JPH0740086B2 - Projection television - Google Patents

Description

The present invention relates to a projection lens suitable for a projection television device.

[Background of the Invention]

As the demand for obtaining a powerful image that cannot be obtained with a small screen by enlarging the image reproducing surface of the television receiver, the image reproduced on the fluorescent screen of the cathode ray tube is projected by a lens, a reflecting mirror, etc. It was enlarged on the screen by the optical system to obtain a large television image.
A so-called projection television device (hereinafter abbreviated as PTV)
Has become widely used. As a result of various improvements made to such a PTV, the image quality is greatly improved compared to several years ago. In this PTV, the lens plays a very important role, and recently plastic lenses have been used, and lenses with a bright F number of 1.0 and low cost have been put into practical use.

In addition, by incorporating a mirror in this lens,
A compact set has also been realized.

An example of realizing a compact set with a built-in mirror is shown in Figs. 1 and 2 (both side views).

The lens 1 has a three-element structure, and one mirror 2 is incorporated therein. In addition, two reflecting mirrors 3 and 4 are provided, realizing a compact set. An image on the cathode ray tube 5 is enlarged and projected by a lens to obtain an image on the screen 6. FIG. 1 shows a lens and mirror arrangement for reducing depth. FIG. 2 shows a lens and mirror arrangement for reducing the height. In the example that is commonly used in the past, the effective raster size of the CRT phosphor screen is 4.8 inches, the screen size is 45 inches, and the magnification is 9.37 times.
The projection distance from the lens tip to the screen is 1220 to 12
It is 70 mm.

As mentioned above, PTV has significantly improved image quality compared to the past and has been made compact, but it is still inferior to direct-vision shadow mask type TV and it is also popular because of its high price. The current situation is that the rate has not significantly improved.

[Object of the Invention]

The object of the present invention is to realize a compact lens with a short projection distance, despite the fact that the number of lenses is small.
It is to realize a compact and low-cost PTV.

[Outline of Invention]

In the present invention, the fluorescent surface glass itself of the cathode ray tube is made into a lens, and in addition, a three-lens constitution plastic lens using an aspherical surface is used, and the constitution is composed of a convex lens, a convex lens and a plano-concave lens from the screen side. By using this cathode ray tube and lens, the lens and the projection distance are shortened, the number of mirrors in the set is one, the structure is very simple, and a compact set can be realized and the cost is low.

Example of Invention

The present invention uses a cathode ray tube and a lens as shown below for such an object.

First, a schematic view of a cathode ray tube and a lens is shown in FIG. 3 (plan view or side view). The CRT phosphor screen has a curved surface with the center of curvature on the screen side, and as the lens configuration, in addition to this phosphor screen glass, a convex lens, a convex lens, and a concave lens are arranged in this order from the screen side. According to the present invention, by using this lens and the cathode ray tube, the projection distance from the tip of the lens to the screen is greatly shortened as compared with the conventional one, and the optical system has a simple structure and realizes a compact set. The features of the present invention will be described below.

In the explanation, a case where a raster image on a 4.5-inch CRT is magnified 10 times by a lens to obtain a 45-inch image on the screen will be described. All signs are positive in the direction from the screen to the cathode ray tube.

Assuming that the cathode ray tube phosphor screen shape is the outermost angle of view and the axial deviation Zp of the center, the distance from the lens tip of the conventional optical system to the screen is 1220.
Compared to ~ 1270mm, the present invention is 800 ~ 850mm, the conventional optical system has an angle of view of about 25 °, while the optical system of the present invention has an angle of view of about 35 °. , Vastly large.

This condition is a condition necessary to realize such an optical system having a large angle of view.

The fluorescent surface of the cathode ray tube is an aspherical surface, and its shape is
The center of curvature near the central axis is located opposite to the electron gun side, and the radius of curvature is larger in the peripheral portion than in the central portion.

Generally, a lens has an aberration called field curvature, and this condition is for reducing this aberration. The fluorescent screen glass of the cathode ray tube is generally manufactured by a press, and the fluorescent screen side is coated with the fluorescent material with a rough surface to some extent without being ground. In such a manufacturing method, there is no big difference whether the fluorescent surface is a flat surface, a spherical surface, or an aspherical surface, and the cost does not change.

As the lens used in the present invention, in addition to the fluorescent surface glass, a convex lens, a convex lens, and a concave lens are arranged in this order from the screen side, and the power of the first convex lens (the reciprocal of the focal length), the power of the second convex lens, and the lens When the powers of the entire system are P ₁ , P ₂ , and P ₀ , respectively, the following relationships are established.

0.6> P ₁ / P ₀ > 0.45 0.85> P ₂ / P ₀ > 0.75 Generally, the projection distance D from the lens tip to the screen is given by the following formula.

DM · f M: Magnification f: Focal length Therefore, in order to shorten the projection distance when the magnification is constant, it is necessary to shorten the focal length and increase the power. In this lens, the total power is shared by the two convex lenses. When this condition is achieved, the intervals between the first lens and the second lens and between the second lens and the third lens are inevitably narrowed. Therefore, this condition is essential for realizing a compact lens.

The screen side surface of the third concave lens is an aspherical surface, and its curvature is made larger in the peripheral portion than in the central portion. Similarly, the CRT side surface of the second convex lens is also aspheric. When the aspherical amounts of these two surfaces are g ₂ and g ₁ and the spherical amounts are S ₂ and S ₁ , respectively, these values and the axial deviation Z _{1 from} the center at the outermost periphery of the lens, It has the following relationship between Z ₂ .

_{Z 1 = g 1 + S 1} Z 2 = g 2 + S 2 -8mm> g 1> -12mm 2mm> g 2> 0.5mm of these conditions reduces the coma aberration and 50% angle of maximum angle This is a necessary condition.

When both surfaces of the first convex lens are aspherical surfaces and the aspherical surface amounts g ₃ and g ₄ and the spherical surface amounts S ₃ and S ₄ on the screen side and the cathode ray tube side, respectively, these values and the center of the lens outermost periphery The following relationship is established between the axial deviations Z ₃ and Z _{4 from} the parts.

Z ₃ = g ₃ + S ₃ Z ₄ = g ₄ + S ₄ −5 mm> g ₃ > −10 mm −3 mm> g ₄ > −5 mm This condition is necessary to eliminate distortion and spherical aberration.

The following relationship is established between the distance T ₂₃ between the second convex lens and the third concave lens and the focal length of the entire system.

0.4f> T _23> 0.35f This is a condition necessary for reducing the distortion, and coma.

The following relationship is established between the distance T ₁₂ between the first convex lens and the second convex lens and the focal length f of the entire system.

0.45f> T ₁₂ > 0.37f This is to make the entire lens system compact and to eliminate astigmatism in relation to the condition (2).

The thickness t ₂ of the second convex lens is set to the following values.

0.25f> t ₂ > 0.2f This is a condition for distortion reduction. Lens data for Examples according to the present invention are shown in Tables 1 to 7.
Make a table. The results of MTF corresponding to each are shown in the characteristic charts in FIGS. In the figure, the vertical axis is MTF (%) and the horizontal axis is the relative angle of view. MTF is the value of 1pl / mm green monochromatic wavelength (λ = 5400Å) on a cathode ray tube. All have good MTFs.

Regarding the lens data shown in Tables 1 to 4, the distortion is a little bad at 3 to 10%. Regarding PTV distortion, even if the lens system is bad, it is possible to reduce the distortion on the screen by back-correcting with the distortion of the cathode ray tube. Therefore, even if the distortion is bad, it is not a big problem when it is used. The lenses in Tables 5 to 7 are designed so that the distortion is well corrected. In all of Tables 1 to 7, good MTF characteristics are obtained even though the F number is as small as about 1.0 and the angle of view and the angle of view are very large at about 35 °. The aspherical expressions in Tables 1 to 7 are coefficients when the lens shape is expressed by the following expression.

Table 11 lists each value described in (1) to (8) for each example, and supports each value.

[Advantages of the Invention] As described above, according to the present invention, a lens having a short projection distance can be configured, so that the mirror arrangement and the like in the set have the structure shown in FIG. 11 (side sectional view), and a very simple set can be obtained. realizable. In addition, the size of the set is the same in height and the depth can be reduced by about 10 cm compared to the configuration shown in Fig. 2 (projection distance 1235 mm) which has been mainly used in the past. Also, since the number of mirrors in the set can be reduced from three to one, the reflection loss of the mirrors is reduced and the clarity is improved by 20%. Compared with the lens shown in FIGS. 1 and 2, the built-in mirror is eliminated, the lens barrel is simplified, and at the same time, the size of the single lens element is reduced, resulting in a very low cost. For example, the diameter of the lens closest to the screen side shown in FIGS. 1 and 2 was 140 to 150φ, but according to the present invention, it is 80 to 90φ. Also, the axial length of the lens is reduced to about half.

[Brief description of drawings]

 1 and 2 are sectional views showing the optical structure of a conventional PTV, FIG. 3 is a plane or side sectional view of a lens system of the present invention, and FIGS. 4 to 10 are characteristic diagrams of an embodiment of the present invention. FIG. 11 is a side sectional view showing the structure of the PTV of the present invention. 1 ... Lens, 2 ... Built-in mirror, 3,4 ... Mirror, 5 ... CRT, 6 ... Screen.

Claims (1)

[Claims] 1. A projection type television device for enlarging and projecting an image on a fluorescent screen of a cathode ray tube onto a screen by a projection lens, wherein the projection lens acts as a convex lens as a whole from the screen side, and a focusing action in the central part and a peripheral part. In the section, a first lens having a diverging action, a second lens acting as a biconvex lens, and a third lens acting as a concave lens are arranged in that order. The power of the first lens, the power of the second lens, and the entire system When the powers of P ₁ , P ₂ , and P ₀ are respectively defined, the relation of 0.6> (P ₁ / P ₀ )> 0.45 0.85> (P ₂ / P ₀ )> 0.75 holds between them, and The CRT side surface of the two lenses and the screen side surface of the third lens are aspherical surfaces, and the following relationships (1) and (2) are established between the aspherical surface quantities g ₁ and g ₂ and the spherical surface quantities S ₁ and S ₂ of each surface. Holds,
Moreover, both surfaces of the first lens are aspherical surfaces, and the aspherical surface amounts on the screen side and the cathode ray tube side are g ₃ and g ₄ , respectively, and the spherical surface amount is S ₃ ,
When S ₄ is set, the following relations (3) to (6) are established between these values and the axial deviations Z ₃ and Z _{4 from} the center portion at the outermost circumference of the lens, and the second relation The following relationship (7) is established between the surface distance T ₂₃ between the lens and the third lens and the focal length f of the entire system, and the surface distance T ₁₂ between the first lens and the second lens and the focal point of the entire system are Projection characterized in that the following relationship (8) is established with the distance f, and the following relationship (9) is established between the thickness t ₂ of the second lens and the focal length f of the entire system. Type television device. Note −8mm> g ₁ > −12mm (1) 2mm> g ₂ > 0.5mm (2) where the aspheric amount g _i (where i = 1,2) is the lens center at the effective outermost circumference Rm of the lens. It is defined by the following equation, where Z _i is the axial deviation of and. g _i = Z _i −S _{i and the} spherical amount S _i are defined by the following equation. However, C is the curvature near the central axis, and the sign is positive in the direction from the cathode ray tube to the screen side. _{Z 3 = g 3 + S 3} (3) Z 4 = g 4 + S 4 (4) -5mm> g 3> -10mm (5) -3mm> g 4> -5mm (6) 0.4f> T 23> 0.35f (7) 0.45f> T ₁₂ > 0.37f (8) 0.25f> t ₂ > 0.35f (9)