JPH04174811A - Retro-focus type lens - Google Patents

Abstract

PURPOSE:To obtain a high optical performance projected image with aberrations such as distortion aberration and flare corrected properly by providing an aspherical lens which has an aspheric surface on one side thereof in the first group and second group. CONSTITUTION:A plurality of lenses are divided into three groups, the first group I, second group II and third group III, in the order from the first conjugate point side of longer distance bordering the broadest lens interval and the next broadest lens interval, and the focal distance of a total system being taken as fT, back focus SK is set so as to be SK>0.7fT, and each of the first group I and second group II is provided with at least one aspherical lens, the surface of at least one side of which is aspheric. By thus setting lens constitution, a high optical performance projected image with aberrations such as distortion aberration and flare corrected properly can be obtained.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a retrofocus type lens, in which, for example, a plurality of images having different color information are combined using a combination mirror and then enlarged and projected onto a screen surface. The present invention relates to a retrofocus lens suitable for a projection lens for a color liquid crystal projection television.

(Prior Art) Various color liquid crystal projection systems have been proposed in which images displayed on multiple color liquid crystals (liquid crystal light valves) are optically superimposed and projected onto a screen using a projection lens. There is.

FIG. 7 is a schematic diagram of the main parts of a color liquid crystal projection television that projects an image formed on a general color liquid crystal onto a screen surface (not shown).

In the figure, reference numeral 1 indicates a white light source that emits a collimated light beam. 2a, 2b, and 2c are liquid crystal display elements for red, edge, and blue, respectively, on which projected images are displayed. 3a, 3
Reference numerals b denote reflecting mirrors, and 4 a red reflecting dichroic mirror, which illuminates the liquid crystal display element 2a for red. 5 is an edge reflection dichroic mirror that illuminates the liquid crystal display element 2b around the edge.

The blue liquid crystal display element 2c is illuminated with blue light that has passed through the edge of the red reflective dichroic mirror and the edge reflective dichroic mirror 5. , 6 is a blue-transmitting dichroic mirror. 7 is a projection lens.

In the figure, the white light from the white light source 1 is separated into red, edge, and blue color light by the tichroic mirrors (4, 5), and these red, green, and blue color lights are used for red, green, and blue, respectively. liquid crystal display elements (2a, 2b.

2c), and the images of the liquid crystal display elements (2a, 2b, 2c) based on these colored lights are superimposed and projected onto a screen surface (not shown) by a projection lens 7, thereby obtaining a color image.

The projection lens in this configuration requires a retrofocus type lens with a long back focus because it is necessary to arrange various optical members such as a reflective mirror and a dichroic mirror between the final lens surface and the liquid crystal display element (between the back focuses). is often used.

(Problem to be solved by the invention) Generally, a retrofocus lens has a lens group with negative refractive power arranged on the object side (longer distance conjugate point side), and a lens group with negative refractive power on the image side (longer distance conjugate point side).
It consists of a lens configuration in which a lens group with positive refractive power is arranged on the conjugate point side (with a shorter distance). Therefore, it has the advantage that a relatively long back focus can be easily obtained.

However, since the lens structure is asymmetrical, a lot of asymmetric aberrations such as distortion and astigmatism tend to occur.

When a retrofocus lens is used in a color LCD projection TV, a dichroic mirror for color separation is placed on the image plane side as shown in Figure 7, which eliminates color unevenness and improves color reproduction across the screen. In order to achieve this, it is necessary that the angle of incidence of the light beam onto the dichroic mirror be approximately equal over the entire screen. That is, it is necessary to put the lens system in a state close to a so-called exit telecentric system in which the chief ray exiting from the image plane side is substantially parallel to the optical axis.

However, when an exit telecentric system is used, the off-axis light beam enters the lens group with positive refractive power on the image plane side at a high position on the optical axis. For this reason, many barrel-shaped (negative) distortions and flares occur, resulting in the problem that it is very difficult to obtain a good projected image.

Therefore, in Japanese Patent Application No. 1-286058, the present applicant has developed a projection lens for color LCD projection televisions that properly corrects various aberrations such as distortion and flare by appropriately setting the lens configuration. We proposed a retrofocus lens with a lens configuration close to a suitable exit telecentric system.

The present invention further improves the retrofocus type lens previously proposed by the applicant, and makes it possible to easily obtain a predetermined back focus while reducing the number of lenses, and to effectively correct various aberrations such as distortion and flare. The object of the present invention is to provide a retrofocus type lens suitable for a projection lens for a color liquid crystal projection television.

(Means for Solving the Problems) The retrofocus type lens of the present invention has a plurality of lenses, and the first conjugate point side with the longest distance between the widest lens spacing and the next widest lens spacing. 1st group, 2nd group in order from
Divided into three lens groups, the third group and the third group, the focal length of the entire system is fT, and the distance from the lens surface on the side of the second conjugate point, which is shorter in distance, to the second conjugate point is SK> 0.7fT ・・・・・・・・・(
1), and is characterized in that each of the first group and the second group includes at least one aspherical lens having at least one aspherical lens surface.

In particular, in the present invention, the focal length of at least one aspherical lens provided in the first group and the second group is fAL1. fA
When L2 is set, l fALl 1>3fT ・・・・・・・・・(
2) l fAL21>3fT ・・・・・・・・・
(3) It is characterized by satisfying the following condition.

(Example) 1st. FIG. 2 is a cross-sectional view of the lens of Numerical Example 1.2 of the present invention, taking as an example a case where it is applicable to projection, and also shows a part of the lens.

In the figure, LFL is a retrofocus type lens of the present invention. S is a screen, and the first conjugate point side (
Hereinafter, this will be referred to as the "object side." ). F is a projection plane, which is arranged on the second conjugate point side (hereinafter referred to as "image plane side") where the distance is short. On this image plane side, in the case of a color liquid crystal projection as shown in FIG. 7, for example, various elements such as a liquid crystal display element which is a projected image, a light source, a filter, etc. are arranged.

■ is the first group, ■ is the second group, m is the third group, and each of these lenses is divided into three lens groups in order from the object side with the widest air gap and the next widest air gap as boundaries. I'm staying.

By appropriately setting the refractive power, lens shape, etc. of a plurality of lenses, the retrofocus type lens of the present invention has a back focus (the lens surface on the second conjugate point side) when the focal length of the entire lens system is fT. The distance #) SK from a certain final lens surface to the second conjugate point is set such that SK>0.7 fT.

At least one aspherical lens with an aspherical surface on at least one lens surface is arranged in each of the first and second groups, which effectively corrects various aberrations such as distortion and flare, resulting in high optical performance. The projected image is obtained.

In particular, the aspherical lens (eleventh aspherical lens) provided in the first group mainly corrects off-axis aberrations such as distortion and flare. In addition, an aspherical lens (second
1 aspherical lens) to mainly correct astigmatism, coma, etc.

At this time, the refractive powers of the 11th aspherical lens and the 21st aspherical lens are made to satisfy conditional expressions (2) and (3).

Conditional expressions (2) and (3) are used to prevent the refractive index of the material from changing due to environmental changes such as temperature changes when aspherical lenses are made by molding, for example, plastic materials, thereby preventing focus shifts and optical performance from deteriorating. It is for the purpose of

In other words, by applying an aspherical surface to a lens with a relatively small refractive power (power) that satisfies conditional expressions (2) and (3), a predetermined optical performance can be maintained while reducing optical performance due to environmental changes. is effectively prevented.

In this embodiment, in order to maintain a long back focus SK and achieve a compact lens system, the focal length of the first group is set to f, and the composite lens of the second and third groups (hereinafter also referred to as r rear group). ) is the 23rd focal length, f + <0, 23rd>O...(7)
I try to satisfy the formula.

At this time, if the principal point interval between the first group and the rear group is e, the back focus SK of the thin type is SK=(1e/f,).tz3''(s).

In this embodiment, in order to achieve a compact lens system with a long back focus SK (that is, a small principal point distance e), it is preferable that the absolute value 1f11 of the focal length of the first group is as short as possible. However, if the value of 1fI 1 becomes too small, a large barrel-shaped distortion will occur on the image plane side from the first group, and a large amount of astigmatism will occur on the over side, making it difficult to maintain good optical performance. It becomes difficult.

Therefore, in this example, 0.5<I f+ /f l<2.0 (
9) The following formula is satisfied.

The aspherical shape of the aspherical surface described above is specified as shown in FIG. That is, on the R1 optical axis, which is the paraxial radius of curvature at the apex on the optical axis 31 of the aspheric surface, from the first conjugate point side with a longer distance to the second conjugate point side with a shorter distance (from the object side to the image plane side) ) Take the H-axis perpendicular to the X-axis through the X-axis and the apex of the lens surface, and define the aspherical shape with B, C, D, and E as the aspherical coefficients + DH'+ EHlo... ( 4) It is expressed by the following formula.

In equation (4) that expresses this aspherical shape, the second term and subsequent terms give the amount of the aspherical surface, and the coefficient B of the second term is based on the third-order aspherical aberration coefficient type and the following relational expression % formula % (10) (However, N' and N are the refractive indices of the medium on the object side and image side of the lens surface.) Furthermore, the aspheric aberration coefficient type has the following change in the third-order aberration coefficient, that is, aspherical This brings about the amount of change in third-order aberration caused by this.

However, ■ represents a spherical aberration coefficient, ■ represents a coma aberration coefficient, ■ represents an astigmatism coefficient, and ■ represents a distortion aberration coefficient.

Also, h and h are the paraxial ray tracing amounts shown in Fig. 4, and h
represents the height (distance from the optical axis) at which the ray that travels along the optical axis and forms an image on the optical axis (paraxial ray 11) intersects each lens surface, and h represents the distance from the oblique direction to the lens surface. It shows the height at which a ray (li paraxial ray It2) that enters and passes through the center of 11 (aperture sp) (the position where the central ray of the off-axis ray intersects the optical axis) intersects each lens surface.

Now, in order to effectively correct the barrel-shaped distortion aberration and excessive astigmatism that occur in the first group with an aspheric surface, ΔV in equation (11) representing the amount of change in the third-order aberration coefficient is AV = hh "I'<O----(12) Δ
■ or Δm=h” b2v>o ・・・・・・・・・(1
3).

Here, in the first group, h>o and h<o! 〉0 ・・・・・・・・・・・・・・・
...(14) may be used. From equation (10), what is the third-order aspherical aberration coefficient type of the aspherical lens in the first group? -8(NALI-1)・(BAL11 -BAL1
2) ...(15) (where NALI is the refractive index of the material of the aspherical lens).

In addition, in order to maintain a long back focus SK and achieve a compact lens system, the absolute value of the focal length of the first group is 1f11, and the focal length of the third group is 3〉0.・・・・・・・・・・・・・・・
In a retrofocus lens that satisfies (16), the shorter the focal ratio j11f3, the longer the back focal length and a more compact lens system can be achieved.

However, if the focal length f3 becomes too short, a large amount of under spherical aberration will occur on the image side in the third group.
Correction becomes difficult. The spherical aberration occurring in the third group is well corrected by arranging at least one aspherical lens in the second group as described above.

In order to effectively correct the under spherical aberration that occurs in the third group with an aspheric surface, the amount of change in the third-order aberration coefficient is shown (11
), ΔI is ΔI=h”!'<0 ・・・・・・・・・(1
7).

From equation (10), the third-order aspherical aberration coefficient for the aspherical lens in the second group! teeth! -8 (NAL2-1)・(BAL21- BAL22
) ...(18) (where NAL2 is the refractive index of the material of the aspherical lens).

Therefore, in the present invention, when providing aspherical lenses in the first and second groups, the aspherical surfaces are set as follows, taking into account the above equations (17) and (18), to ensure a predetermined focus of y < However, a projected image with good optical performance is obtained.

That is, the first group includes an eleventh aspherical lens having an aspherical surface on both the first conjugate point side and the second conjugate point side, and the second group has an The 21st aspherical lens has an aspherical surface on both lens surfaces on the side of the 2nd conjugate point, and the 4th order aspherical surface of the aspherical surface on the side of the 1st conjugate point and the side of the 2nd conjugate point of the 11th aspherical lens is provided. Each spherical coefficient is BALI 1. BALI
2. The fourth-order aspherical coefficients of the aspherical surfaces on the first conjugate point side and the second conjugate point side of the 21st aspherical lens are determined by BAL21. B
When setting AL22, BAL 12< BAL 11 ---
(5a) B A L 21 < B A L 22
The following condition (6a) is satisfied.

Alternatively, the first group is an eleventh group with an aspherical surface provided on the first conjugate point side.
It has an aspherical lens, and the second group has a first conjugate point side and a second conjugate point side.
A 21st aspherical lens is provided with an aspherical surface on both lens surfaces on the conjugate point side, and the 4th order aspherical coefficient of the aspherical surface on the 1st conjugate point side of the 11th aspherical lens is BAL11, 21 Aspherical surface 4 on the first conjugate point side and second conjugate point side of the aspherical lens
Each of the following aspherical coefficients is BAL21. When BAL22, 0<BALI 1
... ... (5b) BAL21<BAL22
...... (6b) The following condition is satisfied.

In addition, in order to obtain good optical performance over the entire screen in the present invention, from the object side, the first group is a meniscus-shaped positive lens with a convex surface facing the object side, and a meniscus-shaped negative lens with a convex surface facing the object side. The second group consists of three positive lenses, the second group consists of one positive lens, the third group consists of a negative lens, a meniscus positive lens with a convex surface facing the image plane, and a positive lens. It is better to configure it with three lenses.

Further, in the present invention, when the center of the off-axis light flux is taken as the principal ray, the angle θ between the principal ray of the maximum off-axis light flux emitted from the lens surface on the image plane side and the optical axis is θ<20 degrees. Each element is set. That is, when the height from the optical axis of the principal ray of the maximum off-axis luminous flux emerging from the lens surface on the image plane side is h, the maximum image height on the image plane is D, and the back focus is SK (D
-h)/SK<0.364 (5C) is satisfied. As a result, a predetermined telecentric system is obtained.

Next, numerical examples of the present invention will be shown. In numerical examples R
i is the radius of curvature of the i-th lens surface in order from the object side, D
i is the i-th lens thickness and air distance from the object side, Ni
and νi are numbers corresponding to the refractive index of the glass of the i-th lens in order from the object side.

Note that the aspheric coefficient is expressed based on the above-mentioned equation (4).

In addition, the aforementioned conditional expressions (1), (2), (3),
(5a), (5b).

Table 1 shows the relationship between (5c), (6a), (6b) and numerical examples.

Numerical Example I F-92,7FNo=1:4.6 2ω-60,9
゜R1-71,15D I-5,37N 1-1.79
19111-25.7R2-119,8302-4,0
082-1,69979ν2-55.5R3-34,7
203-9,88 R4-134,4204-4,00N 3-1.493
75 v 3-57.4R5-145, 3005-57
,75 R6=1913.49 06-3.00 N 4-1
．． 49375 v 4-57.4R7-664,750
7-35,46 R8--85,9808-5,00N 5-1.812
65shi5-25.4R9--1914,4309-0,
61RIO-419, 57DIO-5, 8486-1,
69979 Jiro 55.5R11--62,30Dll
-0,20R12-7065,46Dl2・5.41
N 7-1.69979 Shear 55.5R13--9
1,27, aspheric coefficient R4 surface B4-6.445x 10-' C4-2
,324x 10-”D4--3.611xlO-12
E4-2.252 xlO-”R5 side B5-5.6
58x 1G-' C5-2,678x 10-'D
5--5.922x 10-” R5-2,697x
10-"R6 side 86-2.923X 10-'
6-7.218 x 10-” D6 Maro-1,465
X10-” E6--2.667 Xl0-1
4R7 side B7-4.681x 10-' C7-
8,221x 10-'D7--1.444X 1G-
"R7-1,708X 10-" Numerical Example 2 F=94.4 FNo-1:4.6 2ω-
60,0°R1-406,18D I-4,0081-
1,49375ν]-57,4R2-2292,66
02-0,20 R3m 170.63 D 3-4.00
N 2-1.77621 v 2-49.
684- 39.16 D 4-15.59R5-1
02,2005-6,70N 3-1°85501ν3
-23.9R6-352,35D 6-44.14R
7=-1322,39D 7-3.00N
4J, 49375 v 4-57.4R8-4
56,8808136,53 R9= -86,7109-5,00N 5-1.81
265 v 5-25.4RIO-794,360IG
-0,99R11=-497,15Dll-5,53N
6-1.69979 v 6-55.5R12--6
6,25012-0,20R13-394,22C13
-6,3387-1,69979 sear 55.5R14
--99,92 Aspheric coefficient R1 surface B! -9,549x IP' Cl-2
, 138x 10-'.

Dl-2,870x 10-” El-7,951x
10-” R7 side B7- 2.375x 1
o-7C7--3,144X 10-"D7--4.5
85x 10-”R7-1,248x 10-”R
8 sides B8-3.988x lo-' C8-3,
102x tO-'D8--2.801X 10-"
R8-9,074 By correcting the present invention, it is possible to achieve a retrofocus type lens having high optical performance and having a long back focus and suitable for a projection lens for a color liquid crystal projection television.

Further, according to the present invention, by setting the refractive power of the aspherical lens provided with the aspherical surface as described above, it is possible to effectively prevent focus shift due to temperature changes when the lens is constructed from a plastic molded lens such as acrylic. It is possible to achieve a retrofocus lens having features such as the ability to reduce the weight of the entire lens system.

[Brief explanation of the drawing]

1 and 2 are cross-sectional views of a lens according to numerical example 1°2 of the present invention, FIGS. 3 and 4 are explanatory diagrams regarding the aspherical shape, and FIGS. FIG. 3 is a diagram of various aberrations on the image plane side at a projection magnification of −3/100 times in Numerical Examples 1 and 2. FIG. FIG. 7 is a block diagram of a conventional color liquid crystal projector. In the figure, LFL is a retrofocus type lens, S is a screen, F is a projected image, ■ is a first group, ■ is a second group, ■ is a third group, 1 is a light source, 2a, 2b. 2c is a liquid crystal display element, 3a and 3b are mirrors, 4 is a red-reflecting dichroic mirror, 5 is a green-reflecting dichroic mirror, 6 is a blue-transmitting dichroic mirror, 7 is a projection lens, ΔS is a sagittal image plane, and ΔM is a meridional image plane. be. Patent Applicant: Canon Co., Ltd.■-------- 111'-1111'-1111 Figure 3

Claims (3)

[Claims] (1) It has multiple lenses, and the first, second, and third groups are arranged in order from the first conjugate point side with the longest distance between the widest lens interval and the next widest lens interval. Divided into lens groups, where fT is the focal length of the entire system, and SK is the distance from the lens surface on the shorter second conjugate point side to the second conjugate point, SK>0.7fT is satisfied, and A retrofocus type lens characterized in that each of the first group and the second group includes at least one aspherical lens having at least one aspherical lens surface. (2) When the focal lengths of at least one aspherical lens provided in the first group and the second group are respectively fAL1 and fAL2, the following conditions are satisfied: |fAL1|>3fT |fAL2|>3fT The retrofocus type lens according to claim 1. (3) The first group includes an eleventh aspherical lens having an aspherical surface on both the first conjugate point side and the second conjugate point side; It has a 21st aspherical lens in which both lens surfaces on the second conjugate point side are provided with aspherical surfaces, and the fourth order of the aspherical surfaces on the first conjugate point side and the second conjugate point side of the 11th aspherical lens is provided. The aspherical coefficients are BAL11 and BAL12, respectively.
, the fourth-order aspherical coefficients of the aspherical surfaces on the first conjugate point side and the second conjugate point side of the 21st aspherical lens are BAL21 and BAL, respectively.
3. The retrofocus lens according to claim 1, wherein the retrofocus lens satisfies the following conditions when BAL12<BAL11 BAL21<BAL22. (4) The first group includes an eleventh aspherical lens having an aspherical surface on the first conjugate point side, and the second group has lens surfaces on both the first conjugate point side and the second conjugate point side. It has a 21st aspherical lens provided with an aspherical surface, and the fourth-order aspherical coefficient of the aspherical surface on the side of the first conjugate point of the 11th aspherical lens is BAL11, and the first conjugate point of the 21st aspherical lens is The retrofocus according to claim 1 or 2, characterized in that, when the fourth-order aspherical coefficients of the aspherical surfaces on the side and the second conjugate point side are respectively BAL21 and BAL22, the following conditions are satisfied: 0<BAL11 BAL21<BAL22 type lens.