Patent Publication Number: US-5890788-A

Title: Anamophic optical system

Description:
BACKGROUNG OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an optical system and, more particularly, to an anamophic optical system for use in projection devices. 
     2. Discussion 
     To project a virtual image to a viewer, various types of projection systems have been utilized. Ordinarily, a projection system includes an image source such as a cathode ray tube (CRT). Projection optics receive the radiation from the CRT and transfer the radiation to a screen. A reflective surface such as a spherical mirror is used to recollimate the radiation and provide a virtual image to the user. The image appears to the viewer as being at or near infinity. In this type of system, the projector is only capable of projecting high resolution imagery onto an axially symmetric screen. When the screen is rotationally symmetric, the resolution of the image will be greatly degraded. Drawbacks associated with this type of projector with a rotational screen are small numerical aperture, low resolution due to excessive field curvature, small field of view coverage and the system requires non-spherical mirror to obtain a large vertical field of view. 
     Ordinarily, in an attempt to obtain wide field of view coverage, either the mirror or the screen has to deviate from a simple geometry. Unfortunately, in order to accomplish this, a significant cost is incurred when the mirror is deviated from a spherical shape. Additionally fabricating a large non-symmetric mirror presents a tremendous technical risk as well as a program schedule risk. 
     In current simulators like those used for training pilots, the projector ordinarily includes a CRT, projective optics, a screen and a reflective surface such a spherical mirror. To avoid any osculation to the pilots, the screen is located above the pilot. Ordinarily, five projectors are utilized to cover a 200° horizontal field of view with each projector covering approximately a 50°×50° field of view. However, when the field of view becomes greater than about 40°, the performance of the projector degrades significantly due to the presence of a significant mount of astigmatism and field curvature. Thus, it would be desirous to have an optical system which would overcome the above drawbacks and provide a projection system which overcomes these drawbacks. 
     SUMMARY OF THE INVENTION 
     According to the teachings of the present invention, an optical system is provided which will provide a projector with a very wide field of view coverage. The field of view coverage is approximately 50° vertical by 225° horizontal field of view. The present invention provides a relatively large numerical aperture as well as high resolution without utilizing non-spherical collating mirrors. The present invention provides a generally anamophic screen which is used to match field curvature of the mirror. Also, the optical system generates a field curvature which matches the shape of the screen. 
     In the preferred embodiment, an optical system is comprised of the following. An image source such as a CRT generates radiation. A first optical element is positioned adjacent the image source. The first optical element corrects the anamophic field curvature of the radiation generated by the image source. Ordinarily, the first optical element is an anamophic field lens which is positioned adjacent the image source to receive radiation from the image source. A second optical element is positioned to receive the radiation passing through the first optical element. The second optical element corrects the astigmatism of the radiation passing through the first optical element. Since the astigmatism generated by the first optical element is almost constant across the entire field of view, the second optical element with some cylindrical power located near the projector&#39;s aperture stop can compensate for the astigmatism generated by the first optical element. The described optical system to be used in an infinite display optical system where a screen is positioned to receive radiation from the exit pupil. Also, a mirror is positioned to reflect the radiation from the screen of a virtual image to a viewer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various advantages of the present invention will become apparent to those skilled in the art after a study of the following specification and by reference to the drawings in which: 
     FIG. 1 is a schematic view of an infinite display optical system. 
     FIG. 2 is an enlarged view of the projector optics of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning to the figures, particularly FIG. 1, an infinite display optical system is illustrated and designated with the reference numeral 10. The system includes an image source 12, projector optics 14, a screen 16, and a mirror 18. The optics are positioned adjacent the CRT screen and project the radiation from the image source 12 toward the screen 16. The mirror 18 reflects the radiation of the screen 16 to a viewing plane 20. The image source may be a cathode ray tube (CRT) which generates the radiation. The image source 12 ordinarily radiates its rays in a substantially 180° field. 
     The projector optics 14 are best illustrated in FIG. 2. As can be seen in FIG. 2, the optics 14 are positioned very close to the image source 12. Ordinarily, a face plate 30 is positioned abutting the image source 12. A space 32 is present between the image plane 30 and the projector optics 14. The space 32 is ordinarily small, however a coolant such as fluid may fill the space between the projector optics 14 and the image plate 30. This is due to the fact that the image source generates a significant amount of heat which should be dissipated. 
     Turning to the projector optics 14, a first optical element 34 is positioned next to the face plate 30. The optical element 34 is an anamophic field lens. The anamophic field lens is of a thin shell shape and its geometry can be either saddle shaped, generalized Zernike polynomials, toroid or torus. The anamophic lens generates a field curvature to match the shape of the screen 16. 
     A second lens 36 is positioned to receive the radiation from the anamophic lens 34. The second lens 36 focuses radiation from the lens 34 and passes the radiation through the projector optics. 
     The radiation is next passed into a lens group 38 which includes four lenses 40, 42, 44 and 46. The lens group 38 provides the most of the projector&#39;s optical power to achieve high numerical aperture. 
     The optical element 50 is a cylindrical lens which corrects for the astigmatism generated by the anamophic field lens 34. The cylindrical lens 50 may be a simple cylindrical lens, a generally aspheric cylindrical lens and have either a positive or negative optical power. The cylindrical lens 50 is positioned very close to the aperture stop 52. 
     The front lens optical group 66 which includes lenses 54, 56, 58, 60 and 62 corrects the aberration generated by the front end optics of the projector optics 14 (i.e. lens 50, 38, 36, 34, 32, 30). In fact, the front focus group 66 provides a very compact wide angle optical system. The front lens group 66 is capable of providing the optical system with a large vertical field of view of 50°. The group 66 and lens 36 enables the projector optics 14 to be focused onto the screen 16. 
     The screen 16 is designed to match the field curvature of the spherical mirror 18. Thus, the screen 16 may have a simple spherical shape, a conic shape, a torus shape, a generally bilateral symmetric shape or it may be described with Zernike polynomials. For smaller field of view, said 30°, a simple spherical screen is generally adequate. However, when a field of view is greater than 40°, a spherical screen is no longer adequate to match the spherical mirror&#39;s field curvature. A screen shape such as aspheric or bilateral symmetric shape is necessary to obtain good image quality. 
     The mirror 18 which reflects the radiation from the screen is preferred to be a spherical shape for cost consideration. Certainly, when required superior display quality, the mirror 18 may have a conic shape, a torus shape, described with Zernike polynomials, or have a generalized bilateral symmetric shape. 
     The ten lens projector optics 14 enable the projector system to achieve high numerical aperture (i.e. F/1.25). The optical system 10 with the anamophic field lens 34 and the cylindrical lens 50 enable the projector 14 to protect a flat CRT image onto a non-rotationally symmetric screen. The lens group 66 and lens 36 provides focus adjustment. 
     Table 1 below provides a detailed optical prescription for the projector optics 14 with the surface numbers starting from the wide angle lens group 66 towards the image source 12. The lens surfaces are designated with a and b. 
     
                                           TABLE 1
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OPTICAL PRESCRIPTION AND ASPHERIC COEFFICIENTS
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                      Thickness
Radius of Curvature
                Clear After Surf
                                Glass
Surf       +/- tol
                Aper (1/2) +/- tol
                                Type
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62a 381.513     86.73 14.663    BACDS
62b -2254.041   86.30 0.500     AIR
60a 211.300     83.60 13.359    BACDS
60b 500.735     82.90 0.500     AIR
58a 138.249     77.90 15.013    BACED5
58b 159.851     73.90 18.102    AIR
56a 1005.185    72.90 6.400     EF2
56b 133.508     67.30 49.024    AIR
54a -168.549    66.20 9.814     EF2
54b 4760.699    69.20 3.156     AIR
50a -12406.094  69.70 24.802    BACD5
50b -198.863    72.00 0.500     AIR
46a 201.911     81.00 26.170    BACD16
46b *           81.00 -43.173   AIR
52  0.000       79.55 44.157    AIR
44a 125.008     84.00 8.500     FD60
44b 114.766     81.00 26.745    AIR
42a 261.155     82.00 25.179    BACD16
42b -414.898    82.00 0.913     AIR
40a -458.746    82.00 8.849     EF2
40b 2188.502    81.00 58.293    AIR
36a 185.734     78.00 12.421    BACD5
36b 351.448     77.00 46.117    AIR
34a -122.261          4.000     PMMA
34b -126.347          8.700     FLUID
30  0.000             6.500     CRT
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Aspheric Equation:
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 ##STR1##
 ##STR2##
d,e,f, = radial aspheric coefficients  w.sub.a = bilateral aspheric
coefficients
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Rotationally Symmetric Aspheric Coefficients
                          Asymmetric Coefficients
Surf
   k    d     e     f     w5    w12
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34a
   -1.61500
        .44247e-07
              -.25898e-11
                    .32837-15
                          -.491931e-3
                                .384557e-7
34b
   -1.61500
        .44247e-07
              -.25898e-11
                    .32837-15
                          -.491931e-3
                                .384557e-7
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 *cylindrical lens
 X-Z plane radius  5250.09
 Y-Z plane radius  ∞-
 (+) curvatures are to the right
 (+) thickness to the right
 all dimensions are in mm unless specified otherwise.
 
    
     The present invention provides a high resolution anamophic projection optical system with an anamophic screen. The present invention is capable of high numeric aperture, wide field of view, and high resolution with a spherical collimating mirror. The projector optical systems will be utilized in simulators to simulate aircraft. Likewise, these projection systems may be utilized in the entertainment field. 
     It should be understood that while the invention has been described in connection with particular examples hereof, that various modifications, alterations and variations of the disclosed preferred embodiment may be made after varying the benefits of a study of the specification, drawings and the following claims.