Imaging lens, image reading apparatus and image forming apparatus

Abstract

An imaging lens with a perfect imaging quality and a low manufacture cost is provided for using in an image reading apparatus or an image forming apparatus by controlling a surface shape error thereof in a predetermined range disclosed by the present invention. The imaging lens includes a plurality of lenses, and an aperture stop, wherein a vertical interval between a convex in one direction with respect to a lens surface and a concave in the other direction reverse to the one direction on at least one surface of a lens disposed adjacent to the aperture stop is controlled not greater than ½ with respect to a wavelength in a wave range used, the vertical interval is a surface shape error which is set as a deviation between the lens surface and a proximal spherical surface thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a pattern diagram illustrating one example of an imaging lens according to the present invention.

FIG. 2 is a pattern diagram illustrating one example of an image reading apparatus according to the present invention.

FIG. 3 is a pattern diagram illustrating a laser printer as one example of an image forming apparatus according to the present invention.

FIG. 4 is a pattern diagram illustrating one embodiment of the imaging lens according to the present invention.

FIG. 5A to FIG. 5D are aberration diagrams for designed values.

FIG. 6A to FIG. 6D are aberration diagrams after a manufacture error.

FIG. 7A to FIG. 7D are aberration diagrams after an interval is adjusted.

FIG. 8A is an explanation diagram illustrating a sectioned central portion in order to explain a surface shape error of an imaging lens according to the present invention.

FIG. 8B is a diagram illustrating a difference between a spherical surface and a lens surface.

FIG. 9 is a diagram illustrating a 20^th order even function polynomial of a surface shape error of the 8^th aspherical lens surface of the lens illustrated in FIG. 4.

FIG. 10 is a diagram illustrating M-D characteristic curves which express MTF characteristics for a lens without a designed surface shape error with respect to wavelength having respective image height ratio of 1.0, 0.9, 0.75 and 0.5.

FIG. 11 is a diagram illustrating M-D characteristic curves for the 8^th aspherical lens surface illustrated in FIG. 9.

FIG. 12 is a characteristics diagram illustrating one example of a wavelength characteristics combined from an emitting wavelength characteristic of a xenon lamp and a spectral transmission characteristic of a CCD filter for reading an image.

FIG. 13 is a surface shape error characteristic curve illustrating a relationship between a surface shape error on an aspherical lens and a radial position.

FIG. 14 is an M-D characteristic curve diagram illustrating the lens in FIG. 4 having an aperture stop diameter of 5.5 mm, the 8^th aspherical lens surface of the lens with the surface shape error in FIG. 13 having a convex-concave interval of 3.1 mm and a convex-concave height of 0.2 μm.

FIG. 15 is a surface shape error characteristic curve illustrating a surface shape error having a convex-concave interval of 1.1 mm and a convex-concave height of 0.2 μm.

FIG. 16 is an M-D characteristic curve illustrating a surface shape error having a convex-concave interval of 1.1 mm and a convex-concave height of 0.2 μm.

FIG. 17 is a surface shape characteristic curve having a ratio of 0.45 of a convex-concave interval of the surface shape with respect to an aperture stop diameter having a convex-concave interval of 2.5 mm and a convex-concave height of 0.2 μm.

FIG. 18 is an M-D characteristic curve illustrating a surface shape error having a convex-concave interval of 2.5 mm and a convex-concave height of 0.2 μm.

FIG. 19 is a surface shape characteristic curve having a ratio of 0.25 of a convex-concave interval of the surface shape with respect to an aperture stop diameter having a convex-concave interval of 1.4 mm and a convex-concave height of 0.2 μm.

FIG. 20 is an M-D characteristic curve illustrating a surface shape error having a convex-concave interval of 1.4 mm and a convex-concave height of 0.2 μm.

FIG. 21 is an M-D characteristic curve of the imaging lens illustrated in FIG. 4 having the 8^th aspherical lens surface with a ratio of 0.25 of a convex-concave interval of the surface shape with respect to an aperture stop diameter and its central portion being a convex.

FIG. 22 is an M-D characteristic curve of the imaging lens illustrated in FIG. 4 having the 8^th aspherical lens surface with a ratio of 0.38 of a convex-concave interval of the surface shape with respect to an aperture stop diameter and its central portion being a convex.

FIG. 23 is an M-D characteristic curve of the imaging lens illustrated in FIG. 4 having the 8^th aspherical lens surface with a ratio of 0.38 of a convex-concave interval of the surface shape with respect to an aperture stop diameter and its central portion being a convex.

Claims

1. An imaging lens comprising: a plurality of lenses, andan aperture stop,wherein a vertical interval between a convex in one direction with respect to a lens surface and a concave in the other direction reverse to the one direction on at least one surface of a lens disposed adjacent to the aperture stop is controlled not greater than ½ with respect to a wavelength in a wave range used, the vertical interval is a surface shape error which is set as a deviation between the lens surface and a proximal spherical surface thereof.

2. The imaging lens set forth in claim 1, wherein for DS and DL which meet a mathematical expression (1): 0.2<DL/DS<0.5   (1)the vertical interval is controlled not greater than ½ with respect to a wavelength in the wave range used, setting DL as a radial interval between a convex in one direction and a concave in a reverse direction thereto and DS as a diameter of the aperture stop.

3. The imaging lens set forth in claim 1 has a vignetting factor of about 100%.

4. The imaging lens set forth in claim 1, wherein the plural lenses have a 5-lens in 4-group configuration including in an order starting from an object side of the imaging lens a first group having a positive first lens, a second group which has a negative refractive power and is jointed from a positive second lens and a negative third lens, a third group having a negative fourth lens and a fourth group having a positive fifth lens; the aperture stop is disposed between the second group and the third group; and the fourth lens is an aspherical lens having at least one aspherical surface.

5. The imaging lens set forth in claim 1, wherein the lens disposed adjacent to the aperture stop is an aspherical lens having an aspherical surface, and a deviation between the aspherical surface and a proximal spherical surface thereof is set as the surface shape error.

6. The imaging lens set forth in claim 1, wherein the plurality of lenses are glass lenses containing no harmful substances such as lead or arsenic.

7. An image reading apparatus comprises the imaging lens set forth in claim 1 as an image reading lens.

8. An image forming apparatus comprises the imaging lens set forth in claim 1 as an image reading lens.

9. The image reading lens set forth in claim 7 reads a draft image in fill color.

10. The image reading lens set forth in claim 8 reads a draft image in full color.