RASTER OUTPUT SCANNER

Abstract: A circuit is disclosed for correcting the write power of a laser raster output scanner as it scans across a write drum (15). During calibration, a piece of glass (25) reflects part of the beam into a photo-multiplier tube (16), the output of which is converted into a digital signal. This is compared with a reference signal. The difference for each segment of the scan is stored in RAM and also is converted back to an analog signal to modulate an acousto-optic modulator in the laser optics to correct the write power. After the system has settled to a point where the write power at the drum does not vary across the scan, the glass may be removed, and the system thereafter operates using the correction signals stored in RAM.

Description: The present invention relates to a circuit for correcting for fluctuations in laser beam power as a raster output scanner travels from one end of its scan to the other, and specifically comprises a memory for storing and compensating for variations for a plurality of segments of the scan. Laser xerographic printing systems for color are very sensitive to the amount of illumination applied by the laser beam when printing halftoned images. The effect of too much illumination is to make the diameter of the halftoned dots oversized, resulting in color shades too light and mismatched with the original. Too little illumination causes unwanted background to be developed in the white areas. The causes for this problem are various. Some examples are: 1) Side-of-drum to side-of-drum, and drum-to-drum, xerographic sensitivity variations ; 2) Optical scanner side-to-side efficiency changes. The changes result from design, fabrication and alignment abnormalities, as well as day-to-day minute variations believed caused by such things as thermal gradients, mechanical straining of the optical bed, vibration and noise from machine mechanisms, etc., and 3) Laser radiation power output level. The useful life of a laser tube is usually determined by the length of time that a laser can furnish sufficient output light. For example, a typical laser may typically start at output levels of 18 to 22 milliwatts, and is usable down to about the 5 to 8 milliwatt level. The problem of keeping the intensity of a light beam substantially constant as it sweeps across a charged photoconductive surface is addressed in GB-A-1 515 606. The present invention differs from this disclosure primarily by using a stored calibration curve to modulate the incident beam. Accordingly, the present invention provides a raster output scanner as claimed in the appended claims. To permit the adjustment of the effective drum exposure level in small increments along the laser scan line, a drum exposure circuit can be installed. The circuit contains a small, dedicated microprocessor package which includes: an analog-to-digital video converter which can implement the direct reading of an illumination level falling at a position that represents the surface of the drum, and a circuit which receives start-of-scan and end-of-scan signals, as well as timing clock pulses. To calibrate the system, either a single curved mirror, or a series of small mirrors, is used to collect a percentage of light from the scan line on to the surface of a light-sensitive photomultiplier tube (PMT). This optical assembly may be either temporarily located in the write beam at a location that represents the drum surface, or permanently located out of the normal beam path, so as not to interfere with the normal writing of the engine. A simple transmission-dividing piece of glass could cause 1 or 2 percent of the total laser light power to fall on the PMT assembly. In actuality, this transmission-divider mirror could be a piece of uncoated plain window glass. The PMT output represents the illumination level incident on the PMT surface that would be falling on the drum surface at any instant in time, which the microprocessor can then translate to any position along the drum surface. This analog signal is connected to the analog-to-digital converter of the microprocessor when calibration is desired. The microprocessor has a built-in standard curve stored in permanent read-only memory (ROM). When a new drum is installed, the ROM curve can be loaded into RAM and a temporary correction curve applied to the system. By then using the PMT signal it is possible to generate the exact correction curve desired and then to upgrade more permanently the curve used by the microprocessor, using electrically-alterable read-only memory (EAROM). The EAROM saves the data so when power is turned off and back on the next day, the previous curve is still available if it is to be used. It is also an option to aim the light collection system at the printed copy as it leaves the machine and, with appropriate color filters and by using three PMTs and a set of red, blue and green filters, to correct on the fly for total machine printing changes. A corrective curve is stored in memory as an 8-bit word for every 625 µm along the length of the scan line on the drum. A memory address generator is started by the start-of-scan signal and driven by a clock signal which has a controlled number of pulse counts existing between start-of-scan and end-of-scan signals. This address generator is configured and constructed to access the EAROM containing the memory curve during the scan line scan time by using DMA. The readout correction word from the memory is reconverted back to an analog siginal by a digital-to-analog (D/A) converter. It is this conversion signal that is used to modulate the transmission efficiency to the write laser optical path for correction purposes. In this system there are two locations where the transmission correction signal can be applied to the optical system. However most laser printing systems are likely to have only one. The described system uses both an electro-optical (E/O) modulator and an acousto-optical (A/O) modulator. The E/O unit is used for impressing either data or video intelligence on the write laser beam, while the A/O unit is used for facet tracking to conserve laser writing power. In this described system, the bandwidth capability of the E/O unit is totally employed for passing the required high-speed modulation intelligence. Consequently, the transmission correction signal had to be applied to the A/O unit. The normal modulation signal applied to any A/O modulator in facet-tracking service consists of a frequency-modulated (FM) AC sinewave carrier with a sweep frequency that might start at 25 MHz, and change in a preselected way to some typical upper frequency of, for example, 160 MHz. In facet-tracking service, the frequency of the applied carrier determines the angle of the deflected beam, and must not be varied by the DMA transmission correction signal. The DMA analog signal 15 is therefore applied to the A/O modulator package in such a manner so as to control the amplitude of the appied FM modulation signal to the A/O crystal head. The techniques used to combine the AM signal with the FM signal must not produce sidebands that can upset the A/O crystals ability to maintain the beam properly centered on the rotating prism facets. The FM signal must remain uninterrupted (CW), never being allowed to diminish to a zero level, while still being modulated in either class A or class A-B₁. This includes the period between start-of-scan and end-of-scan during retrace. To set up the transmission correction system, it is critical that the system be designed so that the DMA subsystem reads the exact word that corresponds to the location along the scan line that the laser write system is scanning. By simultaneously applying the saved correction word, reading the PMT output which measures actual transmission of the optical path, and comparing the difference, if any, to an internal reference, a dynamic correction signal can be generated, saved in EAROM on the fly, and the system made to track small changes in the optical writing subsystem. If the correction is to include the developed image, then it is necessary to use the upper or lower margin time on the copy when no intelligence is being impressed on the laser writing beam. If it is desired to include the development process and a replacable drum sensitivity, then the sampling must be taken on the finished copy after fusing, instead of ahead of the write drum just before the xerographic surface. However, in either case the laser scanning and pickup techniques used can easily be the same. If an input PMT light measuring system is not present, then an option is to replace the input reading mechanism with data read from the printed copy using a spectrophotometer and substituting read values along the scan line for PMT values. The present invention will now be described by way of example with reference to the accompanying drawings, in which: Fig 1 is a block diagram of the system, and Figs 2, 3 and 4 are schematic diagrams of the circuit. Figure 1 is an overall block diagram of the system. The laser scan is originally generated by a laser 10 producing a beam L of coherent radiation which is then modulated by an electro-optic modulator 11 to impart data to the beam. After travelling through optics 12, the beam is made to track the current facet by acousto-optic modulator 13. The resultant beam is processed through additional optic elements 14, and is then used to illuminate the drum 15. As the beam travels from one end of its scan to the other, it scans across a start-of-scan detector 28 and an end-of-scan detector 29. These are standard devices used in the industry to generate start and end signals. These signals are typically used to start and stop a counter which counts a data clock, so that data can be clocked out to the write electronics in synchronism with the scan. In this system, the data clock pulses generated by the clock generator 33 are used to drive a counter address generator 23, which in turn clocks out the values stored in memory. Memory output is converted to an analog signal which is then used to modulate the FM signal applied to the modulator 13, thereby varying in a controlled manner the A/O modulator transmission. A fraction of the light is reflected by partial mirror 25 into a photo-multiplier tube (PMT) 16, the analog output of which is converted into a digital signal by analog-to-digital converter (A/D) 17. That digital signal is compared to a reference level at comparator 26, the difference being stored in memory 18. At this point, switch 20 can be set in its "up" position to read out the current value (in the calibrate mode, with the mirror 25 in the beam path) or set to its "down" position to read out a stored value (in the operational mode with the mirror 25 removed from the beam path). In the 'calibrate' mode, the closed loop will result in the memory finally containing a set of values which will result in a non-varying beam across the length of the drum. Then, in the 'operational' mode, these values are used for the actual write scanning. Finally, the memory output is converted into an analog signal at D/A converter 19 and is used as an input to driver 27 to modulate the amplitude of the beam to correct for all variations. The address generator 23 is a counter which is driven by the system clock, turned on by the start-of-scan signal and terminated by the end-of-scan signal. Data can also be entered into the memory 18 by means of a terminal 21 and a microcomputer package 22 which can load data into the memory 18 through DMA channel 24. Fig. 2 is a detailed schematic diagram of the address generator. The main element is a 12-bit counter, implemented from three four-bit parts 30, 31, 32, which is used to generate address signals A0 through A11. The counter is driven by oscillator 40 through buffers 54, 55. The counter is started at the beginning of the scan by the start-of-scan signal being coupled through line receivers 33 and 34, turning on flip-flops 35 and 36, which apply an enabling voltage to line 37. Transistors 38 and 39 then generate a voltage which allows the oscillator 40 to begin generating pulses. At the end-of-scan, the end-of-scan signal from line receivers 52, 53 will turn off flip-flop 35. Flip-flop 36 will continue to stay on until a short time after the end-of-scan signal, which is coupled through line receivers 52, 53, since the end-of-scan signal is supplied to flip-flop 36 through one-shots 41, 42, which generate a slight delay. Therefore, the counter 30, 31, 32 will continue to count until an overflow is produced at line 43. This overflow is coupled to flip-flop 44 which therefore will stay on from the start-of-scan to the overflow of the counter, an amount of time which is not the same as the total time from the start-of-scan to the end-of-scan. In other words, flip-flop 35 turns off at the end-of-scan, while flip-flop 44 turns off at the end of the count, which may be earlier or later. The faster the counter is running, the sooner flip-flop 44 will turn off, resulting in a difference signal which is either high or low depending on whether the counter is too fast or too slow. This difference signal is generated by gates 45, 46, 47 and used as an input to integrator 48, which will generate an analog signal which rises or falls as a function of the time difference, and is therefore an indication of whether the counter is running faster or slower than the prism is rotating (the variation in rotation speed of the prism resulting in more or less time between start and end-of-scan signals). Therefore, as the prism varies in speed, the output of the integrator will also vary. This output signal at line 49 is applied to variable capacitors 50, 51 to vary the rate of oscillator 40. In this way the data rate, as determined by the oscillator, will track the prism speed variations. Fig 3 is a schematic diagram of the memory comprising RAM 60 and PROM 61, addressed through buffers 62 - 65. Data are loaded into the memory through buffer 66. Data are taken out through buffer 67, which is enabled from start-of-scan to end-of-scan, and converted into a zero to one volt analog voltage in D/A converter 68 for input to amplifier 69, which, in turn, generates a zero to five volt signal to drive the acousto-optical facet tracker. During fly-back time (from end-of-scan to start-of-scan), a dummy input, representing a constant gray value, is generated at register 70 and applied to the amplifier 69 through D/A converter 68 to maintain all operating voltages in the following circuits in their normal operating ranges. Unity-gain amplifier 71 uses a voltage divider comprising resistors 84, 85 to generate the minus two volt reference voltage of a D/A converter. In the calibrate mode, the data are written into memory 60, and the same word is immediately read out to the buffer 67. In the operate mode, no data are written in. The memory contents are sent out, as stored, to the buffer. Fig 4 is a schematic diagram of the modulator controller circuit, the main component of which is the acousto-optic modulator driver 80. The zero to five volt output from amplifier 69 of Fig 3 is shifted to a more positive voltage by zener diodes 81, 82, 83 to about 2 to 7 volts, and applied as the signal input to the driver 80, while a voltage divider reference is adjusted to plus 2 volts by potentiometer 81 at the other input. The driver 80 amplifies the voltage , resulting in an output which varies from approximately 2 volts to 26 volts. Since this is a correction voltage to compensate for power variations, and since power typically falls off at the ends of the scan, the output will typically be higher at start and end-of-scan, and relatively lower during the middle of the scan. This signal results in a facet tracker output power that is correspondingly lower during the middle of the scan.

Claims: A raster output scanner comprising: means (10) for generating a beam (L) of coherent radiation; a write drum (15); an optical system (11,12,13,14) for scanning the drum (15) with the beam; a beam output power regulation circuit, comprising means (25) located between the optical system and the drum for picking-off a fraction of the beam that would otherwise fall on the drum; means (16) for receiving the picked-off radiation and converting it into an electrical signal; means for generating a reference signal, and means (26) for comparing the said electrical signal with the reference signal to produce a difference signal, characterized by; a detector (28) for generating a start-of-scan signal; a detector (29) for generating an end-of-scan signal; a generator (33) of clock pulses; a counter (23) for counting the clock pulses, the counter using the start-of-scan signal to start counting, and the end-of-scan signal to stop counting, to produce a set of counts which correspond to scan positions across the drum; a memory (18) addressed by the set of counts for storing the difference signal for each scan position; means for enabling or disabling the loading of the memory with the difference signals so that the content of the memory location is either the difference signal of each scan position for the current scan or for a previous scan, and means (19) responsive to the output of the memory for controlling the amount of radiation falling on the write drum as a function of the position of the beam impact point along the scanline. The scanner as claimed in claim 1, wherein the reference signal is a digital voltage; the conversion means (16) generates an electrical analog signal; the memory (18) is digital, and further including an analog-to-digital converter (17) for converting the analog output to a digital signal, and a digital-to-analog converter (19) for converting the memory output to an analog signal. The scanner as claimed in claim 1 or 2, wherein the picked-off radiation is converted into an electrical signal by means of a photo-multiplier tube (16). The scanner as claimed in any preceding claim, wherein the control means includes an acousto-optic modulator (13). A system for regulating the intensity of a beam (L) of coherent radiation as it is scanned along a line on a write drum (15), comprising: an acousto-optic modulator (13) positioned in the path of the beam; a mirror (25) located between the optical system and the drum for picking-off a fraction of the beam that would otherwise fall on the drum; a photomultiplier tube (16) for converting the picked-off radiation into an analog signal; an analog-to-digital converter (17) for converting the analog signal into a digital signal; means for generating a digital reference signal, and a comparator (26) for comparing the reference signal with the digital signal to produce a difference signal, characterized by; a detector (28) for generating a start-of-scan signal; a detector (29) for generating an end-of-scan signal; a generator (33) of clock pulses; a address generator (23) for counting the clock pulses, which generator is adapted to be started by the start-of-scan signals, and to be stopped by the end-of-scan signals, for generating an address; a memory (18) responsive to receipt of the address for loading the output of the comparator into the memory location given by the address, and for outputting the contents of the memory at the given address; a digital-to-analog converter for converting the output of the memory into an analog signal; a driver (27) responsive to the output of the digital-to-analog converter for supplying to the acousto-optic modulator a signal for varying the amount of the beam which is transmitted through it, and means for enabling or disabling the writing of the comparator output into memory, so that the content of the memory location will be the output of the comparator for either the current scan or a previous scan.

Inventors: FROELICH, RONALD W.
Applicants: XEROX CORPORATION
Application year: 1987
Main classification: H04N1
IPC codes: H04N1, G03G15, G02B26, B41J2