Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1998-09-16
2002-02-19
Evans, F. L. (Department: 2877)
Optical: systems and elements
Diffraction
From grating
Reexamination Certificate
active
06349000
ABSTRACT:
FIELD OF THE INVENTION
First Invention
The first invention relates to a diffraction lens (also referred to as “lens with grating element” or “lens with diffraction element” in the following), in particular to a calculation (simulation) technique for calculating a diffraction efficiency of a diffraction lens that is cut with a diamond bit or molded using a die that is cut with a diamond bit, and a design technique for designing achromatic lenses.
Furthermore, the first invention relates to a lens with a grating element, particularly to a small size imaging apparatus such as a board camera or a monitoring camera etc. and a reading apparatus such as a bar code reader etc.
Second Invention
The second invention relates to an optical system for reading in which chromatic aberration is excellently corrected, and to an image reading apparatus and a bar code reader using the same.
BACKGROUND OF THE INVENTION
First Invention
In an optical system for imaging or an optical system for reading, imaging performance is of great importance. As factors that influence the imaging performance, there are those inside the optical system such as aberration of lenses, diffraction and dust, and those outside the optical system such as environmental conditions. Particularly, chromatic aberration due to different refractive indices of a lens at different wavelengths is one cause of deteriorating imaging performance.
Accordingly, conventional techniques try to reduce the chromatic aberration by combining several lenses having different Abbe numbers, and among other technologies, it is known that an anomalous dispersion glass may be used as an achromatic lens system.
Also, recently, as another technology for reducing chromatic aberration, a lens with diffraction element where a relief for providing diffractive effect is formed on a surface of the lens to correct chromatic aberration has been proposed. For example, in Publication of Unexamined Japanese Patent Application (Tokuhyo) No. Hei 8-508116, it is proposed to correct chromatic aberration in the entire visible spectrum by a single lens with diffraction element.
Recently, a large number of achromatic lenses and dual-focus lenses have been proposed where lens functionality is enhanced with diffraction lenses (see e.g. Publication of Unexamined Japanese Patent Application No. Hei 8-171052 and Japanese Patent Application No. Hei 8-290080). Most of these diffraction lenses are so-called relief-type diffraction lenses having a periodic relief on a surface of a lens or flat plate of, for example, glass.
There are basically two methods for forming a relief-type diffraction lens. One method is to cut the lens with a diamond bit. In this case, a saw-tooth-shaped relief (relief profile) can be cut. The other method involves photolithography and approximates this saw-tooth-shaped relief with a step relief. This is also called “binary method”.
Diffraction efficiencies are important parameters for the utilization and the design of diffraction lenses.
It is widely known, that according to Swanson et al (G. J. Swanson and Wilfrid B. Veldkamp, “Diffractive optical elements for use in infrared systems”, Optical Engineering, Vol. 28, No. 6, (1989)), the relation between the number of masks used during manufacturing and the diffraction efficiency can be calculated for the binary method.
The retardation of the wave front passing a periodic relief-type diffraction grating with a grating ring interval (pitch) that is sufficiently longer than the wavelength and a phase shift of about one wavelength can be calculated from the refractive index of the grating material on the basis of its cross-section. It is widely known (see e.g. M. C. Hutley, “Diffraction Grating”, Academic Press, Chap. 2, 1982) that when the retardation is Fourier-transformed, the diffraction efficiency of the diffraction grating can then be obtained as the Fourier coefficients (scalar diffraction theory).
FIG.
49
(
a
) outlines how a die for the diffraction lens is cut with a diamond bit. A die
1901
, which rotates in the arrow direction, is cut by a diamond bit
1902
. The diamond bit has a pointed tip, which is suitable for cutting diffraction lenses or dies for diffraction lenses.
FIG.
49
(
b
) is a magnification of FIG.
49
(
a
) showing a cutting region A. The tip
1903
of the diamond bit describes a circular arc with a certain curvature radius (nose radius)
1904
. Even when the designed shape is a saw-tooth shape as indicated by the chain double-dashed line
1905
, the dent left by the diamond bit is a circular arc
1906
that has almost the same radius as the curvature radius of its tip.
FIG. 50
shows cross-sections outlining how a diffraction lens and a die are cut. For the sake of simplicity, the diffraction lens is formed on a planar substrate.
When the designed shape of the lens is as shown in FIG.
50
(
a
), the designed shape of the die for manufacturing the lens is as shown in FIG.
50
(
b
). However, when the die is cut with a diamond bit
2001
whose tip is a circular arch with a certain curvature radius, the convex angular portions in the cross-section of the die will be rounded out, as shown in FIG.
50
(
c
). As a result, lenses that are formed with that die have a relief profile as shown in FIG.
50
(
d
).
FIG.
50
(
e
) is a magnification of the cross-section shown in FIG.
50
(
c
), showing the microscopic features of the cutting region A after the cutting. Depending on the feed speed and the curvature radius of the cutting bit, cutting traces
2002
amounting to a tiny undulation remain on the cut surface. These cutting traces are transferred to the lens surface.
Since the diffraction efficiency of the diffraction lens is influenced by the relief profile, it may turn out to be quite different from the designed value, if the relief profile degenerates like this during the manufacturing step.
As mentioned above, the cutting bit can have a pointed tip to avoid a change of the diffraction efficiency, but then, many technically difficult problems arise. For example, the necessary cutting distance becomes long, the degeneration due to abrasion of the cutting bit becomes large, and the cutting bit chips more easily. As a result, the productivity becomes considerably worse.
If the relationship between the curvature radius of the cutting bit and the diffraction efficiency of the obtained diffraction lens were known, it could be decided before the cutting process which cutting bit should be chosen to keep the decrease in diffraction efficiency during the manufacturing process in a tolerable range, and the used bit would not have to be sharper than necessary, which would be very useful for the production efficiency.
If, at the design stage, the diffraction efficiency of the lens could be calculated with consideration to the processing method, then the processing method could be taken into account as one of the lens design parameters and lenses that are easier to manufacture could be designed. Consequently, there is a need for an easy calculation method for calculating, at the design stage, the finally attained diffraction efficiency with consideration to the processing method.
A typical example of an application for a diffraction lens is the use as an achromatic lens to correct the chromatic aberration of a refractive lens with the chromatic aberration of the diffraction lens. Such lenses are known, for example from Publications of Unexamined Japanese Patent Publication No. Hei 6-242373 and No. Hei 8-171052. In the lenses disclosed in both of these publications, the number of grating rings is large, so that it is difficult to cut a die for the lens using, for example, a diamond bit. Moreover, the diffraction efficiency can decrease due to deterioration of the shape because of the curvature at the vertex of the cutting bit. In the above publications, these problems were not addressed by the design considerations, so that it was difficult to ensure both diffraction efficiency and productivity.
Furthermore, in the above-mentioned conventional technologies, the pitches of the relief rings th
Boku Kazutake
Ono Shusuke
Sasano Tomohiko
Tanaka Yasuhiro
Yamagata Michihiro
Evans F. L.
Merchant & Gould P.C.
Smith Zandra V.
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