JPH04335610A - Projection lens - Google Patents

Abstract

PURPOSE:To obtain a >=47 deg. wide view angle and back focus which is >=2 times as large as the focal length of the whole system by specifying the composite refracting power of a 1st and a 2nd group, the arrangement of the 2nd group, and the arrangement and refracting power of a 3rd and a 4th group. CONSTITUTION:The projection lens is equipped with the 1st group G1 with negative refracting power, the 2nd group G2, the 3rd group G3 with positive refracting power, the 4th group G4, and a stop S between the 2nd group G2 and 3rd group G3. Further, the 2nd group G2 is equipped with a cemented lens of a positive and a negative lens. Then conditions shown inequalities I-III are sartisfied, where f12 is the composite focal length of the 1st group G1 and 2nd group G2, d2 the on-axis distance from the lens surface of the 1st group G1 which is closest to a liquid crystal display body to the stop S, d4 the on-axis distance from the lens surface in the 2nd group G2 which is closest to the liquid crystal display body to the stop S, f3 and f4 the focal lengths of the 3rd group G3 and 4th group G4, e3 the on-axis distance from the stop S to the rear principal point of the 3rd group G3, and e4 the on-axis distance e4 from the stop S to the rear principal point of the 4th group G4.

Description

Detailed Description of the Invention [0001] [Industrial Application Field] The present invention relates to a projection lens that enlarges and projects an image of a liquid crystal display such as a liquid crystal display element, a so-called liquid crystal light valve, onto a screen to project a good screen. It is related to. [0002] Conventionally, as a method for obtaining reproduced images on a CRT such as a large-screen television, B (blue) and
2. Description of the Related Art Projection tube type video projectors that enlarge and project the reproduced image of a CRT onto a screen using three projection lenses corresponding to the three colors of G (green) and R (red) have become popular. On the other hand, in recent years, light from the same light source divided into three parts has been applied to each of the liquid crystal display bodies (hereinafter referred to as liquid crystal light valves) corresponding to the three colors of B, G, and R. There is active development of liquid crystal projectors that transmit illumination, combine the light from each liquid crystal light valve, and enlarge and project the image on the three color liquid crystal light valves onto a screen using a single projection lens. There is. [Problems to be Solved by the Invention] Unlike conventional video projectors that project CRT images, liquid crystal projectors use a single projection lens to project light from liquid crystal light valves for each color of B, G, and R. To guide the light, the B, G, and R colors are combined using dichroic mirrors, prisms, etc. Therefore, in order to secure a space for arranging these dichroic mirrors, prisms, etc., a projection lens with a long back focus is required. [0005] Furthermore, it is required to maintain wavelength characteristics by changing the angle (field angle) of each color's light relative to a dichroic mirror, etc., and to maintain a high contrast image by using light emitted perpendicularly from each color's liquid crystal light valve. . For this reason,
A telecentric projection lens whose chief ray has an inclination of several degrees or less is required. Furthermore, in order for each pixel of the liquid crystal light valve to be faithfully reproduced for each color on the screen, it is necessary to sufficiently correct lateral chromatic aberration in the projection lens. Furthermore, unlike a video projector, it is difficult for a liquid crystal projector to electrically process distortion of the screen, so it is necessary to sufficiently correct this distortion (distortion aberration) using a projection lens. This invention was made in view of the above-mentioned problems, and has a wide angle of view of 47 degrees or more, a back focus that is more than twice the focal length of the entire system, and excellent results while maintaining telecentricity. The purpose of the present invention is to provide a projection lens suitable for a liquid crystal projector having image performance. Means for Solving the Problems In order to achieve the above object, the present invention, as shown in FIG. a second group G2 having refractive power; a third group G3 having positive refractive power;
a fourth group G4 having positive refractive power; and an aperture S between the second group G2 and the third group G3;
has a cemented lens composed of a positive lens and a negative lens, the focal length of the entire system is f, the combined focal length of the first group G1 and the second group G2 is f12, and the first group G1 The axial distance from the lens surface closest to the liquid crystal display (liquid crystal light valve) in the second group G2 to the aperture S is d2, and the distance from the lens surface closest to the liquid crystal display (liquid crystal light valve) in the second group G2 to the aperture S is d2. The on-axis distance is d4, and the focal length of the third group G3 is f3.
, the focal length of the fourth group G4 is f4, and from the aperture S to the third group G
The axial distance to the rear principal point of 3 is e3, and from the aperture S to the 4th
When the axial distance to the front principal point of group G4 is e4,
It is designed to satisfy the following conditions. (1) 0.55<f12/f<0
．． 75(2) 0.1 <d4 /
d2 <0.4(3) 2.5 <
f3 e4 /f4 e3 <3.5 [Operation] In the projection lens of the present invention, first, in order to maintain a back focus of at least twice the focal length of the entire system and telecentricity, a negative refractive power is required. The following conditions (1 ). (1) 0.55<f12/f<0
．． 75 However, f: focal length of the entire system, f12: composite focal length of the first group G1 and second group G2. When the upper limit of condition (1) is exceeded, the first group G1
The combined refractive power of the lens and the second group G2 becomes weaker, making it difficult to secure a sufficient back focus, and the overall length of the projection lens and the lens diameter of the first group become larger, leading to an increase in the size of the projection lens itself. Become. On the other hand, if the lower limit of condition (1) is exceeded, although it is advantageous in securing back focus,
Since the combined refractive power of the first group G1 and the second group G2 becomes too strong, the value of the Petzval sum becomes an excessively negative value,
Significant curvature of field occurs. Moreover, the front group (first group G1
Since the distance between the principal points of the second group G2) and the rear group (the third group G3 and the fourth group G4) becomes shorter, it becomes impossible to secure a space in which the diaphragm S should be placed. Furthermore, as described above, in the projection lens for a liquid crystal projector, in order to faithfully reproduce each color of each pixel of the liquid crystal light valve on the screen, lateral chromatic aberration must be sufficiently corrected. Therefore, in the present invention, the following condition (2) was found in order to correct the chromatic aberration of magnification in a sufficiently well-balanced manner. (2) 0.1 <d4 /d2
<0.4 However, d2: On-axis distance from the lens surface closest to the liquid crystal display (liquid crystal light valve) in the first group G1 to the aperture S, d4
:The most liquid crystal display (liquid crystal light valve) in the second group G2
This is the axial distance from the side lens surface to the aperture S. Chromatic aberration correction in the lens group on the screen side of the aperture S is mainly performed by the second group G2, which has a cemented lens made of a negative lens and a positive lens. However, when the upper limit of condition (2) is exceeded, the cemented lens in the second group G2 is too far away from the aperture S, and the effect of correcting lateral chromatic aberration due to the angle of view of the cemented lens in the second group G2 is reduced. The difference becomes larger. As a result, the lateral chromatic aberration is under-corrected at small angles of view, and over-corrected at large angles of view, resulting in significant deterioration of the imaging performance of the projection lens. On the other hand, if the upper limit of condition (2) is exceeded, the second
Since the cemented lens in group G2 is too close to the aperture S, the effect of correcting lateral chromatic aberration by the cemented lens in the second group G2 is weakened, and the rear group (third The burden of correcting the chromatic aberration of magnification in the group G2 and the fourth group G4 increases. Therefore, this rear group (third group G2
When correcting chromatic aberration of magnification with the 4th group G4), the axial chromatic aberration becomes overcorrected, and as a result, the correction balance between lateral chromatic aberration and axial chromatic aberration is greatly disrupted, making it difficult to ensure excellent imaging performance of the projection lens. becomes. Furthermore, as mentioned above, unlike CRT video projectors, it is difficult for liquid crystal projectors to electrically process screen distortion;
It is necessary to sufficiently correct distortion (distortion aberration) using the projection lens. Therefore, in the present invention, the following condition (3) has been found for properly correcting the distortion aberration of the projection lens. (3) 2.5 <f3 e4 /
f4 e3 <3.5f3: Focal length of the third group G3, f4: Focal length of the fourth group G4, e3: On-axis distance from the aperture S to the rear principal point of the third group G3, e4: From the aperture S This is the axial distance to the front principal point of the fourth group G4. Condition (3) defines the optimal distribution of refractive power between the third group G3 and the fourth group G4, and the appropriate positions of the third group G3 and the fourth group G4 with respect to the aperture S. This compensates for the correction of distortion aberration. Since the present invention is based on the configuration of a retrofocus lens,
Negative distortion occurs in both the front group (first group G1 and second group G2) with negative refractive power and the rear group (third group G3 and fourth group G4) with positive refractive power. Therefore, correction of this negative distortion is important. In the projection lens for a liquid crystal projector according to the present invention, as described above, the angle of each color light (image It is required to maintain wavelength characteristics due to angles) and to maintain high contrast images using light emitted perpendicularly from the liquid crystal light valves of each color. Therefore, in a projection lens for a liquid crystal projector, it is necessary to ensure that the principal ray is telecentric with an inclination of several degrees or less. However, if the projection lens is simply made telecentric, the chief ray will be
Since the position where the lens passes through the third group G3 and the fourth group G4 becomes higher, it is inevitable to suppress the occurrence of negative distortion. If an attempt is made to correct this negative distortion, a bending of the distortion aberration will occur, that is, a cast-shaped distortion aberration will occur, resulting in unnatural distortion and unavoidable deterioration of the imaging performance of the projection lens. . Therefore, in the present invention, according to condition (3), the positive refractive power of the rear group (third group G3 and fourth group G4) is combined with the lens group (third group G3) close to the aperture S and the aperture S. It is possible to reduce the occurrence of negative distortion by arranging the lens group separately from the aperture S (fourth group G4) and weakening the refractive power of the lens group (fourth group G4) distant from the aperture S. I'm finding out. If the upper limit of condition (3) is exceeded, the rear group (3rd group G3 and 4th group
The positive refractive power of the lens group (fourth group G4) located away from the aperture S of the group G4) becomes large. As a result, the back focus becomes short, making it impossible to secure a space for arranging a dichroic mirror, a prism, etc. for combining the light from the liquid crystal light valves of each color and guiding it to the projection lens. Moreover, the exit pupil position of the projection lens approaches the liquid crystal light valve side, making it impossible to maintain telecentricity. As a result,
The wavelength characteristics are maintained by the angle (angle of view) of the light of each color relative to the dichroic mirror or prism that combines the light from the liquid crystal light valves of each color and guides it to the projection lens, and the light is emitted perpendicularly from the liquid crystal light valve of each color. It becomes difficult to maintain high contrast images using light. In the present invention, in order to achieve more sufficient correction of negative distortion, the first group G1 includes a positive lens with a surface of stronger curvature facing the screen side, and a positive lens with a convex surface facing the screen side. It is desirable to use a negative meniscus lens. As a result, the chief ray is incident on the liquid crystal light valve side surface of the positive lens in the first group G1 at a relatively large incident angle, so that distortion can be generated in the positive direction. Furthermore, in order to correct lateral chromatic aberration in a better balance, it is more desirable to satisfy the following conditions. (4) −9<ν11−ν12<25(
5) −40<ν21−ν22<−18 However, ν11: Abbe number of the positive lens in the first group G1 with the surface of stronger curvature facing the screen side, ν12: The surface of the positive lens facing the screen side in the first group G1 The Abbe number of the negative meniscus lens with the convex surface facing, ν21: The Abbe number of the positive lens in the cemented lens of the second group G2 is ν21, ν22: The Abbe number of the negative lens in the cemented lens of the second group G2 is ν22, be. When the upper limit of condition (4) is exceeded, the first group G1
The effect of chromatic aberration correction by the positive lens and negative meniscus lens in the lens becomes too strong, and as the angle of view becomes larger, lateral chromatic aberration suddenly becomes significantly overcorrected. On the contrary, the condition (4
) is undesirable because lateral chromatic aberration will be under-corrected. If condition (5) is exceeded, lateral chromatic aberration will be overcorrected, and conversely, if condition (5) is exceeded,
This is not preferable because the chromatic aberration of magnification will be insufficiently corrected. In addition, in order to correct astigmatism in a well-balanced manner, the fourth group G4 is cemented with a positive lens whose surface has a stronger curvature toward the liquid crystal light valve, a negative lens, and a positive lens. It is more preferable to have a cemented lens with a surface having a stronger curvature facing the liquid crystal light valve side. In other words, the positive lens and the cemented lens are shaped so that the convex surface with a strong curvature faces toward the liquid crystal light valve side and the convex surface or concave surface with a weak curvature faces toward the screen. Since the incident angle of the chief ray can be made small on average, the occurrence of astigmatism can be minimized. Embodiments FIG. 1 shows lens configurations of first and second embodiments of the present invention. As shown in FIG. 1, in order from the screen side, there is a first group G1 having a negative refractive power, a second group G2 having a negative refractive power, and a third group G2 having a positive refractive power.
group G3, a fourth group G4 having positive refractive power, and a second group G
2 and the third group G3, the first group G1 and the second group G2 constitute a front group with negative refractive power, and the third group G
3 and the fourth group G4 constitute a rear group with positive refractive power. That is, the projection lenses of each embodiment of the present invention have a retrofocus type lens as their basic structure. The first group G1 consists of a positive lens L11 with a surface of stronger curvature facing the screen, and a meniscus lens L12 with a convex surface facing the screen, and the second group G2 includes a positive lens L21 with a biconvex shape. and a biconcave negative lens L22 cemented thereto. The third group G3 consists of a positive meniscus lens L31 with a convex surface facing the liquid crystal light valve side, and a negative meniscus lens L32 bonded to this with a convex surface facing the liquid crystal light valve side. , a positive lens L41 having a surface with a stronger curvature facing the liquid crystal light valve side, a negative lens L42 having a concave surface facing the liquid crystal light valve side, and a biconvex positive lens L43 cemented thereto. Here, the positive lens L21 and the negative lens L2
2 has a meniscus shape with a convex surface facing the screen side, and the cemented lens consisting of a positive meniscus lens L31 and a negative meniscus lens L32 has a convex surface facing the liquid crystal light valve side. It has a oriented shape. The cemented lens composed of the negative lens L42 and the positive lens L43 has a biconvex shape with a surface of stronger curvature facing the liquid crystal light valve side. Now, the values of the specifications and the numerical values corresponding to the conditions of the first and second embodiments of the present invention are listed below. However, the number on the left side represents the order from the object side, r is the radius of curvature of the lens surface, d is the distance between the lens surfaces, ν is the Abbe number, n is the refractive index at the d-line (λ = 587.6 nm), and f is the refractive index at the d-line (λ = 587.6 nm). Focal length of the entire system, FNO is F number, 2ω is angle of view (°)
, ds represents the axial distance from the side surface (seventh surface) of the liquid crystal light valve to the aperture S in the third group. [First embodiment] f=128.6, FNO=8.0,
2ω=48°No r
d ν n1 15
1.119 7.00 64.10 1.5
18722 ∞ 0.50
3 93.512 5.00 6
4.55 1.808224 41.4
99 37.505 88.050
6.50 33.75 1.652866
-84.023 5.00 60.
14 1.622877 47.914
43.008 -124.341
11.00 82.52 1.499269
-40.066 5.00 51.35
1.5292610 -62.099
38.0011 -1102.910
14.30 82.52 1.4992612
-88.050 2.2013
1108.160 6.00 39.82
1.8751314 107.827
19.00 82.52 1.4992615
-157.736 (Bf)Bf=280
．． 00,ds=14.00f1/f=0.617
, d4 /d2 =0.30, f3 e4 /f4
e3 =2.755 ν11-ν12=17.6, ν
21-ν22=-26.4 [Second embodiment] f=128.6, FNO=7.9,
2ω=48°No r
d ν n1 16
5.037 7.00 64.10 1.5
18722 -1678.660 0.5
03 124.341 5.00
64.55 1.808224 45.
153 34.005 72.310
8.00 33.75 1.652866
-220.073 6.00 67
．． 87 1.595277 44.14
8 47.008 -121.136
8.50 82.52 1.499269
-39.999 5.00 51.3
5 1.5292610 -63.390
38.0011 -559.900
15.00 82.52 1.4992612
-85.727 1.0013
792.450 6.00 39.82
1.8751314 109.110
19.00 82.52 1.4992615
-159.009 (Bf)Bf=27
9.59, ds = 15.00f1 /f = 0.654
, d4 /d2 =0.23, f3 e4 /f4
e3 =3.023 ν11-ν12=17.6,
ν21-ν22=-34.1 As shown in the specification table above, the comparison has a wide angle of view of 47 degrees or more, and a back focus that is more than twice the focal length of the entire system while maintaining telecentricity. It is clear that a compact projection lens has been achieved. FIGS. 2 and 3 show various aberration diagrams of the first and second embodiments of the present invention, respectively. E in each aberration diagram is the E line of the reference light beam (λ = 546.1 nm), F
is F-line (λ=486.1 nm), C is C-line (λ=656
．． 3 nm) are shown. Comparison of each aberration diagram shows that in all examples, various aberrations are corrected well and in a well-balanced manner, and in particular, distortion aberration and lateral chromatic aberration, which are problems in projection lenses for liquid crystals, are very well corrected. it is obvious. [0028] As described above, according to the present invention, the angle of view is 4
Relatively compact projection suitable for high-definition LCD projectors with a wide viewing angle of 7 degrees or more, a back focus that is more than twice the focal length of the entire system, and excellent imaging performance while maintaining telecentricity. lenses can be achieved. [0029] As a result, space for arranging dichroic mirrors, prisms, etc. for combining the light from the B, G, and R color liquid crystal light valves and guiding it to one projection lens is secured, and the arrangement space for each color to the dichroic mirror, etc. is secured. The wavelength characteristics are maintained by the angle of light (angle of view), and the maintenance of high contrast images is ensured by using the light vertically emitted from the liquid crystal light valves of each color. Furthermore, since lateral chromatic aberration can be corrected in a well-balanced manner, each pixel of the liquid crystal light valve can be faithfully reproduced for each color on the screen.
Furthermore, since distortion can be well corrected, excellent image quality can be obtained without causing screen distortion.

[Brief explanation of drawings]

FIG. 1 is a lens configuration diagram of first and second embodiments according to the present invention.

FIG. 2 is a diagram showing various aberrations of the first embodiment according to the present invention.

FIG. 3 is a diagram showing various aberrations of a second embodiment according to the present invention.

[Explanation of symbols of main parts]

Claims (1)

[Claims] Claim 1: In order from the screen side, a first group G1 having a negative refractive power, a second group G2 having a negative refractive power, a third group G3 having a positive refractive power, and a positive refractive power. and a diaphragm S between the second group G2 and the third group G3, and the second group G2 is a cemented lens made of a positive lens and a negative lens. The focal length of the entire system is f, the combined focal length of the first group G1 and the second group G2 is f12, and the axis from the lens surface closest to the liquid crystal display in the first group G1 to the aperture S Upper distance d2, axial distance d4 from the lens surface closest to the liquid crystal display in the second group G2 to the aperture S, f3 the focal length of the third group G3, f4 the focal length of the fourth group G4, from the aperture S When the axial distance to the rear principal point of the third group G3 is e3, and the axial distance from the aperture S to the front principal point of the fourth group G4 is e4, it is characterized by satisfying the following conditions. projection lens. (1) 0.55<f12/f<0
．． 75(2) 0.1 <d4 /
d2 <0.4(3) 2.5 <
f3 e4 /f4 e3 <3.5