Optical: systems and elements – Diffraction – From zone plate
Reexamination Certificate
2001-09-04
2004-02-17
Juba, John (Department: 2872)
Optical: systems and elements
Diffraction
From zone plate
C359S569000, C359S570000, C359S738000
Reexamination Certificate
active
06693744
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an imaging element for making an off-axis beam with a field angle become incident on a diffraction optical element and, more particularly, to an imaging element suitable for a variety of optical systems such as a photographic camera, video camera, binocular, projector, telescope, microscope, and copying machine, in which beams in the wavelength range used concentrate on a specific order (design order) and a high diffraction efficiency can be obtained in the wavelength range used.
The present invention also relates to an image reading apparatus which has a controller for generating various control signals in an apparatus used for a copying machine, image scanner, facsimile apparatus, multifunctional printer and the like each of which has an imaging element having a diffraction optical element.
2. Related Background Art
An optical system has a variety of aberrations, and optical elements are so assembled as to correct these aberrations. Of all the aberrations generated in an optical system, chromatic aberration is conventionally reduced by combining glass materials having different dispersion characteristics. In the objective lens of a telescope or the like, a low-dispersion glass material and a high-dispersion glass material are used to form positive and negative lenses, respectively, and these lenses are combined to eliminate on-axis chromatic aberration. When the number of constituent lenses is limited, or usable glass materials are limited, chromatic aberration cannot be satisfactorily eliminated.
As opposed to the conventional method of reducing chromatic aberration by a combination of glass materials, methods of reducing chromatic aberration by arranging a diffraction optical element (to be referred to as a diffraction grating hereinafter) having diffraction action on a lens surface or in part of an optical system are disclosed in SPIE Vol. 1354 International Lens Design Conference (1990), Japanese Laid-Open Patent Application Nos. 4-213421 and 6-324262, and U.S. Pat. No. 5,044,706. Such a method uses a physical phenomenon wherein the refraction and diffraction surfaces of an optical system exhibit chromatic aberration outputs in opposite directions with respect to a light ray having a given reference wavelength.
This will be briefly described with reference to
FIG. 9. A
diffraction optical element
11
is placed in air with a refractive index of 1 and is perpendicular to an optical axis
13
. Diffracted light emerges in a diffraction direction &thgr; of a light ray A parallel to the optical axis
13
:
P sin &thgr;=m&lgr; (1)
where P is the periodic pitch of the diffraction grating
12
, m is the order of diffracted light, and &lgr; is the wavelength.
FIG. 9
shows the periodic structure in only one direction. When such a periodic structure is built rotationally symmetrically about the optical axis or the like, and the periodic pitch of the diffraction grating is gradually changed, the resultant annular structure having this periodic structure serves as a lens. A lens using such diffraction action has a larger diffraction angle at a longer wavelength with a given order according to equation (1). With this lens, the positional relationship between the imaging points depending on wavelengths is opposite to that of a refractive lens having power in the same direction. The above references mainly use this principle to correct aberration (chromatic aberration).
In refraction, one light ray is one light ray upon refraction. In diffraction, however, one light ray is diffracted into light components of the respective orders. When a diffraction optical element is used as a lens system, the diffraction grating structure is determined so that the beams in the wavelength range used concentrate on a specific order (to be referred to as a design order hereinafter). When the intensities of light beams concentrate on the specific order, the direction of remaining diffracted light is represented by equation (1), but its intensity is low. When the intensity is zero, no diffracted light is present.
To increase the diffraction efficiency of mth-order diffracted light, if a phase difference of 2 &pgr;m is imparted to the optical-path light rays in the diffraction direction, the light rays are brought to interference and strengthened.
To impart a phase difference of 2 &pgr;m to mth-order diffracted light in a transmission diffraction grating, the following condition must be satisfied:
2
&pgr;m
=2
&pgr;d
(
n
−1)/&lgr; (2)
where d is the height of the grating and n is the refractive index of the material of the grating. When condition (2) holds between the respective pitches, the diffraction efficiency is maximized.
The detailed structure of a diffraction optical element for obtaining this diffraction action is called a kinoform. Known examples of the kinoform are a kinoform with a continuous portion for imparting a phase difference of 2 &pgr;, a kinoform having a binary shape approximating a continuous phase difference profile stepwise, and a kinoform obtained by approximating a fine periodic structure into a triangular shape. Such a structure is formed on the surface of a flat plate or the surface of a lens to exhibit the diffraction effect. Such a diffraction optical element is manufactured by cutting or a semiconductor process such as lithography.
Such a diffraction optical element is excellent in effect for particularly correcting chromatic aberration occurring on a refraction surface upon glass material dispersion. The period of the periodic structure is changed to exhibit the effect of an aspherical lens. The periodic structure can greatly reduce the aberration.
The known examples reduce various aberrations and particularly chromatic aberration due to the diffraction effect. The effect of incorporating a diffraction optical element in an optical system can be confirmed on an aberration chart. When the diffraction efficiency of diffracted light contributing to a reduction in aberrations is not high, this diffracted light is not present in practice. The diffraction efficiency of a light ray for reducing aberration must therefore be sufficiently high. When light rays with an order different from the design order are present, these light rays form an image at a position different from that formed by the light ray of the design order to cause flare or ghost, thereby reducing the image contrast. The diffraction efficiency profile and the behavior of light rays with orders different from the design order must also be carefully considered.
FIG. 10
shows the spectral transmission characteristic of a general optical system. In
FIG. 10
, the wavelength is plotted along the abscissa and the spectral transmittance is plotted along the ordinate. This spectral transmission characteristic is determined by light absorption and reflection at a refraction surface of glass. A spectral transmission characteristic matching an evaluation target in the wavelength used is required for this optical system.
FIG. 11
shows a diffraction efficiency characteristic with respect to a specific diffraction order when a diffraction optical element is formed on a given surface. In
FIG. 11
, the wavelength is plotted along the abscissa and the diffraction efficiency is plotted along the ordinate. This diffraction optical element is designed so that the diffraction efficiency maximizes in the first order (indicated by a solid line in
FIG. 11
) in the wavelength range used. That is, the design order is the first order. The diffraction efficiencies of the diffraction orders (first order±first order) adjacent to the design order are also shown in FIG.
11
. As shown in
FIG. 11
, the diffraction efficiency in the design order maximizes at a given wavelength (to be referred to as a design wavelength hereinafter) and gradually decreases at remaining wavelengths due to the following reason. Although the thickness of the grating which makes the phase difference 2&pgr; is exhibited in equatio
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