Abstract:
An imaging system includes a first off-axis illumination source for providing a first illumination field at a surface of an illumination modulator such that when the modulator is in a first non-activated mode a zero-order reflected illumination field is directed toward a first illumination blocking device, and when the modulator is in a second activated mode one first order reflected illumination field is directed toward an imaging surface while another first order reflected illumination field is directed toward a second illumination blocking device.

Description:
BACKGROUND OF THE INVENTION 
   The invention generally relates to imaging systems, and relates in particular to imaging systems that employ an illumination modulator. 
   Imaging systems such as those disclosed in U.S. Pat. No. 6,433,934, may include an illumination source, a field lens system, an illumination modulator, imaging optics and an imaging surface. During imaging, the field lens system directs the illumination field onto the light modulator and the light modulator reflects the illumination field toward the imaging surface in one mode and reflects the illumination field away from the imaging surface in another mode. The modulator may, for example, include a Grating Light Valve (GLV) as sold by Silicon Light Machines of Sunnyvale, Calif., and the system may direct, via the imaging optics, either the zero order reflection or the first order reflection toward the imaging surface in various embodiments. 
   In particular, the image may be recorded from the zero order reflection  10  of the illumination field  12  from the modulator  14  at the imaging surface  16  as shown in  FIG. 1A  during imaging. As shown in  FIG. 1B , when the modulator  14  is activated, the illumination field  12  is reflected at first order reflections  18 A and  18 B, and to a lesser extent at further order reflections (not shown). The first order reflections  18 A and  18 B are blocked from reaching the imaging surface  16  by energy absorbing blocking filters  20 A and  20 B. Imaging occurs, therefore, when the modulator  14  is not activated as shown in  FIG. 1A . Ideally, no illumination should be directed along the path of the zero order reflection when the modulator  14  is activated as shown in  FIG. 1B . In practice, however, it may be difficult to completely remove illumination from the zero order direction. The illumination field  12  and reflected fields  10 ,  18 A and  18 B may also be coplanar as long as the source is protected from the reflected signal, for example by using a directional blocking filter and a beam splitter. 
   Another conventional imaging system may employ an energy absorbing block filter  22  in the zero order direction  10  as shown in  FIG. 2A , and a lens  24  to direct the first order reflections  18 A and  18 B toward the imaging surface  16  as shown in  FIG. 2B . Such an imaging system, however, requires very precise alignment of the components to ensure that the first order reflections  18 A and  18 B converge at a common focal point at the imaging surface  16 . Any mis-alignment of these components may significantly reduce image quality. Further, off-axis imaging limits the available depth of focus in the imaging system. 
   Also, many imaging systems employ an illumination field that is generally in the shape of a line of illumination, permitting a line of picture elements (or pixels) to be imaged simultaneously. The illumination fields  10 ,  12 ,  18 A and  18 B may, therefore, be in the shape of a line that extends a short distance along the modulator  14  and along the imaging surface  16  respectively. The use of an illumination field in the shape of a line may further complicate certain of the off-axis imaging constraints. 
   In certain applications it is desirable to provide an imaging system having a high contrast ratio that does not image when the power is not applied to the modulator, yet does not require highly precise alignment of numerous components and may be readily adjusted to optimum image quality. 
   SUMMARY OF THE INVENTION 
   The invention provides an imaging system that includes a first off-axis illumination source for providing an first illumination field at a surface of an illumination modulator such that when the modulator is in a first non-activated mode a zero-order reflected illumination field is directed toward a first illumination blocking device, and when the modulator is in a second activated mode one first order reflected illumination field is directed toward an imaging surface while another first order reflected illumination field is directed toward a second illumination blocking device in accordance with an embodiment. 
   The imaging system may further include a second off-axis illumination source for providing a second illumination field at a surface of the illumination modulator such that when the modulator is in the first non-activated mode a zero-order second reflected illumination field is directed toward a second illumination blocking device, and when the modulator is in the second activated mode a first order second reflected illumination field is directed toward the imaging surface in accordance with a further embodiment of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description may be further understood with reference to the accompanying drawings in which: 
       FIGS. 1A and 1B  show illustrative diagrammatic views of a prior art imaging system; 
       FIGS. 2A and 2B  show illustrative diagrammatic views of another prior art imaging system; 
       FIGS. 3A and 3B  show illustrative diagrammatic views of an imaging system in accordance with an embodiment of the invention; 
       FIGS. 4A and 4B  show illustrative diagrammatic views of an imaging system in accordance with another embodiment of the invention; 
       FIGS. 5A and 5B  show illustrative diagrammatic views of an imaging system in accordance with a further embodiment of the invention; 
       FIG. 6  shows an illustrative diagrammatic top view of an imaging system in accordance with another embodiment of the invention; 
       FIGS. 7A and 7B  show illustrative diagrammatic top views of an imaging system in accordance with a further embodiment of the invention; 
       FIG. 8A  shows an illustrative diagrammatic end view of a portion of the imaging system shown in  FIG. 7A  taken along line  8 A— 8 A thereof; 
       FIG. 8B  shows an illustrative diagrammatic end view of a portion of the imaging system shown in  FIG. 7B  taken along line  8 B— 8 B thereof; and 
       FIG. 9  shows an illustrative diagrammatic side view of the imaging system shown in  FIG. 7B  taken along line  9 — 9  thereof. 
   

   The drawings are shown for illustrative purposes only and are not to scale. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Systems of the invention may be used in a variety of imaging systems such as, for example, thermal imaging systems that include an illumination field, an illumination modulator and an imaging surface (e.g., an external imaging drum). The modulator receives the illumination field via a field lens system and directs a modulated illumination field toward the imaging surface via imaging optics. The illumination source, field lens system, modulator, imaging optics and imaging surface may be as disclosed in U.S. Pat. No. 6,433,934, the disclosure of which is hereby incorporated by reference. The modulator may include a Grating Light Valve (GLV) as sold by Silicon Light Machines of Sunnyvale, Calif. 
   As shown in  FIG. 3A , an imaging system in accordance with an embodiment of the invention involves directing an illumination field  30  toward an illumination modulator  32  along an off-axis direction such that the zero order reflected illumination field  34  is directed toward an energy absorbing block filter  36  when the modulator is not activated. In this case, no illumination should be directed toward the imaging surface  38  when the modulator is not activated. During imaging as shown in  FIG. 3B , the +/− first order reflected fields  40  and  42  are directed toward the imaging surface  38  and another energy absorbing block filter  44  respectively. The first order reflected field  40  is directed toward the imaging surface  38  along a direction that is normal to the surface  38 . Although the reflected field  42  is not used in imaging, reducing throughput efficiency, the system provides a high contrast ratio. 
   Another system of the invention may further include an additional illumination field  50  that is directed along an off-axis (e.g., a negative off-axis direction with respect to the field  30 ) toward the modulator  32  such that the zero order reflected illumination field  52  is directed toward an energy absorbing block filter  54  when the modulator is not activated as shown in  FIG. 4A . Again, no illumination should be directed toward the imaging surface  38  when the modulator is not activated. During imaging as shown in  FIG. 4B , the +/− first order reflected fields  56  and  58  are directed toward the imaging surface  38  and another energy absorbing block filter  60  respectively. The first order reflected field  56  is coincident with the first order reflected field  40  and both are directed toward the imaging surface  38  along a direction that is normal to the surface  38 . The system of  FIGS. 4A and 4B  may employ two independent illumination sources or may employ optics for splitting the illumination field from one illumination source to provide the illumination fields  30  and  50 . The modulator may have a plurality of modulator elements along a horizontal direction across the modulator to provide a two first order reflections as shown in  FIG. 4A . 
     FIGS. 5A and 5B  show a further system of the invention similar to the system of  FIGS. 4A and 4B  in which the incident and reflected signals are coplanar. The system involves directing illumination fields  70  and  72  via beam splitters  74  and  76  toward an illumination modulator  78  along off-axis directions such that the zero order reflected illumination fields  80  and  82  are directed toward energy absorbing block filters  84  and  86  when the modulator is not activated as shown in  FIG. 5A . In this case, no illumination should be directed toward the imaging surface  88  when the modulator is not activated. During imaging as shown in  FIG. 5B , the first order reflected fields  90  and  92  are directed toward the imaging surface  88  and the first order reflected fields  94  and  96  are directed toward energy absorbing block filters  98  and  100  respectively. The first order reflected fields  90  and  92  are coincident and are directed toward the imaging surface  88  along a direction that is normal to the surface  88 . Again, although the reflected fields  94  and  96  are not used in imaging, the system provides a high contrast ratio. A benefit of using coplanar incident and reflected fields is that for illumination fields that are in the shape of a line, the coplanar system reduces any aberrations that may result in reflected fields when an incident field is directed toward the modulator from a high angle of incidence. 
     FIG. 6  shows an imaging system in which illumination fields  102  and  104  are directed via a field lens system  106  toward a GLV  108  having a grating spacing of, for example 8.5 μm. The illumination fields are each folded into collimated space at the GLV  108 . During actuation, the first order reflected fields from the GLV  108  are directed coincident with one another as shown at  110  and are directed through imaging optics  112  onto the imaging surface of an imaging drum  114 . 
     FIGS. 7A and 7B  show an illumination system in accordance with a further embodiment of the invention in which the modulator is off in  FIG. 7A  and on in  FIG. 7B . In particular, the illumination system includes a plurality of illumination sources  120  that direct illumination via optics  122  through a broad mask  124  onto a modulator  126 . When the modulator is off, the reflected illumination is dispersed in many directions, but when the modulator is turned on, the reflected illumination is directed via imaging optics  128  onto a surface  130  of an imageable medium as shown in  FIG. 7B . As shown in  FIG. 8A , the system may include many illumination sources  120  that are positioned around the zero order direction ( 132 ). The system, in effect, floods the modulator with illumination, but reflects only the illumination along the zero order using the imaging optics  128  as shown in  FIG. 8B . The illumination sources  120  may include one or more lens bars with a slow axis diffuser for single axis uniform angular diffusion. The illuminated line at the modulator may be broader than the active area and the mask without affecting the contrast ratio, although a loss of throughput may result.  FIG. 9  shows a side view of the imaging system of  FIG. 7B  taken along line  9 — 9  in which the width of the imaging line on the modulator  126  and on the imaging surface  130  may be seen. 
   Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.