Abrading – Precision device or process - or with condition responsive... – Computer controlled
Reexamination Certificate
2002-04-10
2004-11-09
Shakeri, Hadi (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
Computer controlled
C451S042000
Reexamination Certificate
active
06814650
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to polishing optical surfaces.
2. Description of the Prior Art
A lens, for example an ophthalmic lens, has two opposite optical surfaces connected by an edge surface that is generally inscribed in a circular cylinder.
At present a distinction is drawn between four categories of optical surfaces, namely:
spherical surfaces, which are well known in the art,
aspherical surfaces, which are derived from spherical surfaces,
toric surfaces, and
atoric surfaces, which are derived from toric surfaces
To facilitate an understanding of the following disclosure, one example of the geometrical construction of a toric surface is described next, with reference to FIG.
1
.
A torus T, only a portion of which is shown, is obtained by rotation of a circle of radius R
2
about an axis Al in the plane of said circle.
The point on the circle at the greatest distance from the axis A
1
traces out a circle of radius R
1
. The radii R
1
and R
2
are respectively referred to as the larger radius and the smaller radius of the torus T.
In this representation, R
1
is much greater than R
2
.
The circles of radius R
1
and R
2
are respectively in a plane P
1
perpendicular to the axis Al and a plane P
2
containing the axis Al, and the planes P
1
and P
2
intersect along a straight line A
2
.
A cylinder with axis A
2
and of radius R
3
(which here is much less than the radius R
2
) intersects the torus T along a curve C delimiting a toric surface S, which has two plane symmetries: one with respect to the plane P
1
, and the other with respect to the plane P
2
.
The intersection of the toric surface S with the plane P
1
is a circular arc of radius R
1
, referred to as the larger meridian M
1
of the toric surface S, and the intersection of the toric surface S with the plane P
2
is a circular arc of radius R
2
, referred to as the smaller meridian M
2
of the toric surface S.
The larger meridian M
1
has a curvature C
1
whose value is equal to the reciprocal of the larger radius R
1
and the smaller meridian M
2
has a curvature C
2
whose value is equal to the reciprocal of the smaller radius R
2
.
Clearly the curvatures of the meridians M
1
and M
2
, which are referred to as the main meridians, are sufficient for a complete definition of the shape of the toric surface S, which is concave in the direction of the axis Al and convex in the opposite direction.
If the toric surface is that of a lens made from a material having a refractive index n, two dioptric powers D
1
and D
2
for the surface S are defined, on the basis of the curvatures C
1
and C
2
, by the following equations:
D
1
=(n−1)C
1
, and
D
2
=(n−1)C
2
.
In the following disclosure, a given surface is considered to be atoric if there is a toric surface which has an offset at any point relative to said atoric surface whose absolute value is less than a chosen value. Here this value is chosen arbitrarily as 0.2 mm for a diameter of 80 mm, but it can be slightly different without departing from the scope of the invention.
At present, optical surfaces have extremely severe constraints on their accuracy, on the one hand with regard to their shape, for which the tolerances are of the order of one micrometer (1 micrometer=10
−6
meter), and on the other hand with regard to their roughness, for which the tolerances are of the order of one nanometer (1 nanometer=10
−9
meter).
After roughing out the atoric surface by appropriate machining, the roughness of the roughed out surface is reduced by a polishing step, possibly preceded by a clear polishing step.
The polishing is delicate because it must reduce the roughness of the surface without deforming it.
An optical surface with circular symmetry, such as a spherical surface, can be polished by means of a tool having a polishing surface with a shape complementary to that of the optical surface, the tool and/or the lens being rotated about the axis of symmetry of the optical surface so that the polishing surface rubs against the optical surface.
On the other hand, polishing other types of optical surface gives rise to more problems.
A distinction is drawn between two categories of clear polishing and polishing tools, namely a first category of tools whose diameter is small compared to that of the lens and a second category of tools whose diameter is close to, or possibly greater than, that of the lens. The two categories of tools give rise to totally different clear polishing and polishing techniques, respectively.
Illustrating the first category, the Japanese document JP-09 396 666 discloses a clear polishing tool for an aspherical convex lens and which comprises:
a basic substrate,
an elastic member adhering to the surface of the substrate, and
a surface member adhering to the surface of the elastic member.
The curvature of a spherical surface for the basic substrate, the elastic member and the surface member is identical to a spherical surface of which the working surface of an aspherical surface lens is an approximation.
During the clear polishing process, the lens is rotated and the tool is simultaneously pressed against the working surface.
Because the tool is small compared to the lens, it is necessary to provide a complex kinematic system so that the tool is swept over the whole of the working surface. This proves to be a long and complicated process.
Furthermore, given the relative rotation of the tool and the lens, the tool tends to deform the surface of the lens and impart its own spherical shape to it, at least locally, and the tool is therefore difficult to use on toric or atoric surfaces.
The invention aims to propose a polishing tool and a polishing method using that tool to polish an atoric surface quickly and uniformly whilst at the same time conforming to the constraints on accuracy mentioned above.
Machining mineral glass lenses requires the removal of more material than machining organic glass lenses and causes subsurface microcracks to appear, requiring a longer polishing time to eliminate them, which leads to deformations and inaccuracies in the final shape of the surface of the lens.
The invention is therefore preferably applied to organic glass lenses, which do not have the drawbacks of mineral glass lenses previously cited.
SUMMARY OF THE INVENTION
In a first aspect, the present invention proposes a tool for polishing an optical surface of a lens, the tool including:
a rigid support including a support surface,
a first layer called the buffer, made from an elastic material, and covering at least part of the support surface and including:
a first surface adhering to the support surface, and
a second surface opposite the first surface,
a second layer called the polisher, covering at least part of the buffer, and including:
a first surface adhering to the second surface of the buffer, and
a second surface called the polishing surface opposite the first surface and adapted to polish the optical surface of the lens by rubbing against it,
wherein the polishing surface is a toric surface and has two circular main meridians with respective curvatures C
1
, C
2
such that the curvature C
1
is much less than the curvature C
2
, and, to be able to polish an atoric optical surface, the buffer is adapted to be compressed elastically and the polisher is adapted to be deformed to espouse the atoric surface.
During polishing, the tool and the surface to be polished are moved relative to each other with two movements in two perpendicular directions, each of which follows one of the meridians of the polishing surface.
According to other features of the tool:
the buffer has a uniform thickness e
T
normal to its second surface and the polisher has a uniform thickness e
P
normal to its polishing surface;
the thickness e
T
of the buffer is from 4 mm to 6 mm;
the thickness e
P
of the polisher is from 0.5 mm to 1.1 mm.
In a preferred embodiment the support surface is a toric surface and has two main meridians coplanar with the main meridians of the polishing surface
Bernard Joël
Huguet Joël
Jeannin Christophe
Essilor International
Shakeri Hadi
Young & Thompson
LandOfFree
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