Projection Lens System

The present invention relates to a projection lens system suitable for a projection type television set.

The picture quality of a projection type television set has been improved remarkably as compared with that several years ago and is now approaching that of a direct viewing type. Particularly, the development of a projection lens as a key device which determines the quality of picture is outstanding. Under such circumstances we have developed a set using a projection lens of a short projection distance capable of attaining both improvement in the quality of picture and a compact construction of the set. As U.S. Patent Applications relating to this type of projection lenses there are Series No. 715,950 (filed Mar. 25, 1985), No. 911,070 (filed Sept. 24, 1986) and No. 948,251 (filed Dec. 31, 1986).

The projection lenses disclosed in the above applications are superior in focus performance and compactness.

Also as to contrast performance, improvement has been made, as disclosed in U.S. Ser. No. 911,070, by filling the space between the surface of a cathode-ray tube and a concave lens positioned in front thereof with a medium having a refractive index close to that of the concave lens and the CRT glass to reduce reflection at the surface of the concave lens on the side of the cathode-ray tube.

However, in the working examples of the said U.S. Ser. No. 911,070 no consideration is given to reflected light at the air-side surface of the concave lens and at other lens interfaces, so the reflected light at a lens interface returning to the phosphor surface is not fully removed. To avoid this inconvenience, in the projection lens disclosed in U.S. Ser. No. 948,251, the shape of the phosphor surface is changed to make the occurrence of reflected light difficult. But a still further means is desired to attain a higher contrast performance.

Moreover, a system structure which permits easy mounting of plural lens elements is desired in order to realize a less expensive projection lens system.

It is an object of the present invention to provide a projection lens system superior in both focus performance and contrast performance.

It is another object of the present invention to provide a projection lens system which is compact and affords a high picture quality.

It is a further object of the present invention to provide a projection lens system which is compact and superior in both picture quality and mass productivity.

Among the above objects, a high contrast is attained by eliminating light rays which are reflected by the air-side surface of a concave lens, return to the phosphor surface and lower the contrast on the same surface, as well as light rays which are reflected by the inner wall surface of a lens barrel. Higher focus performance can be attained by selecting suitable lens shape, spacing and glass material in the design of lens. Further, higher mass productivity can be achieved by selecting a lens shape which permits assembling in one direction (image side or object side)

FIGS. 1 and 2 are each a longitudinal sectional view showing an embodiment of the present invention;

FIG. 3 is a diagram showing reflections occurring between the phosphor surface of a cathode-ray tube and a concave lens;

FIG. 4 is a diagram showing a relation between the phosphor surface - concave lens spacing and contrast;

FIGS. 5 and 25 are diagrams each showing a surface shape of lens;

FIGS. 6 to 22 are characteristic diagrams showing MTF characteristics of examples set forth in Tables 1 to 17;

FIG. 23 is a characteristic diagram showing an emission spectrum distribution of a phosphor;

FIGS. 24 and 28 are diagrams showing the results of having traced light rays emitted from the phosphor surface of a projection lens;

FIGS. 26 and 27 are each a longitudinal sectional view of a projection type television set using a projection lens;

FIGS. 29 to 36 are each a longitudinal sectional view showing a lens barrel; and

FIG. 37 is a perspective view showing a retaining member for a lens barrel.

FIGS. 1 and 2 are each a sectional view showing a principal lens portion of an optical system for a projection type television set according to an embodiment of the present invention, respectively showing examples based on later-described Tables 1 and 7. In the figures, P.sub.1 denotes a phosphor surface of a cathode-ray tube; numeral 6 denotes a CRT panel; numeral 5 denotes a coolant; and numerals 4, 3, 2 and 1 denote fourth, third, second and first lens groups, respectively. The optical systems shown in FIGS. 1 and 2 are constructed so that the best performance is attained when a raster of 5.4 inches on the CRT phosphor surface is enlarged to 45 inches (8.4 X) on a screen. The distance from the first lens group 1 to the screen is 778.5 mm and the field angle is 36 degrees. The first lens group 1 is in an aspherical shape to eliminate a spherical aberration based on the aperture. The second lens group 2 is a glass lens group to reduce focus drift caused by a change in temperature and has as large a power as possible.

The third lens group is of an aspherical construction to remove coma aberration of high order and astigmatism and has as small a power as possible.

The fourth lens group 4 is for correcting a field curvature and their air-side surfaces are aspherical to correct an off-axis sagittal aberration.

Moreover, the coolant thickness is set at a sufficient value to increase the foregoing H/L.sub.1 ratio for attaining a high contrast and to radiate the heat from the cathode-ray tube by convection.

Further, the CRT phosphor surface P.sub.1 has a curvature to correct a field curvature. Particularly, if it is made aspherical to correct a field curvature of high order, it becomes possible to effect a more exact correction.

Generally, the phosphor surface P.sub.1 of the CRT panel 6 is formed by press forming and not subjected to post-processing. Therefore, no matter whether the shape to be formed is spherical or aspherical, the manufacturing method per se does not change.

At the central part of the picture plane, a spherical aberration is corrected to a satisfactory extent and there occurs only a chromatic aberration.

On the other hand, the lenses of the lens system of the invention are designed so that the power of plastic lenses is kept to a minimum, to afford thin-walled lenses with as small a difference in wall thickness as possible between the central portion and the edge portion, thereby improving the formability.

In order to overcome the aforementioned problem of the prior art the present inventors have determined by calculation a position of a light ray reflected totally by an air-side surface of a concave lens and returning to the phosphor surface out of light rays emitted from the center of the phosphor surface P.sub.1, according to the model shown in FIG. 3. The calculation was made as follows. In the figure, R.sub.D3 represents an average radius of curvature of the air-side surface of the concave lens; R.sub.D0 represents an average radius of curvature of the phosphor surface P.sub.1 ; and l.sub.1 represents the distance from the phosphor surface to the air-side surface of the concave lens.

If the refractive index of the CRT face panel is N.sub.3, and the angle of a ray emitted from the center of the phosphor surface P.sub.1 is .theta., the following is a condition for this ray to be reflected totally by the air-side surface of the concave lens:

wherein N.sub.1 represents a refractive index of the concave lens.

The h in the same figure is determined according to equation (1): ##EQU1##

Further,

wherein ##EQU2##

Substitution of equations (1) and (3) into equation (2) gives ##EQU3##

Further, ##EQU4##

If the H obtained in equation (6) is 80% or more of an effective raster region L.sub.1 of the phosphor surface P.sub.1, the lowering of contrast on the screen scarcely causes a practical problem. A value of H of 100% or more is nearly perfect.

In the present invention, therefore, the average radius of curvature R.sub.D0 of the phosphor surface, average radius of curvature R.sub.D3 of the air-side surface of the concave lens and the distance l.sub.1 from the phosphor surface P.sub.1 to the air-side surface of the concave lens are determined so that the H satisfies the following relationship with respect to L.sub.1 :

FIG. 4 shows results obtained by an experiment which was conducted to check changes in contrast of a video on the screen with l.sub.1 used as a variable. The axis of ordinate represents the ratio of contrast which is normalized assuming that the value of H/L.sub.1 =0.5 is 1. From this figure it is seen that the larger the H/L.sub.1 ratio, the more improved the contrast, and that there is obtained a contrast of a level scarcely causing a practical problem provided the above condition (7) is satisfied.

Concrete lens data capable of being adopted by this lens system are shown in Tables 1 to 17. All the lenses are not larger than 1 in F number, affording a very bright lens system. The field angle is 36 degrees. How to read Table 1 will now be explained. The data are separated into a spherical system which handles the lens region on and rear the optical axis and aspherical system which handles the outside lens region. First, it is shown therein that the screen has a radius of curvature of .infin. (that is, plane); an effective radius (clap radius) which is guaranteed optical performancewise is 580 mm; the distance (intersurface spacing) on the optical axis from the screen to a surface S.sub.1 of the first lens group 1 is 778.5 mm; and the refractive index of the medium (air) filled in the space therebetween is 1.0. It is also shown that the radius of curvature of the surface S.sub.1 of the first lens group 1 is -92.857 mm (the center of curvature is on the phosphor surface side; the spacing (intersurface spacing) on the optical axis between lens surfaces S.sub.1 and S.sub.2 is 9.0 mm; and the refractive index of the medium filled therebetween is 1.49345. Then, it is shown that the phosphor surface P.sub.1 of the CRT panel 6 has a radius of curvature of 341.28 mm; the clap radius is 72.4 mm; the thickness on the optical axis of the CRT panel 6 is 13.4 mm; and refractive index is 1.53994. Further, there are shown aspherical coefficients for the surfaces S.sub.1 and S.sub.2 of the first lens group, surfaces S.sub.5 and S.sub.6 of the third lens group 3, a surface S.sub.7 of the fourth lens group and the phosphor surface P.sub.1. The "aspherical coefficient" is a coefficient in a surface shape expressed by the following equation: ##EQU5## wherein, as shown in FIG. 5, Z represents the height (function of r) of the lens surface when the optical axis is plotted along Z axis and the lens radius plotted along r axis; r represents a radial distance; and R.sub.D represents a radius of curvature. Thus, once coefficients of CC, AE, AF, AG and AH are given, the height of the lens surface, that is, the shape thereof, is determined according to the above equation. In Table 1, moreover, it is shown that a surface S.sub.8 of the first lens group 1 and the fourth lens group 4 is zero in all aspherical coefficients, proving to be spherical. The above are how to read the data shown in Table 1. Tables 2 to 17 show concrete examples of other lens data. In the objection lens system of the present invention, as set forth above, the distance from the phosphor surface to the air-side surface of the concave lens (the fourth lens group), an average radius of curvature of the air-side surface of the concave lens and that of the phosphor surface are optimized, whereby not only an enlarged image of a high contrast is obtained on the screen but also the focus performance is improved.

Referring now to FIGS. 6 to 22, there are shown evaluation results of focus characteristics using MTF (Modulation Transfer Function), obtained when a 5.4 inch image on the phosphor surface is enlarged onto a screen through the projection lens system of the present invention. The phosphor emission spectrum shown in FIG. 23 was used. The characteristic diagrams of FIGS. 6 to 22 correspond to the Examples of Tables 1 to 17, respectively. The illustrations are of the case where 300 TV pc. is taken as a frequency of a black and white stripe signal on the screen. It is seen that FIGS. 6 to 22 show good MTF characteristics.

With respect to the Examples shown in Tables 1 to 17, if the focal length of the entire system is f.sub.0, the focal distances of the first, second, third and fourth lens groups are f.sub.1, f.sub.2, f.sub.3 and f.sub.4, respectively, there exist the relationships shown in Table 18, namely:

In order to increase the brightness ratio of the image field edge while maintaining the focus performance, it is necessary to satisfy the following condition:

As this ratio increases, the quantity of light in the middle region tends to decrease, so it is desirable to satisfy the following condition:

Under this condition, upper and lower limited rays of rays emitted from an object point other than the optical axis are allowed to pass through the lens system in a well-balanced state to thereby realize a good aberration correction. Another means for increasing the brightness ratio of the image field edge is to widen the clap diameter of the lenses which define the upper and lower limited rays. In the lens system of the present invention, as shown in FIG. 24, the lower limited ray is defined by a fourth lens group 4-side surface diameter D.sub.6 of the third lens group 3 and the upper limited ray is defined by a second lens group 2-side surface diameter D.sub.2 of the first lens group 1. On the other hand, F.sub.N0 of lens is determined by a third lens group 3-side diameter D.sub.4 of the second lens group 2, so in order to obtain a good brightness ratio of the image field edge it is necessary to select appropriate values of lens diameter ratios D.sub.2 /D.sub.4 and D.sub.4 /D.sub.6. In the lens system of the present invention, there exists the following relationship on the lower limited ray as shown in Table 18:

On the other hand, as to the upper limited ray, there exists the following relationship as in Table 18:

Where the D.sub.2 /D.sub.4 ratio is smaller than 1.19, the quantity of light at the edge portion of the screen is deficient, while when it exceeds 1.30, there occurs a coma aberration of high order and so the focus performance is deteriorated. Where the edge brightness is not required so much, only the following limitation may be adopted:

It is known that a sufficient edge brightness can be obtained by determining the clap diameter of each constituent lens of the projection lens system so that a video light emitted from an object point off the optical axis can pass sufficiently therethrough. However, it is extremely difficult to make aberration correction for rays which pass through the outer peripheral portion of each unit lens. Usually the clap diameter is determined on the basis of a balance between focus and the edge brightness ratio. In the projection lens system embodying the present invention, an upper limited ray passing through the projection lens among video rays from the aforementioned object point located off the optical axis is defined by the clap diameter D.sub.2 of the second lens group 2-side surface of the first lens group 1. As to the ratio of this D.sub.2 to the overall lens length L.sub.0, it is seen from Table 18 that there exists the following relationship:

Further, the larger the spacing l.sub.23 between the first lens group 1 and the second 2, the smaller the quantity of passing light. Therefore, in order to obtain a good value of the edge brightness ratio, it is necessary to select an appropriate value of l.sub.23. As to the ratio of the spacing l.sub.23 to the focal length f.sub.0 of the entire system, it is seen from Table 18 that there exists the following relationship:

If importance is attached to the edge brightness ratio, the following condition will do:

On the other hand, when viewed from the aspect of aberration correction, the following condition is desirable:

Satisfying the above relationships, the projection lens system embodying the present invention exhibits good values of edge brightness not smaller than 30% (at an object height of 90% for the outermost portion of the screen).

The shape of the lens surface will now be explained. The following can be said with respect to the aspherical shape of both surfaces S.sub.1 and S.sub.2 of the first lens group 1 and both surfaces S.sub.5 and S.sub.6 of the third lens group 3.

Referring to FIG. 25, there is illustrated an aspherical lens shape. When the optical axis direction is plotted along Z axis and the radial direction of the lens plotted along r axis, if the lens surface height of the spherical system or R.sub.D alone is Is(r) and the case where the aspherical coefficients of CC, AE, AF, AG and AH are substituted into equation (1) is As(r), it is seen from Table 20 that if the clap radius is substituted into r, the As(r) to Is(r) ratio of the fourth lens group 4-side surface of the third lens group 3 is in the following relationship:

It is also seen that the ratio of As to Is of the second lens group 2-side surface of the third lens group 3 is in the following relationship:

Likewise, it is understood from Table 19 that the As to Ss ratio of the screen-side surface of the first lens group 1 is in the following relationship:

Further, it is understood that there exists the following relationship with respect to the As to Ss ratio of the second lens group 2-side surface of the first lens group 1:

As to the shape of the phosphor surface, the data of Tables 6 and 11 are of a spherical shape, while the other data are of a surface shape having a radius of curvature which becomes larger from the central portion toward the peripheral portion.

Referring to FIGS. 26 and 27, there are illustrated common arrangements of optical components of projection type television sets, which optical components are turn-around mirrors 8, 9, a projection lens 11 and a cathode-ray tube 10. Where the screen-projection lens spacing (projection distance) is long, two turn-around mirrors are disposed to attain a compact construction of the set, as shown in FIG. 27.

In the projection lens system of the present invention, the projection distance at 45 inch projection is as short as 778.5 mm, so it is possible to attain a compact construction of the set using a single turn-around mirror as shown in FIG. 26.

Features of the present invention have been described above on the basis of the lens data thereof. Another embodiment of the invention for attaining a high contrast will now be described with reference to FIG. 28 which is a sectional view of the lens system of the invention as mounted in a lens barrel. Out of rays emitted from an object point Q located off the optical axis, rays which are incident on an inner barrel 7 are in the range of R.sub.0 to R.sub.4, of which rays in the ranges of R.sub.0 to R.sub.1 and R.sub.3 to R.sub.4 are reflected in the ranges of P.sub.0 to P.sub.1 and P.sub.4 to P.sub.5, respectively, of the barrel, but are shut off by the clap of the third lens group 3. After being reflected by the barrel, those rays do not become unnecessary rays. Further, rays in the range of R.sub.1 to R.sub.2 are reflected at P.sub.1 and P.sub.2 of the barrel, but due to inclination formed on the inner surface of the barrel, the reflected rays are refracted at the second and first lens groups 2 and 1 and then travel in a direction other than the screen, so do not become unnecessary rays at the image point. Further, after refracted by the second lens group, the ray R.sub.2 is refracted by the first lens group without being incident on the inner wall surface of the barrel and is focused on the image point as a normal ray. Consequently, the lens system of the present invention does not produce unnecessary rays, permitting the realization of a high contrast. As noted above, the inner wall surface of the lens barrel in the present invention is inclined with respect to the axis of the barrel so that the inside diameter of the barrel is larger gradually from the third lens group toward the first lens group. In conformity with such inside diameter of the barrel, the constituent lenses are also gradually larger in diameter in the order of the third, the second and the first lens group. Therefore, unnecessary rays are difficult to be produced, thus preventing the lowering of contrast.

The following description is now provided about the lens barrel and lens assembling method in the present invention with reference to FIGS. 29 to 36.

FIG. 29 illustrates lenses as mounted in a lens barrel embodying the invention and FIG. 30 illustrates in what order the lenses are mounted in the barrel. Lens retaining pawls 10, 11 and 12 are formed, projecting from the inner wall surface of a lens barrel 7 made of a resin. The pawls 10, 11 and 12 are each inclined toward the axis of the barrel. Further, the inner wall surface of the resinous lens barrel 7 is formed with lens receiving faces 13, 14 and 15. The unit lenses (hereinafter referred to as the "lens elements") which constitute the lens groups are held grippingly by the thus-formed lens retaining pawls 10, 11 and 12 and lens receiving faces 13, 14 and 15. The lens receiving faces also serve as claps of the lens elements. In assembling the lenses, as shown in FIG. 30, the third lens group 3, the second 2 and the first 1 are inserted in this order into the lens barrel from an image field-side opening of the barrel. As the lens elements are inserted, the lens retaining pawls 10, 11 and 12 are spread out, and when the lens elements reach the lens receiving faces 13, 14 and 15, those lens retaining pawls substantially revert to the original state. At the same time, the front ends of the lens retaining pawls 10, 11 and 12 press the other faces of the lens elements, whereby the lens elements can be held grippingly. Such lens barrel of the present invention permits the lenses to be mounted from one direction (image field side) and thus it is superior also in point of mass productivity. Moreover, even in comparison with the conventional bonding using an adhesive, the assembling time can be greatly reduced, thus permitting reduction of the manufacturing cost.

FIG. 31 illustrates lenses as mounted in a lens barrel according to another embodiment of the present invention and FIG. 32 illustrates in what order the lenses are mounted. The lenses are mounted in the order of the third, the second and the first lens group. The third lens group 3 and the second 2 are held grippingly by lens retaining pawls 11, 12 and lens receiving faces 14, 15 in the same manner as in the previous embodiment. The first lens group 1 presses against a lens receiving face 13 and a lens retaining member 10 is pushed against an inner barrel 7. At this time, the lens retaining member is spread out, but a return 16 thereof engages a stepped portion of the inner barrel 7, thus allowing the retaining member to revert its original state. In this way the first lens group 1 is held grippingly by lens receiving faces 13 and 17 of the inner barrel 7 and the lens retaining member 10, respectively. Also in this embodiment the lenses can be mounted from one direction (the image field side) and bonding is not required.

FIG. 33 illustrates lenses as mounted in a lens barrel according to a further embodiment of the invention and FIG. 34 illustrates in what order the lenses are mounted. The lenses are mounted in the order of the third, then the second lens group, then a spacing ring 11 and the first lens group 1. The third lens group 3 is held grippingly by a lens retaining pawl 12 and a lens receiving face 15. Then, the second lens group 2 is pushed against a lens receiving face 14 and the spacing ring 11 also pressed to hold the second lens group between both lens receiving faces. Lastly, the first lens group 1 is pushed against the other face 13 of the spacing ring 11 and is held between a lens receiving face 17 of the lens retaining member 10 and the lens receiving face 13 of the spacing ring 11. Also in this embodiment the lenses can be mounted from one direction (the image field side) and bonding is not required.

FIG. 35 illustrates lenses as mounted in a lens barrel according to a still further embodiment of the invention, in which a spacing ring 12 is provided between the third lens group 3 and the second 2. The lens mounting order is as shown in FIG. 36. That is, the third lens group 3, the spacing ring 12, the second lens group 2 and the first 1 are inserted in this order into the lens barrel from an image field-side opening of the lens barrel and fixed by a lens retaining member 10. Also in this embodiment the lenses can be mounted from one direction (the image field side) and bonding is not required.

FIG. 37 illustrates the lens retaining member 10 as seen from an oblique angle.