WO1998052076A1

WO1998052076A1 - Wide angle large aperture imaging lens

Info

Publication number: WO1998052076A1
Application number: PCT/US1997/008122
Authority: WO
Inventors: William G. Peck
Original assignee: Peck William G
Priority date: 1997-05-15
Filing date: 1997-05-15
Publication date: 1998-11-19

Abstract

Imaging or projection lenses (10) of small size and excellent perormance are disclosed. Either triplet or doublet in form, the lenses include aspheric (12a, 12b, 14a, 14b, 16b) and diffractive surfaces (14b), the diffractive surfaces being formed on selected ones of the aspheric surfaces. The fastest F/numbers of the lenses are from about 1.8 to about 1.9, and the diffractive surfaces are placed next adjacent to the aperture stops of the systems to allow for maximum correction of the axial chromatic aberrations with minimum effect on the lateral chromatic corrections.

Description

Wide Angle Large Aperture Imaging Lens

This invention relates to imaging or projection lens systems including aspherical refracting/diffracting surfaces, and having advantageous optical properties including wide fields of view together with large numerical apertures and high resolution. Description of the Prior Art:

Lens systems of the invention may be regarded as of the general kind described in United States patent No. 5.251.069. issued to Iizuka on October 5. 1993 intended for use in picturephone systems and the like. In a typical system of this type it is usually desired to provide an objective lens system that has a wide field of view, a large numerical aperture, and reasonably good resolution and freedom from aberrations. It should also be rugged, very compact, and relatively inexpensive to manufacture.

Summary of the Invention:

Lens systems of the invention are characterized by advantageous optical properties together with relatively simple mechanical design such that their optical performance is achieved at relatively low cost. Their half-angle field of view is approximately 35°. and their fastest practical F/numbers are from about 1.8 to about 2.4. Their optical properties such as resolution and freedom from excessive aberrations and distortion are highly advantageous, and they are fully capable of operating with high resolution photo receiving surfaces such as a VX charge coupled device having definition of 640 x 480 pixels. Brief Description of the Invention:

Doublet and triplet embodiments of the invention are described in this application, each system having aspheric optical refracting surfaces, and at least one refrac- tive/diffractive surface. The refractive/diffractive surface is placed next adjacent to the aperture stop of the system allowing correction of axial chromatic aberrations with minimum effect on lateral chromatic aberrations.

In addition, the optical design of the lenses is chosen to allow fast and efficient assembly of the systems, so that, for example, the lens elements may be injection molded of acrylic plastic with integrally formed axially extending annular flanges, the elements being spaced apart by the flanges.

Brief Description of the Drawings:

Figure 1 is a simplified cross section of an imaging lens system according to a first embodiment of the invention;

Figure 2 is a pair of graphs illustrating the field curvature and distortion of the lens system shown in Fig. 1 ;

Figure 3 is a set of graphs showing the transverse ray fan plot of the lens system shown in Fig. 1 ;

Figure 4 is a set of curves illustrating the polychromatic, through focus, modula- tiong transfer function (MTF) of the lens system shown in Fig. 1 ;

Figure 5 is a simplified cross sectional diagram of a lens system according to a second embodiment of the invention;

Figure 6 is a pair of graphs showing the field curvature and distortion of the lens system shown in Figure 5;

Figure 7 is a set of graphs showing the transverse ray fan plot for the lens system shown in Fig. 5.; Figure 8 is a graph showing the polychromatic, through focus, modulation transfer function (MTF) for the lens system shown in Fig. 5;

Figure 9 is simplified cross sectional view of a third embodiment of the in¬

vention;

Figure 10 is a pair of graphs illustrating the field curvature and distortion of the lens system shown in Fig. 9;

Figure 1 1 is a set of graphs illustrating the transverse ray fan plot for the lens system shown in Figure 9;

Figure 12 is a graph illustrating the polychromatic, through focus, modulation transfer function for the lens system shown in Figure 9;

Figure 13 is a simplified cross sectional view of a fourth lens system of the

invention;

Figure 14 shows a pair of curves illustrating the field curvature and distortion of the lens system shown in Fig. 13;

Figure 15 is a set of graphs illustrating the transverse ray fan plot for the

system shown in Fig. 13; and

Figure 16 is a partly simplified cross sectional view of the first embodiment of the invention, particularly showing the mechanical spacing feature of the invention, the use of relatively short, shoulder-like flanges instead of separate spacer elements for providing the proper distances between the air spaced surfaces of the assembly. Description of the Preferred Embodiments:

Representative embodiments of the invention will now be described in conjunction with the drawings.

Conventions and notations herein, unless otherwise noted, conform with those generally accepted in the optical industry as described, for example, in the ZEMAX optical Design Program, User's Guide, Version 5.0, published by Focus Software. Incorporated. P.O. Box 18228. Tucson, Arizona. The calculations and numerical data for the lens systems of the invention as given herein are based on the even asphere surface as defined in that program. The diffractive surfaces are given in conformity with the Binan optic 2 notation therein. Dimensions are in millimeters unless otherwise noted. In all cases for purposes of calculation the object distance is assumed to be 600 mm.

Referring first to Figure 1 , a triplet lens system 10 according to a first, and presently preferred embodiment of the invention is shown having three lens elements 12, 14, and 16, five of their six optical surfaces being aspheric. The surfaces 12a and 12b of the first lens element, reading from the object, or long conjugate end, are both aspheric, and also both surfaces 14a and 14b of the second, main focusing element. The fourth surface 14b, however, is both refractive and diffractive. being circularly grooved for diffraction. The fifth surface 16a is spherical, and the sixth surface 16b is aspherical.

A piano cover plate 20 and the surface 22 indicating the image plane are also shown in the drawing and listed in TABLE 1.

The shape of an even aspheric surface can be expressed in terms of its sag. or deviation, z, from a plane tangent to the vertex of the surface, as given by: α,r⁸ + α ¹⁰ + a_brⁿ + a₇r + α_sr¹⁶

where z is the sag, c is the curvature (1/R) of the vertex radius, R is the radius of curvature of an individual lens surface, r is the radial aperture coordinate (distance from the axis of rotational symmetry), d is the lens thickness or the air spacing between lens sur¬

faces along the central optical axis, K is the conic constant, and ot| . α₈ are the asphericity

coefficients.

For a diffractive surface there are additional polynomial terms which represent the variation in phase of a transmitted light wavefront (rather than surface height) across the op¬

tic surface. The coefficients of these terms have units of radians (2π radians = one wave¬

length of light at the predominant wavelength) rather than lens units. The base shape of the

diffractive surface is an even asphere surface. The diffractive optic surface adds phase, Φ,

to the transmitted wavefront according to the polynomial expansion:

where N is the number of polynomial coefficients in the series, A, is the coefficient of the

2ith power of p, which is the normalized radial aperture coordinate.

The numerical values that characterize the arrangement of the first example of the imaging lens system of the present invention are listed in TABLE 1 , wherein F_NO is the F/number of the system, f is the focal length at the predominant wavelength; na is the index of refraction of the lens material; and EFL is the effective focal length of the lens element. TABLE 1

Predominant wavelength = 550 nm.; F_NO = 1 : 1.9 : f = 4.838 Petzval radius = -22.5 Half field angle = 35°

Surface R d rid K. EFL

1 -6.3588 2.39 1.4918 -05450 4.909

2 4.4008 0.9799 0.9661713 r> 2.7112 5.474 1.4918 -1.870605 4.559 4 -4.9144 0.05 -2.580518

Stop 0.2

3.3979 1.975 1.4918 0 147.42

6 2.8797 2.665 -3.876037 cover glass 0.5 1.5163 8 0.478

Image 0

Asphericity Coefficients: Surface 1: Surface 5: spherical α₂ = 0.003184457 α₃ = -0.0001425256 Surface 6: α₄ = 8.827576 x 10^'6 α₂ = 0.02420209 a, = -2.657098 x 10 -7 α₃ = -0.007269247 α₄ = 0.002835906

Surface 2: α₅ = -0.0004149545 ₂ = -0.002151056 α₃ = 0.000532806 Coefficients of phase equation θ4 = 3.222623 x 10 -5 for surface 5 (diffractive):

A, = -66.87387

Surface 3: A₂ = 3.272176 α₃ = 0.0003920896 A₄ = 2.559262 α₂ = 0.002056374 A₃ = -5.06125

A₅ = -0.410263

Surface 4: c-₂ = 0.002446787 α₃ = -0.0003589343 α₄ = 7.1 17641 x 10^"5 αs = 2.611741 x 10 -5 The aperture stop is 3.06 mm. in diameter, and is spaced 0.05 mm. after the second lens (surface 4). The lens elements with optical power in this and in all of the following embodiments are preferably molded of acrylic plastic material, poly methyl meth¬

acrylate (PMMA). which has a refractive index of 1 .4918 and an Abbe number, υ. of

57.4. The index of the glass cover plates is 1.5163. and their Abbe number is 64.1.

The lens assembly is small, light weight, and highly effective. Performance for the lens system of the preferred Example 1 as shown in Figs. 2, 3. and 4.

A second embodiment of the invention is shown in Figure 5. It is a triplet having three lens elements 24. 26. and 28 and four aspherical surfaces 24a. 24b. 26a. and 26b. the fourth one 26b of which, counting from the object, or long conjugate end, is also diffractive. The numerical values that characterize its arrangement are listed in TABLE 2:

TABLE 2

Predominant wavelength = 550 nm.; FNO ⁼ 1 1.9 : f = 4.801 Petzval radius = -20 .8 Half field ang le = 35°

Surface R d rid K EFL

1 -16.1835 1.9435 1.4918 0 -5.874

2 3.6723 0.9002 1.137052

3 3.6278 4.6091 1.4918 -1.134623 4.678 4 -4..0031 0.05 -2.448985

Stop 0.2

5 3.3191 2.1082 1.4918 0 75.26 6 2.8797 2.665 -2.717832

7 cover glass 0.5 1.5163 8 0.546

Image 0 TABLE 2 (cont'd) Asphercity Coefficients: Surface 1: spherical Surface 5: ₂ = 0.02307228

Surface 2: α₃ = -0.008899306 α₂ = 0.003122377 α₄ = 0.003548783 α₃ = 0.0006904625 α₅ = -0.0004837645 ₄ = 3.508308 x 10^"5

Surface 6: spherical

Surface 3: a₂ = 0.03616276 Coefficients of phase equation α₃ = 0.0006125425 for Surface 4 (diffractive):

A, = -60.72823

Surface 4: A₂ = -4.03179 α₂ = 0.002992498 A₃ = -3.865499 α₃ = -0.001267482 A₄ = 4.667682 α = -0.0002025849 A, = -0.993969 α. = 0.0001463354

A third embodiment of the invention is shown in Figure 9 comprising a doublet having only two lens elements 30 and 32, all the refractive surfaces of which are a- spheric, and the .fourth one 32b of which is also diffractive.

The numerical data characterizing this embodiment are given in TABLE 3:

TABLE 3

Predominant wavelength = 550 nm.; F_No = 1 :2.4 : f = 4.818 Petzval radius = -26.22 Half field angle = 35°

Surface R d n_d K. EFL

1 -2.3948 2.45 1.4918 -4.44209 4.004 2 15.1335 0.8 21.45605

3 1.8832 5.459 1.4918 -2.542302 3.770 4 -14.6946 0.3029 14.76696

Stop 0.5

5 cover glass 0.5 1.5163

6 1.719

Image 0 TABLE 3 (cont'd) Asphercity Coefficients: Surface 1: Surface 4: ₂ = 0.003893814 α₂ = -0.04275283 α₃ = -0.0002080592 α₃ = 0.05764789 α₄ = 6.733405 x 10^'6 α₄ = -0.02247332 cu = -9.683855 x 10^"8 α₅ = 0.001813895

Surface 2: Coefficients of phase equation α₂ = 0.007512316 for Surface 4 (diffractive): α₃ = 0.0001455065 A, = -234.7858 α₄ = -3.034508 x 10^"5 A₂ = 318.8353 α₅ = 4.541006 x 10^'6 A₃ = -303.8566 A₄ = 100.9584

Surface 3: ₂ = 0.0186629 α₃ = -0.00156672 α₄ = 0.0001 131472

A fourth example of a lens system according to theinvention is the second doublet shown in Figure 13. Its characterizing numerical data are listed in TABLE 4. and it includes two optical elements, the bi-concave lens 40 and the bi-convex lens 42. All of the optical surfaces in the system are aspherical. and the fourth surface 42b is also diffrac¬

tive.

TABLE 4

Predominant wavelength = 550 nm.; FNO ⁼ 1 :2.4: f = 4.40 Petzval radius = -27.80 Half field angle = 35°

Surface R d rid K EFL

1 -2..2173 2.45 1.4918 3.7988038 -3.819

2 17.1739 0.9 15.98805

3 1.9311 6.45 1.4918 -2.131674 4.049 4 -9.9751 0.338 21.66639

Stop 2.08 TABLE 4 (cont'd)

Surface R d rid K EFL

6 1.554 Image 0

Asphercity Coefficients: Surface 1: Surface 4: α₂ = 0.003018955 ₂ = -0.02028169 α₃ = -0.0001303863 ₃ = 0.02442391 α₄ = 3.716952 x lO^-6 α₄ = -0.003997133 α₅ = -4.726771 x 10^"8 α₅ = -0.001628156

Surface 2: Coefficients of phase equation ₂ = 0.0007437566 for Surface 4 (diffractive) : α₃ = 0.00071 15328 A, = -191.2439 α₄ = -6.939278 x 10^"5 A₂ = 206.9009 α₅ = 4.222148 x lO^"6 A₃ = -179.073 A₄ = 54.69428

Surface 3: ₂ = 0.008024102 α₃ = -0.0003531334 α₄ = 2.453634 x 10^"5

Figure 16 illustrates the assembly of a practical embodiment of the invention, especially showing the integrally molded, annular, axially extending flanges 50 and 52 on selected ones of the lens elements of the system for spacing the elements apart in accor¬

dance with the specified dimensions to provide the desired optical characteristics. This arrangement effectively reduces the cost of the system relative to previous systems that included air spaces that are too long to be filled by practical, integrally molded flanges, and, therefore, required separate spacer rings.

Claims

WHAT IS CLAIMED IS:

1. An imaging or projection lens system comprising a field reducing lens element on the long conjugate side of the system, and a positive lens element optically aligned with said reducing lens element on the side thereof toward the short conjugate side of the system, each of said lens elements having two optically operative surfaces, at least one of said optically operative surfaces of at least one of said elements being aspheric, and one of said optically operative surfaces being diffractive.

2. An imaging or projection lens system according to claim 1 wherein one of said optically operative surfaces is both aspheric and diffractive.

3. An imaging or projection triplet lens system comprising in order from its long conjugate end a field reducing lens element, a focusing lens element, and a third lens element at the short conjugate end, each of said lens elements having two optically operative surfaces, at least four of the total of six optically operative surfaces being aspheric, and at least one of the six surfaces being diffractive.

4. A triplet lens system according to claim 3 wherein said at least one surface is both refractive and diffractive.

5. An imaging or projection triplet lens system comprising in order from its long conjugate end a field reducing lens element, a focusing lens element, and a third lens element at the short conjugate end, each of said lens elements having two optical surfaces, at least five of the total of six optical surfaces being aspheric, and at least one of the six surfaces being diffractive

6. A triplet lens system according to claim 5 wherein said at least one surface is both refractive and diffractive.

7. An imaging or projection doublet lens system comprising a field reducing first lens element at its long conjugate end. a focusing lens element next adjacent to said first element on the short conjugate side thereof, and means defining an aperture stop on the short conjugate side of said focusing element, all of the optically operative surfaces of said elements being aspheric. and one of said surfaces being also diffractive.

8. A doublet lens system according to claim 7 wherein the diffractive one of said surfaces is the surface of said focusing lens next adjacent to said stop means.

9. A refractive/diffractive imaging or projection lens system comprising optical elements including a focusing lens, a field reducing lens, and means defining an aperture stop, said focusing lens being of positive power and positioned between said stop means and said field reducing lens, both surfaces of said focusing lens being refractive, and the surface of said focusing lens next adjacent to said stop means also being diffrac┬¼

tive.

10. A refractive/diffractive imaging or projection lens system comprising a field reducing lens element, a focusing lens element of positive power, and means defining an aperture stop, means mounting said elements and said stop means with said focusing element between said field reducing element and said stop means, both surfaces of said focusing element being aspherically curved, and the surface of said focusing element next adjacent to said stop means also being diffractive.

1 1. A lens system according to claim 1 wherein said lens elements are molded of acrylic plastic and at least one of said lens elements includes an integrally molded, peripheral, axially extending flange for spacing said one element from the next adjacent element of the system.

12. A refractive/diffractive triplet imaging len system comprising counting from the long conjugate end a first lens element having an effective length of about - 4.909 mm., a second lens element having an effective focal length of about 4.559 mm. and spaced about 0.797 mm. from said first element, means defining an aperture stop spaced about 0.05 mm. from said second lens element, and a third lens element, said third lens element having a focal length of about 147.42 mm. and being spaced about 0.2 mm. from said aperture sto, the surface of said second lens element next adjacent to said aperture stop means being both refractive and diffractive.

13. A refractive/diffractive triplet imaging lens system comprising counting from the long conjugate end a first lens element having effective focal length of about - 5.874 mm., a second lens element having an effective focal length of about 4.678 mm. and spaced about 0.9002 mm. from said first lens element, means defining an aperture stop spaced about 0.05 mm. from said second lens element, and a third lens element having a focal length of about 75.26 mm. and spaced about 0.2 mm. fom said stop means, the surface of said second lens element next adjacent to said aperture stop means being both refractive and diffractive.

14. A refractive/diffractive doublet imaging lens system comprising counting from the long conjugate end a first lens element having an effective focal length of about -4.004 mm., a second lens element having a focal length of about 3.770 mm. and spaced about 0.8 mm. from said first element, and means defining an aperture stop spaced about 0.3029 mm. from said second element, the surface of said second element next adjacent to said stop means being both refractive and diffractive.

15. A refractive/diffractive doublet imaging lens system comprising counting from the long conjugate end a first lens element having an effective focal length of about -3.819 mm., a second lens element having a focal length of about 4.049 mm. and spaced about 0.9 mm. from said first element, and means defining an aperture stop spaced about 0.338 mm. from said second element, the surface of said second element next adjacent to said stop means being both refractive and diffractive.

16. An imaging or projection lens system comprising a plurality of refractive lens elements arranged along a common optical axis, one of said lens elements being both diffractive and refractive, and all of said elements consisting essentially of the same light transmitting material.

17. An imaging or projection lens system comprising a plurality of refractive lens elements arranged along a common optical axis, one of said lens elements being both diffractive and refractive, and all of said elements consisting essentially of poly methyl methacrylate.

18. An imaging or projection lens system comprising a plurality of refractive lens elements arranged along a common optical axis, one of said lens elements being both diffractive and refractive, and all of said elements consisting essentially of a light transmitting material having an index of refraction of 1.4918