Optical lens

An optical lens made of plastic includes an optically functioning part, a flange part and disposed around the optically functioning part. The flange part has an outer circumferential face having a cylindrical surface about a lens optical axis. A gate-removed part is located at the outer circumferential face as an outward-projecting quadratic surface. The gate-removed part is located between a virtual outer circumferential face and a plane passing through a tolerance limit on the lens optical axis side. The virtual outer circumferential face is defined when a residual gate part is cut along a cylindrical surface coaxial with the lens optical axis and having a radius identical to that of the outer circumferential face. The plane is set on the lens optical axis side when the residual gate part is removed by cutting the flange part.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an optical lens made of plastic, which is 
usable in a variety of products ranging from objective lenses of CD-ROM 
drives or the like to large-aperture lenses for CRT or the like. 
2. Related Background Art 
Optical lenses made of plastic, such as the one shown in FIGS. 5 and 6, 
have conventionally been known and widely used for eyepieces of 8-mm video 
cameras, objective lenses of CD-ROM drives, and the like. The optical lens 
101 of FIGS. 5 and 6 is made by injection molding, transfer molding, or 
the like. For making the optical lens 101, a molten resin such as PMMA 
(polymethyl methacrylate) is caused to flow into a cavity from a gate. 
Then, the resin is solidified by cooling. After the resin is solidified, 
the part of resin solidified within the gate orifice and pulled out of the 
die, or both the gate orifice and the part of resin solidified within the 
gate orifice (hereinafter referred to as "residual gate part" 
collectively) are removed. After surface treatment carried out as 
required, the optical lens 101 is completed. 
For processing the above-mentioned residual gate part, it will essentially 
be ideal if the residual gate part 110 is removed alone from the optical 
lens 101 by moving a cutting tool such as end mill along the outer 
circumferential face 104 of the optical lens 101. In general, however, in 
view of the processing accuracy of the processing machine, reduction in 
manufacturing cost, and the like, a part of the outer circumferential part 
(flange part 106) of the optical lens 101 is cut flat as shown in FIG. 5, 
thereby removing the residual gate part 110. Consequently, the outer 
circumferential face 104 of the optical lens 101 completed as a product 
includes a flat, gate-removed part 105 which is defined when the residual 
gate part 110 is removed. 
On the other hand, in general, an optical lens made of plastic absorbs 
moisture (water) existing in the air, due to the hygroscopicity inherent 
in the plastic material. Also, it is known that the moisture is absorbed 
into the optical lens 101 radially from the outer circumferential face 104 
of the optical lens 101 toward the optical axis (a) thereof. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an optical lens which 
exhibits stable optical performances with a uniform refractive index. 
The optical lens in accordance with the present invention is an optical 
lens made of plastic. This optical lens comprises an optically functioning 
part, a flange part formed around the optically functioning part, and a 
gate-removed part formed at the outer circumferential face. The flange 
part has an outer circumferential face formed as a cylindrical surface 
about a lens optical axis. The gate-removed part is defined by removing a 
residual gate part projecting from the outer circumferential face and is 
formed as an outward-projecting quadratic surface. The gate-removed part 
is located between a virtual outer circumferential face and a plane 
passing through a tolerance limit on the lens optical axis side set when 
the residual gate part is removed by flatly cutting the flange part. The 
virtual outer circumferential face is defined when the residual gate part 
is cut along a cylindrical surface which is formed about the lens optical 
axis and has a radius identical to that of the outer circumferential face. 
In the conventional optical lens 101, as shown in FIG. 5, the residual gate 
part 110 is cut together with the outer circumferential part (flange part 
106) of the optical lens 101, whereby the flat, gate-removed part 105 is 
formed at the outer circumferential face 104. The gate-removed part 105 is 
located on the lens optical axis (a) side of the original outer 
circumferential face 104 of the optical lens 101, which is a cylindrical 
surface, for example. As a consequence, the outer circumferential face 104 
of the optical lens 101 as a whole including the gate-removed part 105 
would not be symmetrical about the lens optical axis (a). Namely, within a 
plane passing through the gate-removed part 105 and including the optical 
axis (a) of the optical lens 101, as shown in FIG. 6, the distance (r1) 
from the gate-removed part 105 to the optically functioning part 102 
through which light is effectively transmitted becomes shorter than the 
distance (r2) from the outer circumferential face 104 other than the 
gate-removed part 105 to the optically functioning part 102. 
When the optical lens 101 absorbs moisture (water) in the air in this 
state, the moisture absorbed into the optical lens 101 from the outer 
circumferential face 104 other than the gate-removed part 105 would hardly 
reach the optically functioning part 102. Since the distance from the 
gate-removed part 105 to the optically functioning part 102 is shorter, by 
contrast, the moisture absorbed into the lens from the surface of the 
gate-removed part 105 would permeate into the optically functioning part 
102 in a relatively short time (rapidly). Consequently, at the part of 
optically functioning part 102 near the gate-removed part 105, density 
would change due to the moisture absorbed into the optical lens 101, 
thereby increasing the refractive index. As a result, the refractive index 
in the optically functioning part 102 in the optical lens 101 as a whole 
may fail to become uniform. When the refractive index of the optically 
functioning part 102 is thus not uniform, it may affect aberration, 
thereby failing to yield expected optical performances. In particular, 
this phenomenon would occur remarkably in PMMA (polymethyl methacrylate) 
which is widely used as a material for optical lenses. 
In the optical lens in accordance with the present invention, by contrast, 
the gate-removed part is formed as a quadratic surface, such as 
cylindrical surface, elliptic cylindrical surface, or conical surface, 
which projects outward (toward the side where the residual gate part 
existed). Let a plane passing through a tolerance limit on the lens 
optical axis side set when removing the residual gate part by flatly 
cutting the flange part be a first plane. Also, let a plane including the 
lens optical axis and the center line of the residual gate part be a 
second plane. Then, a boundary line between the outer circumferential face 
of the optical lens and the gate-removed part is located between a line of 
intersection between the outer circumferential face of the optical lens 
and the first plane, and a line of intersection between the virtual outer 
circumferential face and the residual gate part. Also, a line of 
intersection between the gate-removed part and the second plane is located 
between a line of intersection between the virtual outer circumferential 
face (residual gate part) and the second plane, and a line of intersection 
between the first and second planes. Consequently, as compared with the 
case where the residual gate part is removed by flatly cutting the flange 
part, the shortest distance between the gate-removed part and the lens 
optical axis can be made longer more easily. 
As a result, when compared with an optical lens having a flat gate-removed 
part, the moisture absorbed into the lens from the gate-removed part is 
restrained from reaching the optically functioning part. Hence, density 
can be prevented from changing in the optically functioning part due to 
the moisture absorbed into the lens from the gate-removed part, whereby 
the refractive index of the optical lens can be kept uniform. 
Preferably, in this case, the gate-removed part is formed like a 
cylindrical or conical surface. Preferably, the plastic is a polymethyl 
methacrylate resin. 
The present invention will be more fully understood from the detailed 
description given hereinbelow and the accompanying drawings, which are 
given by way of illustration only and are not to be considered as limiting 
the present invention. 
Further scope of applicability of the present invention will become 
apparent from the detailed description given hereinafter. However, it 
should be understood that the detailed description and specific examples, 
while indicating preferred embodiments of the invention, are given by way 
of illustration only, since various changes and modifications within the 
spirit and scope of the invention will be apparent to those skilled in the 
art from this detailed description.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In the following, preferred embodiments of the optical lens in accordance 
with the present invention will be explained in detail with reference to 
the accompanying drawings. 
FIG. 1 is a perspective view showing an optical lens in accordance with the 
present invention. The optical lens 1 shown in FIG. 1 is manufactured, for 
example, by injection molding, transfer molding, or the like using a 
plastic resin such as PMMA (polymethyl methacrylate), polystyrene, 
polycarbonate, amorphous polyolefin, or the like, and thus can be made 
inexpensively. The optical lens 1 has a diameter of about 3 to 8 mm, for 
example. It can also be formed as a large-aperture lens having a diameter 
of about 100 mm to 150 mm, employed in CRT or the like. 
Disposed at the center part of the optical lens 1 is an optically 
functioning part 2 adapted to function as a convex lens, through which 
light is effectively transmitted. Around the optically functioning part 2, 
a flange part 3 is formed. The upper and lower faces of the flange part 3 
are made flat, thereby functioning as reference surfaces when the optical 
lens 1 is mounted on a lens holder section (not shown) incorporated in an 
optical instrument such as 8-mm video camera or CD-ROM drive. 
As shown in FIGS. 1 and 2, the outer circumferential face 4 of the flange 
part 3 is formed as a cylindrical surface. A part of the outer 
circumferential face 4 includes a gate-removed part 5 which is formed as a 
quadratic surface projecting outward (toward the side where the residual 
gate part existed, see FIG. 2). Here, the quadratic surface refers to both 
complete quadratic surface and approximate quadratic surface. The 
approximate quadratic surface refers to a set of minute planes formed by 
appropriately changing the moving direction of a linearly-moving cutting 
tool. Such a set of minute planes is practical in view of the accuracy in 
the processing machine and the fact that it is practically difficult to 
form the gate-removed part 5 as a complete quadratic surface. Without 
being restricted to a cylindrical surface, the gate-removed part 5 may 
also be formed as a quadratic surface such as elliptic cylindrical surface 
or conical surface. 
The gate-removed part 5 is defined by removing the residual gate part 10 
projecting from the outer circumferential face 4 in order that so-called 
burrs may not project from the outer circumferential face 4 of the optical 
lens 1. The residual gate part 10 is formed upon injection molding or the 
like and indicates the part of resin solidified within the gate orifice 
and pulled out of the die, or both the gate and the part of resin 
solidified within the gate orifice (see FIGS. 2 and 3). In this optical 
lens 1, the gate-removed part 5 is formed as a cylindrical surface whose 
radius of curvature is greater than the radius R of the optical lens 1, 
with its center line of curvature located on the lens optical axis (a) 
side. 
In order to form the gate-removed part 5 by removing the residual gate part 
10 from the optical lens 1, a cutting tool such as end mill is used. The 
cutting tool is moved, like an arc, by a NC processing machine or the 
like. As shown in FIGS. 2 and 3, the cutting tool cuts the part being 
defined between a virtual outer circumferential face 4a and a first plane 
6 like a quadratic surface (cylindrical surface). 
Here, as with the outer circumferential face 4 of the optical lens 1, the 
virtual outer circumferential face 4a is a cylindrical surface having a 
radius R formed around the lens optical axis (a). Namely, the virtual 
outer circumferential face 4a refers to an ideal curved surface defined 
when the residual gate part 10 is removed alone from the optical lens 1. 
On the other hand, the first plane 6 is a plane passing through a tolerance 
limit on the lens optical axis (a) set when the residual gate part 10 is 
removed by flatly cutting the flange part 3. Namely, when the gate-removed 
part is made flat, an optical lens whose gate-removed part is located on 
the lens optical axis (a) side of the first plane 6 would be treated as a 
defective item. The tolerance limit is defined by appropriately setting 
the distance (d) between the virtual outer circumferential face 4a and the 
first plane 6 on a second plane 7. The second plane 7 is a plane including 
the lens optical axis (a) and the center line of the residual gate part 10 
(see FIG. 2). In the optical lens 1 having an outside diameter of 3 to 8 
mm, the distance (d) is typically 0.3 to 1.0 mm, though depending on the 
width of the residual gate part 10. On the other hand, in the optical lens 
having an outside diameter of 100 to 150 mm, the distance (d) is typically 
1.0 to 6.0 mm. 
When the gate-removed part 5 is thus defined, boundary lines 45 between the 
outer circumferential face 4 and the gate-removed part 5 are located 
between an intersecting line 46 and an intersecting line 41. The 
intersecting line 46 is a line of intersection between the outer 
circumferential face 4 and the first plane 6. The intersecting line 41 is 
a line of intersection between the virtual outer circumferential face 4a 
and the residual gate part 10. On the other hand, an intersecting line 57 
between the gate-removed part 5 and the second plane 7 is located between 
an intersecting line 47 and an intersecting line 67. The intersecting line 
47 is a line of intersection between the virtual outer circumferential 
face 4a (residual gate part 10) and the second plane 7. The intersecting 
line 67 is a line of intersection between the first plane 6 and the second 
plane 7. Hence, the surface of the gate-removed part 5 is positioned on 
the lens optical axis (a) side of the original outer circumferential face 
4 (virtual outer circumferential face 4a) of the optical lens 1, or is 
positioned on the same surface as the outer circumferential face 4. As a 
consequence, so-called burrs would not project from the outer 
circumferential face 4 of the optical lens 1. 
When the residual gate part 10 is removed by flatly cutting the flange part 
3, by contrast, the tolerance limit on the residual gate part 10 side 
(outside) is a third plane 8. The third plane passes through the 
intersecting line 41 and is orthogonal to the second plane 7 (center line 
of the residual gate part 10). An optical lens whose gate-removed part is 
located outside the third plane 8 (the residual gate part 10 side) would 
be treated as a defective item since burrs are likely to project from the 
outer circumferential face 4 of the optical lens. 
Now will be considered is a case where the gate-removed part 5 is defined 
by cutting off, like a quadratic surface, the part between the first plane 
6 and the third plane 8. 
In this case, when the gate-removed part 5 is located closest to the lens 
optical axis (a), the boundary lines 45 between the gate-removed part 5 
and the outer circumferential face 4 coincide with the intersecting lines 
46 between the outer circumferential face 4 and the first plane 6. Since 
the gate-removed part 5 of the optical lens 1 is a quadratic surface 
(cylindrical surface) projecting outward, the intersecting line 57 between 
the gate-removed part 5 and the second plane 7 would be located outside 
the line of intersection 67 between the first plane 6 and second plane 7. 
When the gate-removed part 5 is located most outside (on the residual gate 
part 10 side), the boundary lines 45 between the gate-removed part 5 and 
the outer circumferential face 4 coincide with the intersecting line 41 
between the virtual outer circumferential face 4a and the residual gate 
part 10. Since the gate-removed part 5 is a quadratic surface (cylindrical 
surface) projecting outward, the intersecting line 57 between the 
gate-removed part 5 and the second plane 7 is located outside a line of 
intersection between the third plane 8 and second plane 7. 
Consequently, as compared with the case where the residual gate part 10 is 
removed by flatly cutting the flange part 3, the shortest distance between 
the gate-removed part 5 and the lens optical axis (a) as a whole can be 
made longer in the optical lens 1 as a product. 
Hence, the permeation time (speed) of the moisture absorbed into the 
optical lens 1 from the outer circumferential face 4 and the gate-removed 
part 5 can be made substantially the same throughout the outer periphery. 
Therefore, the moisture absorbed into the optical lens 1 from the 
gate-removed part 5 can be prevented from permeating into the optically 
functioning part 2. As a result, the density in the optically functioning 
part 2 can be prevented from changing due to the moisture which is 
absorbed from the gate-removed part 5 into the optical lens 1 and reaches 
the optically functioning part 2. Accordingly, the refractive index of the 
optical lens 1 can be made uniform. Hence, it is possible to realize an 
optical lens which can be made easily and inexpensively and yields stable 
optical performances with a uniform refractive index. 
FIG. 4 is a side view showing another embodiment of the optical lens in 
accordance with the present invention. Disposed at the center part of the 
optical lens 11 shown in FIG. 4 is an optically functioning part 12 
adapted to function as a convex lens, through which light is effectively 
transmitted. Around the optically functioning part 12, a flange part 13 is 
formed. The upper face 13a and lower face 13b of the flange part 13 are 
made flat, and would function as reference surfaces when the optical lens 
11 is mounted on a lens holder section (not shown). The outer 
circumferential face 14 of the optical lens 11 is formed as a cylindrical 
surface. A gate-removed part 15 is formed at the outer circumferential 
face 14 as a conical face projecting outward. In order to form the 
gate-removed part 15 by removing the residual gate part from the optical 
lens 1, a cutting tool such as end mill is used. The cutting tool is moved 
in a slightly tilted state, like an arc, by a NC processing machine or the 
like. The cutting tool cuts a part of the flange part 13 together with the 
residual gate part. 
When the gate-removed part 15 is thus formed as a conical surface, quite 
favorable results can be obtained in practice. In the vicinity of the 
gate-removed part 15 of the optical lens 11, molecular orientation may be 
biased under the influence of resin flows at the time of molding. Optical 
distortion is likely to occur in the part where the molecular orientation 
is biased. Therefore, in a lens holder section incorporated in an optical 
instrument such as CD-ROM drive, the position of the gate-removed part is 
preset in view of the direction of polarization of light. Here, since the 
gate-removed part 15 is formed as a conical surface, the gate-removed part 
15 becomes an oblique, curved surface. As a result, the gate-removed part 
15 is clearly distinguished from the other outer circumferential face 14. 
For example, when the gate-removed part 15 is detected by a photosensor or 
the like, the direction of light reflected by the gate-removed part 15 
greatly differs from the direction of light reflected by the other outer 
circumferential surface 14. Consequently, when placing the gate-removed 
part 15 at a predetermined position in the lens holder section, it can be 
detected quite easily. 
From the invention thus described, it will be obvious that the invention 
may be varied in many ways. Such variations are not to be regarded as a 
departure from the spirit and scope of the invention, and all such 
modifications as would be obvious to one skilled in the art are intended 
for inclusion within the scope of the following claims.