Original reading lens and original reading apparatus using the same

An original reading lens for imaging the image information on the surface of an original on the surface of a sensor, includes an original reading lens having, in succession from the original side, a meniscus-shaped positive first lens having its convex surface facing the original side, a meniscus-shaped negative second lens having its convex surface facing the original side, a stop, a meniscus-shaped negative third lens having its convex surface facing the sensor side, and a meniscus-shaped positive fourth lens having its convex surface facing the sensor side, and the surface of a diffracting optical element disposed near the stop.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an original reading lens and an original reading
 apparatus using the same, and is particularly suitable for an apparatus
 for imaging the image information on the surface of an original such as a
 document, literature, a drawing sheet or film on a reduced scale on the
 surface of a sensor such as a line sensor and reading it, for example, an
 image scanner, a film scanner, a digital copying apparatus or the like.
 2. Related Background Art
 There have heretofore been proposed various original reading apparatuses
 designed to image the image information on the surface of an original,
 such as a document or literature, on a reduced scale on the surface of a
 sensor by an original reading lens, and read the image information from a
 signal from the sensor as electronic information.
 As the original reading lens at this time, a lens system is desired in
 which the number of constituent lenses is small and the entire lens system
 is compact and moreover, relatively high optical performance is easily
 obtained. As an original reading lens satisfying such a desire, there is,
 for example, a symmetrical type lens system in which a plurality of lenses
 are disposed substantially symmetrically about a stop. As such symmetrical
 type lens system, there is, for example, a Tessar type lens of a
 three-unit, four-lens construction or a Gauss type lens of a four-unit,
 six-lens construction.
 Generally the symmetrical type lens system has the feature that
 particularly images of high resolving power and high quality can be
 obtained easily.
 When the image information on the surface of an original is to be imaged on
 a reduced scale on the surface of a sensor by an original reading lens and
 the image information is to be read as electronic information by a signal
 from the sensor, it becomes important to image the entire surface of the
 original on the surface of the sensor with high resolving power.
 As an original reading lens, for example, a Tessar type, original reading
 lens of a three-unit, four-lens construction relatively sufficiently well
 corrects various aberrations, such as spherical aberration, curvature of
 the image field, and distortion. In this lens, however, astigmatism at a
 medium angle of view and coma off the axis have had the tendency to be
 great and remain. Therefore, the Tessar-type original reading lens is used
 in an image reading apparatus having low resolution and a narrow angle of
 view.
 Also, this Tessar-type original reading lens is corrected to a certain
 level with respect to chromatic aberration, particularly on-axis chromatic
 aberration, but could not always be said to be sufficiently satisfactory
 as a lens system for a color image reading apparatus.
 Particularly, of the on-axis chromatic aberration, the short wavelength
 side of the visible wavelength region becomes over-corrected and the long
 wavelength side becomes under-corrected, and in the wide range of the
 visible wavelength region, it has been difficult to correct well and it
 had remaining chromatic aberration (secondary spectrum). Therefore, when
 such an original reading lens is used in a color image reading apparatus,
 such as an image scanner, there has been the problem that the focus
 positions in various colors, such as R(red), G(green) and B(blue) somewhat
 differ from one another and the deterioration of the quality of read image
 occurs.
 On the other hand, as the original reading lens, the Gauss type original
 reading lens of four-unit, six-lens construction sufficiently corrects
 various aberrations, such as spherical aberration, coma and curvature of
 image field. Therefore, it is used in an original reading apparatus having
 relatively high resolution and a wide angle of view.
 However, with respect to chromatic aberration, the Gauss type original
 reading lens, like the Tessar-type original reading lens, has remaining
 chromatic aberration (secondary spectrum). Therefore, it has suffered from
 the problem that the focus positions in the color lights R, G and B differ
 from one another and the deterioration of the read image occurs.
 To correct this remaining chromatic aberration relative to a wavelength of
 a wide band, it is necessary to use glass having an abnormal partial
 dispersing property, but there has been the problem that the glass of this
 kind is generally expensive and difficult to work.
 SUMMARY OF THE INVENTION
 The present invention has as its object the provision of an original
 reading lens in which a plurality of lenses are appropriately disposed
 substantially symmetrically about a stop and the surface of a diffracting
 optical element is utilized to thereby correct particularly chromatic
 aberration of various aberrations well over a wide hand and in spite of a
 small number of constituent lenses, so that the image information of the
 entire surface of an original can be imaged on a reduced scale on the
 surface of a sensor with high resolving power and which can highly
 accurately read the image information as electronic information, and an
 original reading apparatus using the same.
 The original reading lens of the present invention is an original reading
 lens for imaging the image information on the surface of an original on
 the surface of a sensor, characterized by, in succession from the original
 side, a meniscus-shaped positive first lens having its convex surface
 facing the original side, a meniscus-shaped negative second lens having
 its convex surface facing the original side, a stop, a meniscus-shaped
 negative third lens having its convex surface facing the sensor side, a
 meniscus-shaped positive fourth lens having its convex surface facing the
 sensor side, and a diffracting optical element surface disposed near the
 stop.
 The original reading apparatus of the present invention is characterized by
 imaging the image information of the surface of an original on the surface
 of a sensor by the use of the original reading lens, and reading the image
 information.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIGS. 1 to 3 are cross-sectional views of the lenses of Numerical Value
 Embodiments 1 to 3, respectively, of the original reading lens of the
 present invention, which will be described later, and show the original
 reading lens as it is applied to an original reading apparatus. FIGS. 4
 and 5 show the aberrations when the imaging magnification .beta. of
 Numerical Value Embodiment 1 of the present invention is .beta.=-0.19,
 FIGS. 6 and 7 show the aberrations when the imaging magnification .beta.
 of Numerical Value Embodiment 2 of the present invention is .beta.=-0.19,
 and FIGS. 8 and 9 show the aberrations when the imaging magnification
 .beta. of Numerical Value Embodiment of the present invention is
 .beta.=-0.19.
 In FIGS. 1 to 3, PL designates an original reading lens. T denotes the
 surface of an original, and image information is formed on that surface.
 IP designates a sensor such as a line sensor or a CCD. Gi denotes the ith
 lens constituting the original reading lens PL, and SP designates a stop.
 GP denotes a plane parallel plate provided near the stop SP. In the ith
 lens Gi constituting the original reading lens PL, G1 denotes a
 meniscus-shaped positive first lens having its convex surface facing the
 original side, G2 designates a meniscus-shaped negative second lens having
 its convex surface facing the original side, G3 denotes a meniscus-shaped
 negative third lens having its convex surface facing the sensor side, and
 G4 designates a meniscus-shaped positive fourth lens having its convex
 surface facing the sensor side.
 In Numerical Value Embodiments 1 and 2 of FIGS. 1 and 2, the surface of a
 diffracting optical element is provided on at least one lens surface of
 the second lens G2 or the third lens G3, which is adjacent to the stop SP.
 Also, in Numerical Value Embodiment 3 of FIG. 3, the surface of a
 diffracting optical element is provided on at least one surface of the
 plane parallel plate GP. The phase of the surface of the diffracting
 optical element is appropriately set, whereby in spite of a simple
 four-unit, four-lens construction, on-axis chromatic aberration is well
 corrected over a wavelength range of a wide band.
 Specifically, in Numerical Value Embodiment 1 of FIG. 1, the surface of the
 diffracting optical element is provided on the lens surface R4 of the
 second lens G2, which is adjacent to the stop SP, and in Numerical Value
 Embodiment 2 of FIG. 2, the surface of the diffracting optical element is
 provided on the surface R6 of the third lens G3, which is adjacent to the
 stop SP, and in Numerical Value Embodiment 3 of FIG. 3, the surface of the
 diffracting optical element is provided on the surface R6 of the plane
 parallel plate GP, which is adjacent to the stop SP.
 In the present embodiment, the correction of on-axis chromatic aberration
 is effected by the utilization of the optical characteristic that the
 optical action of the surface of the diffracting optical element is
 negative conversely to the sign of the Abbe number of an ordinary optical
 material. That is, utilization is made of the optical characteristic that
 the way in which chromatic aberration appears for a ray of a reference
 wavelength becomes converse to that of a refraction type lens.
 The surface of the diffracting optical element is provided near the stop,
 whereby by the utilization of the fact that the passage position of an
 off-axis light beam from the optical axis is low, the adverse effect onto
 off-axis aberration is minimized to thereby efficiently correct on-axis
 chromatic aberration. Also, by appropriately combining the refracting
 action of a refraction type lens and the diffracting action of the surface
 of the diffracting optical element together, off-axis chromatic aberration
 is well corrected with the other aberrations.
 In the embodiments of FIGS. 1 to 3, the surface of the diffracting optical
 element is not limited to one, but a plurality of surfaces may be
 provided, and according to this, the refractive power of each surface of
 the diffracting optical element can be made small and the manufacture
 becomes easy, and the on-axis chromatic aberration can be corrected better
 over a wavelength range of a wide band.
 In the present invention, when the refractive power of the surface of the
 diffracting optical element is defined as .phi..sup.d and the focal length
 of the entire system is defined as f,
EQU 0.02&lt;f.times..phi..sub.d &lt;0.06 (1)
 is satisfied.
 Conditional expression (1) is for appropriately setting the refractive
 power .phi..sub.d of the surface of the diffracting optical element, and
 correcting chromatic aberration well while reducing the influence of the
 other diffracted lights than 1st-order diffracted light used as an imaging
 action.
 If the refractive power .phi..sub.d of the surface of the diffracting
 optical element becomes too great beyond the upper limit value of
 conditional expression (1), on-axis chromatic aberration will become
 over-corrected and the influence upon off-axis aberration will increase
 the grating pitch difference between the central portion, and the
 peripheral portion of the surface of the diffracting optical element will
 increase and manufacture will become difficult, and this is not good.
 On the other hand, if the refractive power .phi..sub.d of the surface of
 the diffracting optical element becomes too small beyond the lower limit
 value of conditional expression (1), on-axis chromatic aberration will
 become under-corrected and the influence of the other diffracted lights
 than the 1st-order diffracted light will increase and flare, and this is
 not good.
 The original reading lens of the present invention is comprised of a lens
 system comprising two lenses disposed substantially symmetrically on the
 opposite sides of the stop SP.
 Thereby the various aberrations, particularly coma, astigmatism,
 distortion, etc. occurring in the front lens unit (the first lens and the
 second lens) provided more adjacent to the original side than the stop SP
 are well-balancedly corrected by the rear lens unit (the third lens and
 the fourth lens).
 Also, spherical aberration is corrected well with the lens surface in which
 the passage positions of an on-axis light beam and an off-axis marginal
 light beam are separate from the optical axis, e.g. the lens surface (R3)
 of the second lens, which is adjacent to the original, as an aspherical
 surface.
 The diffracting optical element in the present embodiment is binarily
 manufactured by the lithographic technique, which is a manufacturing
 technique for a holographic optical element (HOE). The diffracting optical
 element may also be manufactured by binary optics. In this case, in order
 to further increase the diffraction efficiency, it may be made into a
 saw-like shape called quinoform. It may also be manufactured by molding by
 a mold manufactured by one of these methods.
 When the reference wavelength (e-line) is defined as .lambda. and the
 distance from the optical axis is defined as h and the phase coefficient
 is defined as C.sub.2i (i=1, 2, . . . ) and the phase is defined as
 .phi.(h), the shape of the diffracting optical element in the present
 embodiment is represented by the following expression:
EQU .phi.(h)=2.pi./.lambda.(C.sub.2.multidot.h.sup.2 +C.sub.4.multidot.h.sup.4
 + . . . C.sub.(2i).multidot.h.sup.2i)
 As the construction of the diffracting optical element used in the present
 embodiment, the one-layer construction of a one-layer quinoform shape as
 shown in FIG. 10, or the two-layer construction as shown in FIG. 13,
 wherein two layers of different (or equal) grating thicknesses are
 laminated, is applicable.
 FIG. 11 shows the wavelength dependence characteristic of the diffraction
 efficiency of the 1st-order diffracted light of the diffracting optical
 element 101 shown in FIG. 10. The construction of the actual diffracting
 optical element 101 is such that ultraviolet-setting resin is applied to
 the surface of a base material 102 and a layer 103 of such a grating
 thickness d that the diffraction efficiency of 1st-order diffracted light
 is 100% for a wavelength 530 nm is formed on the resin portion.
 As is apparent from FIG. 11, the diffraction efficiency of the design order
 number is reduced away from the optimized wavelength 530 nm, and on the
 other hand, the diffraction efficiency of O-order diffracted light and
 2nd-order diffracted light which are the order number in the vicinity of
 the design order number increases. The increase in the diffracted lights
 of the other order numbers than the design order number causes flare, and
 this leads to a reduction in the resolution of the optical system.
 FIG. 12 shows the MTF characteristic for a spatial frequency when the
 grating shape of FIG. 10 is applied to Numerical Value Embodiment 1. In
 FIG. 12, y' indicates the image height. In FIG. 12, the MTF in the low
 frequency area is somewhat reduced.
 FIG. 14 shows the wavelength dependence characteristic of the diffraction
 efficiency of the 1st-order diffracted light of a diffracting optical
 element of a laminated type shown in FIG. 13 wherein two layers 104 and
 105 are laminated.
 In FIG. 13, a first layer 104 comprising ultraviolet-setting resin
 (nd=1.499, .nu.d=54) is formed on a base material 102, and a second layer
 105 comprising discrete ultraviolet-setting resin (nd=1.598, .nu.d=28) is
 formed thereon. In the combination of these materials, the grating
 thickness d1 of the first layer 104 is d1=13.8 .mu.m and the grating
 thickness d2 of the second layer 105 is 10.5 .mu.m.
 As can be seen from FIG. 14, by using a diffracting optical element of
 laminated structure, the diffraction efficiency of the design order number
 is as high as 95% or greater in the entire wavelength range used.
 FIG. 15 shows the MTF characteristic for a spatial frequency when the
 grating shape of FIG. 13 is applied to Numerical Value Embodiment 1. If a
 diffracting optical element of laminated structure is used, the MTF of a
 low frequency is improved and a desired MTF characteristic is obtained. If
 as described above, the laminated structure is used as the diffracting
 optical element according to the present invention, the optical
 performance can be further improved.
 As the aforedescribed diffracting optical element of laminated structure,
 the material thereof is not restricted to ultraviolet-setting resin, but
 other plastic materials or the like can also be used, and depending on the
 base material, the first layer 104 may be directly formed on the base
 material. Also, the grating thicknesses need not always differ from each
 other, but depending on the combination of materials, the grating
 thicknesses of the two layers 104 and 105 may be made equal to each other
 as shown in FIG. 16.
 In this case, the grating shape is not formed on the surface of the
 diffracting optical element and therefore, the diffracting optical element
 is excellent in dustproof property and the assembling work thereof can be
 improved.
 The numerical value embodiments of the present invention will be shown
 below. In the numerical value embodiments, .gamma.i represents the radius
 of curvature of the ith lens surface from the object side (the original
 side), di represents the thickness and air space of the ith lens from the
 object side, and ni and vi represent the refractive index and Abbe number,
 respectively, of the glass of the ith lens from the object side. .beta. is
 the imaging magnification. Also, the relation between the aforementioned
 conditional expression and each numerical value embodiment is shown in
 Table 1 below.
 When the X-axis is taken in the direction of the optical axis and the
 distance from the optical axis in a direction perpendicular to the optical
 axis is defined as H and the direction of travel of light is positive and
 the paraxial radius of curvature is defined as R and A, B, C, D, E and F
 are aspherical surface coefficients, the aspherical shape is represented
 by the following expression:
 ##EQU1##