Method for imaging a continuously moving object

[0002] The invention relates to imaging, and specific embodiments of the invention relate more particularly to projecting stationary images onto a moving object.

[0003] In various industries employing photo-lithography, there is a desire to use two-dimensional light valves to project images onto moving objects. One typical use for such image projection involves projecting a UV image in order to expose a UV sensitive material, such as photoresist or photopolymer. One advantage of two-dimensional light valves (as compared to lasers) is that low brightness sources, such as arc lamps, can be used with light valves. The best known device using a light valve to project an image is the video projector, which is commonly used to project video and computer generated images onto a screen. Prior art systems for projecting images onto moving objects use one of the following methods:

[0004] a) Using a step-and-repeat technique, wherein the object is stepped over a certain movement and then is held stationary at the moment of imaging. The stepping and imaging may then be repeated a number of times to complete the image;

[0005] b) Synchronizing the image to the motion of the object by shifting the data in the light valve. See, for example, U.S. Pat. Nos. 5,049,901; 5,208,818; and 5,132,723.

[0006] c) Moving both the light valve and the object at the same time. See, for example U.S. Pat. Nos. 5,870,176 and 6,060,224;

[0007] d) Dividing the image into columns and using an acousto-optic modulator to synchronize the image with the motion one column at a time. See, for example, U.S. Pat. Nos. RE37,376E and 5,923,359.

[0008] None of these techniques involve using a stationary light valve based projector to project a series of stationary images in a rapid sequence to image a continuously moving object.

[0009] The invention uses a scanning device between an image source, typically a commercial video projector, and a moving object, typically a planar object which may be coated with photoresist. The scanning device allows the projector and the projected image to remain stationary while the scanning device tracks the motion of the moving object until one frame is exposed. At the end of each frame the scanning device resets to the initial position while the image source changes the image to the next frame. The process repeats itself until a complete stripe of images is recorded. For imaging in the UV, the video projector lamp and the optics may be modified in order to enhance the UV output. After one stripe of images is recorded, further stripes may be imaged using a conventional two-dimensional scanning method. Either stepping motion or continuous helical motion can be used to scan the second dimension of the object.

[0010] The invention allows the use of a low cost, off-the-shelf video projector as a lithographic imaging device, without requiring step-and-repeat type motion of the object or the imaging source. One advantage of a continuous mechanical motion over a stepped motion is in a smoother, more accurate and more reliable mechanical system. The present invention is particularly useful in the following applications: the production of printing plates (both lithographic and flexographic), the production of printed circuit boards (also known as “direct imaging”), the production of displays and the production of solid objects from photonolymer.

[0011] FIG. 1 is a schematic representation of a particular embodiment of the invention.

[0012] FIGS. 2-ato 2-cshow a sequence of tracking one frame and switching to the next frame in the embodiment of FIG. 1.

[0013] FIG. 1 shows a generalized view of a commercial video projector 1. For clarity, FIG. 1 depicts a cut-away view of projector 1 showing some of its components which include: a light source 2, typically a SHP type or UHP type arc lamp, a reflector 3, typically of the dichroic type, a rotating filter wheel 4, typically containing Red, Green and Blue filters, various mirrors 5 and a light valve 6. In a preferred embodiment, light valve 6 comprises a DMD (Digital Mirror Device), such as the one-made by Texas Instruments of Plano, Tex.). The two-dimensional image formed by light valve 6 is projected by lens 7 and mirror 8 onto moving object 10. In the illustrated embodiment, moving object 10 is mounted on continuously rotating drum 11. Light valve 6 forms an image 12 on object 10. In one particular embodiment, object 10 is a lithographic printing plate comprising a light sensitive coating. Mirror 8 is mounted on a fast galvanometer 9. Video projector 1 can be configured for various light spectra. For example, if object 10 is a printing plate comprising a UV sensitive coating, it is advantageous to maximize the light output of projector 1 in the UV range. The light output of projector 1 in the UV range may be maximized by replacing the filters in filter wheel 4 with filters that transmit UV light and block visible and infra-red light and/or changing the dichroic coating on reflector 3 to a coating which reflects UV and transmits other wavelengths. If reflector 3 is effective to provide a sufficient amount of UV output from projector 1, the filters on wheel 4 can be simply removed. In some lithographic imaging applications, it may also be desirable to replace lens 7 with a lens which is optimized for UV and which has a desired magnification ratio. When object 10 is a lithographic printing plate, lens 7 Preferably has a magnification of about 1:1.

[0014] Projector 1 is connected to a data source 13 in a conventional manner. Signal 14 is used to synchronize the projected frames (typically 60 per second) to data source 13, and is also used to synchronize galvanometer 9 via saw-tooth generator 15. All components of the system shown in FIG. 1 are well known, commercially available components which can be purchased as complete functional assemblies. As such, no further details regarding these components are provided herein.

[0015] The operation of the FIG. 1 system is shown in FIG. 2-ato FIG. 2-c, which should be viewed in conjunction with FIG. 1. Mirror 8 is rotated at a velocity which makes the projection of any image point on light valve 6, stay stationary relative to moving object 10. As shown in FIG. 2a, image point A on light valve 6 is projected onto object 10 as point A′. Projected point A′ stays stationary relative to moving object 10. Since object 10 is moving at a constant velocity, mirror 8 preferably rotates in an approximately linear fashion. At the end of the travel of mirror 8, shown in FIG. 2-b, mirror 8 has to return as fast as possible to the starting position, as shown in FIG. 2-c. At the moment between FIG. 2-band FIG. 2-c, the image on light valve 6 is changing. In FIG. 2-c, mirror 8 has returned to its starting position and the image on light valve 6 has changed. As shown in FIG. 2-c, new image point B is projected onto object 10 as point B′, again stationary relative to moving object 10. The images containing points A and B can be imaged in a contiguous manner on object 10 or in an overlapping manner. The degree of overlap is determined by the ratio of the image size to the range of motion of object 10 during one image (frame) interval. The amount of overlap of successive images may be controlled by adjusting the speed of object 10 relative to the frame rate of projector 1. Obviously the image data has to be changed in data source 13 to reflect the correct overlap.

[0016] Saw-tooth waveform generator 15 is preferably a digital device including provisions for a look-up table. Such a table allows modification of the profile of the saw-tooth waveform to compensate for optical distortion introduced by the scanning process. Galvanometer drivers including look-up tables are well known in the art and commercially available.

[0017] The motion of object 10 may be a linear motion instead of a rotary motion. A linear motion may be used when scanning flat and rigid objects which cannot be wrapped around a drum. Examples of applications wherein this invention may be applied to image objects that move linearly include: the imaging of glass panels for electronic displays and the imaging of printed circuit boards, when imaging a flat object moving back and forth, the imaging process can be performed in both directions by reversing the direction at the galvanometer when motion direction is reversed.

[0018] One example embodiment of the invention involves a computer-to-plate machine used in the printing industry. Projector 1 is Model 340B made by InFocus (Wilsonville, Oreg.). All of the filters are removed from filter wheel 4 and dichroic reflector 3 is replaced with a UV-enhancing reflector optimized from 200 nm to 450 nm. The lens 7 is replaced with a fused-silica UV lens with a 1.04:1 magnification ratio of the double-gauss configuration. Mirror 8 and galvanometer 9 are made by Cambridge Instrument (Cambridge, Mass.), Model 6230. The frame rate used is 60 Hz, with a saw-tooth rise time of 15 mS and a retrace time of about 1.6 mS. The saw-tooth is generated by a standard function generator. The moving object 10 is a lithographic printing plate of the projection type having a sensitivity of about 5 mJ/cm 2 for UV light. The projected image is 600×800 pixels or about 6.34 mm×8.46 mm. The speed of the drum 11 is adjusted to cover 6.34 mm in 15 mS, giving 0.42 m/S. For a plate size of 50 cm×70 cm and a drum circumference of 60 cm, a complete image is written in about two minutes.

[0019] It is also clear that the relative motions of the projector 1 and object 10 can be reversed, and have projector 1 as well as galvanometer 9 and mirror 8 continuously moving while object 10 is stationary. Such a configuration is advantageous when object 10 is bulky, such as a container of liquid photopolymer.