Collimator lens

A collimator lens meets conditions that: PA1 (i) at least the outgoing surface is spherical; PA1 (ii) the lens comprises at least an index-varying region in which the refractive index varies in the axial direction but does not vary in the direction perpendicular to the axial direction, and a constant index region all over which the refractive index is fixed; PA1 (iii) the point having the maximum refractive index n.sub.oo in the index-varying region is located at an apex of the profile of the lens; PA1 (iv) when the refractive index n(Z) on the optical axis at a distance Z from the apex is represent by n(Z)=n.sub.oo +n.sub.1 Z+n.sub.2 Z.sup.2, n.sub.oo is within the range of 1.50 to 1.77, n.sub.1 within the range of -0.14 to -0.02 mm.sup.-1 and n.sub.2 within the range of -0.03 to +0.03 mm.sup.-2 ; and PA1 (v) the inded-varying region is formed in the range of Z=0 to at least a depth d represented by an equation EQU d=D.sup.2 /{4R(1+.sqroot.1-(D/2R).sup.2)} PA1 where D is the aperture of the lens and R is the radius of curvature of the lens surface which includes the point having the maximum refractive index n.sub.oo.

BACKGROUND OF THE INVENTION 
1. Field of the Invention: 
This invention relates to a collimator lens which is useful to an optical 
system for reading from or writing in a recording medium. 
2. Description of the Prior Art: 
Recently, information processing using a high-density recording medium such 
as a compact disc, an optical disc or the like, has been rapidly advanced, 
in which an optical system is generally employed for reading from or 
writing in the recording medium. 
In such an optical system, diffused light from a light source such as a 
semiconductor laser is converted to parallel rays by a collimator lens and 
then converged onto the surface of a recording medium by an objective 
lens. In addition to such an optical reading or writing system, a 
collimator lens has been generally and widely used for the purpose that 
diffused light is converted to parallel rays. When a collimator lens 
having reduced spherical aberration and coma is required, a compound lens 
consisting of plural spherical lenses has been used. In that case, 
however, it is hard to reduce the cost of the system. 
In order to solve the above problem of such a compound lens system, system 
using an aspherical lens or a gradient index lens having refractive index 
gradient in the radial direction has been developed. Aspherical lenses 
are, however, unsuitable for mass-production because the aspherical 
surface is difficult to process or take an accurate measurement. On the 
other hand, a radial gradient index lens is made by an ion-exchange 
process of glass, however, the ion-exchange process has need of a long 
process time so that only a small lens having the effective aperture less 
than 5 mm can be obtained in practice. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a 
collimator lens which is suitable for an optical reading or writing 
system. 
It is another object of the present invention to provide a collimator lens 
whose spherical aberration and coma can be reduced without remarkably 
increasing the cost. 
It is still another object of the present invention to provide a method of 
making a collimator lens for an optical reading or writing system with 
facility for a relatively short time. 
The above and other objects can be attained by the invention as follows. A 
collimator lens according to the invention meets conditions that: 
(i) at least the outgoing surface is spherical; 
(ii) the lens comprises at least an index-varying region in which the 
refractive index varies in the axial direction but does not vary in the 
direction perpendicular to the axial direction, and a constant index 
region all over which the refractive index is fixed; 
(iii) the point having the maximum refractive index n.sub.00 in the 
index-varying region is located at an apex of the profile of the lens; 
(iv) when the refractive index n(Z) on the optical axis at a distance Z 
from the apex is represented by n(Z)=n.sub.00 +n.sub.1 Z+n.sub.2 Z.sup.2, 
n.sub.00 is within the range of 1.50 to 1.77, n.sub.1 within the range of 
-0.14 to -0.02 mm.sup.-1 and n.sub.2 within the range of -0.03 to +0.03 
mm.sup.-2 ; and 
(v) the index-varying region is formed in the range of Z=0 to at least a 
depth d represented by an equation 
EQU d=D.sup.2 /{4R(1+.sqroot.1-(D/2R).sup.2)} 
where D is the aperture of the lens and R is the radius of curvature of 
the lens surface which includes the point having the maximum refractive 
index n.sub.00. 
The above collimator lens can be made by the following method. An oxide 
glass plate containing at least a kind of monovalent cations is brought 
into contact with a molten salt containing monovalent cations which are to 
serve to increase the refractive index of the glass material, and the 
cations of the molten salt are diffused into the glass plate so that an 
index distribution decreasing gradually from the surface to the inside of 
the glass plate is established to a predetermined depth and the deeper 
part than that depth remains a fixed refractive index, and subsequently, 
at least the surface of the glass plate at the side of the index-varying 
region is processed to a spherical surface. 
Other further objects of the invention will become obvious upon an 
understanding of the illustrative embodiments about to be described or 
will be indicated in the appended claims, and various advantages not 
referred to herein will occur to one skilled in the art upon employment of 
the invention in practice.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Hereinafter, embodiments of the invention will be described in detail with 
reference to attached drawings. 
Referring first to FIG. 1, a collimator lens 1 has a first refracting 
surface 2A which is formed into a spherical surface having the radius of 
curvature R.sub.1. The refractive index n(Z) in the plane perpendicular to 
the optical axis 3 at a distance Z from the origin 0 which coincides with 
the central point of the spherical surface 2A, is represented by an 
equation 
EQU n(Z)=n.sub.00 +n.sub.1 Z+n.sub.2 Z.sup.2 (1) 
where n.sub.00 is the refractive index at the origin 0, n.sub.1 and n.sub.2 
are constants, and all of them are values in relation to the wavelength of 
a light source such as a semiconductor laser used in an optical system. 
An index-varying region 1A having the above index distribution is formed in 
the thickness range of the origin 0 to a depth Z.sub.0. The refractive 
index is maximum at the origin 0 and decreases gradually as the distance Z 
increases. The values of n.sub.00, n.sub.1 and n.sub.2 fall within the 
ranges of 1.50 to 1.77, -0.14 to -0.02 mm.sup.-1 and -0.03 to 0.03 
mm.sup.-2, respectively. When the thickness of the lens is represented by 
t, the refractive index is fixed all over a region 1B in the thickness 
range of t minus Z.sub.0. The radius of curvature R.sub.2 of a refracting 
surface 2B at the side of the constant index region 1B is determined so as 
to correct coma. The radius R.sub.2 depends on the refractive index of the 
constant index region 1B and has a value within one of the ranges of 
1/R.sub.2 &gt;0, 1/R.sub.2 =0 and 1/R.sub.2 &lt;0. 
In the lens 1, the distance S.sub.0 on the optical axis between the central 
point 0 of the spherical surface 2A and the point corresponding to the 
outerest periphery of the sperical surface 24 is called "sag". When the 
aperture of the lens is represented by D, the sag S.sub.0 is given by an 
equation 
EQU S.sub.0 =D.sup.2 /{4R.sub.1 (1+.sqroot.1-(D/2R.sub.1).sup.2)}(2 
In this embodiment, the index gradient is established in the range of the 
central point 0 to the point distant from the central point 0 by a length 
Z.sub.0 which is larger than the sag S.sub.0. 
FIG. 2 shows another embodiment of the invention, in which an index-varying 
region 1A having the same index gradient as that of the embodiment of FIG. 
1 is formed in the range of the central point 0 of one refracting surface 
2A to a depth Z.sub.0, and another index-varying region 1C having index 
gradient represented by the above-mentioned equation (1) is formed in the 
range of the central point 0' of the other refracting surface 2B to a 
depth Z.sub.0 '. A constant index region 1B is intermediate between them. 
Also in the region 1C, the distance Z.sub.0 ' is larger than sag S.sub.0 ' 
given by an equation 
EQU S.sub.0 '=D.sup.2 /{4R.sub.2 (1+.sqroot.1-(D/2R.sub.2).sup.2)}(3) 
FIG. 8 shows an embodiment in which a collimator lens of the invention is 
used in an optical reading apparatus for an optical disc. Diffused light 5 
emitted from a semiconductor laser 4 is introduced to a collimator lens 1 
through a beam splitter 6. Parallel rays collimated by the collimator lens 
1 are converged onto an optical disc 8 by an objective lens 7. Reflected 
light from the optical disc 8 is introduced through the objective lens 7, 
collimator lens 1 and beam splitter 6 to an optical detector (not shown). 
Alternatively, the beam splitter 6 may be disposed between the collimator 
lens 1 and the objective lens 7. 
Next, a suitable method of making a collimator lens of the invention will 
be described with reference to FIGS. 9A through 9D. 
An oxide glass plate 10 containing at least a kind of monovalent cations is 
used as a base material, which is dipped in a molten salt 11 at a 
temperature near the transition temperature of the glass as shown in FIG. 
9A. The molten salt contains monovalent cations to increase the refractive 
index of the glass material, for example, at least a kind of cations 
selected from the group of Li ions, Cs ions, Tl ions and Ag ions. 
In the dipping process, the cations of the molten salt are diffused into 
the glass plate from both surfaces thereof by ion-exchanging with the 
cations of the glass plate. As the result, as shown in FIG. 9B, an index 
distribution 12 decreasing gradually and evenly from the surface to the 
inside of the glass plate 10 is established by an ion concentration 
distribution. 
Subsequently, as shown in FIG. 9C, a large number of disk-like lens 
materials 10A are cut from the glass plate 10. One or each of both flat 
surfaces of each lens material 10A is processed into a spherical surface 
having a predetermined radius of curvature so that the lens thickness is 
adjusted into a predetermined value. In the case that a single lens is 
formed from a single glass plate, if the above-mentioned dipping process 
is effected without masking the edge portion of a glass plate, an index 
distribution layer is formed also in the edge portion. In that case, it is 
preferable that processing to form a spherical surface is effected after 
removing the index distribution layer of the edge portion because the 
processing becomes easy in view of the change of the hardness of the 
surface to be processed. 
A glass plate subjected to the above-mentioned ion-exchange process must 
contain at least a kind of monovalent cations and have a high refractive 
index within the range of 1.50 to 1.77. 
For making a glass having a high refractive index, oxide such as TiO.sub.2, 
BaO, PbO and La.sub.2 O.sub.3 is generally used as an additive for 
increasing the refractive index. Those kinds of additives, however, may 
cause a change in quality of glass in the process of ion exchange or may 
lower the rate of ion exchange. In contrast to them, Tl.sub.2 O is 
preferable because it can increase the refractive index by 0.010 to 0.015 
per mole % without the above problems. 
Tl ions are most preferable as cations contained by a molten salt for an 
ion-exchange process of the invention. As a molten salt, nitrate, sulfate, 
halide, etc. can be used. 
The ion-exchange process of the invention is preferably effected at a 
temperature as high as possible because the ion-exchange rate increases in 
general as the temperature rises. Too high temperature, however, causes a 
deformation of glass. For these reasons, a temperature near the transition 
temperature of the glass to be processed is used in practice. Generally, 
the dipping process is preferably effected at a temperature within the 
range of the transition temperature plus or minus 50.degree. C. 
Hereinafter, experimental results will be described. 
EXAMPLE 1 
A disk-shaped glass plate having the diameter of 8 mm and the thickness of 
3.2 mm was dipped in a molten salt at a temperature of 505.degree. C. for 
444 hours. The composition of the glass plate is shown in the following 
table 1. The molten salt consisted of 10 mole % Tl.sub.2 SO.sub.4, 30 mole 
% K.sub.2 SO.sub.4 and 60 mole % ZnSO.sub.4. 
TABLE 1 
______________________________________ 
Composition (weight %) Characteristics 
SiO.sub.2 
B.sub.2 O.sub.3 
ZnO Na.sub.2 O 
K.sub.2 O 
Tl.sub.2 O 
Sb.sub.2 O.sub.3 
Tg (.degree.C.) 
nd 
______________________________________ 
39.2 4.1 19.0 7.2 5.5 24.8 0.2 478 1.600 
______________________________________ 
Tg: transition temperature 
Tl ion concentration distribution in the thickness direction of the glass 
plate measured with an X-ray microanalyzer is shown in FIG. 3. 
After grinding each flat surface of the glass plate by a thickness of 0.1 
mm, the index distribution in the thickness direction was measured. The 
result showed an index distribution which evenly and equally descends from 
each flat surface to a depth of 0.8 mm. The refractive index at each flat 
surface was 1.641. The index distribution from each flat surface to the 
inside of the glass plate in the range of 0.ltoreq.Z.ltoreq.0.8 mm was 
given by n(Z)=1.641-0.0514Z. 
The refractive index in the intermediate region between both index-varying 
regions in the thickness range of 1.4 mm was not changed by the 
ion-exchange process and had a constant value 1.600 which was the same as 
that of the original glass material. 
Subsequently, both flat surfaces of the glass plate were ground to 
spherical surfaces having the radii of curvatures of R.sub.1 =7.90 mm and 
R.sub.2 =-249.8 mm, respectively, so that the aperture of the lens was 
adjusted to 7.2 mm. 
The focal length and numerical aperture (NA) were 12.0 mm and 0.3, 
respectively. A measurement result of spherical aberration (on the axis) 
of this optical system is shown in FIG. 4a. The maximum spherical 
aberration and coma of this system were 2 .mu.m and less than 5 .mu.m, 
respectively. 
EXAMPLES 2 TO 6 
Glass materials having different refractive indexes were provided. Each of 
them was subjected to an ion-exchange process fundamentally similar to 
that of Example 1. As the result, lens materials having various index 
distributions in the thickness directions were obtained. Each lens materal 
was then processed to form spherical surfaces. Index distributions mesured 
and conditions of lenses finally obtained are shown in the following 
tables 2A and 2B, respectively. 
TABLE 2A 
______________________________________ 
Ex- Distance Z (mm) 
ample on the axis Index distribution 
______________________________________ 
2 0.about.0.4 
n(Z) = 1.556 - 0.138Z - 0.00513Z.sup.2 
0.4.about.2 n(Z) = 1.50 
3 0.about.0.4 
n(Z) = 1.650 - 0.124Z 
0.4.about.1.6 
n(Z) = 1.6 
1.6.about.2.0 
n(Z) = 1.6 + 0.124(Z - 1.6) 
4 0.about.0.15 
n(Z) = 1.608 - 0.0514Z 
0,15.about.2 n(Z) = 1.6 
5 0.about.0.4 
n(Z) = 1.687 - 0.0297Z 
0.4.about.2 n(Z) = 1.675 
6 0.about.1 n(Z) = 1.631 - 0.0309Z 
0.about.3 n(z) = 1.6 
______________________________________ 
TABLE 2B 
______________________________________ 
Focal 
Ex- R.sub.1 R.sub.2 t D NA length 
ample (mm) (mm) (mm) (mm) (mm) (mm) 
______________________________________ 
2 3.23 -13.76 2 3 0.3 5 
3 3.42 -50.85 2 3 0.3 5 
4 7.56 -181.40 2 2.4 0.1 12 
5 13.19 303.66 2 6 0.15 20 
6 12.82 -694.13 3 10 0.25 20 
______________________________________ 
Spherical aberrations of the lenses are shown in FIGS. 4B through 4F, 
respectively, and other characteristics are shown in the following table 
3. 
TABLE 3 
______________________________________ 
Maximum spherical 
coma 
Example aberration (.mu.m) 
(.mu.m) 
______________________________________ 
2 0.3 0.4 
3 0.7 1.5 
4 0.1 0.3 
5 0.1 0.8 
6 0.6 4 
______________________________________ 
EXAMPLE 7 
A disk-shaped glass plate having the diameter of 10 mm and the thickness of 
1.6 mm was dipped in a molten salt at a temperature of 472.degree. C. for 
500 hours. The composition of the glass plate is shown in the following 
table 4. The molten salt consisted of 5 mole % Tl.sub.2 SO.sub.4, 40 mole 
% K.sub.2 SO.sub.4 and 55 mole % ZnSO.sub.4. The transition temperature 
(Tg) of the glass was 490.degree. C. 
TABLE 4 
______________________________________ 
Composition (weight %) Characteristics 
SiO.sub.2 
B.sub.2 O.sub.3 
ZnO ZrO.sub.2 
Na.sub.2 O 
K.sub.2 O 
Tl.sub.2 O 
Tg (.degree.C.) 
nd 
______________________________________ 
37.2 4.0 18.3 2.0 5.7 4.9 27.9 490 1.621 
______________________________________ 
Tl ion concentration distribution in the thickness direction of the glass 
plate measured with an X-ray microanalyzer is shown in FIG. 5. 
After grinding each flat surface of the glass plate by a thickness of 100 
.mu.m, the index distribution in the thickness direction was measured. The 
result showed an index distribution that the refractive index n(Z) on the 
optical axis at a distance Z from one flat surface was given by 
n(Z)=1.638-0.034Z in the range of Z=0 to 500 .mu.m, n(Z)=1.621 (constant) 
in the range of Z=500 to 900 .mu.m and n(Z)=1.621+0.034Z in the range of 
Z=900 to 1400 .mu.m. 
Subsequently, one flat surface of the glass plate was processed into a 
spherical surface having the radius of curvature R.sub.1 =10.84 mm and the 
aperture of the lens is adjusted to 4.76 mm. The focal length of the lens 
obtained, which had a spherical surface at one side and a flat surface at 
the other side, was 17.0 mm and the numerical aperture NA thereof was 
0.14. 
The lens was arranged so that parallel rays enter from the spherical 
surface into the lens. A beam splitter made of an optical glass BK7 and 
having the thickness of 5 mm was disposed in the rear of the lens. When 
the working distance WD is defined by (distance between the outgoing 
surface and the focal point) minus (thickness of the beam splitter), WD 
was 12.77 mm. 
A measurement result of spherical aberration (axial aberration LSA) of this 
optical system is shown in FIG. 6A. The maximum hight of image was 0.6 mm 
and the third-order plus fifth-order coma was less than 4 .mu.m. 
EXAMPLES 8 TO 11 
Glass materials having different refractive indexes were provided. Each of 
them was subjected to an ion-exchange process similar to that of Example 
7. As the result, lens materials having various index distributions in the 
thickness directions were obtained. Then, lenses were made from the lens 
materials, respectively. Index distributions in the lenses measured and 
conditions of the lenses are shown in the following tables 5A and 5B, 
respectively. Each lens had a spherical surface having the radius of 
curvature R at the side of higher refractive index and a flat surface at 
the other side. In Example 8, a sufficiently thick glass plate was 
subjected to an ion-exchange process and then the surface portion was cut 
for a sampling lens from the glass plate. In Example 10, a glass plate 
having the thickness of 2 mm was subjected to an ion-exchange process for 
a long time so that both index-varying regions formed by ion-diffusing 
might intercross to each other. 
TABLE 5A 
______________________________________ 
Distance Z (mm) 
Example 
on the axis Index distribution 
______________________________________ 
8 0.about.0.5 
n(Z) = 1.68 - 0.0345Z 
0.5.about.1.5 
n(Z) = 1.663 
9 0.about.0.5 
n(Z) = 1.635 - 0.041Z + 0.040Z.sup.2 
0.5.about.1.0 
n(Z) = 1.624 
1.0.about.1.5 
n(Z) = 1.624 + 0.021(Z - 1) 
10 0.about.1.0 
n(Z) = 1.66 - 0.034Z 
1.0.about.2.0 
n(Z) = 1.625 + 0.034(Z - 1) 
11 0.about.0.5 
n(Z) = 1.627 - 0.041Z 
0.5.about.1.0 
n(Z) = 1.606 
1.0.about.1.5 
n(Z) = 1.601 + 0.041(Z - 1) 
______________________________________ 
TABLE 5B 
______________________________________ 
Lens conditions 
Example 
R (mm) t (mm) D (mm) NA (mm) Fl (mm) 
______________________________________ 
8 11.56 1.5 4.76 0.14 17.0 
9 10.80 1.5 5.5 0.16 17.0 
10 11.88 2.0 5.4 0.15 18.0 
11 9.40 1.5 5.7 0.19 15.0 
______________________________________ 
Each lens obtained was evaluated with an optical system shown in the 
following table 6. Spherical aberrations of the lenses are shown in FIGS. 
6B through 6E and other characteristics are shown in the table 6. In the 
table 6, "BS" represents a beam splitter made of BK7 glass and having the 
thickness of 5 mm. 
TABLE 6 
______________________________________ 
Ex- Maximum Coma of 
am- Optical system for height of 3rd plus 
ple measurement image (mm) 5th order 
______________________________________ 
8 with BS WD = 12.79 mm 0.6 less than 2 .mu.m 
9 with BS WD = 12.81 mm 0.6 less than 10 .mu.m 
10 without BS 
WD = 16.78 mm 0.6 less than 1 .mu.m 
11 without BS 
WD = 14.07 mm 0.6 less than 7 .mu.m 
______________________________________ 
EXAMPLE 12 
A disk-shaped glass plate having the diameter of 16 mm and the thickness of 
10.5 mm was dipped in a molten salt at a temperature of 525.degree. C. for 
45 days. The composition of the glass plate is shown in the following 
table 7. The molten salt consisted of 22 mole % TlNO.sub.3 and 78 mole % 
KNO.sub.3. 
TABLE 7 
__________________________________________________________________________ 
Composition (weight %) Characteristics 
SiO.sub.2 
B.sub.2 O.sub.3 
ZnO 
Na.sub.2 O 
K.sub.2 O 
Tl.sub.2 O 
Sb.sub.2 O.sub.3 
Al.sub.2 O.sub.3 
Tg (.degree.C.) 
nd 
__________________________________________________________________________ 
39.1 
2.1 
19.0 
7.2 
5.5 
24.8 
0.2 2.1 485 1.600 
__________________________________________________________________________ 
Tl ion concentration distribution in the thickness direction of the glass 
plate measured with an X-ray microanalyzer was the same as that of FIG. 3. 
After grinding each flat surface of the glass plate by a thickness of 0.1 
mm, the index distribution in the thickness direction was measured. The 
result showed an index distribution which evenly and equally descends from 
each flat surface to a depth of 2.1 mm. The refractive index at each flat 
surface was 1.724. The index distribution from each flat surface to the 
inside of the glass plate in the range of 0.ltoreq.Z.ltoreq.2.1 mm was 
given by n(Z)=1.724-0.0600Z. 
The refractive index in the intermediate region between both index-varying 
regions in the thickness range of 6.1 mm was not changed by the 
ion-exchange process and had a constant value 1.600 which was the same as 
that of the original glass material. 
Subsequently, the glass plate was cut and separated at the center of the 
thickness parallel with both flat surfaces so that two glass plates having 
the same index distribution were obtained. Then, the surface at the side 
of index-varying region of each glass plate was processed into a spherical 
surface having the radius of curvature R.sub.1 =7.24 mm and the other 
surface thereof remained flat. The thickness and aperture of each lens 
were adjusted to 4.5 mm and 10.0 mm, respectively. The focal length and 
numerical aperture (NA) of each lens obtained were 10.0 mm and 0.5, 
respectively. 
A measurement result of spherical aberration (axial aberration) of this 
optical system is shown in FIG. 7A. The maximum spherical aberration and 
coma of this system were 4 .mu.m and less than 15 .mu.m, respectively. 
EXAMPLES 13 to 15 
Glass materials having different refractive indexes were provided. Each of 
them was subjected to an ion-exchange process fundamentally similar to 
that of Example 12. As a result, lens materials having various index 
distributions in the thickness directions were obtained. In Examples 13 
and 14, each disk-shaped glass plate after an ion-exchange process was cut 
at the center of the thickness similarly to that of Example 12, 
subsequently, the surface at the side of the index-varying region was 
processed into a spherical surface and the surface at the side of the 
constant index region was polished to remain a flat surface. In Example 
15, both surfaces of a disk-shaped glass plate after an ion-exchange 
process were processed into spherical surfaces. 
Index distributions in the lenses measured and conditions of the lenses are 
shown in the following tables 8A and 8B. In the table 8A, Z represents the 
distance from the origin which is the intersecting point between the 
outgoing surface and the optical axis, toward the inside of the lens. 
TABLE 8A 
______________________________________ 
Example Z (mm) Index distribution 
______________________________________ 
13 0.about.1.6 
n(Z) = 1.765 - 0.0573Z 
1.6.about.5.1 
n(Z) = 1.675 
14 0.about.0.6 
n(Z) = 1.583 - 0.135Z - 0.00554Z.sup.2 
0.6.about.2.2 
n(Z) = 1.5 
15 0-2.7 n(Z) = 1.684 - 0.0314Z 
2.7.about.9.2 
n(Z) = 1.6 
9.2.about.11.9 
n(Z) = 1.6 + 0.0314(Z - 9.2) 
______________________________________ 
TABLE 8B 
______________________________________ 
R.sub.1 R.sub.2 t D NA Fl 
Example 
(mm) (mm) (mm) (mm) (mm) (mm) 
______________________________________ 
13 7.42 149.8 5.1 9 0.45 10 
14 3.33 -14.92 2.2 3.5 0.35 5 
15 14.6 -137.0 11.9 16 0.4 20 
______________________________________ 
Spherical aberrations of the lenses obtained are shown in FIGS. 7B through 
7D, respectively, and other characteristics are shown in the following 
table 9. 
TABLE 9 
______________________________________ 
Maximum spherical 
coma 
Example aberration (.mu.m) 
(.mu.m) 
______________________________________ 
13 2 8 
14 1 1 
15 10 10 
______________________________________ 
According to the invention, a lens material can be obtained by such a 
simple manner that a glass plate is subjected to an ion-exchange process 
in a molten salt, so that, a highly efficient collimator lens with low 
spherical aberration and coma can be obtained by the manner that the lens 
material is only processed to have a spherical surface or surfaces.