Method for imaging a continuously moving object

#The invention relates to imaging, and more particularly to the elimination of motion blur when projecting stationary images on a moving object.

#In various industries employing photo-lithography there is a desire to use two-dimensional light valves to project an image on a moving object. The typical use is projecting a UV image in order to expose a UV sensitive material such as photoresist or photopolymer. The advantage of two-dimensional light valves over a laser is that low brightness sources, such as arc lamps, can be used. The best known device using a light valve to project an image is the video projector, used to project video and computer generated images on a screen. It is an object of this invention to utilize a low cost and readily available video projector as means of projecting images on moving objects. Prior art systems for projecting on moving objects use one of the following methods:

#a) A step-and-repeat motion of the object, thus object is stationary at the moment of imaging;

#FIG. 1 is a schematic representation of the method, as applied to imaging onto a rotary cylinder.

#FIGS. 2-a to 2-c show a sequence of tracking one frame and switching to the next frame.

#FIG. 1 shows a generalized view of a commercial video projector 1. For clarity, the view is shown as a cut-away view showing some of the inside components of projector 1: 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 the preferred embodiment the projector 1 uses a DMD (Digital Mirror Device, made by Texas Instruments of Plano, Tex.) as a light valve. The two-dimensional image formed by the light valve 6 is projected by lens 7 onto moving object 10, mounted on continuously rotating drum 11. The light valve 6 forms an image 12 on the object 10. In one application, object 10 is a light sensitive lithographic printing plate. The light from lens 7 is reflected off mirror 8 before reaching object 10. Mirror 8 is mounted on a fast galvanometer 9. The off-the-shelf video project 1 can be modified to maximizes the spectrum of the light in the region most effective for exposing the light sensitive coating. For example, if object 10 is a printing plate it is advantageous to maximize the light output of the projector in the UV range. This is done by replacing the filters in filter wheel 4 with filters that transmit UV light and block visible and infrared light as well as changing the dichroic coating on reflector 3 to a coating which reflects UV and transmits other wavelength. If reflector 3 is effective, the filters on wheel 4 can be simply removed. Normally it would also be required to replace lens 7 with a lens optimized for UV and for a different magnification ratio. For lithographic plate applications, lens 7 should be replaced with a lens having a magnification of about 1:1.

#The projector 1 is connected to a data source 13 in a conventional manner. The signal 14, used to synchronize the projected frames (typically 60 per second) to the data source, is also used to synchronize the galvanometer 9 via saw-tooth generator 15. All components of the system show in FIG. 1 are well known, commercially available and no further details about them are required, as they can be purchased as complete functional assemblies.

#The operation of the system is shown in FIG. 2-a to FIG. 2-c, which should be viewed in conjunction with FIG. 1. Mirror 8 is rotated at a velocity which makes any image point, (by the way of example point “A” on light valve 6 imaged as point A′) stay stationary relative to the moving object 10. Since object 10 is moving at a constant velocity the mirror 8 has to rotate in 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 this moment the image on light valve 6 is changing and a new point “B” is imaged 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 motion of object 10 during one image (frame) interval. By changing the speed of object 10 relative to the frame rate of projector 1 the amount of overlap is controlled. Obviously the data has to be changed in data source 13 to reflect the correct overlap.