Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
1998-09-23
2001-02-06
Mack, Ricky (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S566000
Reexamination Certificate
active
06185043
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an optical system and an image pickup apparatus including the same. More particularly, the present invention relates to an optical system which includes a relief type diffractive optical element having a protective layer provided on a relief surface and on which band light is incident. The present invention also relates to an image pickup apparatus including the optical system.
Recently, diffractive optical elements, particularly relief type diffractive optical elements, have been widely used for the purpose of correcting aberrations and achieving compact optical systems. As shown in the sectional view of
FIG. 1
, a typical relief type diffractive optical element diffracts light by a relief pattern that is formed on a surface thereof with a depthwise structure. The relief pattern consists of very fine pattern elements. When visible light is used, the relief pattern pitch is generally of the order of from several microns to several hundred microns.
Because of the fine pattern formed thereon, the surface of the relief type diffractive optical element is readily stained. That is, dust or other foreign matter is likely to collect in the bottoms of the grooves. Unlike ordinary refracting lenses, it is difficult to remove stains, e.g. fingerprints, from the surface of the relief type diffractive optical element. Projections that constitute the relief pattern are readily chipped and hence fragile. For this reason, when such a diffractive optical element is used in an image pickup optical system of a camera, for example, the diffraction surface provided with a relief pattern cannot be placed at a first surface on the object side.
One approach to solve such a problem is to form a protective layer on the diffraction surface so as to level the surface. In Japanese Patent Application Unexamined Publication (KOKAI) No. 2-43503, for example, a protective layer is provided on a grating layer that has a relief grating pattern formed on a surface thereof to level the pattern surface.
However, when a protective layer is formed on a diffraction surface, the optical performance may be degraded. In an optical system using such a diffractive optical element, intense flare or ghost is likely to appear, causing the image quality to be degraded.
The relief type diffractive optical element has the nature that the diffraction efficiency for a certain order of diffracted light reaches a maximum at a certain wavelength with respect to a certain groove depth of the relief configuration, and the diffraction efficiency for that order of diffracted light reduces as the wavelength deviates from that wavelength. In the case of a diffractive optical element having a sawtooth sectional configuration as shown in part (a) of
FIG. 1
, for example, the diffraction efficiency for m-th order light at wavelength &lgr;
0
reaches 100% when the diffractive optical element is placed in the air and the groove depth d satisfies the following equation (10):
d=m&lgr;
0
/{n
1
(&lgr;
0
)−1} (10)
where n
1
(&lgr;
0
) is the refractive index at the wavelength &lgr;
0
of the material constituting the relief surface of the diffractive optical element. The wavelength &lgr;
0
at which the diffraction efficiency reaches a maximum will be hereinafter referred to as “optimization wavelength”.
FIG. 2
shows the diffraction efficiency for first-order light in a visible wavelength region when the optimization wavelength &lgr;
0
is 510 nanometers. In this case, the diffractive optical element is assumed to be formed from BSL7 (manufactured by OHARA K. K.; n
d
=1.51633, and v
d
=64.1). As will be clear from
FIG. 2
, the diffraction efficiency is 100% at a wavelength is 510 nanometers, but it reduces in longer and shorter wavelength regions. In these regions, a quantity of light corresponding to the reduction in the diffraction efficiency appears as other orders of diffracted light, mainly zero-order light and second-order light.
FIG. 3
shows the diffraction efficiencies for zero-order light and second-order light in the case of the optimization wavelength &lgr;
0
. For zero-order light, the diffraction efficiency increases in the long wavelength region; for second-order light, the diffraction efficiency increases in the short wavelength region.
Accordingly, if light having a wide wavelength range, i.e. white light, is made incident on the diffractive optical element, unwanted orders of diffracted light (in this case, mainly zero-order light and second-order light) other than a working order of diffracted light (in this case, first-order light) used for image formation occur, causing flare or ghost. Consequently, the image quality is degraded.
Let us consider a case where, as shown in
FIG. 4
, a protective layer made of a material different from the material constituting the relief type diffractive optical element is formed on the diffraction surface of the diffractive optical element. Assuming that the refractive index of the material constituting the protective layer is n
2
, it is necessary for the groove depth d′ to satisfy the following equation (11) in order to allow the diffraction efficiency for m-th order of diffraction to reach 100% at the optimization wavelength &lgr;
0
as in the case of the diffractive optical element having no protective layer:
d′=m&lgr;
0
/{n
1
(&lgr;
0
)−
n
2
(&lgr;
0
)} (11)
In a case where the relief type diffractive optical element is formed from BSL7 and the protective layer is PC (polycarbonate), for example, if the optimization wavelength &lgr;
0
is 510 nanometers, the diffraction efficiency for first-order light in a visible wavelength region is such as that shown by curve {circle around (1)} in FIG.
5
. It should be noted that curve {circle around (2)} in the figure represents the diffraction efficiency for first-order light in a case where a protective layer is not provided, which is the same as the curve in FIG.
2
. As will be clear from
FIG. 5
, when a protective layer is provided, the diffraction efficiency also reduces as the wavelength deviates from the optimization wavelength, and the rate of reduction in the diffraction efficiency is higher than in the case of the diffractive optical element having no protective layer. The rate of reduction in the diffraction efficiency is particularly high in the short wavelength region. The diffraction efficiencies for zero-order light and second-order light are such as those shown in FIG.
6
. The diffraction efficiencies for zero-order light and second-order light increase by an amount corresponding to the reduction in the diffraction efficiency for first-order light.
Thus, when a protective layer is provided on a diffraction surface of a relief type diffractive optical element, the wavelength dependence of the diffraction efficiency increases. Consequently, it is likely that the diffraction efficiency for a working order of diffracted light used for image formation will reduce in short and long wavelength regions with respect to the optimization wavelength and that the diffraction efficiency for unwanted orders of diffracted light, which are unnecessary for image formation, will increase in the short and long wavelength regions. Accordingly, provision of a protective layer may enhance the adverse effect of unwanted-order light on the image. That is, it may intensify flare or ghost, causing the degradation of the image quality to be aggravated.
SUMMARY OF THE INVENTION
In view of the above-described circumstances, an object of the present invention is to provide an optical system capable of preventing degradation of the image quality and minimizing the effect of unwanted-order light even when a protective layer is provided on a diffraction surface, thereby obtaining an image of high quality. Another object of the present invention is to provide an image pickup apparatus including the optical system.
To attain the above-described object, the present invention provides an optical system on whic
Mack Ricky
Olympus Optical Co,. Ltd.
Pillsbury Madison & Sutro LLP
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