Television – Camera – system and detail – Optics
Reexamination Certificate
1998-06-02
2002-09-17
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Optics
C348S335000, C348S241000
Reexamination Certificate
active
06452635
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image pickup apparatus and, more particularly, to an image pickup apparatus having a signal processing device in which an image of an object formed by an image-forming optical system including a diffractive optical element is converted into electric signals by an electronic image pickup device and the electric signals are converted into a displayable image signal.
It is known that a diffractive optical element has the function of diverging an optical path by using a plurality of different orders of diffracted light and also has the function of converging diffracted light through an annular zone-shaped diffraction grating. It is also known that a diffractive optical element arranged to have a light-converging action, for example, can readily produce aspherical waves and is therefore remarkably effective in correcting aberrations, and that because it has substantially no thickness, a diffractive optical element can be disposed in an optical arrangement with a high degree of freedom and is therefore useful to construct a compact optical system, and further that because the dispersion characteristic quantity of diffractive optical elements which is equivalent to the Abbe's number in refracting lenses is a negative value, a diffractive optical element as combined with a refracting optical system is markedly effective in correcting chromatic aberrations. Techniques of improving the performance of optical systems by using these features of diffractive optical elements are described in detail, for example, in Binary Optics Technology: The Theory and Design of Multi-Level Diffractive Optical Element, Gary J. Swanson Technology Report 854, MIT Lincoln Laboratory, August. 1989. Further, Japanese Patent Application Unexamined Publication (KOKAI) Nos. 6-331941 and 6-324262 are known as prior arts that include such a diffractive optical element as an image-forming lens.
Japanese Patent Application Unexamined Publication (KOKAI) No. 4-9803 and Japanese Patent No. 2,524,569 (B2) are known as prior arts that make use of a diffractive optical element to diverge an optical path by using a plurality of different orders of diffracted light. The former prior art uses a diffractive optical element as a low-pass filter for removing moire from an electronic image pickup device. The latter prior art uses a diffractive optical element as a color separation optical system.
In general, light incident on a diffractive optical element is split into a plurality of different orders of diffracted light. In a case where a diffractive optical element is constructed as a lens element, for example, the fact that there are a plurality of different orders of diffracted light is equivalent to that there are a plurality of focal points. This is unfavorable for an image-forming optical system except a special case.
In a case where it is intended to construct an optical system using a specific order of diffracted light and where other orders of diffracted light have an adverse effect on the desired optical performance, it is necessary to remove diffracted light other than the specific order. In this regard, it is known that as shown in the sectional view of
FIG. 1
, the sectional configuration of a relief pattern A for diffraction is formed into a sawtooth shape (blazed), thereby concentrating the energy of light on a specific order of diffracted light and preventing other orders of diffracted light from being produced.
However, even if the sectional configuration of the relief pattern is formed into a sawtooth shape as shown in
FIG. 1
, a wavelength at which the light energy can be concentrated to the full (hereinafter referred to as “optimization wavelength”) varies according to the depth of the sawtooth grooves, and it is impossible to concentrate the energy of light in a band having a wavelength width on a specific order of diffracted light. This phenomenon gives rise to no problem in a case where a monochromatic light source, e.g. laser light, is used. However, in the case of an image pickup apparatus which uses white light, e.g. a camera, the diffraction efficiency reduces at wavelengths other than the optimization wavelength, and the light energy disperses to other orders of diffracted light.
FIG. 2
shows the relationship between the diffraction efficiency of first-order diffracted light, which is a working order of diffracted light, and the wavelength with regard to a diffractive optical element having a sectional configuration such as that shown in FIG.
1
. In this case, the relief pattern is for ed on a substrate of glass BK
7
by determining the groove depth such that first-order diffracted light is 100% when the wavelength &lgr; is 530 nanometers. The wavelength band shown in
FIG. 2
extends over a range of from &lgr;=400 nanometers to &lgr;=700 nanometers, which is generally regarded as a visible wavelength region. The diffraction efficiency reduces as the wavelength deviates from the optimization wavelength &lgr;=530 nanometers.
FIG. 3
shows the relationship between the zero-order diffraction efficiency and second-order diffraction efficiency on the one had and the wavelength on the other in this example. It will be understood from
FIG. 3
that the amounts of zero-order diffracted light and second-order diffracted light increase in the short-wavelength region and the long-wavelength region, in which the amount of first-order diffracted light reduces.
If such a diffractive optical element is used as a lens element of an image pickup apparatus which uses white light, e.g. a camera, diffracted light other than a specific working order of diffracted light appears as a colored flare or ghost, causing the image-forming performance to be degraded.
In a case where a diffractive optical element is used in an image pickup apparatus which uses image formation by an image-forming lens at in the above-described example, it is necessary to remove a flare image formed by diffracted light other than a specific working order of diffracted light at wavelengths other than the optimization wavelength, or to reduce the effect of the flare image.
SUMMARY OF THE INVENTION
In view of the above-described circumstances, an object of the present invention is to provide a high-precision image pickup apparatus in which the degradation of optical performance due to unwanted orders of diffracted light at wavelengths other than the optimization wavelength for a diffractive optical element is electrically minimized at high speed and with high accuracy.
To attain the above-described object, the present invention provides an image pickup apparatus including an image-forming optical system which forms an image of an object, and an electronic image pickup device which converts the image into electric signals, The image pickup apparatus further includes a signal processing device for converting the signals obtained from the electronic image pickup device into a displayable image signal. The image-forming optical system includes a diffraction surface having an image-forming action or an action whereby image-forming performance is improved. In order to reduce a diffraction image formed by an unwanted order of diffracted light, which is unnecessary for image formation, in the image converted into the image signal, the signal processing device has a device for storing a point spread function of the unwanted order of diffracted light, and a device for selecting electric signals having an intensity higher than a predetermined value from the electric signals obtained from the electronic image pick up device and for calculating the convolution of an object image formed from the selected electric signals with the point spread function of the unwanted order of diffracted light to obtain a flare signal. The signal processing de ice further has a device for reducing or removing the flare signal from a signal representing the object image.
The function of the above-described arrangement will be explained. The image pickup apparatus according to the present inv
Ho Tuan
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
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