Eyepiece lens for a display image observation device

An eyepiece lens for a display image observation device is disclosed that has only six lens elements with refractive power, in the order from the eye side, as follows: a first lens group G.sub.1 formed of two lens elements L.sub.1 and L.sub.2, both of positive refractive power, with the lens element L.sub.1 on the eye side being of positive meniscus shape with its convex surface on the eye side; a second lens group G.sub.2 formed of two lens elements L.sub.3 and L.sub.4 which are either separated by air or cemented together, the first of the lens elements on the eye side (i.e., L.sub.3) having negative refractive power and the other having positive refractive power; and a third lens group G.sub.3 formed of two lens elements which are either separated by air or cemented together, the first of the lens elements on the eye side (i.e., L.sub.5) having positive refractive power and the other having negative refractive power. Preferably, one of the surfaces of the lens elements in the first lens group G.sub.1 is aspherical, and specified conditions are satisfied in order to achieve favorable aberration correction in an eyepiece lens that is compact.

BACKGROUND OF THE INVENTION
 Heretofore, various types of display image observation devices have been
 conventionally known which display a desired image from an image display
 element, such as a liquid crystal display panel, and which enlarge the
 image using an eyepiece. An example of such an eyepiece used in a display
 image observation device is the eyepiece disclosed in Japanese Laid-Open
 Patent Application H6-308396. This eyepiece makes it possible to obtain a
 flat and clear image throughout the periphery of the field while using
 only a few lens elements by employing an aspherical surface and by making
 the object surface (i.e., the source image) a curved surface. However,
 this eyepiece is limited to cases where the range for favorably correcting
 the aberration designates an entrance pupil diameter of less than 4 mm
 (since the brightest lens disclosed has an F.sub.No. of 3.7).
 The pupil diameter of most people is in the range from 3 to 4 mm and, when
 observing in a motionless environment, using a lens with an F.sub.No. of
 3.7 gives satisfactory results. However, when the observation is
 accompanied by vibrations or other motion, such as while the observer is
 riding in an automobile or while walking, the entrance pupil of the
 eyepiece and the pupil of the observer can be temporarily offset as much
 as 3 to 5 mm due to the vibrations or other motion. Therefore, if the
 various aberrations are not favorably corrected for an entrance pupil of
 several tens of millimeters, then the viewed image will deteriorate to an
 inferior image quality as a result of the vibrations or other motions.
 This breakdown in the image quality causes favorable observation under
 such conditions to become difficult.
 BRIEF SUMMARY OF THE INVENTION
 The present invention is an eyepiece for a display image observation device
 used when enlarging and observing images displayed in an image display
 device. More specifically, the present invention relates to an eyepiece
 for observing an image of an object that is formed at an image display
 surface of an image intensifier which intensifies low-level light
 collected by an object lens of a night vision device. A first object of
 the present invention is to provide an eyepiece for a display image
 observation device that is light in weight. A second object of the
 invention is to provide such an eyepiece that is also compact. A third
 object of the invention is to provide such an eyepiece that enables
 viewing with high image quality even during unfavorable viewing
 conditions, such as when the observer is riding in a vehicle or while
 walking.

DETAILED DESCRIPTION
 The eyepiece for a display image observation device of the present
 invention includes, in the order from the eye side, a first lens group
 G.sub.1 formed of two lens elements L.sub.1 and L.sub.2, with both lens
 elements being of positive refractive power; a second lens group G.sub.2
 formed either of two lens elements or of a cemented lens having two lens
 elements, where one lens element L.sub.3 has negative refractive power and
 the other L.sub.4 has positive refractive power; and a third lens group
 G.sub.3 formed of either two lens elements or a cemented lens having two
 lens elements, where one lens element L.sub.5 has positive refractive
 power and the other L.sub.6 has negative refractive power. Further, the
 lens element L.sub.1 nearest the eye is of a positive meniscus shape with
 its convex surface on the eye side; and at least one surface of the lens
 elements L.sub.1 and L.sub.2 of the first lens group G.sub.1 is
 aspherical.
 Further, it is preferred that the eyepiece of the invention satisfy the
 following Conditions (1)-(4).
 ##EQU1##
 where
 f is the focal distance of the eyepiece lens,
 f.sub.1 is the focal distance of the first lens group,
 f.sub.2 is the focal distance of the second lens group,
 f.sub.3 is the focal distance of the third lens group, and
 R.sub.i is the radius of curvature of the image display surface.
 In general, to hold the lens outer diameter small, it is beneficial not to
 diverge the light rays while holding positive power on the lens element at
 the eye side (i.e, the observer side) of the first lens group; however,
 with the configuration of lens elements as described above, the light rays
 may be favorably deflected, especially by making this lens element in the
 shape of a positive meniscus with its convex surface on the eye side. On
 the other band, since spherical aberration and coma are generated when the
 light ray is deflected so drastically, the configuration described above
 suppresses the generation of the spherical aberration and coma by
 employing an aspherical lens surface as a lens surface of the first lens
 group.
 Further, Condition (1) above regulates the focal distance of the first lens
 group. When falling below the lower limit, the power of the first lens
 group becomes too strong, thereby making it impossible to correct the
 spherical aberration and the coma; on the other hand, when exceeding the
 upper limit, the power of the first lens group becomes too weak, thereby
 causing the light rays to diverge (since the angle of deflection has
 become too small). Hence, as the outer diameter of the second lens group
 must be increased, compactness is lost.
 In addition, Condition (2) above regulates the focal distance of the second
 lens group. When falling below the lower limit, the negative power of the
 second lens group becomes too strong thereby causing an increase in the
 positive power of the first lens group and the third lens group. Hence,
 correction of the spherical aberration, coma, and distortion becomes
 difficult. On the other hand, when exceeding the upper limit, the positive
 power of the second lens group becomes too strong, making correction of
 the spherical aberration and coma difficult.
 Furthermore, Condition (3) above regulates the focal distance of the third
 lens group. When falling below the lower limit, the positive power of the
 third lens group becomes too strong, thereby increasing the negative
 distortion. On the other hand, when exceeding the upper limit, the
 positive power of the first lens group and the second lens group tends to
 increase, which makes correction of the spherical aberration and coma
 difficult.
 In addition, Condition (4) above regulates the radius of curvature of the
 image display surface. Since an eyepiece serves basically as a positive
 lens, the image curves as a concave surface. The amount of curvature is
 closely related to the power of the eyepiece, and in this way, the
 curvature of the image surface is regulated. Therefore, when exceeding the
 upper limit of Condition (4), the image curvature increases. More
 specifically, the focus slippage at the ends of the field of view becomes
 too large.
 Hereinafter, various embodiments of the invention will be described with
 reference to the drawing figures.
 Embodiment 1
 FIG. 1 shows the basic lens element configuration of Embodiment 1. As shown
 in FIG. 1, the eyepiece of this embodiment is formed of only 6 lens
 elements L.sub.1 through L.sub.6, is for observing an image on the image
 display surface 2 of the image intensifier, and is an eyepiece of a night
 vision optical device. The specific lens element configuration is, in
 order from the eye side: a first lens group G.sub.1 formed of a first lens
 element L.sub.1 and a second lens element L.sub.2, each of which are of
 positive meniscus shape with their convex surfaces on the eye side; a
 second lens group G.sub.2 formed of a third lens element L.sub.3 of
 negative meniscus shape with its convex surface on the eye side and a
 fourth lens element L.sub.4 that is biconvex; and a third lens group
 G.sub.3 formed of a fifth lens element L.sub.5 that is biconvex and
 cemented to a sixth lens element L.sub.6 of a negative meniscus shape with
 its concave surface on the eye side. The eye-side surface of the second
 lens element L.sub.2 is aspherical. Furthermore, the eyepiece is designed
 so as to satisfy the above Conditions (1)-(4).
 According to the eyepiece for a display image observation device composed
 in this manner, the object image formed on the image display surface 2 of
 the image intensifier by way of an object lens is led to the pupil
 position 1 by way of the lens groups 1, 2 and 3 where it is once again
 formed on the retina of the eye.
 Table 1 below gives the surface number #, in order from the eye side, the
 radius of curvature R in mm of each surface, the spacing D in mm between
 each surface, as well as the refractive index N.sub.d and the Abbe
 constant .nu..sub.d at the sodium d-line (i.e. .lambda.=587.6 nm) of each
 lens element of Embodiment 1.
 TABLE 1
 # R D N.sub.d .nu..sub.d
 0 (Pupil position) 25.00
 1 21.5757 6.40 1.71300 53.9
 2 63.5050 0.74
 3* 30.7310 3.00 1.49023 57.5
 4 53.9341 2.36
 5 157.6196 1.80 1.84666 23.8
 6 23.3182 1.44
 7 33.4705 7.10 1.71300 53.9
 8 -34.0300 0.30
 9 72.0373 4.30 1.71300 53.9
 10 -69.2233 1.50 1.84666 23.8
 11 -2338.7947 10.70
 12 -40.0000 (Image display surface)
 f = 25.02 mm F.sub.NO. = 1.79 .omega. = 20.9.degree.
 As shown in the bottom portion of Table 1, the focal distance of the
 eyepiece lens of Embodiment 1 is 25.02 mm, the F.sub.NO. is 1.79, and the
 half-field angle .omega. is 20.9 degrees. Any surface marked with a * to
 the right of the surface number # in Table 1 is an aspherical surface, and
 the aspherical surface shape is expressed by Equation (A) below.
EQU Z=CY.sup.2 /{1+(1-KC.sup.2 Y.sup.2).sup.1/2 }+A.sub.4 Y.sup.4 +A.sub.6
 Y.sup.6 +A.sub.8 Y.sup.8 +A.sub.10 Y.sup.10 (Equation A)
 where
 Z is the length (in mm) of a line drawn from a point on the aspherical
 surface at distance Y from the optical axis to the tangential plane of the
 aspherical surface vertex,
 C (=1/R) is the curvature of the aspherical surface near the optical axis,
 Y is the distance (in mm) from the optical axis,
 K is the eccentricity, and
 A.sub.4, A.sub.6, A.sub.8, and A.sub.10 are the 4th, 6th, 8th, and 10th
 aspherical coefficients.
 The values of each of the constants K and A.sub.4 -A.sub.10 of the
 aspherical surface #3 indicated in Table 1 are shown in Table 2.
 TABLE 2
 K A.sub.4 A.sub.6 A.sub.8 A.sub.10
 1.0000000 -0.3450584 .times. 10.sup.-4 -0.1803346 .times. 10.sup.-7
 -0.8317521 .times. 10.sup.-10 0.4066094 .times. 10.sup.-12
 Embodiment 2
 The eyepiece lens element configuration of Embodiment 2 is shown in FIG. 2.
 This configuration is similar to that of FIG. 1, except that the two lens
 elements L.sub.3 and L.sub.4 of the second lens group G.sub.2 are cemented
 together, and the two lens elements L.sub.5 and L.sub.6 are separated by
 air.
 Table 3 below gives the surface number #, in order from the eye side, the
 radius of curvature R in mm of each surface, the spacing D in mm between
 each surface, as well as the refractive index N.sub.d and the Abbe
 constant .nu..sub.d at the sodium d-line (i.e. .lambda.=587.6 nm) of each
 lens element of Embodiment 2.
 TABLE 3
 # R D N.sub.d .nu..sub.d
 0 (Pupil position) 25.00
 1 22.5279 4.487 1.71300 53.9
 2 36.0209 1.066
 3* 25.9026 4.110 1.49023 57.5
 4 87.0748 1.708
 5 272.3953 1.500 1.84666 23.8
 6 28.6153 9.495 1.62280 56.9
 7 -32.1562 0.300
 8 52.9744 3.996 1.71300 53.9
 9 -121.5460 1.500
 10 -50.0000 1.500 1.84666 23.8
 11 -200.0000 10.240
 12 -32.0000 (Image display surface)
 f = 25.03 mm F.sub.NO. = 1.79 .omega. = 21.5.degree.
 As shown in the bottom portion of Table 3, the focal distance of the
 eyepiece lens of Embodiment 2 is 25.03 mm, the F.sub.NO. =1.79, and the
 half-field angle .omega. is 21.5 degrees. Any surface marked with a * to
 the right of the surface number # in Table 3 is an aspherical surface, and
 the aspherical surface shape is expressed by Equation (A) above. The
 coefficients that relate to the aspherical surface #3 above are indicated
 in Table 4.
 TABLE 4
 K A.sub.4 A.sub.6 A.sub.8 A.sub.10
 1.0000000 -0.2994343 .times. 10.sup.-4 -0.1955347 .times. 10.sup.-8
 -0.2219328 .times. 10.sup.-9 0.3995139 .times. 10.sup.-12
 Embodiment 3
 The eyepiece lens element configuration of Embodiment 3 is shown in FIG. 3.
 This embodiment differs from Embodiment 1 in that, in this embodiment, the
 two lens elements L.sub.3 and L.sub.4 of the second lens group G.sub.2 are
 cemented together, and the third lens element L.sub.3 is of a biconcave
 shape. Further, the image-side surface of the sixth lens element L.sub.6
 is planar.
 Table 5 below gives the surface number #, in order from the eye side, the
 radius of curvature R in mm of each surface, the spacing D in mm between
 each surface, as well as the refractive index N.sub.d and the Abbe
 constant .nu..sub.d at the sodium d-line (i.e. .lambda.=587.6 nm) of each
 lens element of Embodiment 3.
 TABLE 5
 # R D N.sub.d .nu..sub.d
 0 (Pupil position) 25.963
 1 25.5507 5.095 1.71300 53.9
 2 60.9539 1.595
 3* 48.1229 3.454 1.50670 50.6
 4 114.5217 3.000
 5 -107.9667 1.500 1.84666 23.8
 6 46.4779 6.608 1.71300 53.9
 7 -41.5920 0.509
 8 41.8755 6.036 1.71300 53.9
 9 -68.3444 1.500 1.84666 23.8
 10 .infin. 13.796
 11 -40.0000 (Image display surface)
 f = 27.00 mm F.sub.NO. = 1.94 .omega. = 20.0.degree.
 As shown in the bottom portion of the Table, the focal distance of the
 eyepiece lens of Embodiment 3 is 27.00 mm, the F.sub.NO. =1.94, and the
 half-field angle .omega. is 20.0.degree.. Any surface marked with a * to
 the right of the surface number # in Table 5 is an aspherical surface, and
 the aspherical surface shape is expressed by Equation (A) above. The
 coefficients that relate to the aspherical surface #3 above are indicated
 in Table 6.
 TABLE 6
 K A.sub.4 A.sub.6 A.sub.8 A.sub.10
 1.0000000 -0.1888700 .times. 10.sup.-4 0.1217837 .times. 10.sup.-7
 -0.1415869 .times. 10.sup.-9 0.3072742 .times. 10.sup.-12
 Embodiment 4
 The eyepiece lens element configuration of Embodiment 4 is shown in FIG. 4.
 The configuration is similar to that of Embodiment 1 but differs in that,
 in Embodiment 4, the image-side surface of the fourth lens element L.sub.4
 is planar, the surface on the image-side of the sixth lens element L.sub.6
 is planar, and the eye-side surfaces of the first lens element L.sub.1 and
 the second lens element L.sub.2, both in the first lens group G.sub.1, are
 aspherical.
 Table 7 below gives the surface number #, in order from the eye side, the
 radius of curvature R in mm of each surface, the spacing D in mm between
 each surface, as well as the refractive index N.sub.d and the Abbe
 constant .nu..sub.d at the sodium d-line (i.e. .lambda.=587.6 nm) of each
 lens element of Embodiment 4.
 TABLE 7
 # R D N.sub.d .nu..sub.d
 0 (Pupil position) 25.00
 1* 21.2823 4.600 1.71300 53.9
 2 34.8121 0.300
 3* 20.7282 4.600 1.49023 57.5
 4 36.6496 2.080
 5 49.0229 1.500 1.84666 23.8
 6 18.7354 2.450
 7 32.3292 4.600 1.71300 53.9
 8 .infin. 0.300
 9 24.3985 7.000 1.71300 53.9
 10 -60.6419 1.500 1.84666 23.8
 11 182.1882 11.679
 12 -40.0000 (Image display surface)
 f = 27.01 mm F.sub.NO. = 1.93 .omega. = 19.9.degree.
 As shown in the bottom portion of the Table 7, the focal distance f of the
 eyepiece lens is 27.01 mm, the F.sub.NO. is 1.93, and the half-field angle
 .omega. is 19.9 degrees. Any surface marked with a * to the right of the
 surface number # in Table 7 is an aspherical surface, and the aspherical
 surface shape is expressed by Equation (A) above. The coefficients that
 relate to the aspherical surfaces 1 and 3 above are indicated in Table 8.
 TABLE 8
 # K A.sub.4 A.sub.6 A.sub.8 A.sub.10
 1 1.0000000 0.1056292 .times. 10.sup.-6 -0.7819249 .times. 10.sup.-10
 -0.1051185 .times. 10.sup.-12 -0.1014760 .times. 10.sup.-15
 3 1.0000000 -0.1449433 .times. 10.sup.-4 -0.5846082 .times. 10.sup.-8
 -0.9769024 .times. 10.sup.-10 -0.9624148 .times. 10.sup.-13
 The ratios listed in Conditions (1)-(4), in other words, the values of
 f.sub.1 /f, f/f.sub.2, f.sub.3 /f, and R.sub.i /f, as well as the entrance
 pupil diameter, are given for each of Embodiments 1-4 in Table 9. As can
 be seen by comparing these values to the respective Conditions (1)-(4),
 Conditions (1)-(4) are satisfied for each of Embodiments 1-4.
 TABLE 9
 Embodiment Embodiment Embodiment Embodiment
 1 2 3 4
 f.sub.1 /f 1.342 1.538 1.619 1.443
 f/f.sub.2 0.348 0.379 0.195 -0.110
 f.sub.3 /f 4.778 5.554 2.439 1.590
 R.sub.i /f -1.599 -1.278 -1.482 -1.481
 entrance pupil 14.0 14.0 13.9 14.0
 diameter
 Moreover, the spherical aberration, astigmatism, and distortion that occur
 in Embodiments 1-4 are shown in FIGS. 5, 7, 9 and 11 respectively.
 Further, the coma for Embodiments 1-4 is shown in FIGS. 6, 8, 10 and 12,
 respectively. In each of FIGS. 6, 8, 10, and 12, coma in the tangential
 direction T is illustrated by the four curves in the left column, and coma
 in the sagittal direction S is illustrated by the three curves in the
 right column. The curves from top to bottom represent the coma at
 different half-field angles .omega. (i.e., different picture angles), with
 the angle .omega. as indicated. For indicating coma in the sagittal
 direction S, only three curves are given, since the sagittal coma on-axis
 (i.e., at .omega.=0.0) is identical to the tangential coma T on axis. As
 is evident from FIGS. 5-12, favorable correction of each of these
 aberrations is achieved in each embodiment described above.
 As described above, according to the eyepiece for a display image
 observation device of the present invention, and by way of the prescribed
 lens composition, observation is made possible of a good image quality
 even under conditions accompanying vibrations such as when the observer is
 riding in a vehicle or during walking, and while providing a lightweight
 and compact composition, favorable correction of the various aberrations
 is made possible with a large entrance pupil diameter.
 The invention being thus described, it will be obvious that the same may be
 varied in many ways. For example, the focal length, radii of curvature and
 spacings may be readily scaled. Such variations are not to be regarded as
 a departure from the spirit and scope of the invention. Rather the scope
 of the invention shall defined as set forth in the following claims and
 their legal equivalents. All such modifications as would be obvious to one
 skilled in the art are intended to be included within the scope of the
 following claims.