JPS61180213A - Compact projection lens - Google Patents

Abstract

PURPOSE:To obtain a compact projection lens by setting the ratio of the focal length of the 1st lens and the focal length of the 2nd lens to a small value. CONSTITUTION:The projection lens consists of the 1st lens L1 which has positive refracting power, the 2nd lens L2, the 3rd lens L3 with negative refracting power which is arranged closely to a CRT and serves as a field flattener, and an optical transparent medium L4 which fills the gap between the 3rd lens L3 and the face plate of the CRT successively from a screen side; and the lens is compacted by decreasing the value obtained by dividing the focal length f1 of the 1st lens L1 by the focal length f2 of the 2nd lens L2 through paraxial calculation. When the f1/f2 exceeds 1.1, i.e. an upper limit, the value is disadvantage to the compacting of the lens and when the value decreases below 0.8 as a lower limit, the value is advantages to the compacting, but it becomes hard to compensate the coma aberration on condition that the optical power of the 1st lens L1 is too large and the refracting power of the CRT-side surface can not be utilized for aberration compensation effectively.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a compact projection lens with good contrast suitable for a video projector that enlarges and projects a CRT image onto a screen.

Conventional video projectors use lenses to enlarge and project the images of blue, red, and green CRTs, each of which has a narrow spectral width, onto a screen, and the lenses do not require achromatic correction. In addition, as a result of the remarkable progress in plastic molding technology, it has become possible to form aspherical surfaces at low cost and with high precision. A simple projection lens consisting of a third lens that acts as a flattener has been proposed. CR for this kind of lens
There was a problem in that mutual reflection occurred between the T face plate and the opposing surfaces of the third lens, resulting in a decrease in contrast. To counter this, the gap between the CRT face plate and the third lens is
Projection lenses are known in which a method of filling an optically transparent medium with a refractive index approximately equal to each refractive index of the lens, such as a liquid such as ethylene glycol, silicone gel, etc., is used to obtain images with good contrast. (Japanese Patent Application Laid-Open No. 59-219709).

Problem 1 that the invention attempts to solve! This conventional projection lens has the advantage of being simple in construction and producing images with good contrast, but lacks consideration for compactness.

For example, the distance (L) from the front surface of the first lens to the imaging plane
The value (L/f) divided by the focal length [f] of the entire system is called the telephoto ratio.
9, L/f=1°6. Recently, there has been a strong demand for video projectors to be made more compact.
As one means of achieving this, it has become necessary to make lenses more compact.

In consideration of these points, the present invention provides a projection lens that takes advantage of the simplicity of the conventional three-lens structure and achieves compactness by appropriately selecting optical parameters.

Means for Solving the Problems The present invention solves the above problems by changing the focal length of the first lens to f4. When the focal length of the second lens is f2,
By setting the ratio f, /f2 of the two focal lengths to a small value, the projection lens can be made more compact.

A 16th page lens having a positive refractive power, a second lens having a positive refractive power, and a third lens having a negative refractive power serving as a field flattener disposed close to the CRT in order from the working screen side; In a projection lens made of an optically transparent medium that fills the gap with the faceplate, paraxial calculations show that the compactness of the lens can be reduced by reducing the value obtained by dividing the focal length of the first lens by the focal length of the second lens. It can be seen that it is possible to achieve Good aberration correction can be achieved by introducing aspheric surfaces into at least the first and third lenses, appropriately setting optical parameters, and making the shape of the first lens a meniscus with positive refractive power. Note that since the gap between the CRT and the third lens is filled with an optically transparent medium, appropriate optical parameters can be determined within the constraints that the refractive power of the surface of the third lens on the CRT side cannot be effectively used for aberration correction. settings are being made.

Embodiment FIG. 1 is a block diagram showing an embodiment of the projection lens of the present invention. In Figure 1, Ll is the first lens with positive refractive power and has an aspherical surface with the convex surface facing the screen, L2 is the second lens with biconvex positive refractive power, and L3 is the aspherical concave lens with the concave surface facing the screen. The third lens L4 having a negative refractive power is an optically transparent medium that fills the gap between the third lens L3 and the face plate (L5) of the CRT, and the refractive index of the medium L4 is It has a value almost close to each refractive index of L5, and contributes to obtaining a high-contrast image by removing unnecessary reflections that occur between the opposing surfaces of L3 and L5. On the other hand, the disadvantage is that the refractive power of the CRT-side surface of the third lens is zero or very weak, and this surface cannot be used effectively for aberration correction.

The optical power of the entire lens system is almost determined by the first lens L1 and the second lens L2, and the composite lens of the third lens L3, the transparent medium L4, and the face plate L5 of the CRT serves as a field planner. In order to perform good aberration correction with a three-element projection lens that is bright with an F number of approximately 3 or less and a half angle of view of 20° to 3o0, at least the first and third lenses must include an aspheric surface. . In addition, selection of optical parameters is important in order to obtain good optical performance and realize compactness. The projection lens of the present invention satisfies the following conditions, assuming that the focal length of the first lens is f4 and the focal length of the second lens is f2.

o, a(f /f (1, 1-......-(1)
Condition (1) relates to the distribution of optical power between the first lens L1 and the second lens L2, and if the upper limit is exceeded, it is disadvantageous for compactness. If the lower limit of condition (1) is exceeded, it is advantageous for compactness, but the optical power of the first lens L1 becomes too strong and the refractive power of the CRT side surface of the third lens L3 is not effective for aberration correction. It is difficult to correct coma aberration under the restriction that it cannot be used.

Further, the shape of the first lens L1 is desirably meniscus for good correction of on-axis and off-axis aberrations.

If the first lens L1 is plano-convex or biconvex, it becomes more difficult to correct off-axis aberrations even if an aspherical surface is used.

Furthermore, in order to realize a lens that is compact and has good optical performance, it is desirable that the following conditions be satisfied.

o, 6(f/f, (0,9... (2)
0.6 <f / f2 (0, 8... (3)
-1,45< f/f5(-1,o...
(4) 0.26(d, /f(0,46...
(5) Here, f: Ll, L2. L3. L4. Focal length f5 of the entire lens system of L5: third lens L3. Synthetic focal length dA of transparent medium L4 and CRT face plate) L5: The distance condition (2) between the second lens L2 and the third lens L3 on the optical axis is related to the distribution of the optical power of the first lens L1, Exceeding the upper limit is advantageous for compactness, but it becomes difficult to correct off-axis aberrations, and exceeding the lower limit is disadvantageous for compactness. Condition (3) relates to the distribution of optical power of the second lens L2, and if the lower limit is exceeded, it is advantageous for compactness, but the refractive surface on the CRT side of the third lens L3 cannot be used effectively for aberration correction. Under these constraints, it is difficult to correct astigmatism. Exceeding the upper limit of condition (3) will be disadvantageous for compactization. Condition (4) relates to the distribution of optical power of the third lens L3, and if the upper limit is exceeded, correction of the Petzval image plane will be insufficient. If the lower limit of condition (4) is exceeded, the correction of the Petzval image surface becomes excessive, and the curvature of the screen-side surface of the third lens L3 becomes too steep, making it difficult to manufacture. If the upper limit of condition (6) is exceeded, the thickness of the transparent medium L4 becomes too thin, causing practical problems such as the heat dissipation effect of the CRT. If the lower limit of condition (6) is exceeded, it becomes difficult to correct off-axis aberrations.

Furthermore, the CRT side surface of the third lens L3 cannot be expected to have any refractive power because it is adjacent to a transparent medium having approximately the same refractive index, so making it a flat surface improves the economic efficiency of production. If the thickness of the periphery of the third lens L3 becomes too thick and the shape becomes difficult to process, the CRT side surface of the third lens L3 should be shaped to be concave toward the screen side, that is, the third lens should be shaped so that it has a meniscus of 11 -/ It is possible to improve workability by forming a shape having negative refractive power to reduce the thickness at the peripheral portion. This is the third lens L
By taking advantage of the fact that the refractive power of the CRT side surface of No. 3 is weak, this can be achieved without adversely affecting aberrations.

Hereinafter, specific examples 1 to 6 of the present invention will be shown.

In the tables of these examples, f is the focal length of the entire lens system, ω
is the half angle of view, β is the projection magnification, f, , f2 are the focal lengths of the first lens L and second lens L2, respectively, f3 is the combined focal length of the third lens L3, medium L4, and GRT face plate, and L is the combined focal length of the third lens L3, medium L4, and GRT face plate. The distance from the front surface of the third lens L3 to the rear surface of the CRT face plate L6, that is, the imaging plane is shown. In addition, r, , r... are counted sequentially from the screen side, and the radius of curvature of the valley lens surface, dl, d2...
・ is the center thickness and air spacing of each lens, n, + n
2, . . . indicate the refractive index of each lens for the e-line. Furthermore, the lens surface marked with * indicates an aspherical surface, and the aspherical shape of the projection lens according to the present invention has an optical axis direction of X, a Y axis perpendicular to the X axis, and a vertex curvature of C( =1/r), conic constant k, aspheric coefficient AD,
When AE, AF, and AC are used, it is expressed by the following formula.

10AD-Y'+AE-Y +AF-Y +AC-Y"
Example 1 f=107.0920mm F number: 272β=
8 to = 27.6° f/f, = 0
．． 689 Quantity/f2=O, 46 f/f5=
-1,305f2/f, :081d,/f20.373
L/f = 49ψ ψ d = 50.32 d = 39.92 13, -.

Small (margin below) 14 pages Example 2 f = 107.6410 rrtm, F number: 271° β = 8. ω227.5, f/f, 20 completed 04. f/f...

(1), 727, +7'f3= -1,238, f
,,/f,==032.

d, /f-0,355, L/f = 49d2shi2
．． 90 (blank space below) 17, - Example 3 f2 107.2716 cylinder, F number: 272° β = 8
．． ω227.4°, 1/amount, :0.73B, f/f
2=0.709, f/f,=-1,282, f
2/amount, =0.962°d, /f==0.328,,
L/f: 47d2:=53.34 d2 35.18 (margin below) 1B...

19 Cage Example 4 f = 106.7317 岨, F number = = 273°β
=8. ω=27.5°, f/fl-o, 767,
f/f2=0.699, f/f3 knee 345, f2
/f, = 0.923°d4/f-0,318,L/f-
1,46d,, =52.97 d =33.99 (margin below) 21 page Example 5 f = = 106.1668 key, F number:, 274.

β=81ω=27.4°, f/f,=o, 7y5. f/
f2=0.681, amount/f3==-1,388
, f2/f1=0.879. dA/f-0,306,L
/f-1,44d2:52.63 (hereinafter referred to as margin) 23,, Example 6 f2 106.1220 mark, F number = 275° β = 8
．． ω==27.3°, f/f,=o, 7s2. f/pass=
0.682, f/f3=-1,402, f2/f,:
0.872, a4/f=o, soo, L/f:
43d2=586 d=383 Below Margin 26 Beno Figure 2 is for Example 1, Figure 3 is for Example 2, Figure 4 is for Example 3, Figure 6 is for Example 4, and Figure 6 is for Example 4. The figure shows Example 5.
FIG. 7 shows various aberration diagrams of Example 6. In the diagram of astigmatism,
m indicates the field curvature in the meridional direction, and 6 indicates the field curvature in the sagittal direction.

As is clear from the above aberration diagrams, the compact projection lens according to the present invention has well corrected aberrations and has good imaging performance. Also, the telephoto ratio L/f is 43 to 4.
9, which is significantly more compact than the conventional example 6 of JP-A No. 759-219709. In addition,
Although the second lens L2 in each embodiment is a spherical lens, it goes without saying that it may be an aspheric lens to further improve performance. In addition, in order to realize an even thinner projection lens,
The second lens L2 having a large optical power may be divided into two or more lenses.

Effects of the Invention As described above, according to the present invention, optical 26,
.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a projection lens according to an embodiment of the present invention, and FIGS. 2 to 7 are diagrams showing various aberrations for the embodiment of the projection lens according to the present invention. Ll...First lens, L2...Second lens, L3...Third lens, L4...Transparent medium, L5...CRT face plate. Name of agent: Patent attorney Toshio Nakao and 1 other person L3
--Ichiku 3rd switch L4-K1 stub LH--cRpy Husamu 1 2nd figure #Jq's lungs left 3rd figure tsubo side left non-7 times, income F1 Figure 4 Figure 5 Spherical surface qsu9 Paksis A Long curve Figure 6 Sphere q7A Astigmatism A Side angle and Figure 7

Claims (1)

[Scope of Claims] (1) A first lens that is used in a television image projection display device and that has a positive refractive power and has an aspherical surface with a convex surface with a large curvature facing the screen side in order from the screen side, and a biconvex positive lens. A second lens having a refractive power, a third lens having a negative refractive power with an aspherical concave surface facing the screen, a CRT, and an optically transparent medium filling a gap between the third lens and a face plate of the CRT. , a compact projection lens characterized in that the refractive index of the transparent medium is approximately equal to the refractive index of the third lens and the face plate, and the following conditions are satisfied. 0.8<f_1/f_2<1.1 (where f_1: focal length of the first lens f_2: focal length of the second lens) (2) The first lens is characterized by having a meniscus shape with positive refractive power. A compact projection lens according to claim 1. (3) A compact projection lens according to claim 1 or 2, wherein the third lens has a flat surface on the CRT side. (4) A compact projection lens according to claim 1 or 2, wherein the CRT side surface of the third lens is a spherical concave surface having a gentle curvature toward the screen side. (5) The focal length of the first lens, the second lens, and the combined focal length of the third lens, the transparent medium, and the face plate of the CRT satisfy the following conditions. The compact projection lens described in item 2. 0.6<f/f_1<0.9 0.6<f/f_2<0.8 -1.45<f/f_3<-1.0 (where f: first lens, second lens, third lens lens,
Focal length f_3 of the entire lens system including the optically transparent medium and the faceplate of the CRT: composite focal length of the third lens, the optically transparent medium and the faceplate of the CRT) (6) The distance between the second lens and the third lens The compact projection lens according to claim 1 or 2, characterized in that the following conditions are satisfied when the air spacing on the optical axis is d_4. 0.25<d_4/f<0.45