JPS6111716A - Projecting lens - Google Patents

Abstract

PURPOSE:To realize a projecting lens having a wide field angle by providing the 1st aspherical face to the 1st lens, providing >=1 face of aspherical face to the 2nd-4th lenses and satisfying the specific conditions. CONSTITUTION:The projecting lens is provided with one aspherical face to the 1st lens and >=1 face of the aspherical face to the 2nd-4th lenses and is constituted to satisfy the conditions 0.75<phi2.3/phi<0.90, 0.4f<D2<0.6f, -300F<R3 <-5F, 0.45<R5/R6<0.85 where the focal length of the entire system is designated as F, the refracting power of the entire system as phi, the combined refracting power of the 2nd lens and the 3rd lens as phi2.3, the radius of curvature on the screen side face of the 2nd lens as R3, the radius of curvature on the screen side face of the 3rd lens as R5 and the radius of curvature on the original picture image side face of the 3rd lens as R6. Not only the comatic aberration but also the spherical aberration, curvature of field and distortion are thus satisfactorily corrected and the high-performance projecting lens having the larger aperture diameter and wider field angle is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a projection lens, and more particularly to an enlarged projection lens suitable for enlarging and projecting an image projected on an electronic imaging device onto a screen and capable of realizing a large screen. .

In general, projection lenses for displaying images use monochromatic cathode tubes of three colors red, blue, and green to display color images, and each image is projected onto a screen by the projection lens, but the coloring characteristics of the three colors also differ. Since the spectral width is relatively narrow, it does not need to be an achromatic lens.

Desirable conditions for a projection lens include a large aperture and a wide angle of view. The large aperture makes it possible to obtain a bright image, and the wide angle of view makes it possible to obtain a desired projected image at a short projection distance, making it possible to downsize the entire apparatus.

Conventional projection lenses include spherical lenses with only spherical surfaces and aspheric lenses with aspherical surfaces, but with spherical lenses, it is extremely difficult to reduce the number of constituent elements and improve performance, so in recent years, non-spherical lenses that incorporate aspherical surfaces have been used. Spherical lenses are the mainstream.

However, the history of a projection lens itself that uses an aspheric surface to correct aberrations is long, and British Patent No. 593,514 is known. The projection lens disclosed in this patent is a composite lens consisting of a biconvex lens and a biconcave lens, starting from the image side, and has an aspherical surface on the first surface, and a first group that corrects aberrations that mainly depend on the aperture and axial chromatic aberration; 2 with the convex side facing the screen
It is composed of a positive second group consisting of two plano-convex lenses, and a third group having negative refractive power for image plane flattening means. According to this configuration, among the aberrations of spherical aberration, field curvature, and coma aberration, the first group lens corrects spherical aberration and coma aberration, and the third group corrects field curvature and distortion aberration. The correction of spherical aberration and coma aberration corrected by the group is insufficient, and the correction of coma aberration is particularly poor.

This results in a flare that impairs the imaging performance, making it difficult to provide a projection lens with a wide field of view of 25° or more.

An object of the present invention is to realize a wide-angle projection lens, in which coma aberration and off-axis aberration, which affect image quality, are primarily corrected. As will become clear from the description below, correction of comatic aberration and off-axis aberration is achieved by the action of meniscus lenses whose concave surfaces are directed in opposite directions.

The projection lens according to the present invention shown in FIGS. 1 to 4 is
In order from the screen side: a first lens with positive refractive power, a second lens with positive refractive power with a convex surface that has a larger curvature than the side surface of the screen facing the original image side, and a second lens with positive refractive power facing the image side. It consists of a meniscus third lens, a fourth lens with a concave surface facing the screen side and having a larger curvature than the surface on the original image side, the first lens has at least one aspherical surface, and the second lens , the fourth lens is provided with one or more aspherical surfaces, F is the focal length of the entire system, φ is the refractive power of the entire system,
φ28 is the combined refractive power of the second lens and the third lens, Ra is the radius of curvature of the side surface of the screen of the second lens, R6 is the radius of curvature of the side surface of the screen of the third lens, and R6 is the radius of curvature of the side surface of the original image of the third lens. When, (1) 0.75<φ2, 3/φ<0.90(2)
0.4 t<D, <0.6f(3) -300F<
R, <-5F (The negative sign is concave toward the screen) (4) 0
゜45 <R, /R6〈o, s 5 conditions are satisfied.

Next, the effects of the above-described configuration will be described. The first lens has one or more aspheric surfaces in order to correct aberrations that mainly depend on the aperture, and the second lens has one or more aspheric surfaces to correct off-axis aberrations, especially coma, as well as to minimize the occurrence of halos that cause flare. It is a meniscus lens that mainly has positive refractive power for image formation, and the third lens is a meniscus lens that has at least one aspherical surface and has a concave surface facing the picture tube and has weak positive M refractive power. In order to suppress the occurrence of aberrations in off-axis rays as much as possible, the lens has a shape similar to that of a centripetal or concentric ray, and provides excellent correction of aberrations that depend on the angle of view, off-axis ray aberrations, and especially coma aberration. , the fourth lens has a concave surface with a curvature hole facing the screen side, which causes angle-of-view dependent aberrations, especially how many curvatures the image plane has,
This corrects distortion aberration.

That is, the first lens corrects spherical aberration, comatic aberration, field curvature, and distortion, the third lens corrects comatic aberration, and the fourth lens corrects field 9 curvature and distortion. In particular, the meniscus-shaped lens arranged as the third lens effectively corrects coma aberration, and by making it concentric or close to concentric, the occurrence of axial aberration is suppressed as much as possible, resulting in extremely high performance. has been realized.

Furthermore, the meaning of the extreme value of the four conditional expressions is as follows.

Condition (1) relates to the combined power of the second lens and the third lens, and when the lower limit is exceeded, the power is shared by the first lens, making it difficult to correct spherical aberration. When the upper limit is exceeded, off-axis aberrations caused by the second lens and the third lens, mainly the second lens, become large and correction becomes difficult.

For example, m2. When the positive power of the Kuwa 3 lens increases, the negative power of the 4th lens must be increased to correct the curvature, which increases the occurrence of aberrations such as distortion and improves performance. It comes as a troublesome thing.

Condition (2) relates to the distance between the first lens and the second lens, and when the lower limit is exceeded, the imaging power of the off-axis light beam is insufficient, and the distance between the second lens and the second lens is exceeded. Third. This increases the burden of imaging the off-axis light beam on the fourth lens, which becomes a major obstacle to widening the angle of view. When exceeding the upper limit,
The angle of incidence of the off-axis light beam on the surface of the second lens, that is, on the picture tube side, becomes large, and the amount of aberration and difference generated in the off-axis light beam becomes large.

Condition (3) is related to the radius of curvature of the front surface of the second lens. When the lower limit is exceeded, that is, when the surface becomes convex, the difference in refractive power between the upper convergence and lower convergence of the off-axis light east becomes large, and the aberration increases. In particular, the occurrence of halo, which is the cause of flare, increases.When the upper limit is exceeded, the negative refractive power increases, so the positive refractive power of the rear surface of the second lens, or the positive refractive power of the first and third lenses increases. The refractive power of the lens must be increased, which increases the occurrence of spherical aberration, comatic aberration, etc., making it difficult to achieve a large aperture and wide angle of view.

Condition (4) concerns the power sharing ratio between the front and rear radii of curvature of the third lens and the degree of meniscus; when the lower limit is exceeded, the power of the third lens increases, causing aberrations of axial and off-axis rays. The amount increases, and the main purpose of correcting coma aberration becomes an inner circle. If the upper limit is exceeded, the power of the third lens becomes very weak, the burden of power on the second lens increases, the amount of aberration generated by the second lens increases, and aberration correction becomes difficult.

The lens data of the example will be described below.
(,... is the curve radius of each lens surface, D,, D,...
is the wall thickness or air gap between lens surfaces, Nl * Nt...
・E-line of each lens (wavelength 54G, 1 nm light)
refractive index for ν1. . . . is the Atsbe number for the e-line.

The shape of an aspherical surface is defined as the displacement in the optical axis direction in rectangular coordinates with the optical axis direction as the X axis.
-)-C/)(7-1-DI (It is a symmetric aspherical surface filled with @.

However, ■I: Height from the optical axis R: Radius of curvature of the apex A, B, C, D, E, A', B', C', D':
The aspherical coefficient of liquid and P represent the tube glass of the picture tube, respectively.

FIG. 5 to FIG. 8 sequentially show the aberration correction states of Examples 1 to 4. M is a meridional image surface, and S is a sagittal image surface.

Rororo ρ000rororororo! ! phi
II II-1□CI') + 1/)ψto■■0ρρ
Rororo (aOρ口ｑ笽Mr. = 眞view - 朞国国坑) As a result, not only coma aberration, which is a major cause of performance deterioration, but also spherical aberration, field curvature, and distortion can be well corrected, and Fl , 2 or more and a half angle of view of 32 degrees or more, a high-performance projection lens with a large aperture and a wide angle of view.

[Brief explanation of the drawing]

1, 2, 3, and 4 are cross-sectional views of lenses according to embodiments of the present invention, respectively. FIG. 5, FIG. 6, FIG. 7, and FIG. 8 are aberration curve diagrams of Examples, respectively. In the figure, ~L is the lens, R is the radius of curvature of the lens surface, D
is the surface spacing. Applicant: Canon Co., Ltd., 1'Yu゛'zo, ÷ 1 Agent: Gi Marushima -・'Mi■Bay1...C2・S33D 4 Nibuk-myeon AL en 琲Point M臘5M Forehead

Claims (1)

[Claims] (1) In a projection lens for enlarging and projecting an original image onto a screen, in order from the screen side, a first lens having positive refractive power, which has a larger curvature than the side surface of the screen; A second lens with positive refractive power with a convex surface facing the original image side, a meniscus third lens with a concave surface facing the image side, and a screen with a concave surface with a larger curvature than the original image side. It consists of a fourth lens facing toward the side, the first lens is provided with at least one aspherical surface, the second to fourth lenses are provided with one or more aspherical surfaces, and F is defined as the focal length φ of the entire system. The refractive power of the system, φ_2_,_3 is the combined refractive power of the second and third lenses, R_3 is the radius of curvature of the side surface of the screen of the second lens, R_5 is the radius of curvature of the side surface of the screen of the third lens, and R_6 is the third lens. When the radius of curvature of the side surface of the original image is (1) 0.75<φ_2_, _3/φ<0.90(2)
0.4f<D_2<0.6f (3) -300F<R_3<-5F (the negative sign is concave toward the screen) (4) Characterized by satisfying 0.45<R_5/R_6<0.85 projection lens.