LASER BEAM RECORDING APPARATUS

Abstract: In laser beam recording apparatus suitable for recording a computer output signal onto a microfilm contained in an aperture card, a laser beam impinges on a beam splitter (24) which passes a write beam and reflects a reference beam to impinge on a photosensor (28). The output of the photosensor (28) is applied to a plurality of comparators (104-118) one of whose outputs is selected by a multiplexer (140) independence of the microfilm characteristics. The write beam is directed by a scanning mirror (46) onto the aperture card. During the time the write beam is not scanning the microfilm, the multiplexer output is applied to a hold circuit (142) which controls the intensity of an acousto-optical modulator (12) for the write beam. When the write beam commences to scan the microfilm, the hold circuit (142) level is held at its current value to control the recording intensity of the write beam on the microfilm.

Description: LASER BEAM RECORDING APPARATUS Technical Field This invention relates to laser beam recording apparatus of the kind including a laser light source adapted to emit a light beam, acousto- optic modulator means adapted to modulate said light beam into a modulated beam, beam-splitter means adapted to split said modulated beam into a write beam and a reference beam, and light sensing means adapted to generate an intensity signal proportional to the reference beam intensity. This invention has a particular application to a computer-output-microfilm (COM) plotter. Background Art Laser beam recording apparatus of the kind specified is known from U.S. Patent No. 4,477,8'21. The known apparatus includes a high frequency oscillator, the input of which is applied to a modulator for modulation by an information signal. The modulated signal controls an acousto-optical modulator. The reference beam is applied to a peak- holding circuit which holds or maintains the peak value of the write beam signal. The output of the peak-holding signal is applied to a comparator for comparison with a reference signal. The output of the comparator is applied to a loop gain control circuit, the output of which is connected to a phase compensation circuit which adjusts the amplitude of the high frequency oscillator. Disclosure of the Invention It is an object of the present invention to provide laser beam recording apparatus of the kind specified, which is simple, inexpensive and suitable for application to a high resolution COM plotter. Therefore, according to the invention, there is provided laser beam recording apparatus of the kind specified, characterized by comparison means adapted to compare said intensity signal with a predetermined signal dependent on the record medium, scanning means adapted to scan said write beam across a record medium, synchronizing means adapted to provide a scan signal when said write beam is scanning across said record medium, hold circuit means responsive to the output of said comparison means during the time said scan signal is not being produced, to continuously adjust an intensity level control signal in accordance with the value of said predetermined signal, said intensity level control signal being held at its current value during the time said scan signal is being produced, and driver means coupled to said hold circuit means and to said acousto-optical modulation means to control the intensity of said light beam in dependence on said intensity level control signal. Brief Description of the Drawings One embodiment of the present invention will now be described by way of example, with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of the bottom portion of a COM plotter which contains all but the final portion of the optical path. Fig. 2 is a plan view of the bottom portion of the plotter shown in Fig. 1. Fig. 3 is a partially cut-away view of the intensity sensor housing in which the intensity beam¬ splitter is mounted. Fig. 4 is a section view of the mounting block for the intensity and the focus beam-splitters. Fig. 5 is a detailed view taken in the direction of 5-5 shown in Fig. 4 to illustrate the mounting of the intensity beam-splitter upon the block. Fig. 6 is a simplified block diagram of the intensity controller electronic circuit functions. Figs. 7A, 7B, 7C, 7D, 7E and 7F when assembled according to the map of Fig. 7 show a detailed schematic diagram of an intensity controller electronic circuit. Best Mode for Carrying Out the Invention The invention is applicable to a COM aperture card plotter. A plotter of this type is described in U. S. Patent No. 4,613,876, issued September 23, 1986, and assigned to NCR Corporation, which patent describes, among other things, how the film of an aperture card can be bent into an arcuate surface in order to provide a circular scanning path which has a uniform radial focal length, so details of how the microfilm bearing, aperture card is held will not be further addressed in this application. Referring now to Figs. 1 and 2, which illustrate the optical path beneath the microfilm aperture card carrier, a HeNe gas laser 10, such as a Siemens model LGR 7641, emits a beam of 632 nanometer, coherent light at a power level of 1.25 mw. The beam traverses a short distance from laser 10 and subsequently enters an acousto-optic modulator 12, such as a Crystal Technology Division, Siemens Corporation model number 3080-16, where the beam is modulated (as will be explained below) to provide a controllable intensity beam. The modulated beam is emitted from acousto- optical modulator 12, where it is reflected by a first, front surface mirror 14 and then a second, front surface mirror 16. Each mirror is oriented at a 45° angle with respect to the incident beam, and reflects the incident beam by 90°, according to known physical principles, resulting in a total reflected direction change of 180°. The acousto-optic modulator 12 is excited by acousto-optical driver 13, and modulates the beam, in a known way, by the interaction of laser light rays with a vibrating, transparent medium into a composite of a zero order beam, a first order beam, and higher order beams. Each order beam diverges away from the other order beams as a function of the distance- from the modulator 12. Thus, after traveling from the modulator 12 and reflecting mirrors 14, 16, the composite beam is comprised of a group of horizontally-spaced beams, which show up as a series of dots on a beam-splitter housing 18. The first order beam is allowed to enter housing 18 through aperture 22 (shown in Fig. 3), but all other beams are rejected, thus housing 18 and aperture 22 perform the function of a spatial filter on the composite beam. Referring now to Figs. 3, 4 and 5, the operation of the beam-splitters will be described. Inside housing 18, mounting block 20 is secured by a bolt 21 as shown in Fig. 3. Mounting block 20 has an inclined surface of 12° on one end, as shown in Fig. 4, upon which the intensity beam-splitter 24 is mounted. The beam-splitter 24 in the preferred embodiment is optical glass, although those skilled in the art will recognize that other transparent media may be used. Light entering through aperture 22 in housing 18 will strike the first surface of beam¬ splitter 24. Since glass has a higher index of refraction than air, an interface exists at which the beam will be partly transmitted and partly reflected, thus splitting the beam into: a beam which is used for writing information on a microfilm card; and a reference beam, which is proportional to the write beam. The reference beam is sensed to provide an intensity control input. The proportion transmitted and the proportion reflected at the interface is a function of the index of refraction of air, the index of refraction of the beam-splitter glass, and the angle of incidence of the beam as is well known from physical principles. The use of an isotropic, transparent media such as glass has many advantages over other types of beam-splitters. A commonly used other type, is a non- isotropic beam-splitter called a half silvered mirror, which is basically an isotropic, transparent media with a partial reflective coating on the outside surface. Where the reflective coating is on the surface the incident beam is efficiently reflected, and, as discussed previously, where there is no reflective coating, the transparent media interface partially reflects and partially transmits the incident beam. The main problems of a partially reflecting mirror beam-splitter are: expense, inefficiency of transmission of the write beam, and non-uniformity of the reflective, coating deposition on the beam-splitter surface. All of these problems are obviated by a simple, inexpensive, and efficient, isotropic glass beam-splitter. A second effect that occurs, whenever an isotropic, transparent beam-splitter is used, is known from Fresnel's equations. Fresnel's equations provide expressions for the reflection coefficients of incoming polarized light as a function of the orientation of the electrical field vector and as a function of the angle of incidence of the beam upon the beam-splitter surface. According to Fresnel's equations, light of some polarities will be more readily transmitted through ' the glass than reflected. Since the laser 10 of the preferred embodiment emits randomly polarized light, especially during the first few hours after being turned on, the preferred embodiment reduces to insignificant the interaction between a randomly polarized laser beam and the polarization dependent coefficients of the beam- splitter glass. According to Fresnel's equations, the reflection coefficients are primarily a function of the angle of incidence, therefore, by designing a mounting which holds the beam-splitter at a 12° angle from a vertical direction, as shown in Fig. 4, the worst case effect of random polarization upon the beam-splitter would be a change of three percent of the ratio of the reflected light beam intensity to the transmitted beam intensity. Maximum variations of three percent due to random polarization orientations have, by experimental testing, proved to be acceptable for a beam-splitter of an intensity controller. The reflected or reference beam caused by the beam-splitter 24 shines upon a transparent body 30 of a photo-transistor 28 which is mounted in beam¬ splitter mounting block 18, as shown in Fig. 3. The write beam continues through the intensity beam-splitter 24 and exits the second - surface 26, which is treated with an anti-reflective coating. The coating reduces the reflections as the write beam passes from the high index of refraction of glass into the lower index of refraction of air. Reflections from second surface 26 give false intensity indications due to optical interference with the first- surface beam at intensity photo-transistor 28. " Such interference would be very unpredictable in a production environment because it critically depends upon the thickness of the beam-splitter glass and the angle of incidence. As the write beam emerges from second surface 26 of the intensity beam-splitter 24, it is refracted parallel to the optical path direction into a circular cross-section passage way 32. At the end of passage way 32 the write beam enters a focus beam-splitter 36 at an inner surface 34, which as will be explained below, has an anti-reflective coating upon it. Upon emerging from the outer surface 35 of focus beam- splitter 36 the write beam is refracted to travel substantially the path it would have taken had not the beam-splitters 24, and 36 been present. The write beam then exits housing 18 through a passageway 38. Referring again to Fig. 1 and Fig. 2, after leaving the beam-splitter housing 18, the write beam enters the focusing tube 44 which contains lenses for focusing the beam to a five micron diameter spot at the aperture card surface. When the write beam emerges from the focusing tube 44 it strikes a scanning mirror 46, which rotates 360° at a constant rate. The scanning mirror 46 thus sweeps the reflected beam constantly in a 360° circle. The area above the mirror, which carries the aperture card, is closed by a shutter 19 (shown in Figs. 1 and 2) during light beam scans without an aperture card in place to keep the light beam away from the microfilm aperture cards until exposure is desired. Scanning mirror 46 is mounted upon a rotary shaft of a brushless DC motor/shaft-position-encoder 52. Motor/encoder 52 not only provides the power to keep the scanning mirror spinning, but also provides a pulse output defining where the beam is being reflected to, in order to coordinate the scanning of the write beam with the electronic intensity control circuitry shown in Figs. 6, and 7A through 7F. Fig. 6 illustrates in block diagram form the intensity controller circuit 100, one embodiment of which is shown in Figs. 7A, 7B, 7C, 7D, 7E and 7F. The intensity controller 100 is connected to the intensity photo transistor 28, such as a photo transistor type OP-500, which provides an electrical signal that is proportional to the intensity of the reference beam shining upon it. The output of the intensity sensor 28 is connected in parallel to a number of exposure control comparators 104, 106, 108, 110, 112, 114, 116, and 118, such as type LM339. Each exposure control comparator compares the sensor signal to a preset d.c. level. Each preset level is different and corresponds to an intensity of the write beam which will produce the correct exposure for recording on a corresponding type of aperture card microfilm. The output of each of the exposure control operational amplifiers goes to one input of an eight- to-one multiplexer 140; such as Motorola type MC14529B, which has tri-state outputs each of which may provide a logic 0, a logic 1, or a high series impedance according to the input logic levels. The three exposure select lines EXP0, EXP1, and EXP2, determine which of the exposure control amplifiers wili be selected to provide the output of the multiplexer. The output of the multiplexer 140 is connected to a hold circuit 142 which, during the approximately 338° of the 360° rotation of the scanning mirror that the light beam is not recording upon the aperture card microfilm, continuously adjusts the intensity to the fixed value determined by the selected exposure comparator. At the end of this approximately 338° the final value is subsequently held for the remaining, approximately 22° as the intensity operating point. This method is acceptable because the scan time for the 22° representing the picture area is approximately 1 millisecond, while the period of laser output intensity variations is on the order of 15 millliseconds. It should be noted that, during the approximately 338° non-recording portion, the final value in the hold circuit 142 is coupled through an output driver 144 to the acousto-optical driver 13 and the acousto-optical modulator assembly 12. This provides a closed loop control system when the laser beam is on because the beam intensity level is sensed by photo transistor sensor 28, compared to a reference level to determine if it is too bright or too dim, and the intensity is reduced or increased by the result of the comparison driving the modulator 12. During the 22° writing scan of the aperture card microfilm this hold circuit value is held by essentially disconnecting the multiplexer output. This is accomplished by the sync circuit 152 impressing a logic 0 on an input of AND gates 139 and 141 (Fig. 7D) , respectively, which subsequently induces both outputs of MUX, 140, to the high series impedance condition. The held value is one input which is used to define the operating point of output driver 144. A second input to the output driver 144 is the pixel data which is "clocked" out of a computer such as a GAD/CAM system using pulses from a scan enable line 145. The pixel data is also synchronized to the rotation of the rotating mirror 46 by timing pulses on lines 147 and 149 (Fig. 7F) from the encoder portion of motor/encoder 52 via an AND gate 151 and the sync circuit 152. One of the signals derived from the encoder signals is a blanking pulse which drives the driver 13 and the acousto-optical modulator 12 to cut off, momentarily, so that the initial edge of the microfilm image for each scan will be synchronized with the initial edge for all other scans on the microfilm. After the blanking pulse is over, the pixel data for that scan is written onto microfilm. Figs. 7A, 7B, 7C, 7D, 7E and 7F show one embodiment of the intensity control circuit 100. The intensity photo transistor 28 is shown in Fig. 7A connected in a common collector mode with an output resistor consisting of the combination of a 2K resistor 102 in parallel with a negative temperature coefficient (NTC) thermistor 101. NTC thermistor 101 is mounted in close proximity to photo transistor 28 and compensates for the increase in gain of intensity photo transistor 28 as the temperature increases. The output of the intensity photo transistor 28 is connected to the. non-inverting input of comparators 104, 106, 108, 110, 112, 114, 116, and 118. The inverting inputs of each of these comparators is connected to a respective voltage divider 124, 126, 128, 130, 132, 134, 136, and 138 which are supplied a constant reference d.c. level by a potentiometer resistor 120 and a capacitor 122. Each voltage divider 124 through 128 is adjusted to correspond to an exposure index of a type of aperture card microfilm used as a recording medium. The output of each comparator 104-118 is zero volts if the voltage divider reference is greater than the intensity photo transistor voltage, or 5 volts if the intensity photo transistor voltage is greater than the voltage divider reference. The eight operational amplifier outputs which are either 0 or 5 volts are supplied to a dual 4-to-l multiplexer 140, such- as a Motorola type MC14529B, which is connected as a single 8-to-l multiplexer. Select lines EXPO and EXP1 select one of four outputs from each of the dual sections to be switched respectively to output 1 and output 2 which are wire "OR'ed". Select line EXP2 using gates 137, 139 and 141 selects output 1 or output 2, depending upon which of these two outputs is not caused to assume a high impedance state by the logic levels of enable inputs EN1 and EN2 which are connected to the gates 139 and 141. Thus of the eight inputs only one will be selected as the input to the hold network 142, which includes a comparator. The output of the hold circuit is supplied to the base of an output driver 144 which contains two transistors 146 and 148, such as Motorola MPS4285, connected in a Darlington configuration as shown in Fig. 7C. The output driver buffers the hold circuit such that the held value is not appreciably degraded during the hold period of just over 1 millisecond. As mentioned above, this held value is the operating intensity reference point from which the pixel data modulates the intensity to record, scan by scan, the desired image. Cascaded within the output driver is a third transistor 150, such as Motorola type MPS4285, which provides another two inputs. Transistor 150 is used as an "OR'ed" current switch. The first input in the adjustment cycle signed, which is used to turn the laser 10 "ON" continuously in order to adjust the intensity for the 338° non-microfilm scanning portion of each rotation. The clocked pixel data is used to turn the beam "OFF" or "ON" in a purely binary sense by means of the acousto-optical modulator 12 to build up the recorded image, which is the ultimate- object of the intensity control system. The intensity of the modulated write beam when turned "ON" is defined by the value held in the hold circuit 142, which was established by the adjustment cycle of the previous 338° rotation. Also shown at the top of Fig. 7A is a focus photo transistor 40. During set up with the beam on and with a microfilm sheet positioned above the rotary scanning mirror 46, the beam will reflect off of the microfilm back down to the rotary mirror 46, through the focusing tube 44 and be split by the focus beam¬ splitter 36, in much the same way that the write beam is split by intensity beam-splitter 24, into a focus beam and a dissipated beam. The focus beam-splitter 36 reflects the focus beam portion of the reflected beam onto photo-transistor transparent body ' 42 of focus photo-transistor 40 and cause an electrical signal to be generated thereby. Maximizing this output does not produce a perfect focus at the film; however, it is extremely close to.a perfect focus and therefore adjusting the focusing tube 44 for the maximum output of the focusing transistor 40 during initial set up provides a method for quickly adjusting the focusing tube 44 to almost its final setting. This, of itself, saves sufficient set up and testing time to justify any added manufacturing costs. Thus there has been described an intensity controller for an aperture card COM plotter that maintains the operating point of the write beam in reference to a predetermined exposure level for the film type.

Claims: CLAIMS : 1. Laser beam recording apparatus including a laser light source (10) adapted to emit a light beam, acousto-optic modulator means (12) adapted to modulate said light beam into a modulated beam, beam¬ splitter means (24) adapted to split said modulated beam into a write beam and a reference beam, and light sensing means (28) adapted to generate an intensity signal proportional to the reference beam intensity, characterized by comparison means (104-118) adapted to compare said intensity signal with a predetermined signal dependent on the record medium, scanning means (46) adapted to scan said write beam across a record medium, synchronizing means (152) adapted to provide a scan signal when said write beam is scanning across said record medium, hold circuit means (142) responsive to the output of said comparison means (104-118) during the time said scan signal is not being produced, to continuously adjust an intensity level control signal in accordance with the value of said predetermined signal, said intensity level control signal being held at its current value during the time said scan signal is being produced, and driver means (144) coupled to said hold circuit means (142) and to said acousto-optical modulation means (12) to control the intensity of said light beam in dependence on said intensity level control signal. 2. Laser beam recording apparatus according to claim 1, characterized in that said beam splitter means includes an isotropic beam splitter (24) . 3. Laser beam recording apparatus according to claim 2 , characterized in that said light beam has a randomly varying polarization direction. 4. Laser beam recording apparatus according to claim 2, characterized in that said isotropic beam splitter (24) includes a flat, transparent body having: a first surface (25) whereat said modulated beam is split into said reference beam, which is reflected from said first surface (25) and said write beam, which is transmitted through said transparent body; and a second surface (26) whereat said write beam exits from said transparent body. 5. Laser beam recording apparatus according to claim 4, characterized in that said modulated beam enters said first surface at an angle of incidence of substantially.12°. 6. Laser beam recording apparatus according to claim 4, characterized in that said second surface (26) has an anti-reflective coating thereon adapted to reduce reflections as said write beam emerges from said second surface (26). 7. Laser beam recording apparatus according to claim 1, characterized in that said modulated beam includes a zero order beam, a first order beam and higher order beams, and in that spatial filter means (18, ~ 22) are provided which transmit said first order beam and reject all beams of other orders. 8. Laser beam recording apparatus according to claim 4, characterized by focusing means (44) located after said beam splitter means and adapted to focus said write beam onto said record medium. 9. Laser beam recording apparatus according to claim 8, characterized by a further beam splitter (36) adapted to split said write beam into a focus beam and a dissipated beam after said write beam is reflected back from said record medium, and by focus intensity sensor means (40) located in the path of said focus beam and adapted to provide a maximum sensor signal when said focusing means (44) is substantially focused. 10. Laser beam recording apparatus according to claim 1, characterized in that said comparison means includes a plurality of individual comparison devices (104-118) adapted to compare said intensity signal with a plurality of predetermined signals dependent on different record medium characteristics, by multiplexer means (140) coupled to outputs of said comparison devices (104-118), and by selection means (EXP 0, EXP 1, EXP 2) coupled to said multiplexer means (140) and adapted to select the output of a selected one of said comparison means (104-118) for application to said hold circuit means (142). 11. Laser beam recording apparatus according to claim 1, characterized in that said record medium is microfilm contained in a aperture card, and in that said write beam is adapted to plot a computer output on said microfilm.

Inventors: RANGER MICHAEL HARRY, SPEKTOR SEMYON, RANGER, MICHAEL, HARRY, SPEKTOR, SEMYON
Applicants: NCR CO, NCR CORPORATION
Application year: 1988
Main classification: G06K15
IPC codes: B41J2, G06K15, H04N1