A microlens for use with an optical fiber or the like. It comprises a single lens having a diameter of about 0.5 mm. It provides an ideal combination of the length of the lens with the refractive index of its opposite surfaces for a minimum curvature of field.

The present invention relates to a microlens having a diameter of about 0.5 
mm for use with optical and image fibers, and adapted for image formation 
with a minimum curvature of field. 
As optical fibers come into general use in recent years, there is an 
increasing demand for microlenses which are used as coupling lenses for 
optical fibers or as image pickup lenses for image fibers. Especially in 
the medical field where the miniaturization of endoscopes is desired, 
intravascular endoscopes and cardioscopes require lenses having a diameter 
of about 0.5 mm. An endoscope transmits a picture through an image fiber 
and its image pickup portion accommodates a lens or lenses for making the 
image of an object at the end surface of an image fiber. Although it is 
generally prevalent to combine a plurality of lenses in order to minimize 
the aberration, combining a plurality of such tiny lenses poses a 
difficult problem because it is very difficult to mount them in the image 
pickup portion in exact alignment with each other and to fix them in 
position. Therefore, the use of a single microlens is more feasible than 
combining a plurality of microlenses. If the angle of field is large, 
however, the use of a single microlens causes a large curvature of field 
which is one of the aberrations and the picture quality in the peripheral 
region of an image remarkably deteriorates. The large angle of field 
requires a short focal distance which in turn, requires a large angle of 
refraction. With a single lens, a light ray undergoes refraction only 
twice i.e. when entering it and when leaving it. This means that the angle 
of refraction per refraction surface is large. On the other hand, 
generally, the larger the angle of refraction, the larger the aberration. 
It is an object of the present invention to provide a microlens which is 
comprised of a single lens having a very small diameter and can form an 
image with a minimum curvature of field. 
It is another object of the present invention to provide such a microlens 
used to form in the air the image of an object in the water. 
In case of a fiberscope, it is at the end surface of an image fiber that 
the lens forms an image. Thus, an image fiber having a diameter of about 
0.5 mm necessarily requires a lens having a diameter of about 0.5 mm. In 
addition, the refractive index of the lens has to be considerably large so 
that a large angle of refraction can be obtained as mentioned above. Under 
these two conditions, the inventors found such a combination of the axial 
length of lens and the radii of curvature of its opposite surfaces as to 
minimize the curvature of field. 
The microlens in accordance with the present invention is comprised of a 
single lens and has a large angle of field and can form an image with a 
minimum curvature of field. It may be attached to an image fiber for use 
with an endoscope in the medical field, or may be used as a coupling lens 
for optical fibers. It can form a clear image without deterioration of the 
picture quality in the peripheral region of the image. 
Since the microlens in accordance with the present invention is comprised 
of a single lens, it can be easily set and adjusted in the image pickup 
portion of an endoscope and the like.

Referring now to FIG. 1, which is a sectional view of a single microlens, 
the symbols used herein will be defined as follows: 
l: Length of the lens 
d: Diameter of the lens 
n.sub.1 : Refractive index of a medium 2 disposed on the left of the lens 
(i.e., on the side where there is an object to be viewed) 
n.sub.2 : Refractive index of a material of which the lens 1 is made 
n.sub.3 : Refractive index of a medium 3 disposed on the right of the lens 
(i.e., on the side where an image is formed by the lens) 
r.sub.1 : Radius of curvature of the surface of lens facing the medium 2 
r.sub.2 : Radius of curvature of the surface of lens facing the medium 3 
Referring next to FIG. 2, a principal ray 4 incident upon the lens 1 forms 
an angle of .theta. with the axis of the lens. Two rays 5 run parallel 
with the principal ray 4 in close vicinity thereto and, after passing 
through the lens, intersect at a point F' to form an image. The point F' 
deviates from the focal plane (i.e., a plane which is perpendicular to the 
axis of the lens and passes through a focus F) by a distance b, which 
represents the magnitude of curvature of field at an incidence angle of 
.theta.. 
As described above, the single microlens for this purpose has to have a 
diameter of about 0.5 mm and a large refractive index of about 1.8 to meet 
the abovesaid requirements. Under such conditions, the inventors sought an 
optimal combination of r.sub.1, r.sub.2 and l by repeated calculations 
using the following Snell's law and by using other methods for designing a 
lens. 
##EQU1## 
Wherein symbols .theta..sub.1 -.theta..sub.4 represent the angles shown in 
FIG. 3, in which C and C' are the centers of curvature of the opposite 
surfaces. 
As a result, for a microlens used to form in the air the image of an object 
present in the air (thus, n.sub.1 =1.0, n.sub.3 =1.0), such an optimal 
combination for minimum curvature of field was found to be as follows; 
(when d.ltoreq.0.58, 1.75.ltoreq.n.ltoreq.1.85) 
0.58.ltoreq.r.sub.1 .ltoreq.0.60 (mm) 
0.42.ltoreq.r.sub.2 .ltoreq.0.44 (mm) 
0.45.ltoreq.l.ltoreq.0.55 (mm) 
When r.sub.1, r.sub.2 and l are within these ranges, the focal distance f 
of the lens and the magnitude C of curvature of field when the angle of 
incidence is 30.degree. were found to fall within the following ranges: 
0.387.ltoreq.f.ltoreq.0.409 (mm) 
0.057.ltoreq.C.ltoreq.0.064 (mm) 
For a microlens used to form in the air the image of an object in the water 
(i.e. n.sub.1 =1.33 and n.sub.3 =1.00), of optimal combination of r.sub.1, 
r.sub.2 and l for minimum curvature of field was found to be as follows: 
0.29.ltoreq.r.sub.1 .ltoreq.0.31 (mm) 
0.37.ltoreq.r.sub.2 .ltoreq.0.39 (mm) 
0.52.ltoreq.l.ltoreq.0.62 (mm) 
(d.ltoreq.0.58 mm, 1.75.ltoreq.n.sub.2 .ltoreq.1.85) 
When the values of r.sub.1, r.sub.2 and l are within these ranges, the 
focal distance f of the lens and the magnitude C of curvature of field 
when the angle of incidence is 30.degree. were found to fall within the 
following ranges: 
0.361.ltoreq.f.ltoreq.0.401 (mm) 
0.083.ltoreq.C.ltoreq.0.110 (mm) 
EXAMPLE 1 
A microlens having a diameter of 0.5 mm was made of a material having a 
refractive index of 1.8. The following values were given as r.sub.1, 
r.sub.2 and l: 
r.sub.1 =0.59 mm, r.sub.2 =0.42 mm, l=0.45 mm 
When the image of an object in the air was formed in the air, the focal 
distance f of this microlens was found to be 0.387 mm and the magnitude C 
of curvature of field was found to be as small as 0.057 mm when the angle 
of incidence was 30.degree.. 
EXAMPLE 2 
A microlens having a diameter of 0.5 mm was made of a material having a 
refractive index of 1.8. The following values were given as r.sub.1, 
r.sub.2 and l: 
r.sub.1 =0.30 mm, r.sub.2 =0.38 mm, l=0.52 mm 
It was tested for its optical properties with one surface of the lens 
having a radius of curvature of r.sub.1 immersed in the water and the 
other surface having a radius of curvature of r.sub.2 attached to the end 
of an image fiber. The focal distance f of this microlens was found to be 
0.367 mm and the magnitude C of curvature of field was found to be as 
small as 0.083 mm when the angle of incidence was 30.degree..