Projection television lens systems having improved modulation transfer functions

[0002] This application is the U.S. national phase of International Application No. PCT/US99/13795 filed Jun. 18, 1999, and claims the benefit under 35 USC §119(e) of U.S. Provisional Application No. 60/090,518 filed Jun. 24, 1998, the contents of both of which in their entireties are hereby incorporated by reference.

[0003] International Application No. PCT/US99/13795 was published in English as WO 99/67662 on Dec. 29, 1999.

[0004] This invention relates to projection lens systems for use in projection televisions and, in particular, to projection lens systems having improved correction of higher order off-axis aberrations, especially coma and astigmatism. and thus improved overall modulation transfer functions across the field of view of the lens in comparison to prior lens systems having a similar construction.

[0005] U.S. application Ser. No. 08/715,028, now U.S. Pat. No. 5,808,804, the contents of which were published on Mar. 26, 1997 as EPO Patent Publication No. 764,865, discloses projection lens systems for CRT projection televisions which employ five lens elements and a negative lens unit which is associated with the CRT during use of the lens system.

[0006] Although the lens systems of U.S. Pat. No. 5,808,804 work successfully for many applications, it has been found that the level of correction of higher order off-axis aberrations, especially coma and astigmatism, achieved by these prior lens systems result in less than desired overall modulation transfer functions (MTFs) across the field of view of the lens.

[0007] The present invention addresses this observed property of these prior lenses while maintaining their otherwise desirable characteristics.

[0008] In view of the foregoing, it is an object of the present invention to provide improved projection lens systems for use in projection televisions and, in particular, in rear projection televisions.

[0009] More particularly, it is an object of the invention to provide projection lens systems capable of covering a semi-field of view of up to about 40° at a f-number at infinity on the order of 1.0, while at the same time providing an image quality which is sufficiently high to satisfy the requirements of modern consumer projection TV systems, including the requirements of high definition televisions (HDTV).

[0010] It is an additional object of the invention to provide projection lens systems having the foregoing characteristics which are also partially corrected for axial color.

[0011] It is a further object of the invention to provide projection lens systems which can be manufactured at a reasonable cost.

[0012] To achieve the foregoing and other objects, the invention provides a projection lens system for use in combination with a cathode ray tube comprising in order from the image (screen) side:

[0013] (a) a first lens element (L1) which has a positive power, an image side surface (S₁), and an object side surface (S₂), wherein:

[0014] (i) at least one of the image and object side surfaces is aspherical; and

[0015] (ii) the object side surface has a best fit spherical surface which is convex to the cathode ray tube, i.e., the object side surface has a best fit spherical radius R_2BFwhich, under the standard convention for assigning radii to optical surfaces, is less than zero (see Table 5);

[0016] (b) a second lens element (L2) which has a negative power, at least one aspheric surface, and is composed of a high dispersion material (e.g., styrene);

[0017] (c) a third lens element (L3) which has a positive power and provides a substantial portion of the power of the lens system;

[0018] (d) a fourth lens element (L4) which has a weak power, at least one aspheric surface, and an overall meniscus shape which is concave to the cathode ray tube;

[0019] (e) a fifth lens element (L5) which has a positive power and at least one aspheric surface; and

[0020] (f) a lens unit (U_N) which has a strong negative power, is associated with the CRT during use of the lens system, and provides most of the correction for the field curvature of the lens system.

[0021] In certain preferred embodiments, the first element's image side surface (S₁) has a clear aperture CA₁which satisfies the following relationship:

[0022] ½CA₁>Y₁  (1)

[0023] where Y₁, is the height of the axial marginal ray at S₁. By means of this limitation, more effective correction of off-axis aberrations can be achieved using the portion of the first lens element which lies between Y₁and the clear aperture of the lens. Preferably, the first element's image side surface satisfies the following relationship:

[0024] ½CA₁>1.1·Y₁.  (2)

[0025] In other preferred embodiments, the first element's image side surface has a best fit radius R_1BFwhich satisfies the following relationship:

[0026] |R_1BF|>|R_2BF|.  (3)

[0027] That is, the best fit power of the CRT side of the first lens element is greater than the best fit power of the screen side of that element.

[0028] In still further preferred embodiments, the lens system has some or all of the following characteristics: (1) each of the first, second, fourth, and fifth lens elements has two aspheric surfaces; (2) the third lens element is biconvex and provides the majority of the power of the lens system, i.e., the focal length of the third lens element is between about 0.8 and about 1.6 times the focal length of the entire lens system; and (3) the fourth lens element has a positive power.

[0029] As used herein, the term “weak” is used to describe an element whose focal length has a magnitude which is at least about 2.5 times the effective focal length of the entire lens system, and the term “strong” is used to describe an element or unit whose focal length has a magnitude which is less than about 2.5 times the effective focal length of the entire lens system.

[0030] FIGS. 1A, 2A, 3A, and 4A are schematic side views of lens systems constructed in accordance with the invention.

[0031] FIGS. 1B, 1C, 2B, 2C, 3B, 3C, and 4B are monochromatic MTF/OTF plots for the lens systems of FIGS. 1A, 2A, 3A, and 4A, respectively.

[0032] FIG. 5 is a monochromatic MTF/OTF plot. for the lens system of FIG. 3 of U.S. Pat. No. 5,808,804.

[0033] FIG. 6 is a schematic diagram of a projection TV employing a lens system constructed in accordance with the invention.

[0034] Table 6 sets forth the system parameters for the MTF/OTF plots of FIGS. 1B, 1C, 2B, 2C, 3B, 3C, 4B, and 5.

[0035] The foregoing drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention.

[0036] As discussed above the present invention is directed to providing improved projection lenses of the general type disclosed in U.S. Pat. No. 5,808,804, the contents of which are incorporated herein by reference.

[0037] In particular, the lenses of the invention have a first lens element which has an object side surface which is convex, rather than concave, to the short conjugate (object) side of the lens system. In addition, to allow for improved correction of off-axis aberrations, the long conjugate (image) side surface of the first lens element has a clear aperture which is substantially greater than the height of the axial marginal ray at that surface. Although such an increase in clear aperture size and thus increase in element size is in general undesirable, in accordance with the invention, it has been found that the increase in size is well justified by the resulting improvement in overall MTF across the field of view of the lens.

[0038] The second lens element of the lens systems of the invention has a negative power and is composed of a high dispersion material so as to provide the system with at least some axial color correction. In particular, the negative second lens element is composed of a material having a higher dispersion than the dispersion of the material used for the positive third lens element.

[0039] As used herein, a high dispersion material is one having a dispersion like that of flint glass. More particularly, a high dispersion material is one having a V-value ranging from 20 to 50 for an index of refraction in the. range from 1.85 to 1.5, respectively. In contrast, a low dispersion material is one having a dispersion like that of crown glass or, in terms of V-values, one having a V-value ranging from 35 to 75 for an index of refraction in the range from 1.85 to 1.5, respectively.

[0040] A preferred high dispersion material for use in constructing the negative second lens element is styrene and a preferred low dispersion material for use in constructing the positive third lens element is a crown-type glass. In place of styrene, other plastics having flint-like dispersions can be used to prepare the negative second lens element, including polycarbonates and copolymers of polystyrene and acrylic such as NAS. See The Handbook of Plastic Optics, U.S. Precision Lens, Inc., Cincinnati, Ohio, 1983, pages 17-29.

[0041] Each of elements 1, 2, 4, and 5, as well as the field flattener lens unit (U_N), has at least one aspherical surface to facilitate the correction of the aberrations of the lens system and, preferably, each of the lens elements has two aspheric surfaces. Because the lens surfaces are aspheric, the overall shapes of the elements are preferably described in terms of best fit spherical surfaces, rather than the radii of curvature of the elements at the optical axis, although in many cases the description of the overall shapes of the elements in terms of best fit spherical surfaces on the one hand and in terms of the radii of curvature at the optical axis on the other will be the same. See Dunham, C. B., and C. R. Crawford, “Minimax Approximation by a Semi-Circle,” Society for Industrial and Applied Mathematics, Vol. 17, No. Feb. 1, 1980.

[0042] To maintain a relatively low cost of the lens system, all the aspherical elements are designed to be manufactured in optical plastic. In particular, for ease of molding, a fairly uniform thickness is maintained across the aperture of each of these elements. Because they are plastic, it is desirable to keep the power of the elements as low as possible so as to minimize shifts in focus of the lens system with changes in ambient temperature. As discussed above, the power element (element 3) is preferably made out of glass.

[0043] FIGS. 1-4 illustrate various projection lenses constructed in accordance with the invention. Corresponding prescriptions appear in Tables 1-4. HOYA or SCHOTT designations are used for the glasses employed in the lens systems. Equivalent glasses made by other manufacturers can be used in the practice of the invention. Industry acceptable materials are used for the plastic elements.

[0044] The aspheric coefficients set forth in the tables are for use in the following equation: $z=\frac{{\mathrm{cy}}^2}{1+{\left[1-\left(1+k\right)c^2{y}^2\right]}^{1/2}}+{\mathrm{Dy}}^4+{\mathrm{Ey}}^6+{\mathrm{Fy}}^8+{\mathrm{Gy}}^{10}+{\mathrm{Hy}}^{12}+{\mathrm{Iy}}^{14}$

[0045] where z is the surface sag at a distance y from the optical axis of the system, c is the curvature of the lens at the optical axis, and k is a conic constant, which is zero except where indicated in the prescriptions of Tables 1-4.

[0046] The designation “a” associated with various surfaces in the tables represents an aspheric surface, i.e., a surface for which at least one of D, E, F, G, H, or I in the above equation is not zero. The designation “c” represents a conic surface, i.e., a surface for which k in the above equation is not zero. The tables are constructed on the assumption that light travels from left to right in the figures. In actual practice, the viewing screen will be on the left and the CRT will be on the right, and light will travel from right to left.

[0047] The CRT faceplate constitutes surfaces 15-16 in Tables 1, 2, and 4, and surfaces 13-14 in Table 3. A coupling fluid is located between surfaces 14-15 in Tables 1, 2, and 4, and surfaces 12-13 in Table 3. Other than for the CRT faceplate of Table 4 which is composed of FDS9.glass, the material designations for the coupling fluid and the faceplate are set forth as six digit numbers in the tables, where a N_evalue for the material is obtained by adding 1,000 to the first three digits of the designation, and a V_evalue is obtained from the last three digits by placing a decimal point before the last digit. Optional vignetting surfaces are shown at surfaces 3 and 10 in Tables 1, 2, and 4.

[0048] All dimensions given in the prescription tables, as well as in Tables 5 and 6, are in millimeters.

[0049] Table 5 summarizes the ½CA₁, Y₁, R_1BF, and R_2BFvalues for the projection lens systems of Tables 1-4. As shown in this table, the lens systems of the examples satisfy requirements (1)-(3) set forth above. Indeed, for each of these examples, the ratio of ½CA₁to Y₁is at least equal to 1.3.

[0050] For comparison, the ½CA₁, Y₁, R_1BF, and R_2BFvalues for Example 3 of U.S. Pat. No. 5,808,804, the only example of that patent which has a high dispersion second lens element, are 40.095 mm, 40.095 mm, 164.3 mm, and 188.1 mm, respectively. Requirements (1)-(3) are thus not met by this prior example. (Examples 1 and 4 of U.S. Pat. No. 5,808,804 have |R_1BF|>|R_2BF|, but like all of the other examples of that patent, the R_2BFvalues for these examples are not less than zero, i.e., the object side surface of the first lens element is not convex towards the CRT.)

[0051] FIGS. 1B, 1C, 2B, 2C, 3B, 3C, and 4B are geometric MTF plots for the lenses of Examples 1-4. In particular, these plots show the through-focus MTF on the left and the optical transfer function (OTF) at best axial focus on the right. The data are shown for five field points, viz., the axis, 0.35H, 0.70H, 0.85H and 1.0H, where H is the maximum field height on the screen. The actual field heights are shown for the right hand plots. These field heights apply to both the right hand and left hand plots and are in millimeters.

[0052] The through-focus data are at the indicated spatial frequency in cycles per millimeter, i.e., at 2 cycles/mm except for the on-axis plot which is at 2 and 5 cycles/mm. A spatial frequency of 2 cycles/mm is often used in evaluating the performance of projection television systems since it. corresponds to approximately 400 TV lines in the horizontal direction for a five-inch diagonal image on a typical seven-inch CRT tube. A 400. TV line resolution corresponds to the maximum resolution specification for most laser disc players.

[0053] Both the through-focus and best-focus data indicate tangential (solid curves) and sagittal (dashed curves) MTF. The modulus scale is on the left of each block and runs from zero to one. The phase of the OTF is shown as a dotted curve in the best-focus plots. The scale for the phase is indicated on the right of each best-focus block and is in radian measure. All the OTF data are for a wavelength of 546.1 nanometers. The axial focus shifts for the best focus plots of FIGS. 1B, 1C, 2B, 2C, 3B, 3C, 4B, and 5 are 0.02, −0.004, 0.014, −0.018, 0.005, −0.055, −0.006, and −0.077, respectively. These focus shifts are relative to the zero position of the through-focus plots. The best-focus plane is at the peak of the axial through-focus plot.

[0054] FIG. 5 shows a corresponding MTF plot for Example 3 of U.S. Pat. No. 5,808,804. (The plot of FIG. 5 differs from the plot of FIG. 3B of U.S. Pat. No. 5,808,804 in that a diffractive MTF is shown in FIG. 3B, while a geometric MTF is shown in FIG. 5.) An examination of FIG. 5 and FIGS. 1B, 1C, 2B, 2C, 3B, 3C, and 4B shows that the projection lenses of the invention achieve improved overall MTFs across the field of the lens in comparison with the MTFs achieved with the construction of U.S. Pat. No. 5,808,804.

[0055] FIG. 6 is a schematic diagram of a CRT projection television 10 constructed in accordance with the invention. As shown in this figure, projection television 10 includes cabinet 12 having projection screen 14 along its front face and slanted mirror 18 along its back face. Module 13 schematically illustrates a lens system constructed in accordance with the invention and module 16 illustrates its associated CRT tube. In practice, three lens systems 13 and three CRT tubes 16 are used to project red, green, and blue images onto screen 14.

[0056] Although specific embodiments of the invention have been described and illustrated, it is to be understood that a variety of modifications which do not depart from the scope and spirit of the invention will be evident to persons of ordinary skill in the art from the foregoing disclosure.