Projection system with rotationally asymmetric objective

Abstract: The invention relates to objective arrangements for use in projection systems such as overhead projectors. An objective arrangement 10 provides a tilted image plane. The arrangement 10 is rotationally asymmetric by virtue of the third element 16 having a principal optical axis 22 which is displaced to one side, but parallel with the principal optical axes 20 of the first and second elements 12,14. This asymmetry has been found to correct aberrations introduced by a tilted Fresnel lens used in the projection arrangement.

Description: The present invention relates to objective arrangements particularly, but not exclusively, for use in overhead and liquid crystal display (LCD) projection systems. U.S. Patent 5,986,806 relates to LCD projection systems. U.S Patent 5,355,188 discusses tilted field buses in LCD projection systems. Overhead projectors and LCD projectors are well known. In an overhead projector arrangement as shown schematically in Fig. 1, a slide 100 is laid on a horizontal light bed beneath an optical objective 102 having a substantially vertical axis. A mirror 104 above the objective reflects the image out onto a reflective screen 106, for viewing. In many situations, it is necessary to tilt the mirror to project the image higher up the screen, in order for it to be visible by the whole audience. When this is done, the distance over which the image is projected to the top of the screen is longer than the distance over which the image is projected to the bottom of the screen. Hence, the width of the image is greater at the top of the screen than at the bottom and consequently, an ostensibly rectangular image will appear on the screen with the distorted shape 108 shown in Fig. 2. This type of distortion is known as the keystone effect, by analogy with the shape of an architectural keystone. Fig. 3 shows one possible arrangement for dealing with the keystone effect. This arrangement seeks to keep the optical magnification constant across the field. It can be seen from Fig. 3 that the image plane 110 is not perpendicular to the optical axis 112, but is parallel to the object plane 114, with the result that the keystone effect will be corrected, because the ratio L (top) to L (top)' will be equal to the ratio of L (bot) to L (bot)' (where L (top) and L (bot) are object distances at respective points in the field, and L (top)' and L (bot)' are image distances for the relevant field). However, the image plane and object plane will no longer be optically conjugate when a tilted objective is used. Other aberrations of the objective will therefore be very large and this method cannot be used when a large effective focal length is required, or high resolution is required. Similar problems arise in relation to LCD projectors. However, the LCD array plane is preferably arranged to be tele-centric, i.e. not tilted relative to the objective, in order for the LCD array to have more energy output. However, the arrangement in Fig. 4 has been proposed, and interposes a Fresnel lens 120 between the LCD plane 122 and the objective 124, to produce a tilted intermediate virtual image of the LCD plane. The tilt introduced by the Fresnel lens 120 gives the virtual image a keystone shape but the intermediate virtual image can be conjugate with the plane of the screen 126, to reduce other aberrations, and the keystone effect introduced by the Fresnel lens is opposite to that introduced by a tilted screen, so that the overall degree of keystone effect can reduce. The present invention seeks to provide improved projection arrangements, particularly, but not exclusively for use with LCD arrays. The invention provides a projection system operable to cast an image onto a screen, the system comprising; a planar image source; a field lens; an objective arrangement having an optical axis; and a screen; wherein the plane of the image source is perpendicular to the optical axis, and the field lens and the screen are tilted relative to the axis to introduce opposite keystone effects characterised in that arrangement has at least one element off-set from the optical axis to provide rotational asymmetry to the arrangement to compensate for aberrations of the tilted field lens. The field lens is preferably a Fresnel lens. Preferably the objective arrangement comprises a plurality of elements, preferably only three elements. Preferably the off-set element is that element of the objective arrangement closest to the screen. The arrangement may be a fixed focus or vari-focus arrangement, and is preferably a zoom arrangement. Preferably the or each fixed element of the objective arrangement is off-set and the or each movable element of the objective arrangement is aligned with the optical axis. The degree of asymmetry is preferably selectable, preferably substantially to balance rotationally asymmetric aberrations, such as astigmatism, caused by the tilt of the field lens. The present invention will now be described in more detail, by way of example only, and with reference to the accompanying drawings, in which: Fig. 1 is a schematic view of a typical overhead projection arrangement; Fig. 2 is a view at A-A in Fig. 1, showing the keystone effect in the image; Fig. 3 shows an arrangement adjusted to reduce the keystone effect; Fig. 4 shows an arrangement similar to that of Fig. 3, for use with an LCD projector; Fig. 5 illustrates an objective arrangement according to the invention; Fig. 6 illustrates the modulation transfer function of an arrangement not embodying the invention; Fig. 7 shows transverse ray fan plots of the arrangement of Fig. 6; Fig. 8 shows field curvature and distortion plots of the arrangement of Fig. 6; Fig. 9 shows spot diagrams of the arrangement of Fig. 6; and Figs. 10 to 13 correspond to Figs. 6 to 9 but relate to an arrangement according to the invention. Fig. 5 illustrates schematically, in conventional form, an objective arrangement 10 for use in a projection system which provides a tilted image plane. The arrangement 10 is rotationally asymmetric. In more detail, the arrangement 10 consists of three elements 12,14,16. Light passes through the arrangement 10 from the right as shown in Fig. 5, toward the left, and on this account, the elements 12,14,16 will hereafter be called the first, second and third elements respectively. The first element 12 is biconvex. The second element 14 is biconcave. The third element 16 is concave on its upstream face 16a and convex on its downstream face 16b. The first and second elements have coincident principle optical axes 20, but the third element 16 has a principle optical axis 22 which is displaced to one side but parallel with the axes 20. The significance of this displacement will become apparent below. After systematic study and analysis, the inventors have appreciated that although conventional objective arrangements having a single principal optical axis can produce some aberrations which contribute toward a reduction in the aberrations resulting from the tilted Fresnel lens, the magnitude of the aberrations produced by the objectives was much less than that of the aberrations produced by the Fresnel lens and consequently, even if used for correction, the residual rotationally asymmetric (non-linear) aberrations, especially astigmatism, are still unacceptably large. The inventors have therefore adopted the arrangement shown in Fig. 5, in which the off-set of the last element 16 introduces a rotational asymmetry into the objective system, which can be used to further correct the aberrations introduced by the Fresnel lens. Using this arrangement in connection with an LCD projector has resulted in the keystone effect being reduced to a negligible level because the tilted Fresnel lens produces a keystone effect opposite to that caused by the tilted screen and in addition, the off-set of the third element 16 also produces a small keystone effect which can be used to finally balance the overall keystone effect. In addition, the off-set can be chosen to introduce an appropriate amount of rotationally asymmetric aberration to balance the residual rotationally asymmetric aberrations otherwise present in the system. The overall effect is to produce a projection system having a substantially improved performance in terms of keystone and rotationally asymmetric distortions and aberrations. The design of an objective arrangement according to the present invention can be more fully understood by first describing in detail an objective arrangement in which there is no off-set in the third element, and then similarly describing the effect of introducing the off-set in accordance with the invention. Example Without Off-Set Appendix A provides additional lens description data for an objective arrangement generally as shown in Fig. 5, but conventional in having three elements with coincident principle optical axes. This objective arrangement has been used experimentally as the objective of an LCD projector arrangement of the type shown in Fig. 4, and yielded experimental results as shown in Fig. 6. Fig. 6 shows the polychromatic diffraction modulation transfer function for wavelengths from 0.4861 to 0.6563 microns. Fig. 7 shows transverse ray fan plots for the same zoom position. Each plot includes data for three wavelengths, namely 0.486 microns, 0.588 microns and 0.656 microns. There are five pairs of plots, as follows: Plot Identification i IMA: 0.00, 0.00 mm ii IMA: 48.00, 36.00 mm iii IMA: 65.00, 49.00 mm iv IMA: -48.00, -36.00 mm v IMA: -65.00, -49.00 mm Fig. 8 shows field curvature and distortion plots for the same zoom positions, again showing lines for three wavelengths, namely 0.486 microns, 0.588 microns, 0.656 microns. In each case, the maximum field is 81.400 mm. Finally, Fig. 9 shows spot diagrams at the three zoom positions. In these drawings, the reference bar 24 represents 400 microns and other parameters for these images are as follows: Fig. No. Parameters RMS Radius GEO Radius 9 i IMA: 0.000, 19.739 mm 32.876 51.463 9 ii IMA: 48.496, 56.243 mm 76.727 210.327 9 iii IMA: 66.026, 69.784 mm 85.769 233.376 9 iv IMA: -48.263, -16.311 mm 55.541 123.705 9 v IMA: -65.591, -29.503 mm 87.616 220.385 Asymmetric Design Appendix B and Figs. 10 to 13 correspond to Appendix A and Figs. 6 to 9, but relate to an asymmetric arrangement according to the invention and having an effective focal length of 170 mm. The element 16 (Fig. 5) has been off-set from the principal optical axis, in accordance with the invention, by an amount of about 0.75 mm. Appendix B shows the general lens data. Fig. 10 shows the optical transfer function for the same zoom position as illustrated in Fig. 6. It is apparent that there is a substantial improvement in the transfer function. In use, it is the off-set element 16 which remains fixed against rotation during zooming or focus adjustment, while one or both aligned elements 12,14 are movable. Fig. 11 shows transverse ray fan plots corresponding to those in Fig. 7 and again, it can readily be seen that there is an overall significant improvement in performance. Fig. 12 shows the field curvature and distortion obtained from the objective arrangement according to the invention and, when contrasted with the corresponding plots in Fig. 8 shows again a marked improvement in performance. Finally, spot diagrams are shown in Fig. 13, to which the following experimental data applies: Fig. No. Parameters RMS Radius GEO Radius 13 i IMA: 0.000, 20.819 mm 36.798 74.383 13 ii IMA: 48.399, 57.279 mm 35.941 95.500 13 iii IMA: 65.784, 70.737 mm 54.153 136.790 13 iv IMA: -48.166, -15.078 mm 45.024 127.557 13 v IMA: -65.392,-28.149 mm 56.421 148.747 This example may have fixed focus or be modified for variable focus, e.g. by allowing the relative positions of the elements to be changed. However, the off-set element remains fixed against rotation, during focussing or zooming, so that the assymmetry is retained. The examples above off-set the final element of the triplet. It can be shown that when an optical element is small relative to its optical field of view, then off-setting any one element is equivalent to off-setting any one other element. It will be apparent from the above disclosure that many variations and modifications can be made without departing from the scope of the present invention defined in the claims. These modifications would include fixed focus, vari-focus and zoom systems. In particular, the details disclosed are by way of example only and it will be readily apparent to the skilled man how other dimensions can be chosen in order to implement the invention in other embodiments, and to adapt the invention according to the choice of optical material (glass, polymers or the like). APPENDIX A Whole F/# 5.6 Working F/# 9.5 Image heights: (-65, -49), (-48, -36), (0,0), (48, 36), (65, 49) Wavelengths: 0.48613, 0.58756, 0.65627 Surface data summary: Surf. Radius Thickness Glass Diameter Decentr. Tilt Object 1800 8° 1 49.197 9.75 SSKN5 58 2 132.808 11.86 58 3 -150.129 2.60 F2 50 4 51.807 5.11 50 5 108.841 9.75 SK16 52 6 -97.431 138.26 52 FRESNEL aspheric 2.00 ACRYLIC 164 6.5° 8 16.00 164 Image APPENDIX B Whole F/# 5.6 Working F/# 9.5 Image heights: (-65, -49), (-48, -36), (0,0), (48, 36), (65, 49) Wavelengthts: 0.48613, 0.58756, 0.65627 Surf. Radius Thickness Glass Diameter Decentr. Tilt Object 1800 8° 1 47.788 9.75 SSKN5 58 0.75 2 117.960 11.86 58 0.75 3 -160.560 2.60 F2 50 4 50.795 5.11 50 5 101.486 9.75 SK16 52 6 -101.486 138.27 52 FRESNEL aspheric 2.00 ACRYLIC 164 6.5° 8 16.00 164 Image

Claims: A projection system operable to cast an image onto a screen (126), the system comprising; a planar image source (122); a field lens (120); an objective arrangement (10) having an optical axis (20); and a screen (126); wherein the plane of the image source is perpendicular to the optical axis, and the field lens and the screen are tilted relative to the axis to intoduce opposite keystone effects characterised in that the objective arrangement has at least one element (16) off-set from the optical axis to provide rotational asymmetry to the arrangement to compensate for aberrations of the tilted field lens. A system according to claim 1, wherein the field lens (120) is a Fresnel lens. A system according to claim 1 or 2, wherein the objective arrangement (10) comprises a plurality of elements (12,14,16). A system according to claim 3, wherein the objective arrangement (10) comprises only three elements (12,14,16). A system according to any preceding claim, wherein the said off-set element (16) is that element of the objective arrangement (10) closest to the screeen (126). A system according to any preceding claim, wherein the objective arrangement (10) is a fixed focus arrangement. A system according to any of claims 1 to 5, wherein the objective arrangement (10) is a vari-focus arrangement. A system according to any preceding claim, wherein the objective arrangement (10) is a zoom arrangement. A system according to claim 7 or 8, wherein the or each fixed element of the objective arrangement (10) is off-set and the or each movable element of the objective arrangement (10) is aligned with the optical axis. A system according to any preceding claim, wherein the degree of asymmetry is selectable. A system according to claim 10, wherein the degree of asymmetry is selectable substantially to balance rotationally asymmetric aberrations caused by the tilt of the field lens (120). A system according to claim 10 or claim 11, wherein the degree of asymmetry is selectable substantially to reduce or correct astigmatism caused by the tilt of the field lens (120).

Inventors: LU KAI CHANG, LU, KAI CHANG
Applicants: INTERNAT OPTICS LTD, INTERNATIONAL OPTICS LIMITED
Application year: 1999
Main classification: G02B27
IPC codes: G02B13, G03B27, H04N17, H04N5