Internal drum scophony raster recording device

Abstract

An optical recording and data processing system for exposing an image on to a flexible, light sensitive medium, which includes a medium holder having an inner cylindrical wall portion against which is held said medium, and a light source having an approximately rectangular emitting aperture, with a short aperture axis and a long aperture axis, operative to emit a beam of light having a rectangular cross section with a long axis corresponding to the long aperture axis and a short axis corresponding to the short aperture axis. An optical modulator is aligned with the light source so as to intercept light from the beam of light and produce a spatial modulation pattern across the long axis of the beam of light. A pattern shifter for shifting the spatial modulation pattern across the length of the long axis at a constant rate is provided as is a pattern rotator for rotating the spatial modulation pattern at a rate equal to the rate of shifting of the spatial modulation pattern. A scanner for scanning the beam of light onto and across the circumference of the inner cylindrical wall portion and a driver for advancing the scanner mechanism, after scanning a row, to an adjacent row to repeat the scanning are both provided. The relative phase angle between rotation of the spatial modulation pattern and scanning of the pattern is maintained such that the direction of movement of the projected image of said shifting is parallel to the scan motion, but opposite in direction. An optical system is provided to project the modulated beam of light and focus it to produce an image of the shifting modulation pattern at the recording medium so that the rate of shifting motion cancels the scan motion.

Description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 2, a general illustration of the opto-mechanical recording system 10, is comprised of a half cylinder 12 and attached feed tray 14 to permit transverse loading and unloading of the recording media 16. The recording medium 16 is flexible and held in place against the inner surface of the cylinder 12 by mechanical means or by vacuum applied through perforations (not shown) in the surface of cylinder 12. Referring to FIG. 3 the rear of the system 10 discloses the cylinder 12 and location of a source optics assembly 32 adjacent thereto. A micropositioner assembly 30 positioned across the open face of the cylinder 12 is a mechanical drive system which translates the scan prism assembly 20 (see FIG. 4) back and forth across the length of the cylinder 12.

As shown in FIG. 4, a single faceted scan prism assembly 20 is attached to a mechanical spindle 22, which, in turn, is mounted on a carriage 24, such that the spindle axis 26 is coincident with the cylinder axis 28 shown in FIG. 3. The carriage 24 is part of the micropositioner assembly 30, which translates the scan prism assembly 20 back and forth across the length of the cylinder 12. The optical source assembly 32 is mounted on the back of the casting of cylinder 12, as shown in FIG. 3. It modulates the light and generates a collimated beam which is directed at the scan mirror assembly 20 by two turning mirrors 51 and 53 (see FIG. 7). A tachometer assembly 25 is coupled to spindle 22 and has an optical encoder (not shown) which measures the speed of rotation and phase of the rotating prism 21.

Referring now to FIG. 5, the optical source assembly 32 has a laser diode assembly 34 which emits laser light through an elongated rectangular aperture 35 (see FIG. 7). The emitted light passes through a source coupling optic lens 36 and is directed onto an acousto-optic modulator crystal 38 with the long axis of the beam waist in a direction parallel to the direction of travel of the acoustic wave as shown by serpentine arrow 39. The diffracted light from the modulator 38 is reflected off of folding mirror 40, impinges on relay optic lens 47 and reflects off of folding mirror 42 after which it enters rotating dove prism 44. Dove prism 44 rotates about axis 45. After being reflected by folding mirrors 46 and 48 the light is collimated by collimating lens 50.

Referring to FIG. 6, the collimated beam from the optical source assembly 32 is directed along the axis of the cylinder 12 by reflection off of folding mirrors 51 and 53. After reflection off of mirror 53 the light passes through objective lens 52 onto rotating scan prism 21. Scan prism 21 is fixed at 45 degrees relative to its axis of rotation 28 and to the direction of the light beam 49 incident thereon. Scan prism 21 reflects the light radially, and normal to the interior surface 61 of cylinder 12. Objective lens 52 is positioned in front of the spindle 22 so that the focal position of the light beam 60 coincides with the surface of the recording medium 54 (see also FIG. 5) which is mounted against the interior surface 61 of cylinder 12.

As the scan prism 21 rotates, the resulting scanned focused light beam 60 scribes a circular arc of constant velocity on the recording medium 54 when mounted against the interior surface 61 of the cylinder 12. The carriage assembly 24 (see FIG. 4) is then translated along the cylinder axis 28 by means of a motorized mechanical actuator (not shown), at a rate equal to a single raster pitch spacing per rotation of prism 21, to affect complete exposure coverage of the recording medium 54.

After passing through objective lens 52, the light impinges on rotating scan prism 21 which scans the light 60 in raster bands perpendicular to the axis of rotation of scan prism 21. Rotating dove prism 44 rotates at one-half the rate of rotation of scan prism 21 and compensates for rotation of the image by the scanning process. The phase of rotation of the dove prism 44 is maintained relative to the phase of rotation of the scan prism 21 so that the long axis of the beam stripe 41 (see FIG. 7) at the inner cylindrical surface 61 of the recording medium 54 is parallel to the direction of scanning along the surface of that medium.

Referring now to FIG. 7, the operation of the optical source assembly 32 is shown schematically. The acoustic wave in the acousto-optic modulator 38 is shown having a velocity v_a in the direction shown by serpentine arrow 39. The acoustic wave diffracts a fraction of the incident light by an amount which depends upon its amplitude. The amplitude modulation of the wave as produced by an RF driving signal from RF generator 33 spatially modulates the light from source 34. The first order diffracted beam passes through a spatial filter 43 which filters out the higher diffraction orders to allow only the first diffraction order to fill the system aperture. The result is optical modulation controlled by the amplitude of the RF drive level. At any point in time there are a number of acoustic pixel elements 29 within the length of the beam stripe 41 which is revealed once the diffracted beam is separated. With the diffracted beam having passed through the dove prism 44 and focused by collimating lens 50 and objective lens 52 onto rotating prism 31, and scanned along a recording surface 54, the pixels 29 are shifted with time within the beam stripe in the direction 37. By scanning in the opposite direction 21 so that the speed of scanning at the recording plane 54 is equal but opposite to that of shifting, the pixels appear motionless on the recording surface 54. This is in contrast to a conventional internal drum scanning system where a single spot is modulated in intensity as it is scanned. The resultant exposure time of such a system, at any point along the scan line, will be extended by the number of image pixels that can be resolved over the length of the laser stripe 41.

The laser diode assembly 34 radiates from a slit-shaped aperture, shown to be oriented parallel to the mounting plane. The output power level is held constant throughout the imaging process, to provide stable illumination of the optical system. The source coupling optics 36 collect the laser light and form an image of the laser aperture at the plane of the modulator 38, typically with a large magnification ratio. The coupling optics 36 may also provide other beam shaping functions such as compensation for source astigmatism.

The acousto-optic modulator 38 consists of an optically active crystal, such as lead molybdate, with a piezo-electric transducer bonded to one face. When the transducer is excited by an RF electrical signal from RF driver 33, it will launch an ultra-sonic acoustic wave through the bulk of the crystal. The acoustic waves modulate the optical density as they propagate through the crystal, and the resulting phase grating will diffract incident light. Coherent, monochromatic light beams are split up into discrete diffraction orders, which can be spatially filtered to allow only one particular diffraction order to fill the system aperture, while all others are blocked. The result is optical modulation controlled by the amplitude of the RF drive level.

The length of the projected stripe of laser light, at the modulator crystal 38, is many times broader than a single acoustic pixel spacing, working at the intended data pixel bandwidth. The spatial period of a single acoustic pixel is determined by the acoustic velocity and data clock period, as follows:

delta_a=mu_a*Tau

where: delta_a is the acoustic spatial period mu_a is the acoustic velocity of the crystal material Tau is the data clock period This means that at any point in time during the imaging process, the length of the light stripe 41 will illuminate a number of acoustic pixel elements. The acoustic pattern is an analog representation of the amplitude modulation of the RF drive form. The diffracted light is therefore modulated spatially, as well as temporally, and forms a real image which is then projected on to the recording plane 54.

The relay optics 47 are used to form a subsequent beam waist at the rotating dove prism 44, which causes the image of the spatially modulated laser stripe to rotate, at twice the rate of the prism rotation. The dove prism 44 is rotated at half the rate of the final scan deflector 31, so that the orientation between the long axis of the laser stripe 41 and surface of the scan deflector 31 is maintained constant. This results in a projected image of the laser stripe 41, at the recording plane 54, that does not rotate along the scan line. The phase angle between the long axis of the laser stripe and the reflective surface of the scan prism 21 remains constant, and the relative position determines the orientation of the projected stripe 41 relative to the scan direction.

The phase angles of both the scan prism 21 and rotating dove prism 44 are determined electronically by means of optical encoders (not shown), attached to the respective rotors for the scan prism 21 and the rotating prism 44. The encoders each consist of a glass disc with an opaque, fine pitched radial grating, patterned on its surface. A thin beam of light is projected through the grating, which chops the light beam, and generates a tachometer clock signal at the opposing optical detector. A secondary marking and optical transceiver pair (not shown) generate a phase index clock, once per rotation. The tachometer and phase index signals are processed using phase locking circuitry to generate drive waveforms which synchronize the rotations of the two rotors. The phase difference can be controlled electronically, and is aligned so that the projected laser stripe 41 at the recording plane 54 is aligned parallel to the scan direction.

The Scophony velocity matching condition constrains the scan rate, depending only on the acoustic velocity of the modulator crystal 38 and the magnification of the optical system. The scan rate of the present invention is controlled by electronically tuning the rotation rate of the scan prism 21. A tachometer signal generated by a spindle encoder in tachometer 25 shown in FIG. 4 is used to phase lock the drive waveform to a precision tunable crystal oscillator (not shown). That tachometer signal is also used to synchronize the modulation of the pixel data, necessary to accurately place the pixels 29 along the scan line.

Finally, the collimation lens 50 is used to contain the beam divergence, and deliver a parallel beam of light to the final focusing objective 52. A collimated beam is necessary so that the final focus position remains at the recording plane 54 throughout the travel of the carriage assembly 24. The light beam 60 is directed on to the cylinder axis and scan mirror 21 by means of folding mirrors 51 and 53 so that the optical source assembly 32 can be mounted on the rear of the cylinder body 12.

Referring to FIG. 8 an alternative light source 62 consists of a linear stacked array of electrically isolated laser diodes 55 each of which are individually addressable. Light output from diodes 55 passes through an elongated generally rectangular aperture 59. Each diode is coupled by an address line 57 through a delay 56 to an input line 58 so that the light output can be modulated by varying the turn-on time of each of the lasers 55. Thus, by using the light source 62 of FIG. 8, one can eliminate the need for a separate modulator and instead modulate the light by simply varying the delay to each of the individual laser diodes in the array.

Obviously, a linear stacked array of laser diodes could be used in place of the laser diode assembly 34.

Obviously, other types of optical devices could be used to rotate the image other than a dove prism. For example, a Paschen prism or K prism could accomplish the same effect.

Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.

Claims

1. An optical recording and data processing system for exposing an image onto a flexible, light sensitive image recording medium, comprising:

(a) a medium holder having an inner cylindrical wall portion against which is held the image recording medium;

(b) a light source which emits a beam of light from an approximately rectangular emitting aperture, said rectangular emitting aperture having a short aperture axis and a long aperture axis;

(c) a spatial modulator positioned to intercept the beam of light from said light source and operative to produce a time varying spatially modulated pattern which shifts across the length of said long aperture axis at a substantially constant rate;

(d) a rotating beam deflector positioned in the path of said beam of light and rotated so as to produce a rotating spatially modulated pattern;

(e) a scanning deflector rotating about an axis aligned with an axis of an interior cylindrical surface at a rate of rotation equal to a rate of rotation of said spatially modulated pattern and aligned to an axis of said beam of light so as to deflect said beam of light substantially orthogonal to said axis to provide a scanning motion around an inner circumference of said image recording medium;

(f) a scanning deflector advancement assembly coupled to said scanning deflector and operative to advance said scanning deflector, after scanning a row, to an adjacent row to enable scanning of the adjacent row; and

(g) optical focusing components, positioned in a path of said beam of light between said spatial modulator and said interior cylindrical surface, operative to focus said beam of light and project an image of said spatially modulated pattern onto said image recording medium;

wherein orientation of the projected image of said spatially modulated pattern on said image recording medium is maintained parallel to a direction of said scanning motion such that a direction of movement of the projected image due to the shifting is parallel to the scanning motion, but opposite in direction; and

wherein said optical focusing components provide an optical magnification factor equal to a ratio of the scan velocity at image recording medium divided by the shifting velocity of said spatially modulated pattern whereby a scanning velocity at said image recording medium produced by rotation of said scanning deflector is equal to but opposite in direction to the shifting velocity of said spatially modulated pattern so as to maintain said pattern stationary on said image recording medium during exposure.

2. A system according to claim 1, wherein said light source is a linear array of individual lasers individually addressable for spatially modulating said light beam and including a plurality of delays coupled to respective ones of said lasers wherein each delay is different from the other of said delays and including a signal generator so as to cause the light from said lasers to be spatially modulated and to shift the modulation pattern.

3. A system according to claim 1, wherein said spatial modulator is an optical modulator which spatially modulates light from said light source and shifts the modulation pattern.

4. A system according to claim 1, wherein said optical modulator is an acousto-optic modulator and wherein said optical focusing components form a rectangular beam waist having a long axis corresponding to the long axis of the emitting aperture.

5. A system according to claim 1, wherein said scanning deflector is a mirror surface oriented at 45 degrees to a direction of incidence of light traveling along said axis.

6. A system according to claim 1, wherein said light source is a laser diode.

7. A system according to claim 1, wherein said rotating beam deflector which rotates said spatially modulated pattern is a dove prism rotating at half the angular velocity of said scanning deflector.

8. A system according to claim 1, wherein said scanning deflector advancement assembly advances said scanning deflector along said axis at a rate equal to one image track spacing per integer number of scan rotations.

9. An internal drum raster optical recorder having a flexible image recording medium mounted against an inner cylindrical mounting surface, comprising:

(a) a laser diode light source optically coupled to produce a beam of light having an elongated rectangular beam waist, said beam waist having a long axis along a long dimension of said beam waist and a short axis along a short dimension of said beam waist;

(b) an acousto-optic modulator crystal positioned so that an acoustic propagation direction is aligned to intercept said beam of light along the long axis thereof and operative to modulate said beam of light in response to a modulating signal and to produce an image of said modulating signal which shifts in the direction of acoustic travel;

(c) a rotating dove prism positioned to intercept and rotate said image of said modulating signal;

(d) means for directing said beam of light along an axis of said inner cylindrical mounting surface;

(e) an optical mirror surface oriented at 45 degrees to the axis of said inner cylindrical mounting surface, affixed to a spindle rotating on said axis, so as to deflect said beam of light radially to provide a scanning motion around an inner circumference of said recording medium;

(f) an advancement assembly coupled to said spindle and operative to advance said rotating spindle along said axis at a rate equal to one image track spacing per integer number of scan rotations; and

(g) an optical system positioned in a path of said beam of light between said acousto-optic modulator and said image recording medium, operative to project an image of said modulating signal onto said image recording medium;

wherein orientation of the modulated image pattern on the recording medium is maintained parallel to a direction of scanning of said modulated image pattern such that a direction of movement of the image due to shifting is parallel to scanning motion, but opposite in direction thereto; and

wherein said optical system provides an optical magnification equal to a ratio between an acoustic velocity of the acousto-optic modulator crystal and a scanning velocity at said recording medium whereby the scanning velocity along said image recording medium produced by rotation of said optical mirror is equal in magnitude but opposite in direction to a velocity of shifting movement of pixels within said image so as to maintain said modulated image pattern stationary during exposure of said image recording medium.