Optical: systems and elements – Lens – Microscope objective
Reexamination Certificate
2001-11-13
2003-02-11
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
Microscope objective
C359S661000, C359S791000
Reexamination Certificate
active
06519092
ABSTRACT:
This application claims the benefit of Japanese Patent application No. 2000-347095 which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an immersion microscope objective lens, and particularly, to an objective lens which is suitable for an immersion microscope for observing an optical path between the objective lens and an object to be observed in a state that the optical path is filled with liquid such as water or oil.
2. Related Background Art
In an immersion microscope objective lens of high magnification, a piano-convex lens having a relatively low refractive index is embedded in a lens at the tip end in order to correct a curvature of field and a negative refracting power is given to a cemented surface by means of a difference of refractive index between the two lenses, thereby reducing a Petzval sum. Further, a pair of concave surfaces facing each other are disposed at positions with a small diameter of light flux. The Petzval sum is decreased by the strong refracting power of these concave surfaces.
That is, an arrangement which combines the reduction of a Petzval sum by using an embedded lens with that by using a so-called Gauss type lens is known as a publicly known art. An apochromat type objective lens of high magnification which serves as such an immersion microscope objective lens is disclosed in, for example, the Japanese Patent Application Laid-Open No. 2000-35541.
Generally, when a Petzval sum is to be reduced by an embedded plano-convex lens at the tip end, it is advantageous that a meniscus lens into which the plano-convex lens is embedded should have as great thickness as possible and should be formed of an optical material having a higher refractive index. In this case, however, the spherical aberration of a color developed on a convex surface with a strong curvature facing the image side of the meniscus lens in which the plano-convex lens is embedded is increased.
The above phenomenon is caused by the fact that a dispersion (a difference of refractive index by a wavelength) tends to be relatively great when the refractive index of an optical material is high, and the fact that, when the meniscus lens is made thick, the height h of a light flux on the refractive surface on the air side thereof becomes great. Also in the conventional immersion microscope objective lens disclosed in the above patent application, aberrations are not sufficiently corrected and, particularly, a high-order component of a spherical aberration (a different of spherical aberration by a wavelength) of a color remains.
SUMMARY OF THE INVENTION
The present invention has been contrived taking the above problems into consideration, and an object thereof is to provide an immersion microscope objective lens which has a high magnification and a high numerical aperture, with excellently corrected aberrations over the wide field of view, and has excellent imaging performance.
In order to solve the above problems, according to the present invention, there is provided an immersion microscope objective lens comprising a first lens group G
1
having a positive refracting power, a second lens group G
2
having a positive refracting power and a third lens group G
3
having a negative refracting power in the order from the object side, characterized in that:
the first lens group G
1
has a first cemented lens comprising a plano-convex lens with the flat surface facing the object side and a meniscus lens with the convex surface facing the image side cemented together, a positive lens with the surface having a larger refractive index facing the image side, and a second cemented lens comprising a negative lens and a positive lens cemented together;
the second lens group G
2
has a cemented lens comprising a negative lens and a positive lens cemented together; and
the refractive index n12 of the meniscus lens in the first lens group G
1
with respect to the d-line satisfies the following condition:
n12>1.9.
According to a preferred embodiment of the present invention, the following condition is satisfied:
5
<|f
31|
/F<
25,
where the focal length of the negative lens for constituting the second cemented lens in the first lens group G
1
is f31, and the focal length of the entire objective lens system is F.
Also according to a preferred embodiment of the present invention, the second lens group G
2
has a third cemented lens composed of a negative lens and a positive lens cemented together, a fourth cemented lens comprising a negative lens and a positive lens cemented together, and a fifth cemented lens comprising a negative meniscus lens, a biconvex lens, and a biconcave lens cemented together, in the order from the object side.
Further, according to a preferred embodiment of the present invention, the third lens group G
3
has a sixth cemented lens comprising a positive lens with the convex surface facing the object side and a negative lens with the concave surface facing the image side cemented together, and a seventh cemented lens comprising a negative lens with the concave surface facing the object side and a positive lens with the convex surface facing the image side cemented together, in the order from the object side; and the following conditions are satisfied:
n71>1.49; and
n82>1.6,
where the refractive index of the positive lens in the sixth cemented lens with respect to the d-line is n71 and the refractive index of the positive lens in the seventh cemented lens with respect to the d-line is n82.
Also, according to a preferred embodiment of the present invention, the refractive index n11 of the plano-convex lens in the first lens group G
1
with respect to the d-line satisfies the following condition:
n11>1.55.
In an immersion microscope objective lens according to a representative embodiment of the present invention, the first lens group G
1
consists of three lens components. The first lens component is a so-called embedded lens component, which is the first cemented lens comprising a plano-convex lens with the flat surface facing the object side, and a meniscus lens in which this plano-convex lens is embedded cemented together.
In this case, due to an arrangement that the plano-convex lens has substantially the same refractive index as that of the used liquid (immersion liquid), little spherical aberration is generated advantageously on an interface between the liquid and the plano-convex lens. Further, since a strong negative refracting power is applied onto a cemented surface with the meniscus lens in which the plano-convex lens is embedded, a Petzval sum is reduced.
A light flux having passing through the first lens component is given a moderate refractive action by the second lens component which is a single lens having a positive refracting power. On this occasion, an angle of deviation of a marginal light beam is arranged to be as small as possible due to the arrangement that the positive lens for constituting the second lens component has the surface having a greater refracting power facing the image side.
The third lens component is adapted to correct a longitudinal chromatic aberration and a spherical aberration of a color generated on the convex surface having a strong curvature on the image side in the first and second lens components by the use of the second cemented lens which is constituted by the negative lens and the positive lens having approximately the same refractive indexes and different dispersions cemented together.
On the other hand, in the second lens group G
2
, a light emitted from the first lens group G
1
is converted to a convergent light and a cemented lens consisting of a negative lens and a positive lens cemented together is disposed, thereby correcting a spherical aberration and a longitudinal chromatic aberration. In this case, it is the most preferable in terms of the aberration correction mentioned above to form the second lens group G
2
of the two sets of cemented lenses consisting of negative lenses and positive lenses cemented together, th
Lester Evelyn A
Nikon Corporation
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