FIELD OF THE INVENTION

The invention relates to a laser printing system having improved ability to independently adjust the energy level for printing while allowing the power level at the scan position

detector to remain unchanged The improvement is especially applicable to a multiple

resolution system requiring different energy levels to mark the print media More particularly, the invention relates to a mechanism for deriving both the scan position detection beam and the print beam from a single laser light source Further, the invention

also relates to a device for deriving the scan position detection beam from an independent light source while utilizing the laser source for marking the print media

BACKGROUND OF THE INVENTION

Each of the prior-art devices utilize a single laser beam for scan position detection and printing To mark the different print media, scanning laser imaging systems for proofing, image setting, computer-to-plate printing, or for different scanning speeds require different

power levels Therefore, the printing power level must be adjusted and calibrated for each

print media This process also changes the power level of the scan position detection beam

To work properly, the scan position detection beam must be kept within a particular power range that may or may not correspond to the printing power range To obviate this unwanted side effect, the present invention provides means for ad]ustιng the print power level while leaving the scan position detection beam unaffected by printing power

adjustments

SUMMARY OF INVENTION

To adjust the power properly and to simplify this process for the system, the present inventor has found that a separate beam of laser power can be created near the front of the

printing laser optics system and guided beam through the same scanning device, whether using a mirror, polygon, or holographic scanner, solely for scan position signal use. This

separate beam is created by separation from a single source of laser power or from a source

independent of the print beam source. In the former case a small portion of the laser beam power is split off and used to detect scan position while maintaining the major power level for printing.

The laser beam intended for printing goes through beam expansion for printing use. This

beam either is maintained as one beam or is split to form multiple beams for high speed printing use, such as a proofing, an image setting or a computer-to-plate printing system,

where printing speeds are critical. A beam splitter or grating is used to split the single beam

into multiple beams, which then go through a modulator to the image plane.

In a first embodiment of this invention, a beam splitter or grating separates a small portion of the main laser beam for use as a scan position beam. All other energy of the laser beam

can go into the printing beam The energy level of the scan position beam is independent of the printing beam energy and can be preset no matter the number of beams, scanning speed,

or printing material The system does not require changing the power level for the detection of the scan position

Alternatively, a separate light source can be used to go through the scanner for scan position detection This method will perform the same function as described above

For a multiple beam system, after the beam is split to Start-Of-Scan and/or End-Of-Scan

(SOS/EOS), but before the beam goes to the modulator, a beam splitter or grating splits a single beam into multiple beams, which then go through one focusing lens to focus the laser

beam(s) to the modulator The lens is designed with a focal length to match each channel

spacing requirement In order to prevent cross talk problems in the modulator, each channel must maintain a certain spacing If the spacing is too great, the channel cannot make the

beam overlap in the image plane Therefore these multiple beams must be rotated to a proper angle to achieve proper overlap For the simplicity of the optical bench arrangement,

the system can be designed using a modular concept The beam splitter or grating to split the beam, a focusing lens, and an acoustic optic modulator are mounted on the same

rotatable base plate The optical axis is at the center of the mechanical rotation axis The

system can be rotated precisely to the angle required to make the multiple beams overlap at the optimum ratio for best performance Because the optical path center is the same as the mechanical rotation axis, the beam spacing will be kept constant to the modulator With the beam splitter and modulator mounted as one unit on the same base, when the system is

rotated, the optical path center beam remains the same The other beams, symmetric to the

center beam, will tilt oniy to a certain, limited angle Therefore, because all of these

multiple beams will go precisely through the multiple channel modulator, alignment is easy

While rotating this whole sub-assembly (beam splitter and modulator), if the system is sensitive to polarization, one circular polarizer can be installed prior to the beam splitter to control the polarization direction in the system After the modulator, another circular

polarizer can be used to rotate the beam angle back in order to keep the polarization direction the same as the original direction to the scanner

A beam splitter, whether conventional, plain glass or holographic grating, can be used to

create a beam separate from the printing beam for position detection use The beam splitter is located in front of the laser head (before the beam goes to any lens or grating, or modulator), after the modulator, or at any other point prior to the scanner This position detection beam goes through the scanner and is used to detect the position of the printing

beam with the introduction of a beam positional sensor The Start-Of-Scan (SOS) and/or End-Of-Scan (EOS) sensor is installed at any convenient location

A second method to get a position detection beam, for SOS or EOS use, is to use a separate light source This separate light source enters the scanner and, as in the first method, is directed to the SOS/EOS sensor for determination of the printing beam position

To use this second method, a different light source and power supply are required, but they

also can be used to eliminate an otherwise long optical path, for its functionality is the same

This separate beam can be used with either the same optical path as the printing beam or

any other optical path This separate beam for Start of Scan or End of Scan use can either go through the scanner and a lens to the focal plane, or just go through scanner alone

Optionally, the beam then passes through a small focal lens Finally it goes to the SOS/EOS

sensor As will be appreciated by those of ordinary skill in the art to which the present

invention pertains, variations of this arrangement can be devised to accomplish the same function.

BRIEF DESCRIPTION OF THE DRAWINGS

For more complete understanding of the invention, reference is hereby made to the

drawings, in which

FIG.1 is a top view of the system layout presenting the invention with a position detection beam separate from the main beam A neutral density filter can be installed at any

convenient place to control printing beam power and will not affect position detection beam

power

FIG 2 is a side view of the system layout of FIG 1

FIG 4 illustrates use in the system of a separate laser as the light source for the position detection beam

FIG 3 is the system illustration showing a single beam split into multiple beams through one

short focus lens and parallel focus to the modulator They also are mounted on same rotatable base to allow proper setup of the beam overlap ratio A circular polarizer can be

installed on rotatable base to control polarization direction as required

FIG 5 is another modified system for use wherein the laser array provides multiple light beams

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention comprises an optical assembly adapted to a laser image printing system to produce a scan position detection beam providing an absolute reference for start-of-scan

(SOS) and end-of-scan (EOS) which is essential for high quality print products One

embodiment, shown in Figures 1 and 2, includes a scan position detection beam adapted to a single print beam system A second embodiment. Figure 3, includes a scan position

detection beam adapted to a multiple print beam system Both of these embodiments use a single laser light source to derive the scan position detection beam A third embodiment,

Figure 4, includes a scan position detection beam derived from an independent light source adaptable to either a single or multiple print beam

To facilitate understanding, in each of the embodiments, the path of the scan position detection beam and the optical elements comprising its path and the path of the print beam

and its optical elements will be described separately

Referring now to the schematic drawings. Fig I contains optical elements of the scan

position detection beam Laser light beam 40 from laser light source 10, is directed onto beam splitter 2, splitting the original beam to produce the scan position detection beam 50

and the print beam 60 which will be described in later paragraphs Only a small percentage

of power from laser light source 10 is required for the scan position detection beam leaving the majority of the power for printing. As will be understood by one of ordinary skill in the

art to which the present invention pertains, a specific power range is required to affect proper operation of the scan detection beam Deflected beam 50 is then focused on a

mirrored surface 19 diffracting beam 50 through a rotating disc 20 consisting of a grating

element sandwiched between two glass discs. Referring now to side view Fig.2, the scan position detection beam 50 emerging from the scan disc 20 is redirected by compensating prism 21 through lens box 22, folding mirror 23 and mirrors 25, 26 and 27 Alternatively,

scan position detection beam 50 can be created by a grating element on compensation prism

21 and reflected by reflection on the internal surface of prism 21 to folding mirror 23.

Referring again to Fig 1, the beam from mirror 27 is focused through lens 29 from mirror

28 and then to scan position detection sensor 30 Start-of-scan (SOS) and end-of-scan

(EOS) as detected by sensor 30 are used as absolute reference points for printing.

The optical path for the print beam is made different from that of the scan position detection

beam for the purpose of precluding mutual interference These mutually exclusive paths are

one of the primary benefits of this system The print beam is directed to the image plane through a series of optical elements as follows The print beam 60 from beam splitter 2 is redirected by mirrors 3 and 4 through grating element 3 l , focused by lens 5 to acoustic optic modulator (AOM) 6, and expanded by lens 7 The expanded beam passes through

aperture 1 1 to mirror 12 where the beam is redirected and refocused through lens 13 to mirror 15. Mirror 15 again deflects the beam through a collimating lens 16 to mirrors 17 and 18 to scanning disc 20 Referring back to Fig.2 the print beam emerging from the scan

disc is redirected by the compensating prism 21 through the lens box 21. Termination of

the printing beam on the image plane is achieved through folding mirrors 23 and 24

The second embodiment illustrated in Fig 3 is an adaptation of the same invention to

multiple print beams The scan position detection path for this adaption is identical to that

of the first embodiments Mechanical and optical elements required for multiple print beams replace some of the elements of the single print beam Referring to FIG 3, print beam 60

from laser light source 10 is again redirected by mirrors 3 and 4 through a rotatable assembly 70 whose center ne is made coincident with the optical center ne of the print

beam The rotatable assembly 70 containing optical elements 31 5 and 6 to convert the single beam to multiple beams can be modularlv adapted to existing printers or integrated with the original printer design A beam splitter/grating element 3 l splits the single print beam 60 into multiple beams which then are focused on the multi-channel modulator

(MCM) 6 by lens 5 To prevent cross-talk problems in the modulator, specific spacing of

the light channels is maintained to affect proper beam overlap in the image plane Likewise the focal length of lens 5 is designed to match each optical channel s spacing requirements As shown in FIG 3, optical elements 3 l 5 and 6 are rigidly mounted in rotatable assembly

70 which can be precisely rotated to the required angle for the optimum beam overlap ratio As the assembly is rotated, individual beams are focused through aperture 1 1 while others

are masked Hence, at any given instant, only one beam passes through the aperture 1 1 Rotation of the optical assembly 70 may cause polarization sensitivity As shown in FIG 3, circular polarizers 80 and 90 are installed before the beam splitter/grating element 31 and after the multi-channel modulator 6 to control polarization direction in the system The

remaining optical path as it emerges from the rotatable assembly is identical to that

described in the previous embodiment.

The third embodiment, illustrated in FIG.4, adapts an independent light source for the scan position detection beam while maintaining the laser light source 10 for either the single or

multiple print beams

Referring to FIG 4, an independent light source 100 is made to enter scan disc 20 at the same location as the split beam of the first and second embodiments. Emerging from the

scan disc 20 the remaining optical path to the scan position detection sensor is identical to

that of the two previous embodiments.

FIG 5 is another modified system for use wherein the laser array provides multiple light

beams.