JP2798770B2 - Projection lens - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a projection type TV suitable for obtaining a high quality projected image.
For lenses.

[Related Art] In recent years, video projectors have become widespread as one method of obtaining a large-screen television image. However, the performance of a projection lens plays an important role in ensuring the quality of the image. I have. The projector needs to have a large aperture ratio in order to secure brightness, a wide angle of view in order to make the device thinner, and it is necessary to correct aberrations well to the periphery of the screen.

In particular, a high-definition television expected to be practically used in the future requires a projection lens having excellent focus performance.

For example, in order to sharply project an image with a spatial frequency of 1000 TV lines, an MTF (Modulation Transfer Function) of a projection lens at least at the center of the screen is required to be 80% or more.

For the above reasons, the following have been conventionally devised.

These include those composed of 10 or more glass lenses, those that use spherical plastic lenses to reduce costs, and those that use both glass and plastic aspheric lenses.

[Problems to be Solved by the Invention] Conventional projection lenses have been variously devised as described above. However, if only a glass lens is used, the number of images increases, cost is high, and the screen is high in order to obtain desired imaging performance. It is difficult to correct the peripheral performance. Aspherical plastic lenses can easily correct aberrations up to the periphery, but if the amount of aspherical surface is increased for powerful correction, the surface will have an inflection point and processing accuracy will be low, and design performance will be fully demonstrated. There was a problem that it was not possible. Further, the plastic lens has a problem that the refractive index and the shape change due to the temperature change, and the imaging performance is deteriorated due to the occurrence of defocus.

In addition, the conventional projection lens still has insufficient focus performance for high-definition television, and in particular, a character display using high-definition television requires good imaging performance to the periphery of the screen. Whereas
It was not satisfactory.

The present invention has been made in order to solve the above-described problems, and realizes a good focus performance over the entire screen as a high-definition television projection lens, and considers workability and has excellent temperature characteristics. It is intended to obtain a projection lens.

[Means for Solving the Problems] A projection lens according to the present invention includes, in order from the screen side, a first meniscus positive first meniscus lens, a positive meniscus second meniscus lens convex to the screen, and a weak negative first meniscus lens. A third group lens, a biconvex fourth group lens, a negative fifth group lens bonded to the fourth group lens, and a sixth group lens having both sides convex and having a positive refractive power, and a concave surface facing the screen. A seventh meniscus lens having a positive meniscus shape and an eighth lens having a negative refractive power
The second and seventh group lenses are grouped lenses, and the second and seventh group lenses are plastic lenses having at least one aspheric surface, and are configured to satisfy the following conditions.

0.45 <f / f1 <0.65 ... -0.65 <f / f3 <-0.55 ... 0.10 <d4 / f <0.15 ... 1.5 <d6 / d4 <1.8 ... where f: focal length of the entire system f1: Focal length of the first lens group f3: Focal length of the third lens group d4: On-axis distance between the second and third lens groups d6: On-axis distance between the third and fourth lens group [Action] First The group lens and the second group lens have a high-precision correction function of spherical aberration and coma aberration and a weak condensing function, and the third lens group has a weak divergence function and balances spherical aberration and off-axis aberration with other lens groups. Has the action of taking. The fourth lens group and the fifth lens group have a function of correcting spherical aberration, coma, and chromatic aberration that cannot be completely corrected by the first lens group, the second lens group, and the third lens group, and a strong light collecting function. The sixth lens group assists in weak light focusing and correction of astigmatism. The seventh group lens has a function of correcting astigmatism, distortion, and coma aberration with high accuracy, and the eighth group lens corrects field curvature and astigmatism as a field flattener.

In order to sufficiently fulfill such a correction function, both the second group lens and the seventh group lens are made aspherical. Also,
By using a plastic lens material having an aspherical surface, a significant reduction in cost is expected.

The condition in the equation relates to the power of the first lens unit,
If the upper limit is exceeded, it becomes difficult to correct coma. If the lower limit is exceeded, the total length of the lens system becomes longer, which is disadvantageous in terms of cost. For this purpose, the power of the second lens group, which is a plastic lens, must be increased, and at this time, the temperature characteristics deteriorate.

The condition in the equation relates to the power of the third lens unit,
If the upper limit is exceeded, it becomes difficult to correct chromatic aberration and sagittal flare. If the lower limit is exceeded, coma tends to occur.

When the condition of the expression is over the upper limit, it is difficult to correct the coma aberration, and when the condition is below the lower limit, the correction of the spherical aberration becomes difficult and the second condition is difficult.
The aspherical amount of the group lens increases.

When the condition of the expression is exceeded, chromatic aberration of magnification occurs when the value exceeds the upper limit, and when the value exceeds the lower limit, sagittal flare increases. To correct this, the amount of aspherical surface of the second lens unit increases. In the case of an aspherical surface, a surface shape having a larger amount of aspherical surface as it has an inflection point is difficult to machine, and assembling accuracy such as eccentricity becomes severe.

[Example] Hereinafter, the present invention will be described with reference to the drawings.

FIG. 1 is a configuration diagram of a projection lens according to a first embodiment of the present invention.

In the figure, (L1) is a meniscus first lens having a positive refractive power, and the first lens (L1) mainly has a function of correcting spherical aberration and coma.

(L2) is a second lens that is a plastic aspherical lens. This second lens (L2) has a positive meniscus shape with a convex surface facing the screen and has almost no power. To be corrected.

(L3) is a third lens unit having a weak negative refractive power. This third lens unit (L3) has a function of correcting coma aberration and spherical aberration together with the first lens unit (L1) and the second lens unit (L2). Have.

(L4) is a fourth lens, (L5) is a fifth lens, and the fourth lens (L4) and the fifth lens (L5) are cemented lenses, and have a strong light-collecting function and chromatic aberration. Is corrected.

(L6) is a sixth lens having a biconvex positive refractive power, and the sixth lens (L6) assists in correcting astigmatism and higher-order components of coma.

(L7) is the seventh lens, which is an aspheric plastic lens. This seventh lens (L7) has a meniscus shape with the concave surface facing the screen, has little power, and corrects astigmatism and distortion with high accuracy. I do. The seventh lens (L
7) is located close to the eighth lens and at a position where the height of the marginal ray on the axis is small, so that off-axis aberrations can be satisfactorily corrected up to the peripheral portion of the screen.

(L8) is an eighth lens which is a negative lens having a strong concave surface facing the screen, and the eighth lens (L8) mainly corrects field curvature. Furthermore, it is composed of a cooling liquid layer and a spherical CRT face plate.

Both the second lens (L2) and the seventh lens (L7) are plastic lenses, but the amount of aspherical surface on each surface is kept small and the lens has a meniscus shape having no inflection point.
Good workability and faithful reproduction of design performance. Also, since little power is provided, the arrangement tolerance can be set relatively large, and defocus due to temperature change does not pose a practical problem.

The phosphor screen has a spherical shape with a concave surface facing the screen direction, and has a function of capturing more light from the periphery of the screen into the lens.

As a result, the peripheral light amount ratio is 90% of the relative maximum angle of view.
More than 40% has been achieved.

FIG. 2 shows various aberrations of the first embodiment of the projection lens of the present invention, and FIG. 3 shows that 1000 TV lines (71 p / mm) of space are shown in FIG. Denote the MTF in frequency. Sufficient performance was obtained for high-definition television, and a high MTF was achieved for the surroundings as a character display.

 Table 1 shows lens data of the first embodiment.

The second embodiment is a projection lens applied to a front projection type TV. The lens specifications are shown in Table 2, the optical path diagram is shown in FIG. 4, the various aberrations are shown in FIG. 5, and the MTF is shown in FIG. . Angle of view is 1st
Compared to the embodiment, it is small to increase the projection distance, but
Has a high MTF of over 90%.

In the table, r1, r2,... Indicate the radius of curvature of each lens surface in order from the screen side, d1, d2... Are the lens center thicknesses and lens intervals, and n1, n2,. 543n
m), ν1, ν2... represent Abbe numbers.

The aspherical shape has a vertex curvature of c, a conic constant of k, and higher-order coefficients of a ₄ , a ₆ ,
Assuming that a ₈ and a ₁₀ are the rotationally symmetric aspheric surfaces represented by the following equations.

Both the first and second embodiments satisfy the above formulas.

Further, each lens may be divided into a plurality of lens groups.

[Effects of the Invention] The present invention uses an aspherical lens and adopts a shape and an arrangement in consideration of temperature characteristics and workability, thereby obtaining a good focus performance over the entire screen and obtaining an MTF8 at a spatial frequency of 1000 TV lines.
0% or more can be realized. In addition, the use of a plastic lens effectively enabled the cost reduction without using an expensive glass material having a refractive index of 1.7 or more.

[Brief description of the drawings]

FIG. 1 is a configuration diagram of a projection lens according to a first embodiment of the present invention, FIG. 2 is a graph showing an aberration curve of the first embodiment of the present invention, and FIG. 3 is an MTF characteristic of the first embodiment of the present invention. FIG. 4 is a configuration diagram of a projection lens according to a second embodiment of the present invention, FIG. 5 is a graph showing aberration curves of the second embodiment of the present invention, and FIG. 6 is a second embodiment of the present invention. 5 is a graph showing MTF characteristics of an example. L1 first lens, L2 second lens, L3 third lens, L4 fourth lens, L5 fifth lens, L6 sixth
Lens, L7 ... 7th lens, L8 ... 8th lens

Claims (1)

(57) [Claims] 1. A first meniscus lens having a positive meniscus shape, a second meniscus lens having a positive meniscus convex toward the screen, a weak negative third lens group, and a biconvex fourth lens in order from the screen side.
A negative fifth group lens bonded to a group lens and a fourth group lens, a sixth lens group having a positive refractive power with a double-sided convex shape, and a weak positive meniscus seventh lens having a concave surface facing the screen
A projection system comprising a group lens and an eighth group lens having a negative refractive power, wherein the second and seventh group lenses are plastic lenses having at least one aspheric surface and satisfy the following conditions. lens. 0.45 <f / f1 <0.65 −0.65 <f / f3 <−0.55 0.10 <d4 / f <0.15 1.5 <d6 / d4 <1.8 where f: focal length of the entire system f1: focal length of the first lens group f3: D4: axial distance between the second and third lens groups d6: axial distance between the third and fourth lens groups