 
make contact with lines A or C. When it is desired to open one of the 
gates, the switch contacts for the electrodes (e.g., 253 and 254) above 
and below the gate area (e.g., 273) both are moved simultaneously to their 
right-most positions, where they contact lines B and D, respectively. This 
grounds one electrode and applies the full voltage V.sub.1 to the other, 
thus opening the gate. In the meantime, the voltage applied to the 
electrode below electrode 254 is at 1/2.sub.V1 so that gate 274 does not 
open and the degree of polarization change is not enough to allow light to 
leak through and produce fogging of the film. 
The proper value of V.sub.2 can be selected experimentally. A value of 1/2 
of V.sub.1 has proven satisfactory in a machine which has been built and 
tested. 
It should be understood that the switches shown in FIG. 18 are shown for 
the purpose of explanation. Actually, similar functions are performed by 
driving circuitry forming part of the electronic control system to be 
described below. 
Another difference between the arrangement of FIG. 12 and FIG. 18 is that 
the sloping portion of each electrode in FIG. 18 is much shorter than it 
is in FIG. 12. However, the electrodes are shaped to obtain the same 
advantage as in FIG. 12, that is, to avoid open spaces between adjacent 
dots on the film. 
Actually, the tops of the right-hand spaces such as 273 and 277 are higher 
than are the bottoms of the left-hand spaces associated with the adjacent 
electrode by a distance 270 so that vertically adjacent dots overlap one 
another slightly to form sold, dark lines on the film. The horizontal 
spacing of the vertical lines of dots also is selected so that 
horizontally adjacent dots overlap slightly, for the same purpose. 
CHARACTER FORMATION 
FIG. 3 shows schematically an enlarged vertical column 113 of elemental 
areas, each representing a dot or the absence of a dot formed by the 
optical system of the invention, using the modulator 24 and other 
components described above. 
In effect, the column 113 is moved rapidly from left to right, in the X 
direction, as shown in FIG. 3, by the rapidly-moving carriage 46 (FIG. 2). 
The presence or absence of a dot in each elemental area is changed in 
synchronism with timing pulses 111 in accordance with a previously-stored 
set of instructions or program to form characters such as the capital "A" 
and a portion of a lower-case "o" shown in FIG. 3. The same characters are 
shown in the right-hand portion of FIG. 3 as they might appear on the 
film, after an approximately 7 to 1 reduction in size by the optical 
system to effectively smooth the edges of the characters and improve their 
quality. 
The width of the sweep path of the light beam in a machine which has been 
built and successfully tested is approximately five and one half 
millimeters or 15.8 points. Thus, in one sweep a whole line of characters 
of up to over 15 points in size is composed. Larger characters, up to over 
30 points, can be composed in two separate passes, and even larger 
characters can be composed with the use of more passes or larger PLZT 
wafers, etc. If desired, two full lines or more of relatively small (e.g., 
6 points) characters can be composed in one pass. 
Of course, an observer looking at unit 28 through the crossed polarizer 30 
would see only a vertical column of line segments composed of luminous 
dots, with the line segments continuously changing in length and /or 
intensity. Characters would appear only if the observer were rapidly 
moving his head in a horizontal direction. 
OPTICAL SYSTEM--DETAILED DESCRIPTION 
FIGS. 4 and 4A together comprise a schematic cross-sectional view taken in 
the vertical plane of the optical system of FIG. 2. FIG. 4 shows the first 
part of the optical system from the laser source to the modulator 28, and 
FIG. 4A shows the second part, from the modulator 28 to the film 50. 
In FIG. 4, block 58 represents the source of polarized light. In includes a 
laser and other components necessary to produce a polarized narrow beam of 
light 60. This beam first is spread vertically by the lens 24, which is a 
small, short focal-length cylindrical lens, e.g., a quartz rod 2 mm in 
diameter. The rays emerging from this first lens enter the lens 26, which 
is a converging cylindrical lens, to form a tapered, relatively flat 
generally fan-shaped light bundle 64. The virtual light source location is 
shown at 60.sup.1. 
Now referring to FIG. 4A, the beam is twice modulated by unit 28, as it has 
been explained in detail above. The light bundle 65 leaving the modulator 
unit 28 contains all the necessary information to produce one vertical 
slice of character. It then proceeds to the polarizer 30 and the 
cylindrical lens 32, which has practically no effect on the light rays of 
the vertical plane, and is folded back by mirrors 36 and 40. Now the 
modulated bundle goes through spherical collimating lens 38 before 
reaching the travelling lens 42 which can project onto the photosensitive 
medium 50 a vertical column formed of a number of on-or-off spots 
indicated at 72 in FIG. 4A, and at 113 in FIG. 3. 
Because of their size and configuration, the gates of the modulator 28 
diffract the incident light. The three most significant rays are shown in 
FIG. 4A. The rays 66, 67 and 68 respectively represent the zero, plus one 
and minus one orders of the diffracted light coming from the uppermost 
gate of the modulator 28. Likewise, rays 70, 71 and 69 represent rays 
emerging from the lowest gate. 
As it can be seen in the drawings, all of these rays are utilized to form 
an image on the photosensitive surface. A significant feature of the 
invention resides in the optics affecting the rays located in the vertical 
plane so that, as shown, a relatively small traveling lens 42 can be 
utilized to produce long lines (more than 840 millimeters or 200 picas) 
without excessive loss of light or loss of parts of the light beams 
(vignetting). This is quite contrary to the system commonly used in second 
generation machines in which the light rays emerging from a collimating 
lens diverge rather rapidly, thus limiting the maximum length of line 
and/or the maximum size of characters. The maximum length of line, as 
defined by the distance traveled by lens 42 (from 42') is shown at 43. Of 
course, the reflector 44 has been omitted from FIGS. 4 and 4A for the sake 
of simplicity. 
The method by which such long lines can be obtained will now be explained 
with reference to FIGS. 4 and 4A. It is assumed that the optical system 
upstream of the modulator 28 has been constructed so as to obtain a 
substantially uniform and dimensionally acceptable light patch to cover 
the effective electrode area 97 (FIGS. 12 and 18) of the modulator 28. The 
only component of the modulator 28 which may be affected by the maximum 
length of line desired is the optical length of field lens 62. 
The home or initial location of imaging carriage lens 42 is first 
determined. This location, shown at 42' in FIG. 4A, could be as close to 
collimating lens 38 as possible. Then, the extreme location of lens 42, 
shown in solid lines in FIG. 4A, is determined from the maximum length of 
line desired, represented by distance 43. Point 16, on the optical axis 
15, is located at the mid-point of the zone 43 of maximum travel of lens 
42. 
Now, going back to the PLZT wafer 84, the uppermost and lowermost gates are 
identified by reference numbers 11 and 11', respectively. Images of these 
extreme gates will be projected at points 25 and 25' of the final image 
72. The principal central point 13 of collimating lens 38 and gate 11 can 
be connected by a line 59 which gives the orientation of all the 
collimated light rays emerging from gate 11, which behaves somewhat like a 
restricted light source Now, from point 16, a line 59' is traced, parallel 
with line 59, which intersects the principal plane of lens 38 at point 61. 
Line 70, connecting this point 61 to gate 11 represents the zero order 
path of the emerging diffracted rays. The prolongation of the line 70 
beyond point 61, not shown in the drawings for clarity's sake, intersects 
the optical axis at point 16' which represents the virtual imaging point 
of the rays entering the modulator 28. 
Although a graphical representation only of the optical path of some 
extreme rays have been shown, it is clear that, for a maximum length of 
line 43, known location of gate 11, known focal length of collimating lens 
38 and the space between that lens and lens 42 at home position 42', and 
the optical characteristics of lens 38, the location of point 16' can be 
determined by simple mathematics. The distance measured from this point to 
lens 62 and the configuration of the optics located upstream of lens 62 
determine its focal length which is dependant, as shown above, on the 
maximum length of line, but does not substantially affect light rays 
emerging from the gates assembly for which it behaves as a field lens. 
The path followed by the plus one and minus one orders of the diffracted 
light are shown in the figure by dashed lines 69 and 71 for gate 11. The 
angle "i" formed between the zero order ray and the first order diffracted 
ray can be computed from the well-known formula: sin i=.lambda./d, in 
which ".lambda." is the wave length of light and "d" is the pitch of the 
column of gates on wafer 84. Thus, the angle of diffracted rays of the 
first order being know, these rays can be traced and their exact impact on 
the principal plane of carriage imaging lens 42 can be computed to 
determine the clear aperature of that lens. 
It is noteworthy that all of the rays emerging from the modulator 28 
converge to a "waist" located on a plane perpendicular to the optical axis 
at point 16. Thus, contrary, it is believed, to any system described in 
the prior art, the light rays emerging from the imaging lens do not 
continuously diverge, but converge to a point located at mid-course of the 
maximum travel of said lens, from which they diverge again as shown in the 
figure. The above described configuration makes it possible to use a 
relatively small and light imaging lens which enables the projection of 
long lines at great carriage speed for a relatively small cost. 
FIGS. 5 and 5A are similar to FIGS. 4 and 4A, except that FIGS. 5 and 5A 
schematically represent the optics in the horizontal plane. The same 
reference numbers represent the same components as in FIGS. 4 and 4A. As 
shown in FIG. 5A, after bouncing back from the modulator 28, the reflected 
light goes through diverging cylindrical lens 32 and converging spherical 
lens 38. In the plane of the drawings these two lenses constitute a "flat" 
telescope (that is affecting rays located in one plane only) whose output 
is a parallel bundle of light beams as shown at 41. Traveling lens 42 
produces, again in the plane of the drawings, a tiny point or dot on the 
photosensitive media 50, as represented at 74. As noted above, this point, 
in the embodiment shown, is of the order of 20 microns across. 
It is also noteworthy that the column of modulator elements located in the 
vertical plane and the points forming rays located in the horizontal plane 
are subjected to quite different optical treatments. In the vertical 
plane, the diverging "flat" fanning out light rays 64, after twice 
crossing lens 62 (FIG. 11) converge to make an image of the virtual light 
source 60' at 16'. All of the light rays emerging from a gate such as gate 
11, except diffracted rays of second order and higher, which carry very 
little light, are channeled to lens 42. 
In the horizontal plane, the narrow bundle of rays 60 emerging from the 
laser 58 are first slightly spread out by cylindrical lens 33 to increase 
the width of the elongated light patch in order to cover the width 97 
(FIG. 12) of the gates assembly. The rays emerging from unit 28 are spread 
out further in the horizontal plane by cylindrical lens 32 whose purpose 
is to increase the width of the bundle of light rays so that when they 
exit collimating lens 38 their width 41 is sufficient to produce a very 
small point 74 whose size, as is well known in the art, is proportional to 
the focal length of imaging lens 42 and inversely proportional to the 
width of beam 41 for a given wave length of the light. 
The purpose of the optics affecting the light rays of the vertical plane is 
to make an image of the gates unit, whereas the optics affecting the rays 
of the horizontal plane only determine the size and quality of the laser 
beam as it emerges from those optics. 
Going back to the vertical plane, the bundle of rays emerging from lenses 
84 and 26 is shown at an enlarged scale in FIG. 6. It has been 
experimentally found that through the judicious selection of simple 
cylindrical lenses, the uncorrected aberrations contribute to the 
formation of an elongated patch of light as shown in FIG. 7, with an area 
of substantially uniform illumination, shown at 77 in FIG. 7. The height 
78 and width 79 of this area are somewhat greater than necessary to cover 
the effective zone 97 of the gates electrodes shown in FIGS. 12 and 18. 
One feature of the invention concerns the path followed by the light-patch 
forming rays. It has already been stated that light rays pass through the 
lens 62 twice, and that the focal length of the lens 62 is such that the 
PLZT wafer 84 is located in the focal plane of the optical arrangement of 
FIGS. 4 - 4A and 5 - 5A. Because the capacitance of the PLZT gates, they 
normally should be operated by a so-called "totem pole" circuit in order 
to obtain a fast response. However, these circuits are relatively 
expensive and consequently they are to be avoided in a low-cost machine, 
such as is the objective of the present invention. Consequently, the 
present arrangement utilizes lower-cost intergrated circuits, as 
schematically shown at 92 in FIGS. 8 and 10. However, in the present state 
of the art, these circuits cannot produce the voltage which is necessary 
to rotate the polarization plane by a full 90 degrees. If 90 degree 
rotation is not obtained, the light output from the shutter including 
modulator 24 and polarizer 30 is quite limited. According to this feature 
of the invention, the light passes through the cermaic wafer 84 twice in 
order to rotate the plane of polarization by an acceptable amount. The 
total rotation is equal to approximately twice the rotation produced by a 
single pass. 
ELECTRICAL CONTROL SYSTEM 
The upper part 108 of FIG. 19 is a block diagram of the major electrical 
components of the mcdulator 28 of FIG. 10. A shift register 118 is 
connected to a clock source by line 120, and receives data on line 122. A 
latch circuit 116 is provided with its input line 124. A known driver 
circuit 114 (e.g. Texas Instruments Electroluminescent Column Driver Part 
No. SN 75555 FN or SN 755556 FN) controls the 256 gates 112-1 to 112-256. 
Block 110 represents a field inversion control circuit, a known circuit 
for reversing the transmission of the gates so as to produce white images 
on a black background instead of black-on-white, for example. 
The circuitry in section 108 of FIG. 19 is well known. 
In the lower part of FIG. 19 block 128 represents the CPU which controls 
all the functions of the machine. It is connected to the other major 
components, such as the information input circuit 126 for data from an 
external source, to the character generator storage 130, to the photomedia 
control 134 (for example for film feed), to the block 136 which, through 
electrical or optical (filters) means, controls the amount of light that 
can reach the photosensitive material, and finally to the block 32 
representing the character forming, projecting and spacing carriage 
control circuitry. Although not shown in the Figure, it is the motion of 
the carriage, which, through an encoder or a grating system and associated 
electronics, controls the opening or closing of pre-selected gates, as 
called for by the character generator. 
FIG. 20 shows, in greater detail, a preferred embodiment of the electronic 
control circuitry of the machine described herein. Data is input over 
input lines 416 or 417. For example, serial data can be transferred over 
line 416 through a standard connector known as RS-232. For faster and more 
complex transfer of information in parallel, connection 417 is utilized, 
particularly for the transfer to the machine of data pertaining to 
graphical material, including half-tone illustrations. 
The information coming from outside sources is transferred via the 
connections referred to above to the main CPU 395 which interprets the 
information received and organizes the general sequential operations of 
the machine in order to produce the desired output on the photosensitive 
media. The main CPU is connected to a large capacity memory 396 where all 
the necessary data concerning, for example, the shape of alphanumeric 
characters, are stored on discs, PROMS or other mass storage media. For 
the production of alphanumeric characters (or other images), the fonts to 
be utilized during the operation of the machine are transferred to random 
access memory (RAM) 394. 
The main CPU transfers the shapes of images to a signal processor 397 (for 
example, Texas Instrument model TMS 32010) which determines the points 
where the contours of the desired images, for a given size, intersect the 
background screen to be printed. This data is transferred over line 421 
and stored in RAM memory 400 so organized that the ON-OFF changes can be 
identified without any ambiguity. Thus, block 400 represents a "bit map" 
of transitions. Its capacity need not be greater than 64 kilobytes 
representing, on the average, a 2" long line of textual material. This 
capacity is much smaller than the several megabytes usually required by 
comparable prior art photocomposers using spinning polygonal mirrors. The 
reduced storage requirements is one advantage of the present invention. 
Signal processor 397 can also, via line 423, send data to bit map storage 
unit 401, of capacity similar to that of unit 400, which may store 
information on screening, shapes, pictoral components, etc. 
The storage in and retrieval of information from units 400 and 401 is 
controlled by a standard read/write control circuit 402 which receives 
over a line 424 timing pulses 111 (FIG. 3) originating from carriage 46. 
Unit 402, which is controlled by processor 397, can control various 
logical operations from information received from the bit maps of blocks 
400 and 401. For example, logical function NAND enables the output of 
reversed imagery (white on black background) on an area defined by one of 
the bit maps. The data serialized by unit 402 is segregated between odd 
and even and introduced into register 403 which controls the modulator 28 
to operate as described above in connection with FIG. 18. 
Main CPU 395, taking into account the status of the bit-map units 400 and 
401, gives the necessary commands for the operation of carriage 46 through 
CPU 399 which is dedicated to the command of mechanical functions. CPU 
399, subject to manual controls 398, controls the motion of the 
photoreceptor (film) and all the other functions of the output unit such 
as: operation of the laser shutter, status of the photoreceptor, film 
cutter operations, display on information status, instruction keys, 
motors, etc. The light intensity is also adjusted by this unit so that the 
light reaching the film is independent of the carriage speed. This result 
is obtained by adjusting the voltage applied to the electrodes of the 
modulator 28 by means of a known control circuit 404. In certain cases, 
the voltage applied to the modulator depends on the mode of operation, for 
example in the case of reversed output, which generally requires less 
light intensity to avoid affecting the white (or transparent) areas 
located on black background. 
In FIG. 20, the translating carriage assembly is shown schematically at 46, 
its rails at 48, and a grating 57 for generating timing pulses is attached 
to the frame of the machine, as will be explained in greated detail below. 
A motor 407, together with a pulley 408 and a belt 409 which is secured to 
the carriage, can move the carriage along rails 48 in either direction 
under the control of servomechanism control circuit 405 which operates 
under the control of the CPU 399. 
The film 50 (or any other photosensitive media) is driven length-wise for 
leading by a drive roller 413 through gearing by tachometer 411 and motor 
410 controlled by servo mechanism control circuit 406 which, like servo 
405, is itself controlled by CPU 399. Leading is performed in the 
relatively short time it takes the carriage to reverse its direction at 
the end of each line of composition. 
According to another feature of the invention, the accurate measure of the 
displacement of the film (or its position) is obtained by a light-weight 
idler roller 414 which is driven by the film itself against which the 
roller 414 is pressed by springs (not shown). Idler 414, preferably in the 
form of a tube, can be coated with a thin coating of high-friction 
material to minimize slippage between the roller and the film. Thus, the 
roller 424 rotates very freely and transfers its rotation to an encoder 
assembly shown at 415. The encoder assembly 415 sends a position feedback 
signal to the servocircuit 406. Means, not shown in the drawings and 
forming no part of the invention, also are utilized to insure the position 
of the film in its transverse location along the driving roller and the 
idler. 
CARRIAGE CONSTRUCTION 
The carriage 46 supporting the moving optics and its mounting structure are 
shown schematically in FIGS. 21 and 22. 
In FIG. 21, one of the cylindrical guide rails on which the carriage slides 
is shown at 427. According to a feature of the invention, these rails are 
relatively flexible. Their position is maintained accurately by pressing 
each rail, by means of a fastener (not shown) or other means, against an 
accurate angular recess such as the recess 450 located in a rigid frame 
426 which supports the translating carriage assembly 46. The carriage, 
which is very light-weight, has a bearing member 435 with a V-shaped 
groove engaging rod 427. 
A flexible film-like member 428 is secured to the supporting frame 426 by 
the pressure of rail 427. The flexible member 428 has a grating in the 
area 428.sup.1 which extends, as shown, between a light-emitting diode 
(LED) 430 mounted on the frame 429 of the moving carriage 46, and a 
multiple-quadrant sensing detector shown at 436 attached to a printed 
circuit board 434 which also is attached to the frame 429. Thus, as the 
carriage is moved along its supporting rails, the LED 430 in co-operation 
with a mask 431 and the detector assembly 436, creates a series of pulses 
as the equipment moves along the fixed grating 428.sup.1 A small spacer 
449 made of Teflon or similar material is located as shown for the purpose 
of holding the grating at an accurate and fixed distance in relation to 
the photodetector assembly. The spacer is attached to the moving carriage 
frame 429 by a fastener 432. 
FIG. 22 is a perspective view of the complete assembly of the traveling 
carriage 46 and its supporting structure. The guide rails are shown at 425 
and 427, secured to a rigid frame 426 by fasteners such as tapered-head 
screws 447 driven in to the wall of the support 426 beside the rods 427 
and 425. Two of the bearing blocks 435 with V-shaped grooves are used. 
Each is made of a low-friction material such as Teflon and is attached to 
the frame 429 of the carriage 46 so as to engage the rail 427. 
Mounted on the frame 429 is the mirror 44 attached to a prism-like block, 
and the lens 42, mounted on a plate 437 whose position can be adjusted 
lengthwise by a screw 438 co-operating with a slot 451. 
The upper part of the carriage 46 is provided with a tapered, 
frustro-conically shaped roller 442 which is pushed against upper rail 425 
by the combined action of a lever 440 pivoted at 441 on the carriage frame 
and a tension spring 439. The purpose of this arrangement is to hold the 
carriage accurately in place on the rails and, to this end, to take up any 
play between the carriage and its rails. Tapered roller 442 tends to push 
the carriage down against the rail 427, and also holds friction-reducing 
Teflon pads 443 against the rail 425 to hold the carriage accurately in 
contact with its guide rails. 
The use of a relatively low-friction, long-wearing material such as Teflon 
for the bearing surfaces 435, 443 is advantageous in that it provides some 
resistance to travel, which tends to dampen unwanted vibrations, and is 
very accurate. 
The carriage is moved along its rails by a cable 445 co-operating, as 
shown, with a capstan 444 driven by a motor which is not shown in FIG. 22 
but is shown at 407 in FIG. 20. The driving cable 445 is attached to the 
carriage frame by a screw 446. The grating-supporting flexible strip is 
shown at 428 and the grating at 428.sup.1. Flexible electrical cables 448 
are provided to conduct electrical signals to and from the carriage 46. 
Preferably, the frame 426 has the shape of an elongated, generally U-shaped 
metal beam with walls shaped to form the angular recesses 450 into which 
the rods 425, 427 are secured. A lower channel portion 426.sup.1 (FIG. 22) 
is provided to accommodate the lower parts 430 (FIG. 21) etc. of the 
carriage 46. Surprisingly, an ordinary extruded aluminum beam has 
sufficient linearity and accuracy to serve as the support 426 for ordinary 
ground, flexible steel rods used as guide rails 425, 427. This helps to 
keep the cost of the machine low. 
OPTICAL ALIGNMENT 
According to another feature of the invention, means are provided for 
accurate alignment of the optical components shown in FIG. 2. The support 
structure for cylindrical lens 24 is adapted to allow the lens to be moved 
as shown by arrow 51 (FIG. 2) in order to adjust the uniformity of the 
elongated light beam. Similarly, lens 26 also can be moved vertically as 
indicated by arrow 53 in order to vertically position the elongated light 
beam. 
Cylindrical lens 33 can be adjusted in the horizontal plane as shown by 
arrow 35 to position the elongated light beam at the proper position on 
light modulator assembly 28. Collimating lens 38 can be moved in the 
direction of beam 54, as indicated by arrow 55, in order to get the exact 
reduction ration desired to compensate for the tolerances of the 
subsequent optical and mechanical components. 
From the foregoing, it can be seen that the machine and method of the 
invention amply meet the objectives set forth above. The machine is fast, 
compact and relatively simple and inexpensive to manufacture, and yet it 
produces composition of relatively high quality. 
The above description of the invention is intended to be illustrative and 
not limiting. Various changes or modifications in the embodiments 
described may occur to those skilled in the art and these can be made 
without departing from the spirit or scope of the invention.