Abstract:
A three-dimensional imaging system uses apertures through which images are projected. The image through each aperture is obtained by a different camera element. The camera elements can be totally physically separate, multiple receiving parts of a single camera, or a single camera which is effectively spatially modulated so that the single camera receives different images at different times. The outputs from the camera are used to form three-dimensional information about the object.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of the U.S. Provisional Application No. 60/078,750, filed on Feb. 25, 1998. 
    
    
     STATEMENT AS TO FEDERALLY SPONSORED RESEARCH 
     The U.S. Government may have certain rights in this invention pursuant to Grant No. N00014-97-1-0303 awarded by the U.S. Navy. 
    
    
     BACKGROUND 
     Different techniques are known for three dimensional imaging. 
     It is known to carry out three dimensional particle imaging with a single camera. This is also called quantative volume imaging. One technique, described by Willert and Gharib uses a special defocusing mask relative to the camera lens. This mask is used to generate multiple images from each scattering site on the item to be imaged. This site can include particles, bubbles or any other optically-identifiable image feature. The images are then focused onto an image sensor e.g. a charge coupled device, CCD. This system allows accurately, three dimensionally determining the position and size of the scattering centers. 
     Another technique is called aperture coded imaging. This technique uses off-axis apertures to measure the depth and location of a scattering site. The shifts in the images caused by these off-axis apertures are monitored, to determine the three-dimensional position of the site or sites. 
     There are often tradeoffs in aperture coding systems. 
     FIG. 1A shows a large aperture or small f stop is used. This obtains more light from the scene, but leads to a small depth of field. The small depth of field can lead to blurring of the image. A smaller F stop increases the depth of field as shown in FIG.  1 B. Less image blurring would therefore be expected. However, less light is obtained. 
     FIG. 1C shows shifting the apertures off the axis. This results in proportional shifts on the image plane for defocused objects. 
     The FIG. 1C system recovers, the three dimensional spatial data by measuring the separation between images related to off-axis apertures b, to recover the “z” component of the images. The location of the similar image set is used find the in-plane components x and y. 
     The current systems have certain drawbacks which are addressed by the present disclosure. 
     SUMMARY 
     The present system caries out aperture-induced three dimensional measuring by obtaining each image through each aperture. A complete image detector is used to obtain the entire image. The complete image detector can be a separate camera associated with each aperture, or a single camera that is used to acquire the different images from the different apertures one at a time. 
     The optical train is preferably arranged such that the aperture coded mask causes the volume to be imaged through the defocusing region of the camera lens. Hence, the plane of focus can be, and is intentionally outside of, the volume of interest. An aperture coded mask which has multiple openings of predefined shape, not all of which are necessarily the same geometry, and is off the lens axis, is used to generate multiple images. The variation and spacing of the multiple images provides depth information. Planar motion provides information in directions that are perpendicular to the depth. In addition, the capability to expose each of the multiple images onto a separate camera portion allows imaging of high density images but also allows proper processing of those images. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will now be described in detail with the accompanying drawings, wherein: 
     FIGS. 1A-1C show views of different systems for 3 dimensional imaging; 
     FIG. 2 shows a geometric analysis of a specified lens aperture system; 
     FIG. 3 shows a camera diagram with camera components; 
     FIG. 4 shows a drawing of the preferred camera; and 
     FIGS. 5 and 6 shows more detailed drawings of the optical relays of the camera shown in FIG.  4 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 2 shows a geometric analysis in which a camera lens of focal length F is located at z=0. Two small apertures are placed within the lens, separated a distance d/2 away from the optical centerline  200  which also corresponds to the z axis. The apertures are shown as pinholes in this diagram to simplify the model. The theory for larger and more complex apertures would be similar. 
     The following equations can be determined by using lens laws and self similar triangle analysis: 
     
       
           Z= 1/((1/ L )+ Kb ).  (1)  
       
     
     
       
         where  
       
     
     
       
           K =( L−f )/( fdL ).  (2)  
       
     
     The remaining two coordinates x, y are found from the geometrical center (X 0 , Y 0 ) of the image pair B′ using: 
     
       
           X =( −x   0   Z ( L−f ))/( fL )  (3)  
       
     
     
       
           Y =(− y   0   Z ( L−f ))/( fL )  (4)  
       
     
     Solving (1) for the image separation b reveals several interesting performance characteristics of the lens/aperture system: 
     
       
           b= 1 /K ((1/ Z )−(1/ L )).  (5)  
       
     
     The inventors recognized that if all this information was obtained by a single camera, an image crowding problem could exist. This would limit the system to a lower density of number of images. 
     The defocusing masses requires multiple spatially-shaped holes. If there are n holes, then each scattering site has been imaged n times onto a single CCD. Hence, n times as many pixels are exposed. This means, however, that the capacity of the technique, i.e. the number of scattering sites that can be imaged, is correspondingly reduced by a factor of n. 
     The present system addresses this and other issues. 
     A first aspect addresses the image crowding problem by exposing each of the multiple exposures using a separate camera portion. The camera system can be electronic or photographic based. The separate camera portion requires that a whole camera imaging portion is used to obtain the images from each aperture at each time. This can use multiple separate cameras, a single camera with multiple parts, or a single camera used to obtain multiple exposures at different times. 
     Another aspect obtains image information about the objects at a defocused image plane, i.e. one which is not in focus by the lens. Since the image plane is intentionally out of focus, there is less tradeoff regarding depth of field. 
     The first embodiment, as described above, uses image separation to expose each of the multiple exposures to its own electronic or photographic camera portion. The image separation can be effected by color filters, by time coding, by spacial filters, or by using multiple independent cameras. 
     The color filter embodiment is shown in FIG. 3. A color camera and mask combination is shown with three separate CCD cameras  300 ,  302 ,  304 . 
     Light is input through mask  342 , which includes an opaque aperture plate with three apertures formed therein. In this embodiment, the apertures are generally in the shape of a triangle. The light passes to a lens assembly  346 , which directs the light into the chamber that houses the camera. 
     The color camera uses three monochrome CCD cameras, situated around a three way prism  310  which separates the incoming light according to its colors. A micro positioner assembly  312  is provided to precisely adjust the cameras  300 ,  302 ,  304  such that each will view exactly the same area. Once those adjustments are made, the three cameras are locked into place so that any vibration affects each of them the same. Each camera includes an associated band filter. The filter  330  is associated with CCD camera  300 , filter  332  is associated with camera  302 , and filter  334  is associated with camera  304 . Each of these narrow band filters passes only one of the colors that is passed by the coded apertures. The filters are placed adjacent the prism output to correspond respectively to each of the primary colors, e.g. red, green and blue. Hence, the filters enable separating the different colors. 
     This color camera assembly is used in conjunction with an image lens assembly  340  and a aperture coded mask  342 . The system in FIG. 3 shows the aperture coded mask having three mask portions in the form of an equilateral triangle. Each aperture is color coded according to the colors of the camera filters. This color coding can be done by, for example, using color filters on the apertures. 
     The image from each aperture goes to a separate one of the cameras  302 ,  304 ,  300 . The output from the camera is processed by the CCD electronics  350  and coupled to output cables shown as  352 . These three values are processed using a conventional processing software. The three values can be compensated separately. 
     While the system describes using three colors and three apertures, it should be understood that any number of colors or apertures could be provided. 
     A second embodiment separates the images from the different apertures using rapid sequential imaging. An embodiment is shown in FIG. 4. A scene is imaged through a mask  400  that includes multiple apertures. Each aperture has an associated selective blocking means  402 . The blocking means is a device that either allows light to pass through the aperture or blocks light from passing through the aperture under control of an applied control signal  404  from a control element  400 . The aperture blocking means  402  can be a mechanical blocker e.g. a mechanical shutter, solid state optics, such as a liquid crystal which is selectively allowed to pass light, or a digital mirror which selectively reflects the light to the aperture or the like. Light from the scattering sites  410  is allowed to pass through each aperture at a separate time, under control of the controller  406 . The passed light is sent to a single camera  430  that produces an image indicative of the passed light. Three different images are obtained at three different times. Each image is based on passage of the light through a different aperture. 
     Alternate ways of obtaining the three images could be used. A purely mechanical means can be provided to pass light through only a single aperture by rotating the blocking element such that the blocking element is associated with different apertures at different times and hence provides different illuminations at different times. 
     In either case, each of the corresponding cameras is exposed only when the corresponding aperture is allowed to receive light. The system shown in FIG. 4A shows a CCD camera assembly  430  receiving the light from the various apertures. 
     Another embodiment uses spacial filters to separate the different light values. FIG. 5 shows a preferred configuration of a spatially coded camera. The system includes a focusing lens assembly  500 ,  504 , with an aperture system  506  between the two portions of the focusing lens  500 ,  504 . An exploded view of the components is shown in FIG.  6 . Each of the prisms  510 ,  512 ,  514  is directly located behind each aperture orifice. A three CCD camera  520  views the three images through the three aperture orifices, thereby providing three simultaneous views of the image. 
     The lenses within the focusing lens assembly  500 ,  504  direct the scattered light from the scene through each of the three orifices at 120° angles with each other. The light is then collected through the aperture orifices and directed to the separate CCD cameras. Each of the images on each of the three cameras is recorded simultaneously and then processed to provide three dimensional spacial locations of the points on the scene. 
     An alternative, but less preferred embodiment, uses three separate cameras, in place of the one camera described above. 
     The system as described and shown herein includes several advantages. The system allows superior camera alignment as compared with other competing images such as stereoscopic techniques. This system is also based on a defocusing technique as compared with stereoscopic techniques that require that the camera be focused on the area of interest. This system has significant advantages since it need not be focused on the area of interest, and therefore has fewer problems with trade offs between aperture size and other characteristics. 
     Although only a few embodiments have been described in detail above, other embodiments are contemplated by the inventor and are intended to be encompassed within the following claims. In addition, other modifications are contemplated and are also intended to be covered. For example, different kinds of cameras can be used. The system can use any kind of processor or microcomputer to process the information received by the cameras. The cameras can be other types that those specifically described herein. Moreover, the apertures can be of any desired shape.