Fresnel lens

A Fresnel lens in use for a projector lens comprising a glass plate, a Fresnel lens body integrally formed on one surface of the glass plate, and a reflective layer formed on an annularly grooved surface of the lens body. The present invention provides a Fresnel lens, fabricated with a low cost, having sufficient hardness as a document support and free from any of ghost images or image distortions when used in a projector.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a Fresnel lens comprising a glass plate, a 
Fresnel lens body integrally formed on one surface of said glass plate, 
and a reflective layer formed on an annularly grooved surface of said lens 
body. The grooved surface serves as a non-spherical light converging 
reflector with reduced lens material instead of using a thick and bulk 
lens. 
2. Brief Description of the Prior Art 
A conventional Fresnel lens is shown in each of FIGS. 9 and 11. The Fresnel 
lens L.sub.6 shown in FIG. 9 includes a Fresnel lens body 31 of a plastic 
material with a reflective layer 33 formed on an annularly grooved surface 
32, and a hard coating layer 34 provided on the opposite surface of the 
annularly grooved surface 32. 
The Fresnel lens L.sub.6 is fabricated through the following steps. In a 
Fresnel lens mold 35, a plastic resin plate material P1 is placed. After 
being softened, the upper mold 37 is lowered and the plate material P1 is 
press-formed. After being cooled, the press-formed Fresnel lens body 31 is 
derived from the mold 35. On the annularly grooved surface 32, aluminum or 
the like is sputtered to form a reflective surface 33. Finally, a hard 
coating layer 34 is formed on the opposite surface of the annularly 
grooved surface 32. 
FIG. 11 shows another Fresnel lens L.sub.7, in which a Fresnel lens body 39 
of a photo-curing resin is integrally formed on a surface of a plastic 
plate 38. Then, a reflective layer 41 on an annularly grooved surface 40 
and a hard coating layer 42 on the opposite surface of the plastic plate 
38 are formed. 
The Fresnel lens L.sub.7 is fabricated through the following steps. As 
shown in FIG. 12, a photo-curing resin layer is formed on a transfer 
surface 43 of the annularly grooved surface 40 of Fresnel lens body 39, 
and a plastic plate 38 is placed thereon. After hardening of the 
photo-curing layer by illuminating ultra-violet ray, the lens body 39 
formed as an integral member with the plastic plate 38 is taken out from 
Fresnel mold 44. The reflective layer 41 is formed on annularly grooved 
surface 40. Finally, the hard coating layer 42 is formed on the opposite 
surface of the plastic plate 38. 
SUMMARY OF THE INVENTION 
The Fresnel lens of the present invention includes a Fresnel lens body of a 
photocuring or thermosetting resin formed integrally on a glass or 
aluminum plate, or a glass plate is integrally formed with a lens body, 
fabricated with a reduced cost, having sufficient hardness for use as a 
document support, and free from any ghost images when used for a projector 
lens. 
An object of the present invention is to provide a Fresnel lens which 
comprises a glass plate, a Fresnel lens body integrally formed by a 
photo-curing resin on one surface of the glass plate, and a reflective 
layer formed by sputtering of a metal on an annularly grooved surface of 
the lens body. 
Another object of the present invention is to provide a Fresnel lens which 
comprises a glass plate, a Fresnel lens body integrally formed by a 
thermosetting resin on one surface of the glass plate, and a reflective 
layer formed by sputtering of a metal on an annularly grooved surface of 
the lens body. 
A further object of the present invention is to provide a Fresnel lens 
which comprises a glass plate, a Fresnel lens body integrally formed by a 
synthetic resin on one surface of the glass plate, a reflective layer 
formed by sputtering of a metal on an annularly grooved surface of the 
lens body, and a glass plate attached on the reflective layer. 
A still further object of the present invention is to provide a Fresnel 
lens which comprises a Fresnel lens body of a synthetic resin, a 
reflective layer formed on a non-spherical surface of the lens body, and a 
glass plate attached on the reflective layer. 
An object of the present invention is to provide a Fresnel lens, in which 
no hard coating is required since the glass plate itself can be utilized 
as a document support to reduce the process steps for producing Fresnel 
lenses. The glass plate, which is harder than the conventional hard 
coating, will not be scratched when it is used as a document support. 
A still further object of the present invention is to provide a Fresnel 
lens, which will not produce any ghost image (double image), when used in 
a projector using a glass plate, having a higher index of refraction than 
that of any synthetic resins. 
The invention provides a curling-free Fresnel lens which will not produce 
any distortion in the projected image, when used in a projector. 
An object of the present invention is to provide a Fresnel lens, in which a 
glass plate is attached to the lens body of a synthetic resin to prevent 
deformation of the lens body.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The preferred embodiments of the invention will now be described with 
reference to the accompanying drawings. 
FIG. 1 shows an embodiment of a Fresnel lens L.sub.1, in which a reflective 
layer 4 by sputtering of aluminum is formed on an annularly grooved 
surface 3 of a Fresnel lens body 2, which is integrally formed on one 
surface of a glass plate 1. 
The process of forming the Fresnel lens comprises the following steps. 
(1) As shown in FIG. 2, the Fresnel lens press mold 6 has a transfer 
surface 5 for forming an annularly grooved surface 3 of the Fresnel lens 
L.sub.1, which is to be finally obtained. A photo-curing resin 7 is 
applied on the transfer surface 5 of the press mold 6. The thickness of 
the photo-curing resin 7, which is 180 micron meter in the embodiment, is 
actually applicable within the range of 160 to 200 micron meter. 
A glass plate 1 having a thickness of 0.7 mm is then placed on the already 
applied resin layer 7. The applicable range of thickness is 0.5 to 1.0 mm. 
An ultraviolet ray is then illuminated on photo-curing resin layer 7 to 
harden the layer to form the Fresnel lens body 2 which is integral with 
glass plate 1. The glass plate with lens body 2 is separated from the mold 
6. On the annularly grooved surface 3, aluminum is sputtered to form the 
reflective layer 4. 
Advantages of the resulting Fresnel lens L.sub.1 are as follows: 
The glass plate 1 of lens L.sub.1 is used as a document support of a 
projector. Hard coating which was conventionally indispensable is 
therefore unnecessary, thereby reducing process of making Fresnel lens 
L.sub.1. Further, glass plate 1, which is higher in hardness than that of 
the conventional hard coating layer, is free from any scratches, even when 
subjected to a long time use as the support. 
As another advantage, the lens L.sub.1 will not produce any ghost image or 
a double image, because the glass plate 1 is of higher refractive index 
than the resin as material of the conventional Fresnel lens. 
Lens body 2 integrally formed with glass plate 1 is subjected to sputtering 
on its annularly grooved surface 3 in a vacuum chamber, and is formed of a 
photo-curing resin, which can expand when freed from the chamber to the 
air. But such expansion is sustained by the hard glass plate 1 to prevent 
curling of lens body 2. When the Fresnel lens L.sub.1 formed as above is 
used in a projector, the projected image is free from any distortion. 
EXAMPLE 2 
FIG. 3 shows the second embodiment of the invention, in which Fresnel lens 
L.sub.2 is provided with a reflective layer 4 of aluminum, formed by 
sputtering on the annularly grooved surface of lens body 8, which is 
integrally formed of a thermosetting resin on one surface of the lens body 
8. 
In comparison with the first embodiment, the process for forming the lens 
L.sub.2 is different in that a thermosetting resin is used instead of 
photo-curing resin used in the first embodiment. In other points, the 
embodiment is similar to the first embodiment. 
EXAMPLE 3 
FIG. 4 shows the third embodiment of the invention. The Fresnel lens 
L.sub.3 comprises a lens body 12 of a thermosetting resin, a reflective 
layer of aluminum sputtered on the annularly grooved surface 11, and a 
glass plate 13 attached on the reflective layer 11. 
The process for fabricating the lens L.sub.3 is as follows. 
A plate-like material 16 of a thermosetting resin, which is 1.0 mm in 
thickness, is placed within a press mold 15 having a transfer surface 14 
for forming an annularly grooved surface 10 of the lens L.sub.3. 
The resin plate 16 is softened by heating. Then the top mold 17 is moved 
down to press the resin plate 16. 
After cooled, lens body 12 is freed from the mold 15. Aluminum is sputtered 
on the annularly grooved surface 10 to form a reflective surface 11. 
Finally, glass plate 13 is attached on the reflective layer with its 
annular fringe area of the lens body 12. 
The glass plate 12 itself of Fresnel lens L.sub.3 can be used as a document 
support of the projector, so that the conventionally applied hard coating 
is unnecessary, and reduce the steps of manufacturing process. The glass 
plate 13, whose hardness is higher than that of the conventional coating, 
is free from being scratched even when employed for a long time. 
The value of index of refraction of glass plate 13, which is higher than 
that of resin as material of the lens, enables to avoid occurance of ghost 
images (double images), when employed in a projector. 
In addition, even though expanded by absorbing humidity when freed from the 
vacuum chamber to the atmosphere, lens body 12 is prevented from curling 
by rigidity of the attached glass plate 13. 
As Fresnel lens L.sub.3 is of reflective type, the light will not go 
through lens body 12, the curling of which is sufficiently prevented by 
increasing thickness thereof. No distortion can be produced in the 
projected images when employed in a projector. 
EXAMPLE 4 
The fourth embodiment is shown in FIG. 6. The Fresnel lens L.sub.4 
comprises a lens body 18 of a thermoseting resin, and a glass plate 20 
attached on the annularly grooved surface 19 of lens body 18. 
The process for making lens L.sub.4 is similar but different from the third 
embodiment in directly attaching glass plate 20 to lens body 18 without 
providing a reflective layer 11 of the third embodiment. 
The process of sputtering is missing for forming the lens body of the 
embodiment. Therefore, lens body 18 is free from expansion due to 
absorbing humidity. A possible deformation owing to absorption and 
desorption of humidity is restricted by the attached glass plate 20. 
The glass plate 20 may be also attached to the surface opposite to the 
annularly grooved surface of lens body 18. 
EXAMPLE 5 
The fifth embodiment is shown in FIG. 7. As shown, the Fresnel lens L.sub.5 
includes: an annularly grooved surface 23 of the Fresnel lens body 22, 
which is integrally formed on one surface of an aluminum plate 21; a 
reflective layer 24 of aluminum formed on the annularly grooved surface 23 
by sputtering; and a glass plate 25 attached to lens body 22 on its 
reflective layer 24. 
As shown in FIG. 8, the steps for fabricating Fresnel lens L.sub.5 include: 
(1) setting an aluminum plate 21 in the lower mold 26, and a thermo-setting 
resin plate 27 of 1.0 mm in thickness thereon, which is heated until being 
softened; 
(2) lowering the Fresnel mold (the upper mold) 29 for forming the resin 
plate 27 by pressing; 
(3) after cooled, taking out a formed lens body 22 from the Fresnel mold 
29, on the annularly grooved surface 23 of which a reflective layer 24 of 
aluminum is formed by sputtering; and finally 
(4) a glass plate 25 is attached on the reflective layer 24 with its 
annular fringe area adhered to glass plate 25.