JPS63169609A - Projection lens - Google Patents

Abstract

PURPOSE:To improve aberration compensation and to project an image with good contrast by constituting 1st and 3rd lenses of a projection lens which projects an image of a CRT on a screen by using plastic and constituting 2nd and 4th lens by using glass, and specifying the air gaps and focal lengths of those lenses. CONSTITUTION:The 1st lens L1 which has an aspherical surface with positive refracting power and has its convex surface on a screen side, the 2nd biconvex lens L2 which has positive refracting power, the 3rd aspherical lens L3 which has negative refracting power, and the 4th lens L4 which has negative refracting power and has its spherical concave surface on the screen side. Those lenses L2 and L3 are made of plastic and the lenses L2 and L4 are made of glass. Further, the relation between the gap distance d4 between the lenses L2 and L3 along the optical axis and the distance d6 between the lenses L3 and L4 along the optical axis is set that 0.5<d4/d6<1.5. Furthermore, the relation among the focal length f1 of the lens L1, the focal length f3 of the lens L3, and that of the total lens system meets specific requirements.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a projection lens suitable for a bright, high-contrast 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 aberration correction. Furthermore, as a result of the remarkable progress in plastic molding technology, it has become possible to manufacture aspherical lenses at low cost and with high precision. The result is a projection lens that is brighter, has a wider angle of view, and has fewer lenses. As a projection lens using an aspherical lens, a simple projection lens consisting of three lenses (for example, JP-A-55-1241) No. 4, JP-A-Sho 5
7-34515, JP 57-108818, JP 58-125007) and a projection lens composed of four lenses are known (JP 60-4931).
) issue).

Recent efforts to improve the performance of video projectors have focused on increasing contrast and brightness. As a measure to improve contrast, the gap between the CRT face plate and the opposing lens is filled with an optically transparent medium, such as a liquid such as ethylene glycol, having a refractive index approximately equal to that of the CRT face plate and lens. A projection lens is known that is filled with silicone gel or the like to prevent multiple reflections occurring between the face plate of a CRT and opposing lenses (Japanese Unexamined Patent Publication No. 59-219709 1g). Also, the brightness is improved. In response to this, there is a movement to improve the brightness of the projection lens as well as increase the input power of the CRT.

The problem that the invention tries to solve! .

Such conventional projection lenses have the advantage of being simple in construction, but since the lens elements facing the CRT are all plastic lenses with aspherical surfaces, they have poor heat resistance compared to glass. There are some aspects where it lacks durability, waterproofness, and chemical resistance.

When the video projector is operating, the CRT generates heat,
The CRT face plate becomes over 100 degrees Celsius and the CRT
The lens facing the front is exposed to high temperatures even though there is a gap. Polymethyl methacrylate is usually used as a material for plastic lenses, and the upper limit of its operating temperature is about 80°C. If the temperature exceeds this temperature, the deterioration of the optical performance due to the deformation of the plastic lens will be too great to be used for practical use. Therefore, the input power of the CRT is automatically limited, and the brightness is increased by increasing the input power to the CRT. This is an impediment to improvement.

Additionally, to improve the contrast of a video projector, the gap between the CRT and the lens facing it is filled with a liquid such as ethylene glycol. Plastic lenses have poor waterproof properties, and the problem is that the amount of liquid filled decreases over time. Furthermore, plastics lack chemical resistance, which places significant restrictions on the selection of filling liquids.

In consideration of these points, the present invention pursues an optimal combination of an aspherical plastic lens and a spherical glass lens, and has a simple structure with excellent heat resistance and waterproofness suitable for a bright and high-contrast video projector. The present invention provides a projection lens.

Means for Solving the Problems In order to solve the above problems, the present invention provides, in order from the screen side, a first lens having a positive refractive power and an aspherical surface, a biconvex second lens having a positive refractive power, and a second lens having a negative refractive power. a third lens with an aspherical surface, and a fourth lens made of glass with a negative refractive power with the concave spherical surface facing the screen, and the third lens is placed approximately midway between the second lens and the fourth lens. do.

By constructing the fourth lens facing the CRT from glass, a projection lens with excellent heat resistance and waterproof properties can be realized. The disadvantage of not being able to make the fourth lens an aspherical surface due to the fact that the fourth lens is made of glass is that the first lens and the third lens are made of aspherical lenses, and the third lens is made of glass. This can be overcome by placing it approximately in the middle, and a simple projection lens with well-corrected aberrations can be realized.

EXAMPLE Hereinafter, a projection lens according to an example of the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram showing an embodiment of the projection lens of the present invention. In Figure 1, Ll is a first lens with an aspherical surface with positive refractive power with its convex surface facing the screen side, L2 is a second lens with biconvex positive refractive power, and L3 is an aspherical surface with negative refractive power. The third lens L4 is a fourth lens made of glass with a negative refractive power with a spherical concave surface facing the screen side, and G is an optically transparent medium that fills the gap between the fourth lens L4 and the face plate P of the CRT. , for example, composed of a liquid such as ethylene glycol.

Glass lenses usually operate at 200 degrees Celsius and are highly waterproof, so by constructing the fourth lens, which is close to the CRT and in contact with liquid G, with glass, we have created a projection lens with excellent heat resistance and waterproof properties. can. 1st lens L1
is for correction of spherical aberration and coma aberration, the second lens L2 is for correction of spherical aberration and coma, the third lens L3 is for correction of astigmatism and coma, and the fourth lens L4 is for image plane correction as a field toner. Helpful. In order to perform good aberration correction with a projection lens that has a four-element configuration, has a bright F number of 1.2 or less, and a half angle of view of 20° to 30°, the first lens L1
The third lens L3 needs to have an aspherical shape. Furthermore, in order to perform very good aberration correction under the constraint that the fourth lens L4 is a spherical surface, the air distance between the second lens L2 and the third lens L3 is set to d4, and the air distance between the third lens L3 and the fourth lens L4 is set to d4. The following conditions must be satisfied when the air distance between the

0.5 <d4 /da < 1.5 -1・-...
----------・- (1) When the lower limit of condition (1) is exceeded, astigmatism correction becomes insufficient, and when the upper limit is exceeded, coma aberration correction becomes unsatisfactory. Furthermore, in order to realize a projection lens that is compact and has good optical performance, it is desirable that the following conditions be satisfied.

1.6 < f t / f < 2.5 -------
−−−−−−−−−−−−−− (21-Q, 3 <
f / f , < Q −−−−−−−−−−
−−−−−−−−−−+31 Here, fI = Focal length r3 of the second lens L2 : Focal length f of the third lens L3 : Focal length condition (2) of the entire lens system is Regarding the distribution of optical power, if the upper limit is exceeded, it is disadvantageous for compactness, and if the lower limit is exceeded, it is advantageous for compactness, but it becomes difficult to correct off-axis aberrations.

If condition (3) is exceeded, astigmatism correction will be insufficient,
If the lower limit is exceeded, astigmatism will be overcorrected.

Furthermore, in order to prevent the focal point from shifting due to temperature changes, it is desirable that the second lens L2 be made of glass. Also, the second
The shape of the lens L2 is desirably meniscus for good correction of on-axis and off-axis aberrations.

Hereinafter, specific examples 1 to 4 of the present invention will be shown.

In the tables of these examples, f is the focal length of the entire lens system, ω
Half angle of view, β is projection magnification, fl is focal length of the first lens,
f2 is the focal length of the second lens L2, f3 is the third lens L
3 focal length, f4 is the fourth lens L4, media G and C
The composite focal length of the RT faceplate P is shown. Also, 'l+'! . . . is the radius of curvature of each lens surface counted sequentially from the screen side, d,, d2 . . . is the center thickness and air spacing of each lens, nl + n! ...
... indicates the refractive index of each lens for the e-line. Furthermore,
The lens surfaces marked with * indicate aspherical surfaces, and the aspherical shape of the projection lens according to the present invention has an optical axis direction of X.
Take the Y axis perpendicular to the axis, and set the vertex distance as C (=1/r),
When the conic constant is K and the aspherical coefficients are AD, AE, AF, and AG, it is expressed by the following equation.

+AD-Y'+AE-Y'+AF-Y"+AG-Y"(
Example 1) f -1) 8,214w, F number = 1.0. β
=-8,0ω=24.5°, f + =234.8
75mm, f z = 129.2331mfs = -
514,694wm, fa = 126.217
mm (Example 2) f = 103.813 m, F number = 1.0. β
=-8,29ω =27.6°, f l
=217.454 Tobij f z=121.87
Triple layer f3 = 1215.882 yen, f4 = -104,1471 yen: l
= 132.583 d:l = 16.5
0L 2 (nz = 1.66152 r4 - 195.517 d, = 22.66 (Example 3) f = 1) 8.151 inches, F number = 1.0.
β=-8,0ω=24.6°, f,=
224.150 ■■, f 2 = 130
．． 628m f: +-662.454mm, f4=
120.810u+ (Example 4) f = 127.554 dragon, F number = 1.0.
β=-8, 64ω=23.6°, f+=246.1
261). fz=140.3141+1fz=
504.370mm, f4-133.47! ha
Figure 2 (al (b) (C) is of Example 1, Figure 3 (
a) (bl (c) is Example 2, FIG. 4(a) (
b) (C) is Example 3, Figure 5 (al(bl (C)
1 shows various aberration diagrams of Example 4.゛In the diagram of astigmatism, m
indicates the curvature of field in the meridional direction, and S indicates the curvature of field in the sagittal direction. As is clear from the above aberration diagrams, the projection lens of the present invention has well corrected aberrations and has good imaging performance.

Effects of the Invention As described above, according to the present invention, by appropriately combining a spherical glass lens and an aspherical plastic lens and appropriately selecting optical parameters, it is possible to achieve good aberration correction suitable for a bright and high-contrast video projector. A projection lens can be realized.

[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 5 are diagrams showing various aberrations of the projection lens according to an embodiment of the present invention. Ll...First lens, C2...Second lens, C3...Third lens, C4...
4th lens, G, old...transparent medium, P....CR
T face plate. Name of agent: Patent attorney Toshio Nakao, 1st Ll-Ol
Les C2 - 2nd lens C3 - 3rd lens C4 - 4th lens G - Transparent medium P" - CRT face plate Figure 1/Figure 2 - 1,00, +1 1.1) -1.ff 0
．． 0 +, 1+ -10 1). 0 5. O
X! 1. ff1I food envy Oki voice, @ difference Distortion aberration (〃) Fig. 3 -1,0α01.0 -LOO,OLD
-5,00,01,0 Spherical starvation Non-assistance difference King song similarity (phantom 4th figure -1,00,61,1+ -Lo 0.0
1.6 Spherical surface 4 is bent n is seated Figure 5 -1,00,0+,0 -1. o o, o
t, a spherical queen difference Non-round material Aifuku 0 αO engineering O' 1 own expense 4 shi difference (54 no - 10 αOio ! #I4shi, ! (5)

Claims (1)

[Scope of Claims] (1) A first lens used in a television image projection display device, having an aspherical surface with a positive refractive power, with the convex surface facing the screen in order from the screen side, and a biconvex positive refractive power. a second lens having a negative refractive power and an aspherical surface, and a fourth lens made of glass and having a negative refractive power with a concave spherical surface facing the screen side, and satisfying the following conditions. A projection lens characterized by: 0.5<d_4/d_6<1.5 However, d_4: Air distance along the optical axis between the second lens and the third lens d_6: Air distance along the optical axis between the third lens and the fourth lens ( 2) The projection lens according to claim (1), which satisfies the following conditions, where f_1 is the focal length of the first lens and f is the focal length of the entire lens system. 1.6<f_1/f<2.5 (3) When the focal length of the third lens is f_3, the following condition is satisfied.
The projection lens described in section 1). -0.3<f/f_3<0 (4) The first lens and the third lens are made of plastic, and the second lens and the fourth lens are made of glass. Projection lens described in ). (5) The projection lens according to claim (1), wherein the first lens has a meniscus shape with positive refractive power.