JPH0711627B2 - Projection TV lens - Google Patents

Description

Description: FIELD OF THE INVENTION The present invention relates to a lens used in a projection television.

[Background of the Invention]

As the demand for increasing the image reproduction surface of television receivers to obtain powerful images that cannot be obtained on a small screen, the reproduced images on the fluorescent surface of the cathode ray tube such as lenses and reflecting mirrors have been increased. A so-called projection television, which is enlarged by a projection optical system to obtain a reproduced image on a large screen, has been widely used. As a result of various improvements made so far in such a projection television, there is a great progress as compared with several years ago. The lens plays a very important role in this projection television. In such a situation, the lens shown in Japanese Patent Application No. 54-115645 achieves high brightness with an F number of 1.0 even though it has three lenses. This is a design that takes full advantage of ease of asphericalization, which is the greatest advantage of plastic lenses. In addition, the cost is low due to the small number of components of 3 and the plastic lens suitable for mass production. The structure of a set using this lens is shown in FIGS. 1 and 2 (side sectional view). The image on the cathode ray tube 4 is enlarged and projected on the screen by the lens 5, and the mirror 2 is arranged for compactness. In FIG. 2, the mirror 3 is provided inside the lens to realize a compact set. It is no exaggeration to say that the projection television has greatly developed with the lens disclosed in Japanese Patent Application No. 54-115645. However, it is still inferior in terms of room, compactness, and price compared to direct-view TVs.

Each problem is described.

(1) Image quality: Focus, brightness, and contrast are listed as important image quality items for projection televisions. Regarding focus, in general,
Although it can be evaluated by MTF (Modulation Transfer Function), the result of evaluating the above-mentioned Japanese Patent Application No. 54-115645 was very good. However, in an actual CRT, the emission spectrum characteristic of the fluorescent substance has a distribution as shown in FIG. 3, and if this is taken into consideration, the MTF becomes worse, and it is generally used especially in the low frequency region. Inferior to glass lenses for projection TVs. This is because no consideration has been given to the improvement of chromatic aberration for this lens. On the contrary, it is expected that the MTF will be significantly improved by improving the chromatic aberration. Further, since this lens is composed entirely of plastic lenses, there is a drawback that the refractive index changes when the temperature changes, which causes focus deterioration.

As can be seen from FIGS. 1 and 2, a set using the present lens, in order to realize a compact set, a large number of mirrors must be arranged. for that reason,
It causes reflection loss of the mirror and reduces the brightness. There is a problem that stray light occurs and the contrast deteriorates.

(2) Compactness: In the past, there was a tendency to increase the number of mirrors in the set in order to achieve compactness, but the current situation is limited.
It is necessary to shorten the lens length, and at the same time it is necessary to reconsider the arrangement of mirrors.

By the way, the projection distance and the lens length of the lens shown in Japanese Patent Application No. 54-115645 are 1200 to 1300 mm and 200 mm, respectively, when a 45-inch screen is used and the magnification is about 9 to 10 times.
It is around.

(3) As described above, in the related art, it is inevitable that the cost will increase due to the complicated arrangement of mirrors and the large lens.

[Object of the Invention]

It is an object of the present invention to provide a projection television lens in which the projection distance and the lens length are shortened, the image quality such as focus, brightness, and contrast is improved, a compact set is realized, and the cost is reduced.

[Outline of Invention]

In order to achieve the above object, a lens having a configuration capable of realizing the following short focal length is used. In order from the screen side, (a) a lens whose side surface shape has a converging function in the central part and a diverging function in the peripheral part.

(B) Biconvex lens.

(C) Concave lens.

(D) The center of curvature of the central portion is on the screen side, and the radius of curvature increases as it moves to the peripheral portion, or
A CRT glass lens having an aspherical fluorescent surface with the center of curvature on the electron gun side.

When the radii of curvature near the centers of the screen side surfaces of the first lens and the second lens are r ₁ and r ₂ , they have a relationship of 0.9r ₂ <r ₁ <1.1r _2. Then, there is a relationship of 0.3f <T ₁₂ <0.4f between the surface distance T ₁₂ between the first lens and the second lens and the focal length f of the entire system, and the surface distance between the second lens and the third lens is There is a relationship of 0.35f <T ₂₃ <0.45f between T ₂₃ and the focal length f of the entire system, and 0.25f <t ₂ between the thickness t ₂ of the second lens and the focal length f of the entire system. <0.35f.

Example of Invention

 The present invention will be described in detail using examples.

In order to solve the above-mentioned problems, the present invention shortens the focal length by about 35% from the conventional one. To reduce the projection distance while keeping the magnification constant, it is necessary to increase the angle of view.
The half angle of view of the conventional Japanese Patent Application No. 54-115645 is about 25 °, but it is necessary to increase this to 35 ° by 40%. This is very difficult to achieve while maintaining the central and peripheral light levels. On the contrary, if the angle of view is increased and the projection distance is shortened in this way, the focal point can be shortened, the lens can be downsized, and the chromatic aberration can be reduced, which will greatly contribute to the development of the plastic lens. You can

Here, the relationship between the reduction of the focal length and the reduction of the chromatic aberration and the focus shift due to the temperature change will be described.

In FIG. 4 (plan view or side view), it is assumed that the lens 5 is thin with a focal length f, the distance between the fluorescent surface and the lens is a, and the distance from the lens to the screen is b.

If the radii of curvature on both sides of the lens are γ ₁ and γ ₂ , From (ii) Here, due to the temperature change, thermal expansion causes the shape to change and the focal length to change, but it is considerably smaller than the effect due to the change in the refractive index, and when a plastic lens barrel is used,
In order to offset the expansion of the lens barrel, the contribution by terms including Δγ ₁ and Δγ ₂ is omitted. The deterioration Δd of the spot diameter on the screen 6 is given by the following equation using the F number of the lens and the magnification M. Using (i) to (iii) That is, the increase Δd in spot diameter due to chromatic aberration or temperature change is proportional to the focal length f. That is, by reducing the focal length by 35%, the increase in spot diameter due to chromatic aberration and temperature change is also reduced by 35%. In particular, this lens has a very large effect because the MTF at the center of the screen is governed only by chromatic aberration.

Further, in the case of a plastic lens, defocusing due to temperature change was the biggest problem, but reducing it by 35% is a very significant effect. How the actual MTF characteristic of the center of the screen changes with temperature will be described later. The typical configuration of this lens is No. 5
Shown in the figure.

Lens data corresponding to Examples 1 to 5 are shown in Tables 1 to 5. The feature of these lenses is that they are arranged as follows in order from the screen side.

(A) The first lens whose side surface shape of the screen has a converging action in the central portion and a diverging action in the peripheral portion.

That is, this lens is shown in FIG. 5 (plan view or side view).
As you can see, it has a fairly strong aspherical shape.

Biconvex second lens.

(C) Third concave lens.

(D) The center of curvature of the central part is on the screen side, and the radius of curvature is larger toward the peripheral part, or the CRT glass has a fluorescent surface with a curvature center on the electron gun side. lens.

When the first lens of the present invention is used, aberrations of off-axis light of the lens are eliminated, and off-axis rays can be used as compared with the conventional convex lens, so that the light amount is increased and a high-luminance image can be obtained.

By using this lens and the cathode ray tube, the present invention shortens the projection distance from the tip of the lens to the screen to a much larger extent than before, and realizes a simple configuration and a compact set of the optical system. The features of the present invention will be described below.

The explanation is given when 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.

(1) As the surface shapes of the first lens and the second lens on the screen side, when the radii of curvature near the center are γ ₁ and γ ₂ , the following relationships are established.

0.9γ ₂ <γ ₁ <1.1γ _{2 As in the} present invention, in order to realize a short projection distance, it is necessary to shorten the focal length and increase the power. In this condition, the total power is shared by the two convex lenses, and the aberration generated in the power lens is reduced. When this condition is achieved, the first lens, the second lens, and the second lens are inevitably used.
The distance between the lens and the third lens becomes narrower, which is essential for realizing a compact lens.

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

0.3f <T ₁₂ <0.4f This is to make the whole lens system compact and to eliminate astigmatism in relation to the condition (1). At the same time, the aperture of the second lens can be increased, which is indispensable for increasing the amount of peripheral light which becomes a problem when the angle of view is widened.

(3) The following relationship is established between the distance T ₂₃ between the second lens and the third lens and the focal length f of the entire system.

0.35f <T ₂₃ <0.45f This is a necessary condition for reducing distortion and coma. When T ₂₃ is much lower than the lower limit, it is necessary to increase the curvature of the concave lens in order to reduce the Petzval sum, but at the same time, high-order coma aberration occurs.

(4) The following relationship is established between the thickness t ₂ of the second lens and the focal length f of the entire system.

As 0.25f <t ₂ <0.35ft _2, and takes the cooling time during molding exceed this upper limit, the cost. There is a problem that material costs are required. When the value is below the lower limit, the edge thickness of the peripheral portion of the lens cannot be ensured, and when the lens is incorporated in the lens barrel, the peripheral portion of the lens may be deformed by an external force due to thermal expansion or the like.

(5) A clap is provided on the second lens side between the first lens and the second lens. In general, this type of lens can easily correct the aberration near the center, can reduce the F number, and can set it to 1.0 or less. However, at this time, as an image on the screen, the central part is very bright and the peripheral part is dark, which makes it difficult to see. In particular, when the angle of view is increased as in the present lens, as is generally well known, the amount of light in the peripheral portion is extremely small in proportion to cos ⁴ θ when the angle of view is θ.

In consideration of such a situation, a clap is provided in order to reduce the aberration in the peripheral portion and increase the light amount.

(6) The Blau tube fluorescent surface is aspherical, and the shape has its center of curvature near the central axis on the screen side, and the radius of curvature increases toward the periphery, or the curvature center on the electron gun side. It has a flat surface shape.

Generally, a lens has an aberration called field curvature, and this condition is for reducing this aberration. The curvature of field generally bends toward the object side in the vicinity of the center, that is, in the third-order aberration region, but the sign of the peripheral portion, that is, higher-order aberration is opposite to that of the third-order aberration.

The above dimensions corresponding to Examples 1 to 5 are summarized in Table 6 and are used as a basis for the present invention.

The results of the MTFs corresponding to Examples 1 to 5 shown in Tables 1 to 5 are shown in FIGS. 6 to 10 (all are characteristic diagrams). These values are at 1 Pl / mm on a cathode ray tube. Moreover, in the calculation of this MTF, the emission spectrum distribution of the phosphor was also considered. The weight of each wavelength is as shown in Table 7.

The result of MTF at the center of the screen is shown in FIG. 11 (characteristic diagram) in comparison with Japanese Patent Application No. 54-115645. Here, A is the characteristic of the lens of the present invention, and B is the one of the conventional example.

In each example, the F number was 0.91 and high brightness,
It shows a good MTF characteristic.

In addition, the peripheral volume ratio is 30%, which is the same as the lens used in conventional projection televisions with a small angle of view. In this way, although the F-number was made smaller and the angle of view was greatly improved compared to the conventional one, the MTF characteristics could be greatly improved without deteriorating the brightness of the periphery.

Fig. 12 shows the characteristics of MTF change with temperature change by this lens. A is a characteristic of the lens of the present invention, and B is a conventional example. MT due to temperature changes compared to conventional plastic lenses despite the all-plastic lens configuration
It can be seen that the F deterioration is halved.

〔The invention's effect〕

According to the present invention, the following effects can be obtained by using the lens described above.

(1) By using the first lens, the aberration of the off-axis light of the lens can be removed and the off-axis ray can be used, so that the light amount can be increased and a high-luminance image can be obtained.

(2) The lens length and aperture are extremely small, the weight can be reduced, and the cost can be significantly reduced.

(3) The focus of the central portion can be greatly improved.

In addition, the projection distance of this lens has been shortened, and in combination with the reduction in lens length, a compact set can be realized. An example of the mirror 2 in the set shown in FIG. 13 (side sectional view)
There is only one, and the structure is very simple. As a result, (1) the depth and height of the set can be reduced, and compactness can be achieved.

(2) Since the number of mirrors in the set is one, it is possible to reduce 10% of light reflection loss per sheet.

(3) The number of mirrors is reduced and the jig for supporting this is simplified, resulting in cost reduction.

(4) Since the conventional set is composed of many mirrors, various unnecessary lights are generated, which causes a problem that the contrast is deteriorated as compared with the case of a single lens. The set using this lens can greatly reduce such unnecessary light and can significantly improve the contrast.

[Brief description of drawings]

1 and 2 are side sectional views showing the structure of a conventional projection television, FIG. 3 is a characteristic diagram showing the emission spectrum of a fluorescent body, FIG. 4 is a plan view for explaining chromatic aberration, and the like. FIG. 6 is a plan view showing the lens structure, FIGS. 6 to 10 are characteristic diagrams showing MTF characteristics of each embodiment, FIG. 11 is a characteristic diagram showing central MTF characteristics, and FIG. 12 is MTF deterioration characteristics due to temperature change. FIG. 13 is a side sectional view showing a set structure according to the present invention. 1,2,3 …… Reflector, 4 …… Projection cathode ray tube, 5 …… Projection lens.

Claims (1)

[Claims] 1. A projection television for enlarging and projecting an image on a fluorescent screen of a cathode ray tube on a screen, when a screen having a screen size of x inches (diagonal direction) is used,
The distance from the fluorescent screen to the lens tip is (140x) / 45mm or less, and the distance from the lens tip to the screen is (90
0x) / 45 mm or less, the projection television lens as the lens used for the same. From the screen side, (a) the side surface shape of the screen is converging in the central part and diverging in the peripheral part. The first lens that is acting, (b) the biconvex second lens, (c) the concave third lens, and (d) the center of curvature of the central portion is on the screen side, and as it moves to the peripheral portion, , Its radius of curvature is large, or
A CRT glass lens having a fluorescent surface whose surface has a curvature center on the electron gun side, and the radius of curvature near the center of the screen side surface of the first lens and the second lens is r ₁ and r ₂ have a relationship of 0.9r ₂ <r ₁ <1.1r ₂ and 0.3f <between the surface distance T ₁₂ between the first lens and the second lens and the focal length f of the entire system. T ₁₂ <0.4f, and there is a relationship 0.35f <T ₂₃ <0.45f between the surface distance T ₂₃ between the second lens and the third lens and the focal length f of the entire system. A projection television lens, characterized in that the thickness t ₂ of the second lens and the focal length f of the entire system have a relationship of 0.25f <t ₂ <0.35f.