JP2011085744A - Projection lens for small projector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact and bright projection lens for small projector with satisfactory image performance, which reduces cost by using only about five lenses. <P>SOLUTION: The projection lens includes, sequentially from a screen side to the imaging side: a first lens having negative refractive power and made of plastic; a second lens having positive refractive power and having its convex surface facing the imaging side; and a third lens having positive refractive power and having its convex surface facing the screen side. In the second and third lenses, one is made of the plastic and the other is made of a glass. The projection lens further includes: a fourth lens having negative refractive power and made of the plastic; and a fifth lens having positive refractive power. The projection lens is formed to be substantially telecentric toward the imaging side. Among the first lens made of plastic, the second and third lens made of plastic or glass, and the fourth lens made of plastic, at least one surface of the plastic lens is formed in an aspherical shape. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a projection lens for a small projector suitable for enlarging and projecting an image displayed on a liquid crystal display element, DMD, or the like on a screen.

  Conventionally, a transmissive liquid crystal panel has been often used as a display element of a projector. 2. Description of the Related Art In recent years, a small-sized projector uses a reflective liquid crystal (LCoS) panel or a DMD (Digital Micromirror Device) element in place of a transmissive liquid crystal panel. For example, an optical system having a substantially telecentric and long back focus on the image side is known for a three-plate LCD (Liquid Crystal Display) projector as shown in Patent Document 1. Further, the light source of the projector has been changed from a mercury lamp to an LED (Light Emitting Diode) light source. A small projector is required to be small, bright, small in number of lenses and low cost while satisfying image performance.

JP 2003-98431 A

  However, in the projection lens as disclosed in Patent Document 1, the projection lens system becomes large and unsuitable for a projection lens for a small projector because the backside focus is long and substantially telecentric on the image side for a three-plate LCD projector. It is.

  Accordingly, an object of the present invention is to provide a projection lens for a small projector that satisfies the image performance, is small, bright, and has a low cost by reducing the number of lenses to about five.

  In order to solve the above-mentioned problems, the invention of claim 1 is a projection lens for a small projector that enlarges and projects an image of an image display element onto a screen, and has a negative refractive power in order from the screen side to the image plane side. And a first lens made of plastic, a second lens having a positive refractive power and having a convex surface facing the image surface, and a convex surface having a positive refractive power and facing the screen side. One third lens, and one of the second and third lenses is made of plastic, the other is made of glass, and has a negative refractive power and is made of plastic. A lens and one fifth lens having a positive refractive power, are formed substantially telecentric toward the image plane side, and are a plastic first lens, a plastic or glass second and third lens, And plastic Of the four lenses, wherein at least one surface of the plastic lens is formed aspherical.

According to a second aspect of the present invention, in the projection lens for a small projector, when the focal length of the first lens is f1, and the focal length of the second lens is f2,
1.45 <| f2 / f1 | <2.15
It is characterized by.

According to a third aspect of the present invention, in the projection lens for a small projector, the fourth lens satisfies the following condition:
νd <24
Where νd is the Abbe number.

  As described above, the projection lens of the present invention can be used for a small projector having a low number of lenses and a low cost while having a bright and high-definition image.

1 shows an optical system of a projection lens in Example 1 of the present invention. 3 shows longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 1. 2 shows lateral aberrations of spherical aberration of the projection lens of Example 1. FIG. 7 shows an optical system of a projection lens in Example 2 of the present invention. 10 shows longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 2. The lateral aberration of the spherical aberration of the projection lens of Example 2 is shown. 7 shows an optical system of a projection lens in Example 3 of the present invention. 10 shows longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 3. The lateral aberration of the spherical aberration of the projection lens of Example 3 is shown. 7 shows an optical system of a projection lens in Example 4 of the present invention. The longitudinal aberration of the spherical aberration of the projection lens of Example 4, astigmatism, and a distortion aberration are shown. The lateral aberration of the spherical aberration of the projection lens of Example 4 is shown.

  The projection lens for a small projector according to the present invention has, in order from the screen side to the image surface side, one first lens made of plastic having negative refractive power and positive refractive power. One second lens having a convex surface facing the surface side and one third lens having a positive refractive power and the convex surface facing the screen side, one of the second and third lenses being It is made of plastic, the other is made of glass, and has one fourth lens made of plastic having negative refractive power and one plastic lens, and one fifth lens having positive refractive power. Of the first plastic lens, the second or third plastic or glass lens, and the fourth plastic lens, at least one surface of the plastic lens is aspherical.

  Of the first, second, third, and fourth lenses, at least one surface of the plastic lens is aspherical, for example, the first surface on the screen (enlarged) side of the first lens, or the image of the first lens. Any one of the second surfaces on the forming element (reduction) side is an aspherical surface.

  The first lens picks up the light, the second lens converges the light, the third and fourth lenses reduce chromatic aberration, and the fifth lens converges the light.

  A sufficient space is secured between the fourth and fifth lenses so that a folded PBS (Polarization Beam Splitter) mirror for the LCoS panel or an illumination system bending mirror for the DMD element can be arranged.

  The aspherical surface of the first lens is effective in reducing the coma and distortion of light entering at a wide angle. The aspherical surface of the second or third lens is effective in reducing the spherical aberration of the spread light beam. The aspherical surface of the fourth lens is effective for reducing coma aberration of reverse-side rays that cannot be corrected by the first lens and reducing distortion.

When the focal length of the first lens is f1, and the focal length of the second lens is f2,
1.45 <| f2 / f1 | <2.15
Is preferred.

  If the value of | f2 / f1 | is equal to or greater than the upper limit (2.15), the power of the first lens becomes too large, and coma aberration is increased when designing a bright projection lens, and distortion is also large. turn into.

  When the value of | f2 / f1 | is equal to or lower than the lower limit (1.45), the power of f1 is small and the power of f2 is large, so it is difficult to secure a wide space between the fourth lens and the fifth lens. Therefore, in order to secure a necessary size of space, the whole becomes large and a compact configuration becomes difficult.

The fourth lens preferably satisfies the following conditions:
νd <24
Where νd is the Abbe number.

  The fourth lens having a negative refractive power corrects chromatic aberration by a combination with the third lens having a positive refractive power. If the Abbe number νd of the fourth lens is 24 or more, it will approach the Abbe number of the third lens, making it difficult to correct chromatic aberration and degrading the overall performance. Therefore, the Abbe number νd of the fourth lens is suppressed to a value smaller than 24.

  By satisfying the above conditions, it is possible to obtain a projection lens for a small projector that can ensure good performance even with a projection lens as bright as F2.4, which is compact with a small number of lenses.

FIG. 1 shows an optical system of a projection lens in Example 1 of the present invention. The projection lens 10 for a small projector according to the first embodiment includes, in order from a screen side (not shown) toward the image plane side, a first lens L11 having a negative refractive power and made of plastic, and a positive lens. One second lens L12 having refractive power and made of optical glass (nd = 1.7725, υd = 49.6), and one third lens having positive refractive power and made of plastic. A lens L13, a fourth lens L14 having a negative refractive power and made of plastic, and a fifth lens L15 having a positive refractive power and made of plastic. It is formed substantially telecentric toward the image plane side. The material of the fifth lens L15 is not limited to the above, and may be a glass lens. At least one surface (for example, R2 surface of the first lens) of the first lens L11, the third lens L13, and the fourth lens L14 is formed as an aspheric surface, and specific lens data is shown in Tables 1 and 2. Has been. The formula of the aspheric coefficient is as shown below.

  FIG. 2 shows the longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 1. FIG. 3 shows lateral aberrations of spherical aberration of the projection lens of Example 1.

In the lens data of Example 1, when the focal length of the first lens L11 is f1, and the focal length of the second lens L12 is f2, f1 = -10.62 and f2 = 17.98, and the ratio When the absolute value of is obtained, | f2 / f1 | = 1.69. That is, when the focal length of the first lens L11 is f1, and the focal length of the second lens L12 is f2, 1.45 <| f2 / f1 | <2.15
You can see that

  The lens data in Table 1 shows the Abbe number νd and lens thickness d of the fourth lens L14, and satisfies the condition of νd≈23.0, that is, νd <24.

FIG. 4 shows an optical system of a projection lens in Example 2 of the present invention. The projection lens 20 for a small projector according to the second embodiment includes, in order from a screen side (not shown) toward the image plane side, one first lens L21 having a negative refractive power and made of plastic, and a positive lens. One second lens L22 having refractive power and made of optical glass (nd = 1.7725, υd = 49.6), and one third lens having positive refractive power and made of plastic. Lens L23, one fourth lens L24 having a negative refractive power and made of plastic, and one fifth lens L25 having a positive refractive power and made of plastic. It is formed substantially telecentric toward the image plane side. The material of the fifth lens L25 is not limited to the above, and a glass lens may be used. At least one surface (for example, the R2 surface of the fourth lens) of the first lens L21, the third lens L23, and the fourth lens L24 is formed of an aspheric surface, and specific lens data are shown in Tables 3 to 4. Has been. The formula of the aspheric coefficient is as shown below.

  FIG. 5 shows longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 2. FIG. 6 shows lateral aberrations of spherical aberration of the projection lens of Example 2.

In the lens data of Example 2, when the focal length of the first lens L21 is f1, and the focal length of the second lens L22 is f2, f1 = -10.66 and f2 = 18.19, and the ratio When the absolute value of is obtained, | f2 / f1 | = 1.71. That is, when the focal length of the first lens L21 is f1, and the focal length of the second lens L22 is f2, 1.45 <| f2 / f1 | <2.15
You can see that

  Further, the lens data in Table 3 shows the Abbe number νd and the lens thickness d of the fourth lens L24, and satisfies the condition of νd≈21.0, that is, νd <24.

FIG. 7 shows an optical system of a projection lens in Embodiment 3 of the present invention. The projection lens 30 for a small projector according to the third embodiment includes, in order from a screen side (not shown) toward the image plane side, one first lens L31 having a negative refractive power and made of plastic, and a positive lens. One second lens L32 having a refractive power and made of optical glass (nd = 1.7725, υd = 49.6) and one third lens having a positive refractive power and made of plastic. From the lens 33, one fourth lens L34 having negative refractive power and made of plastic, and one fifth lens L35 having positive refractive power and made of optical glass. Thus, it is formed substantially telecentric toward the image plane side. The material of the fifth lens L35 is not limited to the above, and a glass lens may be used. At least one surface of the first lens L31, the third lens L33, and the fourth lens L34 (for example, the R1 surface of the third lens) is formed as an aspheric surface, and specific lens data is shown in Tables 5-6. Has been. The formula of the aspheric coefficient is as shown below.

  FIG. 8 shows the longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 3. FIG. 9 shows lateral aberrations of spherical aberration of the projection lens of Example 3.

Further, in the lens data of Example 3, when the focal length of the first lens L31 is f1, and the focal length of the second lens L32 is f2, f1 = −11.80 and f2 = 22.61, and the ratio When the absolute value of is obtained, | f2 / f1 | = 1.92. That is, when the focal length of the first lens L31 is f1, and the focal length of the second lens L32 is f2, 1.45 <| f2 / f1 | <2.15
You can see that

  Further, the lens data in Table 5 shows the Abbe number νd and the lens thickness d of the fourth lens L34, and satisfy the condition of νd≈21.0, that is, νd <24.

FIG. 10 shows an optical system of a projection lens in Example 4 of the present invention. The projection lens 40 for a small projector according to the fourth embodiment includes, in order from a screen side (not shown) toward the image plane side, one first lens L41 having a negative refractive power and made of plastic, and a positive lens. One second lens L42 having refractive power and made of plastic, and one third lens L2 having positive refractive power and made of optical glass (nd = 1.589, υd = 61.2) From the lens L43, one fourth lens L44 having negative refractive power and made of plastic, and one fifth lens L45 having positive refractive power and made of optical glass. Thus, it is formed substantially telecentric toward the image plane side. The material of the fifth lens L45 is not limited to the above, and a glass lens may be used. At least one surface (for example, R2 surface of the second lens) of the first lens L41, the second lens L42, and the fourth lens L44 is formed as an aspheric surface, and specific lens data are shown in Tables 7 to 8. Has been. The formula of the aspheric coefficient is as shown below.

  FIG. 11 shows the longitudinal aberration, astigmatism, and distortion of the spherical aberration of the projection lens of Example 4. FIG. 12 shows lateral aberrations of spherical aberration of the projection lens of Example 4.

Further, in the lens data of Example 4, when the focal length of the first lens L41 is f1, and the focal length of the second lens L42 is f2, f1 = -10.16 and f2 = 17.107. When the absolute value of is obtained, | f2 / f1 | = 1.684. That is, when the focal length of the first lens L41 is f1 and the focal length of the second lens L42 is f2, 1.45 <| f2 / f1 | <2.15
You can see that

  The lens data in Table 7 shows the Abbe number νd and lens thickness d of the fourth lens L44, and satisfies the condition of νd≈21.0, that is, νd <24.

  From the above, in the small projector of the present invention, it is possible to realize a small projector projector lens that is small in size while satisfying image performance, but is bright and reduces the number of lenses to about five. it can.

  In each lens data, R represents a radius of curvature (mm), d represents a lens thickness (mm), n represents a refractive index (nd) at the d-line, and ν represents an Abbe number (νd) at the d-line.

Claims (3)

  In a projection lens for a small projector that enlarges and projects an image of an image display element on a screen, a first lens made of plastic having negative refractive power in order from the screen side to the image plane side, and a positive lens A second lens having a refractive power of 1 and a convex surface facing the image side, and a third lens having a positive refractive power and a convex surface facing the screen side, One of the third lenses is made of plastic, the other is made of glass, has a negative refractive power and is made of plastic, and a fifth lens having a positive refractive power. The first lens made of plastic, the second lens made of plastic or glass, the third lens, and the fourth lens made of plastic are small in number of plastic lenses. Both small projector projection lens, wherein the one surface is formed aspherical. When the focal length of the first lens is f1, and the focal length of the second lens is f2,
1.45 <| f2 / f1 | <2.15
The projection lens for a small projector according to claim 1. The projection lens for a small projector according to claim 1 or 2, wherein the fourth lens satisfies the following condition:
νd <24
Where νd is the Abbe number.

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