Optical: systems and elements – Lens – With multipart element
Reexamination Certificate
1999-09-24
2001-06-05
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With multipart element
Reexamination Certificate
active
06243215
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a Fresnel lens, in particular, to a Fresnel lens adapted to be placed in front of a display screen of a computer or a television set in order to magnify images displayed thereon for observation.
PRIOR ART
A conventional Fresnel lens has a cross sectional view shown in FIG.
1
. That is, a plurality of annular prism elements Li is coaxially arranged with respect to the optical axis O at a predetermined pitch. The top surface of the annular prism element Li is inclined by an angle &thgr;
i
with respect to the horizontal plane so that said surface forms a part of a spherical surface having a radius of curvature corresponding to the focal distance “f” of the Fresnel lens. When the height of the annular prism element apart from the optical axis O by a distance h
i
is represented by d
i
the smaller h
i
makes the smaller d
i
. A flat circular area having an extremely small diameter is formed on a central part around the optical axis O.
In a Fresnel lens attached before a display screen of a computer or the like in order to magnify images for observation, annular prism elements are arranged at an extremely small pitch in order to prevent Moire fringes from appearing. When a Fresnel lens having annular prism elements arranged at such a small pitch is attached before a display screen, a rainbow-like pattern tends to appear in a specific region of the central part around the optical axis. This rainbow-like pattern distinctively appears when the pitch in arranging the annular prism elements as constituent elements of the Fresnel lens is equal to or less than 0.25 mm. Such a rainbow-like pattern is believed to be produced due to a kind of structure of diffraction grating formed by the fine pitch in arranging the annular prism elements of the Fresnel lens.
OBJECTS AND SUMMARY OF THE INVENTION
The object of the present invention is to provide a Fresnel lens, placed in front of a display screen of a computer or a television set in order to magnify images displayed thereon for observation, which does not cause the rainbow-like pattern to be produced even when the annular prism elements of the Fresnel lens as the constituent element of said Fresnel lens are arranged at a fine pitch.
In the present invention, to solve the problem described above, there is formed a spherical surface having a radius of curvature corresponding to the focal distance of a Fresnel lens formed by annular prism elements of a Fresnel lens within a circular area defined by a predetermined radius around the optical axis, where the rainbow-like pattern would otherwise appear due to diffraction. In the present invention, the predetermined pitch in arranging a plurality of annular prism elements of the Fresnel lens is equal to or less than 0.25 mm and the height of the annular prism elements in said circular area is equal to or less than 5 &mgr;m, preferably equal to or less than 7 &mgr;m. Further, in a specific aspect of the present invention, the Fresnel lens has the focal distance of 500 mm.
For a typical Fresnel lens of
FIG. 1
having the focal distance of 500 mm, relationships between the prism angle &thgr;
i
and the prism height d
i
for various pitches in arranging prism elements are shown in Table
2
. According to the results of the experimental verification conducted by the inventor, the diffraction causing the rainbow-like pattern described above was observed within the range where the prism height d
i
is equal to or less than 7 &mgr;m under the condition that the prism elements are arranged at a pitch equal to or less than 0.25 mm, and it was found that a practically disturbing effect appeared in the range where the prism height d
i
was equal to or less than 5 &mgr;m.
Therefore, a Fresnel lens according to the present invention prevents this diffraction and thereby prevents the rainbow-like pattern from appearing by forming a spherical surface having a radius of curvature corresponding to the focal distance of lens formed by said annular prism elements within a circular area defined by a predetermined radius around the optical axis of the Fresnel lens. In a Fresnel lens having the focal distance of 500 mm, for example, when the prism elements are arranged at a pitch of 0.25 mm, the prism element height d
i
is equal to or less than 7 &mgr;m within the circular area defined by the diameter of 14 mm around the optical axis.
REFERENCES:
patent: 3744882 (1973-07-01), Forster, Jr.
patent: 4775229 (1988-10-01), Ichihara et al.
patent: 4904069 (1990-02-01), Nakata
patent: 5796520 (1998-08-01), Maruyama
patent: 3 827 834 A1 (1990-02-01), None
patent: 9-258010 (1997-10-01), None
patent: 10-232432 (1998-09-01), None
Fukuda Junji
Hirota Kashichi
Seto Susumu
Epps Georgia
Frishauf, Holtz Goodman, Langer & Chick, P.C.
Kyowa Electric and Chemical Co. Ltd.
Lester Evelyn A.
LandOfFree
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