Projection System With a Cathode Ray Tube and a Lens

This invention relates to a projection system adapted to project an enlargement of an image appearing on a cathode ray tube (CRT).

U.S. Pat. No. 4,348,081 discloses a projection system with a high brightness by using lenses having aspheric surfaces. However, the length of the light beam progress path from the face glass of the CRT to a screen in the disclosed projection system is so long that it is not suitable for a compact projection set. In general, if a lens consists of a glass material, it produces a spherical aberration rather than a color aberration, and if it consists of a plastic material, it produces color aberration rather than spherical aberration. Since the lens in the disclosed projection system in U.S. Pat. No. 4,348,081 is aspherical, it consists of the plastic material and therefore color aberration is present. Color aberration is increased in accordance with the light beam progress path, and the color aberration is increased in the disclosed projection system.

The relationship between the focus distortion by thermal variation and the focal length or the relationship between the color aberration and the focal length are as follows. Assuming that the thickness of the lens is negligible in FIG. 1, the following equations are obtained.

where

a is the distance between lens 1 and the phosphor surface of CRT 2,

b is the distance between lens 1 and screen 3,

f is the focal length of lens 1,

r.sub.1 and r.sub.2 are curvature radii of lens 1.

N is the refractive index.

Since the characteristics of the projection system is affected less by the fluctuation for the curvature radius, which is caused by the fluctuation of the lens shape caused by the thermal fluctuation, than the fluctuation for the refractive index which is caused by the fluctuation of the lens shape, and since fluctuation caused by curvature radius may be cancelled with the fluctuation for the lens barrel if they consist of plastic materials, equation (3) is obtained from equation (2) by neglecting the terms including r.sub.1 and r.sub.1.

Deviation .delta. d for the diametered of the beam spot projected on screen 3 is defined by following equation (4).

where

F is the F number of lens 1,

M is the magnification of lens 1,

.alpha. is the common elevation angle for both the beam spot and lens 1.

From equation (1)-(4)

As appearing equation (5), d is proportioned to focal length f, therefore the aberration corresponding to diamater d of the beam spot is reduced if flocal length f is shortened.

An object of the invention is to provide a projection system with a light beam progress path of reduced length.

In accordance with the invention, there is provided from the image end a screen, a first lens in which a core portion including the lens axis accomplishes a convergence function and in which a peripheral portion including a lens edge thereof accomplishes the divergence function, a second lens comprising a convex lens surface, a third lens comprising a concave lens element, and a CRT having a face glass with an inner surface which is coated with a phosphor and being curved to form a fourth lens element with a curvature center within a core portion including the lens axis of the fourth lens that is disposed on the screen side of the projection system, the curvature radius of each part of the core portion of the fourth lens increasing in accordance with increments of the semi-aperture distance of each part from the lens axis.

FIG. 1 shows a side elevation of a general basic projection system;

FIG. 2 shows a side elevation of one embodiment of the invention:

FIGS. 3-9 show characteristics of the embodiment shown in FIG. 2;

FIG. 10 shows a side view of a projection set in accordance with the embodiment of the invention:

FIG. 11 shows a side view of another embodiment of the invention.

It is necessary to enlarge the picture angle of lens 1 for shortening focal length f without changes of magnification M and the brightness. According to the embodiment of this invention, the focal length f is shortened by 35% from that in the projection system which is trial manufactured from the data disclosed in said U.S. Pat. No. 4,348,081. Though the picture angle of the projection system in U.S. Pat. No. 4,348,081 is almost 50 degrees, that of this embodiment is almost 70 degrees.

In FIG. 2, numeral 10 denotes a lens element having surface S1 opposed to screen 3, in which a core portion 10 including the lens axis thereof accomplishes a convergence function and in which a peripheral portion 10" including a lens edge thereof accomplishes a divergence function; numeral 20 denotes a bi-convex lens element having surface S3 opposed to surface S2 of lens element 10: numeral 30 denotes a concave lens element having surface S5 opposed to surface S4 of lens element 20; numeral 40 denotes a liquid layer comprising a silican gel disposed between surface S6 of lens element 30 and surface S7 of shield glass 50 which is sealed by sealing material 42: numeral 60 denotes a cooling liquid filled between surface S8 and surface S9 of face glass 22 of CRT 2; numeral 70 denotes a diaphragm: numeral 80 denotes a lens barrel for supporting lenses 10 and 20, and diaphragm 70; and numeral 90 denotes a heat radiator. In this embodiment, lenses 10, 20 and 30 consist of plastic material.

Concerning the face glass 22, although surface S9 is plano, surface S10 on which a phosphor layer is coated is curved to cause face glass 22 to function as a lens. The curvature center 11 for the core portion 11' of surface S10 is disposed on the screen side of the projection system, and the curvature radius of each part of the core portion is increased in accordance with an increment of the semi-aperture distance of each part from the lens axis to reduce field curvature. Diaphragm 70 is used to reduce the aberration and to increase the brightness in the peripheral portion of the lens system.

Tables 1-5 shows lens data of 5 typical examples for the embodiment as shown in FIG. 2, respectively. In each table, the longitude dimension is "mm". The surface S10 is symmetrical with respect to the curvature center 11 which lies on the optical axis 11" of the system. Each point on the core portion 11' has a curvature radius which is a function of the radius of that point from the curvature center 11 with points located farthest from the curvature center having the greatest curvature radius. As is known, the curvature radius of a point is the length of the radius from the optical axis which intersects the point orthogonally on the surface S10.

In each above tables aspheric coefficients are defined by the following equation. ##EQU1## where Z is the surface sag at a semi-aperture distance r from the lens axis,

RD is the curvature radius of the lens,

CC is a conic constant or other surfaces of revolution.

Following conditions are obtained from above Tables in the case of 10 times enlarging by using a 4.5 inch CRT as CRT 2 and a 45 inch screen as screen 3.

(1) 0.9 r1<r2<1.1 r1

where r1 and r2 are curvature radii of the core portions of the scrren-side surface of lens 10 and 20, respectively, assuming that said portions of spherical.

(2) 0.2 fS<T12<0.4 fs

where

T12 is the distance between lens 10 and lens 20, fS is the focal length of all lens systems.

(3) 0.35 fS<T23<0.45 fS

where T23 is the distance between lens 20 and lens 30.

(4) 0.25 fS<t2<0.35 fS

where t2 is the thickness of lens 20.

Radius and other sizes in each example 1-5 in accordance with the above tables are as below (Table 6).

MTF (Moduration Transfer Function) in each example 1-5 are shown in FIGS. 3-7, respectively. In FIGS. 3-7, a resolution on face glass 22 is 1 pl/mm, and also in the calculation for MTFs weights of spectrums produced from phosphors are considered as below (Table 7).

FIGS. 8 and 9 show the relationship between the resolution and MTF in the core portion, and the relationship between the thermal and MTF. In both FIGS. 8 and 9 A and B are characteristics of an example in accordance with this embodiment and the trial-manufactured example in accordance with U.S. Pat. No. 4,348,081.

In each example, though the lens system maintains high brightness (F number is 0.91), it shows good MTF characteristics as shown in FIGS. 3-9. The brightness ratio in the peripheral portion in the embodiment is 30% which is that in the prior system having a small picture angle. Therefore, the projection system in accordance with the embodiment is suitable for the compact set as shown in FIG. 10, with only one mirror being used in such set.

The projection system of FIG. 10 is described as follows. The projection system has a cabinet 110 having a top side 112, a bottom side 114, a front side 116 and a rear side 118. A screen 3, also illustrated in FIG. 2, is fitted into the upper part of the front side. A mirror 91 is spaced from the screen and inclined to the screen with the lowest edge 119 of the mirror being positioned higher than the lowest edge 121 of the screen 3. The projection system 122 is comprised of a CRT 2, and a projection system 124 in accordance with FIG. 2. As is apparent from FIG. 10, the projection system 124 is mechanically coupled to the CRT 2 by a coupling mechanism. The image projected from the projection system 122 is enlarged upon imaging upon the mirror 91. Light from the upper edge 123 of the projection system 124 closest to the screen 3 falls upon the upper corner 125 of the mirror 91 and is reflected to the upper part 127 of the screen 3. Similarly, light from the upper corner 129 of the projection system 124 closest to the bottom edge 119 of the mirror 91 is reflected from the mirror 91 to the lower edge 121 of the screen 3.

If the CRT whose size is X inches is used for the present invention, the distance between surfaces may be multiplied by X/45 to the longitude values in Tables 1-5.

Lens 20 in FIG. 2 may be divided into two lenses 22 and 24 as shown in FIG. 11. In this embodiment, lens 22 is a convex meniscus lens of weak power. Lens 24 supplies the main share of the multiplication power, so it is desirable to use glass material to reduce the spherical aberration with the other lenses 10, 22 and 30 being plastic material.

Lens data of 6 typical examples for the embodiment shown in FIG. 11 are as follows in Tables 8-13.

The following conditions are obtained from Tables 8-13 in the case of 10 times enlarging by using a 4.5 inch CRT and a 45 inch screen.

(1) Relating to surface S1, the relationship between spheric value Ps and aspheric value Pa is as below:

where ##EQU2## and where rm and zm are the radius from the lens axis and the deviation in lens axis of the outermost effective portion of surface S1. Ps, Pa and Ps/Pa are shown as below (Table 14).

(2) 0<T2<0.04 fS

where

T2 is the distance between surface S34 and surface S42.

MTF characteristics in each example 6-11 have a tendency similar to that as shown in FIGS. 3-9.