WO1991010928A1

WO1991010928A1 - Electronic printer using a fiber optic bundle and a linear, one-dimensional light source

Info

Publication number: WO1991010928A1
Application number: PCT/US1990/005699
Authority: WO
Inventors: Eugene I. Gordon
Original assignee: Photon Imaging Corp.
Priority date: 1990-01-22
Filing date: 1990-10-04
Publication date: 1991-07-25
Also published as: AU6539790A; WO1991010929A1; AU7492991A; US4975729A; AU7241391A; EP0511960A1; CA2074469A1; WO1991010930A1; JPH05506409A

Abstract

An electronic printer, comprising a fiber optic bundle (15) having the fiber ends organized in a linear array in a first face (17) and in an area array in a second face (18), includes a linear array of LEDs or LCSs (19) adapted to image light into the fiber ends in one linear segment of the area face at a time. A rotating mirror (24) is operative to move the light image onto each of the consecutive linear segments on the area face and to provide a digital signal indicative of the position of the light image in each instance. The arrangement provides a low cost, high speed, and small size electronic printer which is easily adapted to high resolution and color printing.

Description

Electronic Printer Using A fiber Optic bundle and a linear, one-dimensional

light source.

Field of the Invention

This invention relates to electronic printers and more particularly to such

printers utilizing a plurality of light conduits such as a bundle of optical

fibe rs .

Background of the Invention

U.S. patent no. 4,760,421 issued July 26, 1988, and now assigned to the

assignee of the present application discloses an electronic printer which

utilizes a noncoherent bundle of optical fibers. The fiber ends at one face

of the bundle are organized in a linear array. The ends of the same fibers

in a second face are organized in an area array which may be, for example,

rectangular, square or circular. There is no predetermined relationship

between the fiber positions in the linear array face and those in the area

array face. Consequently, the bundle is noncoherent.

The fiber bundle is used to transmit light from an array of light sources

optically coupled to the area array face of the bundle to, for example,

photosensitive, photographic, or electrophotographic medium coupled to the linear array face of the bundle. Each of the fibers in the linear array

face represents a pixel in a line of pixels. Illustratively, the light source

comprises a cathode ray tube (CRT) operative to generate a localized area

of light at each of a sequence of electron beam addresses on the face plate

of the tube which is coupled to the area array face and produces the

desired sequence of pixels generated at the linear face of the bundle.

The sequence of tube face piate addresses which corresponds- to the

sequence of pixels in the linear array face of the fiber optic oundie is

obtained in accordance with the above-mentioned patent, during an

initialization procedure in which light is introduced into individual fibers

in the area face of the bundle by a CRT, and a photosensor is moved

incrementally along the linear face of the bundle. Illustratively, the

photo-sensor is covered by an opaque hood with a narrow transparent slit

in it. The slit is narrow compared to a fiber diameter and is moved in

increments also small compared to a fiber diameter. Light passing through

the slit is incident on the photosensor. This procedure results in the

photosensor measuring light unambiguously from only one fiber at a time.

A CRT is used to direct electrons tc each of the preestabiished beam

addresses on the face plate while the photosensor is stationary at a

selected position in the linear face. When the photosensor indicates the presence of light in the fiber, the set of electron address for which light

appears in the fiber is associated with the photosensor slit position, in

each instance, to determine the correspondence between the electron

addresses and a pixel position (presumably a fiber end) in the linear face.

An optimum address for each pixel is chosen from each set. Because the

photosensor is moved from pixel position to consecutive position, a

sequence of associated addresses is thus obtained. The resulting table of

pixel positions vs. electron bea'm addresses is stored in a PROM which is

interrogated during normal operation of the printer. The interrogation of

the sequence of addresses occurs each time a line of pixels is to be

generated.

Pixels are excited, at the positions so obtained, during normal operation

of the printer, to discharge, for example, consecutive (imaginery) linear

segments of a moving electrostatic drum or belt in an electrophotographic

process. The discharged medium containing an electrostatic image moves

past toner, transfer, and fixer stations in a manner common to

commercially available xerog raphic copiers, to transfer the electrostatic

image, so created, to plain paper.

The CRT useful in such a printer is relatively small, essentially about the

size of a conventional cigar. The face plate for such a CRT advantageously k comprises a disc of small diameter optical fibers. The area face of the

fiber optic bundle is abutted against or optically coupled to the fiber optic

face plate and permanently fixed in position with respect to the face plate

so that light generated at a selected one of the face plate addresses

enters the end of a fiber in the area face of the bundle and excites a

corresponding pixel in the linear face.

Although CRT's of this type are available commercially, they are made in

relatively small quantities at present and thus are relatively expensive.

They are presently used primarily for head-up displays in military .

vehicles. Stability of the electron beam position typically requires

elaborate feedback control in such equipment.

Brief Description Of An Embodiment Of this Invention.

Two dimensional commercial displays are available which use a linear

array of light emitting elements to define a horizontal sequence of pixels

within a row with mechanical deflection to achieve a vertical sequence of

rows to simulate a two-dimensional array. Because the TV field rate in

the US is 60 fields per second, a vertical scan rate of 60 per second is

implied and not excessive for mechanical scanners. Such arrangements

produce useful TV, computer and helmet mounted displays . It has been recognized that light signals generated at the addresses

determined during an initialization procedure may be generated at the

same time rather than in sequence. In accordance with the principles of

this invention, the above-noted linear scan display technique is adapted to

replace the CRT of the printer arrangement of patent 4,260,421 mentioned

above to provide, not only the option to generate light signals on a flying

spot basis as in a CRT, but also on a basis of all light signals of a line

being generated simultaneously. The invention specifically is directed at a

printer which includes a fixed linear array of discrete light-generating

elements which is relatively inexpensive, relatively small and low power,

and can be assembled from commercially available components. An

advantage of such an array is that the geometric position of a spot of light

projected onto the area face of the fiber bundle is invariant and is

determined digitally. The size of the spot is also invariant.

The adaptation includes a digitizing arrangement for the movement of the

image of the linear array through a sequence of reproducible vertical

positions to achieve the equivalent of a two-dimensional field of discrete

light sources. The adaptation also includes a unique initialization

procedure which relates the addresses for the light generating elements

of light patterns introduced at an area face of a fiber optic bundle to pixel

positions at a linear opposite face of the bundle. The use of a linear, light-generating array with a fiber optic bundle in a

p^ointer with a continuously moving photosensitive medium as disclosed

herein with a noncoherent fiber optic bundle would be expected to exhibit

vertical tearing. This phenomenon occurs when a vertical line of pixels is

displaced relative to an adjacent vertical line. Vertical tearing occurs

whenever adjacent pixels on a horizontal line are transmitted through the

bundle non sequentially or non simultaneously and because the photosensi¬

tive medium moves vertically as those pixels are transmitted. In this

invention, adjacent pixels in a linear horizontal segment (of the image)

are not transmitted through the fiber bundle consecutively. Hence, pos¬

sibly unacceptable vertical displacement of adjacent pixels could be

expected to occur.

In accordance with the principles of the present invention, a linear array

of light sources such as a linear light emitting diode (LED) array or liquid

crystal shutter (LCS) array is imaged onto a linear segment (i.e. aligned

with one axis) of the area face of a fiber optic bundle. The reason a linear

array of light sources can be used with a noncoherent fiber optic bundle

despite the potential for vertical tearing is that the opposite ends of

those fibers, which have ends which are adjacent in the linear end of the

bundle, are typically positioned relatively close together in the area face as will be discussed more fully below. Because of the row sequential

operation of the light sources, these fibers are excited close together in

time and relative vertical displacement of adjacent pixels in the linear

array face can be kept to an insignificant amount. Thus, vertical tearing

does occur, but it is not visible to the eye. Further, judicious selection

and timing of the light-emitting source corresponding to a fiber end in the

area face of the fiber optic bundle permits compensation for any

distortions due to tearing.

Advantageously, the area face of the bundle is organized in a rectangular

geometry, illustratively a random arrangement nominally almost ninety

fibers wide by thirty fibers high, for a 2550 element linear fiber array

suitable for a three hundred dot per inch resolution over eight and one half

inch wide output common to presently-available, commercially available

printers. The 256 LED or LCS Jinear array is imaged onto consecutive

(imaginery) linear segments of the area face by a lens system and a

moving mirror. The light from the LED or LCS linear array is turned off

between each linear segment to permit entry of data for the LED or LCS

from memory for the next linear segment before movement of the image to

the position of the next linear segment occurs. Then appropriate elements

in the iinear array are turned on simultaneously to inject light into the

appropriate fibers of the area array face. The photosensitive medium, although moving, is for ail practical purposes, made stationary as the

light spots are mirrored consecutively onto the consecutive linear seg^¬

ment in the rectangular face of the fiber optic bundle.

The light spots corresponding to the totality of fibers in the linear end of

the fiber optic bundle are generated during the time the mirrored image

moves vertically through, for example, 100 discrete horizontal rows. This

number actually corresponds to several times the nominally thirty

(virtual) rows of fibers in the area face of the bundle for the, above-

mentioned assumed number of fibers for reasons that will became clear

hereinafter. We will adopt the convention that the width of the

rectangular area face is along an x-axis (the height is along a y-axis) and

the image of the linear LED or LCS array is moved along the y-axis. For

each such position of the mirror, the precise y-axis position of the

mirrored image is determined conveniently by a single large area sensor

or a linear array of photosensors positioned along the y-axis of the area

face of the bundie. This large area photosensor is, for example, covered by

a series of transparent and opaque lines positioned in the path of the

mirrored image of an index LED or LCS which is continuously on. The

mirrored light of the index source impinges on the grid in a manner to

identify the y-axis position of the image of the linear array during the

initialization procedure, thus providing a digital (y axis) position code for the light generated by the LCS or LED array.

The LCS is operative as a shutter of light generated by a backside lamp.

The LCS array thus shutters al! the light from the iamp selectively. The

individual elements in the LCS array shutter light otherwise occurring at

associated addresses in the area face of the fiber optic bundle selectively

during the time period when the LCS image is at a particular vertical

position .

An initialization procedure to obtain the LCS or LED array addresses

associated with the sequence of pixels in the linear face of the fiber optic

bundle is obtained, in accordance with the present invention, conveniently

by activating all the LCS or LED elements, for example, and then rotating

the mirror through the succession of (y) positions corresponding to the

linear segments of fibers of the bundle. The LCS or LED element addresses

are ascertained for each vertical position corresponding only to the

positions of maximum light intensities rather then to all the addresses as

described above. During the initialization procedure, an individual light

source usually is illuminated for more than one vertical position and

consecutive y positions may overlap one another.

The light source addresses so determined are placed in proper sequence by moving the hooded photosensor to consecutive maximum intensity posi¬

tions in the linear array face. At each position for the photosensor, light

is generated at all of the "maximum intensity" addresses in succession

until the photosensor detects light. The horizontal and vertical addresses

of each source of light are stored in a look up table along with the

corresponding position of the photosensor. The photosensor is then moved

to the next maximum intensity position and the process is repeated. Thus,

the proper sequence of addresses is determined to correspond^' to the

sequence of "pixels' encountered by the photosensor as it is moved along

the linear face. A linear array of photo sensor cells such as a charge-

coupled device may be operated electronically to function as a moving

slit during an initialization procedure by interrogating the cells sequen¬

tially (one at a time).

Brief Description of the Drawing

Fig. 1 is a representative block diagram of a printer engine in accordance

with this invention;

Fig. 2 is an expanded representation of a portion of the engine of figure 1 ;

Fig. 3 is a schematic representation of the components of the engine of

figures 1 and 2;

Figs 4 and 5 are representative block and expanded detail views of a

component of the engine of figures 1 and 2; Fig . 6 is a flow diagram of procedure for initializing the optical

subsystem of the printer engine of figures 1 and 2; and

Fig. 7 is a representative block diagram of apparatus used for performing

the method represented in figure 6.

Detailed Description Of An Illustrative Embodiment Of This nvention

Figure 1 shows a printer engine assembly 10 in accordance with the

principles of this invention. The assembly comprises an image formation

subassembly and an optical subassembly.

The image formation subassembly is represented as box 1 1 . Box 1 1 com¬

prises an illustrative, and commercially available, xerographic module

including on electrostatic drum or belt, a toner station, a transfer station

and a fixer station. This subassembly operates in a manner well under¬

stood in the art to transfer to plain paper a charge image formed on the

electrostatic medium by optical exposure. The optical subsassembly is

operative to develop the charge image on the electrostatic medium. The

electrostatic medium is represented by cylinder 12 and is adapted to

rotate about axis 13 as shown.

The optical subassembly for forming the charge image on medium 12 13 includes a fiber optic bundle shown encompassed by imaginary tie 15. The

fiber ends are arranged linearly in a first face 17 of the bundle and

arranged illustratively in a rectangular geometry in a second face 18. The

area face can be represented as having x and y axes with the fibers

arranged roughly in rows along the x axes. These rows are imaginary and

the row representation is used for simplifying the description only. It

will become clear that the fibers are not usually in rows (or columns) but

that it does not matter in any case.

The illustrative optical subassembly also includes a linear array 19 of

liquid crystal shutters (LCS's) operative as light valves. The image of the

array 19 extends across the entire x axis of the area face of the fiber

optic bundle. Array 19 also includes a number, N, of LCS devices which is

large compared to the number, S, of fibers along the x axis of the area

face of the bundle. Thus, for a 300 dpi resolution across on eight and one

half inch page, a rectangular (but random) array of fibers approximately

30 x 90 is used where the ninety fibers are positioned along the x axis and

the thirty fibers are positioned along the y axis, the fiber positions being

entirely random and not actually occurring in rows or columns, in this

instance, a linear array of LCS's would include perhaps 256 individual

shutter elements. The excess number of elements over the number of

fibers in the row will be seen to be important as explained below. Linear array 19 is operated as a line of individual light valves shuttering

light according to data applied to it. Thus, an output of memory 20 is

connected to array 19 and is operative to shift a set of data into array 19

under the direction of control circuit 21. Light, for LCS embodiments, is

provided by a backplane lamp 23 and is selectively shuttered by the LCS

device.

Any light exiting LCS array 19 is directed at a mirror 24. Mirror 24 is

rotated about its axis 25 by a motor 26 also under the direction of control

circuit 21. Axis 25 is aligned with the x axis of area face 18 of the fiber

optic bundle. Thus, when mirror 24 is rotated through a succession of

positions, the light from LCS array 19 is imaged onto consecutive y

positions of the linear segments of the area face where the long

dimension of each linear segment is parallel to the x axis of the area face.

It was noted above that there is an excess number of shutter elements in

array 19 over the number of fibers along the x axis of area face 18. Mirror

24 also is rotated in a manner so that memory 20 operates to move pixel

data into the LCS array 24 many times, say 100 times, during the rotation

of the mirror over the area face. Thus, there are perhaps three times as

many vertical addresses as there are fibers in the columns. The number 15 100 rows times 256 elements per row (25600) is approximately ten times

as many (xy) addresses at which light is generated than there are fibers in

area face 18. This means that there are about ten addresses dedicated for

each fiber. An initialization procedure need select only one series of

addresses for each fiber (one address if sufficient light is provided) and

is operative to determine the series of addresses at which a maximum in

light intensity occurs in each instance. The factor 10 insures that this is

so. Thus, the constraints on the positioning of fibers in rows and columns

in the area face are quite relaxed and, in practice, are totally disregarded

and its fibers can be organized entirely at random. This is important to

achieve a low cost fiber bundle. At least one address is selected from

each series of addresses corresponding to a single fiber, the choice being

made to compensate for tearing.

Control circuit 21 also controls a motor 28. Motor 28 drives electrostatic

medium 12 with which all the functions of the image-formation sub-

assembly are synchronized in a well known manner. Thus, it is clear that

control circuit 21 moves the electrostatic medium properly in synchro¬

nism with the movement of mirror 24 to achieve a vertical pixel height

approximately equal to the pixel width.

Figure 2 shows an enlarged fragment of the electrostatic medium coupled to a corresponding fragment of the fiber optic bundle. The fibers 30, 31 ,

32, 33, - - shown by solid lines, all originate at the same row (^v ₂₎ in

figure 2. It should be noted that the ends of these fibers at linear end 17

are in no predictable positions there. Similarly, fibers 40, 41 , 42, 43 - -

originate at row y₁ in area face 18 and again assume no predictable

positions in linear end 17. Yet because of the address sequence deter¬

mination of the initialization procedure and because of the excess

numbers of addresse.s over fibers and because the y-axis address is

determined digitally and because the x-axis position of the LCS in the

array 19 is known, the memory operates to provide the proper pixel data

for each pixel at linear end 17 of the fiber optic array even though that

array is totally noncoherent.

The y-axis address vector is determined conveniently by including an

extra index element in array 19. The extra index element is designated 50

in figure 2. The extra element is termed "extra" because it does not

correspond in position to any portion of area face 18 of the fiber optic

bundle. Rather, the light exiting element 50 is directed onto a single

sensor 60 through a mask 61 . Mask 61 includes a grid of opaque and

transparent spaces (an optical grating) which responds to the position of

the light exiting element 50 to represent the y-axis address vector corresponding to the image of the imear light source array. Alternatively,

sensor 60 may comprise, for example, a photosensor array to provide the

y-axis vector for each image position. Such an arrangement does not

require mask 61. The x and y addresses for each pixel are determined

during the initialization procedure and stored in a PROM considered part of

control circuit 21. The operation of the PROM is analogous to that of its

counterpart in the above-noted patent.

Figure 3 shows a schematic representation of the various elements of the

optical subassembly of figure 1. Lamp 70 is shown as having a filament

73. The subassembly also includes a reflector or metallized coating (not

shown) used (in practice) to double the light introduced into the system

by the lamp. Light from lamp 70 is transmitted by condensing lens 76 to

LCS array 19. The light pattern passing LCS array 19, in each instance, is

directed to the mirror (24) for reflection onto a linear segment of the

area face 18 of the fiber optic bundle through projection lens 78.

LCS array 19 has been described as a single linear array of liquid crystal

light valves. In practice, array 19 might be arranged in other than a single

linear row, for example, in two rows assembled like bricks in a wall

where the bricks of one row are offset one half brick from the bricks in

the next row. The arrangement is shown in figures 3 and 4 where elements 90 and 91 in row 93 are shown offset from elements 95 and 96

in row 97. Figure 5 shows the array in greater detail showing an LCS cell

size of 37 μ wide by 67 μ high and intercell gaps of 1 1 .15 μ. Such a LCS

array is available commercially from Displaytech inc. of Boulder Colorado.

The arrangement displayed for the liquid crystal shutters in figure 3 is

consistent with the brick-like LCS organization.

The operation of the printer engine of figures 1 and 2 can be summarized

as follows: The data to be printed on a page is already stored in memory ^•

20 which may be a computer memory such as a disk. Memory 20 shifts

shutter (open and closed) data into LCS array 19. Mirror 24 directs the

resulting light pattern to . a linear segment of area face 18 of the fiber

optic bundle for exposing medium 12 to a discharge pattern. Memory 20

shifts a next subsequent pattern into LCS array 19. Meanwhile, mirror 24

rotates to a position corresponding to the next linear segment of area face

18 and medium 12 moves perpendicular to the linear array of pixels. The

process repeats until the entire stored image is recorded on medium 12.

By virtue of encoding the mechanical motion of the mirror to provide y-

axis address information, the arrangement operates as an all-digital

printer engine. If we assume that LCS array 19 comprises a ferroelectric liquid crystal

shutter (FE-LCS) the state of the art time to switch the shutter is 10 μs

or less in a nonmultiplexed configuration such as is used here. In normal

operation of shutters of this type, voltage is applied and the shutter

begins to open at 5 μs. At 10 μs, the shutter is fully open and exposure

has begun. At 20 μs, a pulse is applied to close the shutter; the shutter

begins to close at 25 μs and exposure is over at 30 μs. During the period

of from 5 μs to 30 μs, the shift register (memory 20) is filled with new

data. The cycle is then repeated providing an effective exposure time of

more than 15 μs, although the shutter may remain open for several periods

(exposure times).

The shift register includes 128 elements for one row of LCS array 19.

Thus, the data rate is about 5 MB/s. The necessary exposure energy pas¬

sing out of the FE-LCS array is 2.4 μ watts per pixel during 15 μs

exposure. The energy incident on the shutter continuously is about 10 μ

watts per pixel. The pixel footprint is 50 μ on a side and the pixel area is

2.5 x 10^~5 cm². Thus, the continuous incident optical power density is

about 1 watt/cm². The FE-LCS can sustain an energy density of about 4

watts/cm². The required continuous optical power incident on the shutter

is 2.6 m watts. The required optical power from the lamp is about 0.2

watts. A 100 watt Tungsten filament operated at 3200 degrees k has adequate , brightness to produce this even with filtering.

The shift register driver chips for the shutter can be attached directly to

LCS array 19 as shown in figure 3.

The description has been rendered in terms of a LCS shutter arrangement.

Other arrangements also can be used such as light emitting diodes (LED's),

magneto-optic light valve and edge-emitting, thin film, electrolumines¬

cence devices (EE-TF-EL). Of these, LED and magneto-optic devices are

already being used in existing printer engines and are useful for printer

engines in accordance with the principle of the present invention also.

The suitability of such alternative devices herein is underscored by the

following illustrative calculations: We will assume an exposure value re¬

quirement of 5 ergs/cm² which translates into 3.6 x 10^{" 1 1} J/pixel. The

transmission factor, TF, for a Lambertian emitter such as LED can be

shown to be

TF= [4F² (1 +1 /M)² +1]-¹

in which F is the F-number of the lens and M is the linear magnification,

i.e. the linear dimension of the spot in the plane of the fiber array divided

by the linear dimension at the emitting array. As a matter of interest, since the size of the LED array element is

potentially a variable, one should consider the implications of that choice.

The spot size on the fiber is fixed. Thus, M is the variable. The energy

available from the array element at constant brightness is proportional to

its area. Thus, M^"2 is the available energy variable. The energy trans¬

mitted to the fiber at constant brightness is, thus,

Energy α M-² /[F² (1 +1 /M)² +1 ]

=1/[F² (M+1)+M²/4]

The transmitted energy decreased monotonically as M increases i.e. as the

source pixel gets smaller. See Table 1. In the limit of a large source,

M→O, and the energy transmitted has the maximum value of 1/F².

The consideration above is valid so long as the effective numerical

aperture of the lens is less than that of the fiber. The maximum possible

value for the effective lens numerical aperture is

NA≤1/(1+F² )^1/2

which is the case when M=0. Since the fiber, NA=0.5, for this case (M=0)

the effective NA of the lens is still less so long as F>1 .732. For M=1

F>(3/4) ^{1 2} insures that the effective lens NA is smaller.

It is interesting to note that at constant source brightness, the energy

transmitted to the fiber increases with source size. On the other hand, the optical and electrical power required, speed, cost etc. favor small

source size (within limits). The lowest practical limit corresponds to

fvi=1 , where the energy transmitted is proportional to 1 (4F² +1 ). This is a

factor of 4 lower than for the large source case.

TABLE 1

EXPOSURE FACTOR

M

EXPOSURE FACTOR= 1/[F²(M+1)²+M²/4] ~> s?

In one practical arrangement, there are two rows of LEDs, perhaps 128 in

each row, so that there are a total of 256 pixel addresses covering 90

fibers in a linear segment of the area face of the bundle. The LEDs are

driven on the two sides in a 1 x 256 configuration (i.e. no multiplexing).

When the linear segments are scanned vertically, there will be about 100

address positions covering about 30 fibers in a column. This provides

enough spatial coverage for about 2700 fibers. The linear dimension of

the LED is about equal to the fiber diameter, i.e. 85 microns. We want this

to be about 28.33 microns in the image plane. Thus, the magnification is

28.3/85=.333. Assuming an F:2 lens, the transmission factor is 0.02. For

a large pixel, say four times larger, the magnification is 0.1 and the

transmission factor is 2.1 x 10^~3 or 10 times lower. However, the avail¬

able energy is at least 16 times greater. Hence, we gain a factor of

almost two by using larger LEDs. The actual gain from using larger LEDs

may be much greater. Large LEDs offer substantial opportunities for

improving brightness. The maximum line time is 3 ms corresponding to a

10 second page print time. Thus, 30 μs is available to achieve the neces¬

sary exposure.

The necessary power out of the LED is 60 μ watts. With reflection losses

of about 15% this requirement becomes 70 μ watts. We assume that the

LED emits 20 μ watt/ma. This corresponds to a power efficiency of about 1 .3%, typical of small, diffused-junction LED arrays in GaAIAs. Thus, the

drive current requirement is 3.5 ma to achieve 70 μ watts. This is a

reasonable drive current for such a small LED. The LEDs tend to saturate

with increasing drive current. On the other hand, there are 256 LEDs driv¬

ing 2550 (nominally 2700) fibers. Thus, each LED is used 9 times in each

frame on the average. The duty cycle is, therefore, 270 μs/3000 μs =0.09

under the assumption of uniform usage. Overheating is not a problem be¬

cause the average drive current is only 0.3 ma.

In presently available high speed LED line printers, the LED on time is

probably of order 60 μs. The exposure requirement is the same. The NA of

a suitable selfoc lens array is 0.1 -0.2 at best. Thus, the energy transmis¬

sion factor can be estimated to be less than 0.04 and probably comparable

to the value assumed for the printer of 0.02. Therefore, the LED approach

also appears feasible, the trade off being the extra cost and drive current

requirements of the large LED array versus the smaller number of LEDs

required.

The initialization procedure for the optical subsassembly of the printer

engine of figures 1 and 2 is directed at determining the addresses of light

imposed on the area face 18 of the fiber optic bundle and the relationship

between those addresses and the pixels exiting the linear face 17 of that bundle. In the absence of the establishment of such relationship, light

patterns directed at the area face will be scrambled at the linear face. In

accordance with the principles of the present invention, no two-dimen¬

sional, addressable, light-generating means such as a CRT is present.

Instead, a linear, light-generating means is made two-dimensional by

rotating a mirror which sweeps an image of a linear array of light-

generating means onto consecutive linear segments of the area face of the

fiber optic bundle, the light patterns being changed "on the fly."

In operation of the illustrative optical subassembly, eight bits (x axis

bits) of a sixteen bit address code are supplied by an address register (not

shown) which is part of the linear LCS array 19. The other eight bits (y

axis bits) are supplied by sensor 60 and mask 61 to correspond to the

angular orientation of mirror 24. In an arrangement where mirror 24 of

figure 1 is an elongated polygon as shown in figure 3, for example, and

motor 26 of figure 1 is a stepper motor, the mirror can be maintained in

consecutive fixed positions during an initialization procedure. In this

instance, the relationship between the pixel positions in the linear end

and those addresses are determined during the initialization procedure by

opening all light valves of LCS array 19 and by directing light through all

the open shutters to a iinear segment of the area face. While light is

incident on all of the instant linear segment, a hooded sensor with a transparent slit is moved along the linear face of the bundle in increments

small compared to a fiber diameter. In this manner, all slit positions for

which maximum intensity peaks occur for that linear segment (i.e. see

figure 2) are obtained. The light valves remain on and the process is re¬

peated for the next linear segment. The process continues until all the

linear segments are exposed. At this juncture in the initialization pro¬

cedure, all the maximum intensity positions and associated addresses are

known but not in the order in which they have to occur to correspond to

the order of the pixels desired at the linear face of the bundle. This rela¬

tionship can be understood by an examination of the interleaved fibers

shown in figure 2.

The proper sequence for those addresses is obtained by opening shutter

only those shutters (LCS's) of array 19 at addresses (x and y) which

correspond to one of the maximum intensity positions previously obtained

one at a time while the hooded photosensor is in one of the consecutive

maximum intensity positions. The address for which light exits at the

photosensor position is recorded in each instance as described above. It

takes about 100 ms to move the hooded photosensor to a new fiber (pixel)

position and scan the mirror. Thus, the total line time is 255 seconds for

2550 fibers. The physical organization of the illustrative optical subassembly permits

illumination, by a separate source, of the entire area face of the fiber

optic bundle at the same time rather than by illumination of only one

linear segment of the area face at a time. The illumination of the entire

face at the same time allows all the maximum intensity positions and

related addresses to be obtained with a single scan of the linear face by

the hooded photosensor. For the illustrative system where one hundred

y-axis positions are used, the illumination of the entire area face reduces

the scan time by a factor of one hundred.

Figure 6 shows a flow diagram of the initialization procedure where the

linear face is scanned by the hooded photosensor each time a linear

segment of the area face is illuminated. Block 100 indicates that a linear

segment of the area face is illuminated. Block 101 indicates that the

hooded sensor is moved to determine all addresses at which maximum

light intensity occurs. Block 102 indicates that those addresses are

stored. This procedure is repeated until light has been generated at all

linear segments as indicated by arrow 105. It is helpful to remember that

there are about ten addresses at the light generating end of the optical

subassembly to correspond to each fiber. The sensor selects the set of

addresses, amongst the ten for each fiber, at which the maximum

intensity occurs, it is also helpful to observe from figure 2 that the ad- dresses at which maximum intensity occur in one linear segment of the

area face are interleaved with the addresses from other segments.

After all linear segments are scanned by the photosensor as indicated by

block 1 06, the addresses of maximum intensity positions are placed in a

sequence to correspond the sequence of pixels in the linear face of the

fiber optic bundle. This is accomplished by moving the slit (photosensor)

to a now known position of maximum light intensity and, while the

photosensor is in that position, generating light consecutively at all the

addresses determined by the above procedure until the photosensor

detects light. The photosensor is then moved to the next consecutive

position where maximum light intensity occurred and the procedure is

repeated as indicated by arrow 107. When all elements at which maximum

intensity light is generated are in sequence, the initialization procedure

stops as indicated by block 108. The scrambled addresses at which

maximum light intensity occurred are now in an ordered sequence corres¬

ponding to the desired ordered sequence of the maximum light intensity

(pixels) in the linear face of the bundle. The ordering procedure is repre¬

sented by blocks 109, 110, and 111 of figure 6.

Figure 7 shows an arrangement for carrying out the initialization pro¬

cedure. The fiber optic bundle 120 again extends from an area face 121 to a linear face 122 as in figure 1. A photosensor 124 covered with a hood

having a slit 125 is moved along track 127. The incremental movement of

the sensor is carried out by a motor 130 under the control of control

circuit 131 which may be control circuit 21 of figure 1 . Control circuit

131 typically comprises a relatively fast computer programmed for the

initialization procedure operation. A much less sophisticated control cir¬

cuit can be used for the finished printer. The control circuit (131 ) is op¬

erative, as is control circuit 21 of figure 1 , to control the light generating

means LCS 19 (or LED equivalent), the liquid crystal shutters, and mirror

24 and to synchronize the operation of the light-generating means with

the movement of sensor 124 along track 127. Figure 7 also shows a lamp

135 for illuminating the entire area face of the fiber optic array if

required by the chosen initialization procedure.

The image of the linear LCS light valve (LV) array (19 of figure 3) is

scanned mechanically by a rotating polygon (24) across the area face 18 of

the fiber optic bundle as described above. Particular shutters in the

linear array are turned on to transmit light into particular fibers. The

image of the linear array scans at approximately constant velocity across

the area face (18) of the fiber optic bundle. An index light source at the

end of the linear LCS array generates timing signals from a transmission

grid photodetector in the plane of the area face of the fiber optic bundle immediately adjacent to the fiber array. In response to the timing

signals, the system turns on the particular pixels in the row at just the

right time to excite the appropriate fibers. The incoming pixel stream is

reordered so as to produce pixels at the right position in the written line

on the photosensitive medium. The indexing light source (shutter) is de¬

signated 200 in figure 3. The photodetector is designated 201 and the

timing signals are designated 202. Polygon 24, in one specific embodi¬

ment is a 20 facet, 38 mm radius mirror arrangement and iens 76 is a 50

mm, F/3.1 , 1 :1 focussing lens.

In a typical scan configuration, there are at least 100 linear segments and

256 elements in the segment (and the segments may overlap). Thus, there

are for any segment on average, about 25.5 elements that are in the

correct position to excite a fiber. Each segment is in its position for 25

μs. The 100 segments are executed in time sequence with no dead time.

Thus, the time to write a linear segment of the page is 2.5 ms with a

cycle time of 3 ms to cover (inter-facet) dead time in the polygon. With

3300 linear segments, it takes about 10 seconds to write a page. In a typ¬

ical design, a polygon with 20 sides performs the scanning. The time for

one complete turn of the polygon is 60 ms corresponding to 1000 revo¬

lutions per minute. Each pixel in the linear array is turned on every 10

rows on average or about 10 times during exposure cycle. Thus, 256 light sources write into 2550 fibers. The timing is consistent with 6 pages per

minute.

Groups of fibers in the fiber linear array emit light simultaneously. There

is not a progressive sequence of light spots for exposing the pixels as in a

flying spot (CRT) scanner. As a result there is the possibility that two

adjacent fibers in the linear array would not be excited with a non trivial

time gap as mentioned above. With a continuously moving photosensitive

medium, the pixels would exhibit small, random vertical displacements

from a straight line, never more than one pixel. These displacements re¬

peat from line to line so there would be a small tear between some

vertical lines. This would be undesirable. Fortunately, the distance bet¬

ween two fibers in the area face which are adjacent in the linear face is

typically a small fraction of the length of the fiber optic bundle, area

array side. Thus, the time lapse in writing the two adjacent fibers in the

linear array face is at most a small fraction of the time to write a

horizontal line. During this time the photosensitive medium moves a

distance equal to the height of one pixel. Consequently, the vertical

displacement of adjacent pixels and the related tear is never more than a

small fraction of the pixel height. On the other hand, if the photosensitive

medium is moved stepwise during the dead time of the polygon and is held

stationary during the writing period, then all pixels fall on the same straight line. Based on simulations, the small vertical displacements that

occur with a continuously moving medium are not observable and it is not

expected that step motion will be necessary.

The reason that fiber ends in the area face of a fiber optic bundle, which

have adjacent ends in the linear face, actually end up in closely-spaced

(although random) positions is a result of the manufacturing procedure for

the fiber optic bundle. Optical fibers are drawn and captured on a drum in

a manner well understood in the art. A fiber optic bundle is made from-

these captured fibers by placing an adhesive tape across the fibers, say

every foot along the drum and by cutting the fibers at the tape. The fibers

thus, are captured in a linear arrangement at one end and just hang at the

other.

The LCS array is controlled by dedicated drivers; it is not multiplexed.

The elements comprise a special, high speed liquid crystal shutter with

cross polarizers. Typical contrast ratio exceeds 500:1. Since there are on

average five LCS elements potentially illuminating each fiber core, a non

excited fiber receives about 5 times the dark output of one element. So

the actual contrast ratio of the emission into the fiber is about 100:1.

The excitation wave form is a +15 v pulse for black and a -15 v pulse for white. The light turn-on and turn-off is delayed by about 7 μs and the 10

to 90% rise time is also about 7 μs. These parameters are voltage and

temperature dependent so the voltage and temperature of the cell are

controlled. Because of the rise time, the 25 μs light pulse is roughly

bell-shaped with a flattened top, thereby slightly reducing the available

light. This further reduces the contrast ratio, but not by much, perhaps

20%. The shutter without polarizers has an absorption loss of 1/2% and

can handle a kilowatt/cm² of optical flux. However, even with a polar¬

izer/cell sandwich and no cooling, the rated incident optical power

capability is 4 watts/cm².

The total flux in the pulse can be varied by controlling pulse width or

height, so gray scale is inherient. Gray scale also can be achieved by time

modulation.

Color can be achieved by putting three linear fiber arrays together in

parallel in the same fiber optic bundle using separate drive sources. The

fiber lines are spaced by one pixel height. The color for three adjacent

lines are written simultaneously with appropriate delay so that they

superimpose on the photosensitive medium. Because of the geometry,

registration is automatically achieved. There is no writing time penalty

i.e. the rate is still 5 or more pages per minute. Another option is to use ~ ? time sequential color with stepped motion of the medium. An initial¬

ization procedure for color embodiments requires, for example, three

consecutive procedures, one for each color with light of red, green and

blue, where each procedure is analogous to that described above. The

procedures also may be carried out by using a color wheel.

Although the invention has been described in terms of a linear array of

light sources and a mirror, it should be clear that a matrix array can be

used in a row by row mode to achieve the equivalent operations. Also, one

or more linear array of light sources can be imaged onto an area face of a

fiber bundle which is in the shape of an annulus where the images are

rotated by, for example, a Dove Prism to move from (radial) segmen

the annulus.

Claims

What is Claimed is:

1 ) A combination comprising a plurality of light conduits having the ends

thereof arranged in a linear array in a first face and in an area array in a

second face having first and second axes and a plurality of linear

segments parallel to said first axis, light source means for generating a

plurality of light elements simultaneously along each of said linear

segments of said area face aligned with said first axis and digital means

for controlling the presence or absence of said light elements at said

linear segments along said second axis of said area face each of said

linear segments extending across said area face along said first axis.

2) A combination in accordance with claim 1 wherein said light source

means comprises linear means for generating said plurality of light

elements and digital means for directing said lig ht elements at

consecutive ones of said linear seαments.

3) A combination in accordance with claim 2 wherein said light source

means comprises a linear LCS multidevice light valve (LCS-LV), and

memory means for shifting into said LCS-LV data for controlling the open

or shut states for the shutter elements therein.

4) A combination of elements in accordance with claim 3 wherein said means for controlling a plurality of light elements comprises mirror

means for reflecting light from said LCS-LV to said area face and means

for rotating said mirror means controllably for directing reflected light

to said consecutive linear segments.

5) A combination in accordance with claim 2 wherein said means for

controlling said light elements includes control means for rotating said

mirror to produce consecutive images along said second axis correspond¬

ing to said linear segments.

6) A combination in accordance with 2 wherein said plurality of light

conduits comprises a fiber optic bundle including fibers arranged

randomly in a two-dimensional arrangement of S fibers along said first

axis and T fibers along said second axis, wherein said linear means com¬

prises N devices where N»S, said combination also including second-axis

control means for controlling said means for controlling such that light

from linear means is directed at a number, Z, of consecutive linear

segments where Z»T.

7) A combination in accordance with claim 6 wherein Z-N is ten or more

times σreater than S-T. 5?

8) A combination in accordance with claim 2 wherein said light conduits

comprise a fiber optic bundle, said combination also including a

photosensitive medium coupled to said iinear face and responsive to light

patterns issuing from said fiber ends in said linear face for forming an

image on the photosensitive medium.

9) A combination in accordance with claim 8 wherein said photosensitive

medium comprises an electrostatic medium with associated toner, trans¬

fer and fixer stations for forming an image on plain paper.

10) A combination in accordance with claim 8 wherein said photo¬

sensitive medium comprises photographic paper.

1 1 ) A combination in accordance with claim 2 wherein said light conduits

comprise a fiber optic bundle, wherein said light source means comprises

means for directing light at said linear means, and said linear means

comprises light shutter means for controllably occluding said light.

12) A combination in accordance with claim 8 also including a light

source for directing light at said linear means and light shutter means for

controllably occluding said light. 13) A combination in accordance with claim 6 wherein said linear means

comprises an LED and said means for directing comprises mirror means

responsive to coded -signal for moving said mirror to direct light to said

linear segments.

14) A combination in accordance with claim 2 wherein said linear means

comprise a plurality of LCS light valves arranged in first and second rows

offset from one another.

15) A combination in accordance with claim 2 also including sensor

means for sensing the one of said linear segments illuminated and means

responsive to said sensor means for providing a coded representation for

y - axis position of the illuminated linear segment in each instance.

16) Apparatus for initializing an optical system comprising a fiber optic

bundle which has the ends of the fibers arranged in a linear array in a

first face and in an area array in a second face and which has a linear

arrangement of light sources operative to direct a light pattern at

consecutive linear segments of said area face, said apparatus comprising

sensor means and means for moving said sensor means incremently along

said linear face each time said light pattern is moved to a next one of said consecutive linear segments, means for storing addresses of each light

source and the instant linear segment position at which maximum light

intensity is observed, means for moving said sensor to consecutive ones

of said positions at which maximum light intensity occurs, and means for

generating light at consecutive addresses with which maximum intensity

light was associated each time said sensor is moved to a next position.

17) A method for initializing an optical system comprising a fiber optic

bundle which has the ends of the fibers arranged in a linear array in a

first face and in an area array in a second face and which has a linear

arrangement of light sources operative to direct a light pattern at

consecutive linear segments of said area face, said method comprising the

steps of obtaining the addresses of all of said light sources for which

maximum-intensity light position is indentified in said first face, and

determining the order of said addresses to correspond to the order of said

positions in said first face.

18) A method in accordance with claim 17 wherein said step of obtaining

includes the steps of maintaining a light pattern along a selected linear

segment of said second face, moving a sensor from position to position

along said first face, generating light sequentially from said light sources

for which maximum intensity light positions previously occurred and r. which have an address which matches that of said linear segment and

repeating said steps for each of a series of consecutive linear segments

until the entire second face is completed.

19) A method in accordance with claim 18 wherein said step of

determining the order comprises moving a sensor to each of said succes¬

sion of maximum-intensity positions along said first face and generating

light consecutively from the light sources the addresses of which cor¬

respond to maximum intensity positions for each position of said sensor.

20) Apparatus comprising a fiber optic bundle, said bundle having the

ends of the fibers therein arranged linearly in a first face, said bundle

having the opposite ends of said fibers arranged in an area array in a

second face, means for projecting an image of an array of light sources on

correspondingly shaped segments of said area face, means for moving said

image through a sequence of said segments covering the entire area of

said second face, means for changing the light pattern from said array of

light sources for each of said segments and means for synchronizing the

means for moving and the means for changing in a manner to organize

pixels in said linear face for faithfully reproducing said image of said

area array of light sources at said first face. 21 ) Apparatus in accordance with claim 20 wherein said array of light

sources is a linear array and said correspondingly-shaped segments of

said area face are linear segments.

22) Apparatus in accordance with claim 21 wherein the image of said

array of light sources extends across one dimension of said area face, said

apparatus include digital means for moving said image from one of said

linear .segments to a next linear segment.

^•