Objective Optical System for Endoscopes

(a) Field of the invention

The present invention relates to an objective optical system for endoscopes.

(B) Description of the prior art

Optical systems for endoscopes, especially of flexible endoscopes, comprises an objective arranged at the distal end and an optical fiber bundle arranged in rear of the objective and such optical systems are arranged to observe an image of an object by transmitting it by the optical fiber bundle.

In case of such endoscope comprising the optical fiber bundle, however, loss in intensity of light occurs when the light is transmitted by the optical fiber bundle. The above-mentioned loss in intensity of light becomes the smallest when the light enters the optical fiber bundle vertically to the end face of the optical fiber bundle. However, in order to let the light from the object enter vertically to the end face of the optical fiber bundle and to arrange so that a favourable image can be obtained, it is necessary to use an objective of complicated lens configuration.

It is, therefore, a primary object of the present invention to provide an objective optical system for endoscopes comprising a light-focusing heterogeneous medium in front of the end face of the optical fiber bundle on the incident side.

Another object of the present invention is to provide an objective optical system for endoscopes comprising a light-focusing heterogeneous medium arranged in front of the end face of the optical fiber bundle on the incident side and a negative lens arranged in front of the light-focusing heterogeneous medium and thereby arranged to have a wide field angle and to favourable correct the curvature of field and chromatic aberration.

FIG. 1 shows the passage of light in the light-focusing heterogeneous medium;

FIGS. 2 and 3 respectively show sectional views of respective embodiments of the present invention;

FIG. 4 shows a sectional view of a known objective optical system for endoscopes;

FIGS. 5A, 5B and 5C respectively show graphs illustrating aberration curves of Example I;

FIGS. 6A, 6B and 6C respectively show graphs illustrating aberration curves of Example II; and

FIGS. 7A, 7B and 7C respectively show graphs illustrating aberration curves of known optical system shown in FIG. 4.

Details of the optical system according to the present invention are described below referring to the accompanying drawings.

In the light-focusing heterogeneous medium for which the refractive index distribution is expressed by the formula n = n.sub.0 (1 - 1/2 .alpha. R.sup.2), where the reference symbol n.sub.0 represents the refractive index on the optical axis, reference symbol .alpha. represents a coefficient of refractive index distribution and reference symbol R represents the distance from the optical axis, the passage of light in the light-focusing heterogeneous medium becomes a sine curve as shown in FIG. 1. When such light-focusing heterogeneous medium 2 having the length equal to the length D = .pi./2 .sqroot..alpha., which is equivalent to 1/4 cycle, i.e., .pi./2, is arranged in front of the optical fiber bundle 1 as shown in FIG. 2 and the stop S is arranged on the end face 2a on the entrance side of the light-focusing heterogeneous medium, the principal ray C goes out vertically to the end face 2b on the exit side of the light-focusing heterogeneous medium as shown in FIG. 2. In other words, the exit pupil is at the infinite distance. The optical system according to the present invention is arranged by providing a cylindrical light-focusing heterogeneous medium 2 at the distal end of the optical fiber bundle 1 as shown in FIG. 2.

For this kind of light-focusing heterogeneous medium, the maximum half field angle .omega..sub.M is expressed by the following formula when the outer diameter is represented by reference symbol 2h. ##EQU1## Therefore, the maximum field angle is limited by the outer diameter etc.

In order to obtain a still wider field angle, a negative lens 3 is arranged in the present invention on the object side of the light-focusing heterogenerous medium 2 as shown in FIG. 3. When the negative lens is arranged on the object side of the light-focusing heteronegeous medium as described in the above, the field angle of the optical system becomes wider and, moreover, it is possible to use the negative lens also as the cover glass. In that case, it is preferable to use a plano-concave lens as the negative lens as shown in FIG. 3. Besides, it is also possible to reduce the curvature of field and chromatic aberration by means of the negative lens.

As for curvature of field, Petzval's sum showing the curvature of field is expresed by the following formula as it is known widely. ##EQU2##

In case of the optical system consisting only of the light-focusing heterogeneous medium, the refractive power is always of positive value and, therefore, it is not possible to make the value P small. When, however, the negative lens is added as shown in FIG. 3, it is possible to make the value P small and, consequently, it is possible to remarkably reduce the curvature of field. When the focal length of the added negative lens 3 is represented by reference symbol f.sub.0, the focal length of the optical system as a whole is represented by reference symbol f.sub.t and magnification of the light-focusing heterogeneous medium is represented by reference symbol .beta., their relation becomes f.sub.t = f.sub.0 .vertline..beta..vertline.. If f.sub.t does not change, f.sub.0 becomes small when .vertline..beta..vertline.becomes large. In that case, the curvature of field will be overcorrected. Moreover, the distance from the end face 2b to the image surface becomes large and it is not desirable for the optical system for endoscopes. When, on the contrary, .vertline..beta..vertline. is too small, f.sub.0 becomes large and it will become impossible to make the field angle wide. Taking the above into consideration, it is preferable that the value of .beta. is 0.04 < .vertline..beta..vertline. < 0.07.

Chromatic aberration of the optical system according to the present invention becomes as explained below. In case of ordinary lens systems, Abbe's number .nu..sub.d is used .nu..sub.d is expressed by the formula: ##EQU3## In case of a thin lens, the achromatic condition for paraxial rays is expressed as follows. ##EQU4##

On the other hand, in case of light-focusing heterogeneous medium, .mu..sub.d expressed by the following formula is used as the parameter corresponding to Abbe's number. ##EQU5##

where, K.sub.d = 1/2.alpha..sub.d n.sub.0d

In case of the light-focusing heterogeneous medium corresponding to a thin lens, the achromatic condition for paraxial rays is expressed as follows. ##EQU6##

Generally, the parameter .mu..sub.d is of positive value and the light-focusing heterogenerous medium has chromatic aberration similar to that of ordinary lenses. Therefore, it is possible to treat the light-focusing heterogeneous medium based on the same idea as the ordinary achromatic lenses and, consequently, chromatic aberration is reduced when the negative lens is added as shown in FIG. 3. In case of ordinary lenses, chromatic aberration is expressed by the following parameter. ##EQU7## When the above parameter is applied to the optical system according to the present invention for convenience' sake, it is possible to consider that chromatic aberration is small when .DELTA. is small. To obtain an optical system for which chromatic aberration is favourably corrected in case of the embodiment of the present invention in which the negative lens is added, it is desirable that the value of .DELTA. is .vertline..DELTA..vertline. < 0.04.

Numerical values of preferred examples of the optical system illustrated in FIG. 3 are as shown below.

Aberrations curves of the above Example I are as shown in FIGS. 5A, 5B and 5C and aberrations curves of Example II are as shown in FIGS. 6A, 6B and 6C.

When an optical system, which has performance equivalent to the optical system according to the present invention, is arranged by using ordinary lenses, the lens configuration becomes for example as shown in FIG. 4 and numerical values of that lens system become as follows.

Aberration curves of the above example of lens system are as shown in FIGS. 7A, 7B and 7C.

As it is evident from FIG. 4 and the above example of numerical values, the lens configuration of the lens system becomes complicated when it is attempted to obtain the performance equivalent to the optical system according to the present invention by using ordinary lenses. As the objective for endoscopes should be arranged in a small limited space, it becomes necessary to produce a very small lens system of complicated configuration and it becomes very difficult to manufacture the lenses and to fix them properly. On the other hand, in the present invention, it is possible to manufacture and fix the optical system quite easily.