Light source optical system for endoscopes

A light source optical system for endoscopes includes an elliptical condensing mirror projecting a bright spot of the light-emitting section of a lamp and a relay optical system transmitting the projected image of the bright spot to the entrance end face of a light guide. The elliptical condensing mirror is designed to satisfy the following condition: EQU 2.0 mm<F<16.0 mm where F is the focal length of the elliptical condensing mirror, which is expressed by F=.beta..sup.2 /(2.alpha.) where (the major axis of an ellipse)/2=.alpha. and (the minor axis of the ellipse)/2=.beta.. In this way, a light beam from the light-emitting section can be efficiently condensed on the entrance end face of the light guide and the entire light source optical system can be constructed to be compact.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a light source optical system for endoscopes, 
particularly having an elliptical condensing mirror projecting a bright 
spot of the light-emitting section of a lamp and a relay optical system 
transmitting the projected image of the bright spot to the entrance end 
face of a light guide. 
2. Description of Related Art 
In general, observation with an endoscope needs an illumination system 
including, at least, a light source supplying light to illuminate a 
subject for observation and a light guide transmitting the light emitted 
from the light source to the distal end of the endoscope. The light source 
is constructed with a discharge lamp giving out intense light, such as a 
xenon lamp or a metal halide lamp, and a condensing optical system 
efficiently collecting the light emitted from this lamp on the entrance 
end face of the light guide. As an example, a light source optical system 
set forth in Japanese Utility Model Preliminary Publication No. Hei 
1-135408 is known. This optical system includes an elliptical condensing 
mirror placed so that the center of the light-emitting section of the 
discharge lamp is located at the primary focal point thereof, and first 
and second condenser lenses situated behind the secondary focal point of 
the elliptical condensing mirror to collect light. The optical system has 
the function that the bright spot of the light-emitting section is 
projected in a space by the elliptical condensing mirror and the projected 
image is transmitted to the entrance end face of the light guide by the 
condenser lenses. 
Recently, by the widespread use of endoscopes, their applications to 
observations have been diversified, and endoscope observation systems with 
high versatility which can accommodate such applications have been in 
demand. In keeping with this, the improvements of an observer's work 
efficiency and of ease with which the observer handles apparatuses have 
come into big problems to be solved. Endoscopes are available in different 
kinds, such as an endoscope of relatively large diameter for observing and 
treating the stomach or intestines and an endoscope of extremely small 
diameter for observing the interior of a blood vessel. Such endoscopes 
require light source apparatuses to supply illumination light with 
brightness sufficient for such observations. In view of the observer's 
work efficiency, it is imperative to provide a light source apparatus with 
lightweight and compact design such that it is easy to carry and does not 
occupy much space when placed. 
In the light source optical system of this type, on the other hand, the 
light-emitting section of the lamp has a light-emitting area with a 
certain size, and thus a question arises as to how efficiently the light 
from the lamp is collected on the entrance end face of the light guide. 
Specifically, the question is due to not only how the focal length of the 
elliptical condensing mirror is determined with respect to the 
light-emitting section having a certain dimension along the optical axis, 
but also how the effective aperture diameter of the elliptical condensing 
mirror having the determined focal length is determined to optimize the 
brightness of the light collected on the end face of the light guide and 
the size of the light beam. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a light 
source optical system for endoscopes in which not only can 
light-collecting efficiency be improved with respect to the end face of a 
fine light guide, but also various filters are easily introduced and 
compact design can be achieved. 
In order to accomplish this object, the light source optical system for 
endoscopes according to the present invention includes an elliptical 
condensing mirror projecting a bright spot of the light-emitting section 
of a lamp and a relay optical system transmitting the projected image of 
the bright spot to the entrance end face of a light guide. The elliptical 
condensing mirror is designed to satisfy the following condition: 
EQU 2.0 mm&lt;F&lt;16.0 mm (1) 
where F is the focal length of the elliptical condensing mirror, which is 
expressed by F=.beta..sup.2 /(2.alpha.) where (the major axis of an 
ellipse)/2=.alpha. and (the minor axis of the ellipse)/2=.beta.. 
Further, according to the present invention, a plane mirror is interposed 
on an optical path between the elliptical condensing mirror and the relay 
optical system so that the optical path is bent at an angle P satisfying 
the following condition: 
EQU 30.degree.&lt;P&lt;120 (2) 
Still further, according to the present invention, the plane mirror is 
located so as to satisfy the following condition: 
EQU .vertline.L/tan .theta..vertline..ltoreq.5.5 (3) 
where L is a distance from the secondary focal point of the elliptical 
condensing mirror to the plane mirror and .theta. is the maximum angle of 
incidence of a ray reflected by the elliptical condensing mirror on the 
secondary focal point. 
This and other objects as well as the features and advantages of the 
present invention will become apparent from the following detailed 
description of the preferred embodiments when taken in conjunction with 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with the embodiments shown in the drawings, the present 
invention will be explained below. 
First embodiment 
FIG. 1 shows the arrangement of the first embodiment of the light source 
optical system for endoscopes according to the present invention. FIG. 2 
shows an example of the spectral transmittance characteristic of an 
infrared removing filter used in the first embodiment. In FIG. 1, 
reference numeral 1 denotes a light-emitting section of a lamp; 2, an 
elliptical condensing mirror having two focal points, a primary focal 
point F.sub.1 and a secondary focal point F.sub.2 ; 3, a spherical mirror; 
4, a relay optical system including two or more lens units, for example, a 
pair of plano-convex lenses 4a and 4b; 5, an infrared removing filter 
removably disposed between the pair of plano-convex lenses 4a and 4b; and 
6, a stop interposed between the relay optical system 4 and a light guide 
LG. Since the light-emitting section 1 is situated at the primary focal 
point F.sub.1 of the elliptical condensing mirror 2, light from the 
light-emitting section 1 is reflected and condensed by the elliptical 
condensing mirror 2 and forms the bright spot image of the light-emitting 
section 1 at the secondary focal point F.sub.2. The light of this image is 
further condensed on the entrance end face of the light guide LG by the 
relay optical system 4. The spherical mirror 3, which lies on the same 
spherical surface, is located so that its center of curvature is 
practically coincide with the primary focal point F.sub.1 of the 
elliptical condensing mirror 2 in order that rays emitted from the light 
emitting section 1 and escaping forward through the aperture of the 
elliptical condensing mirror 2 are reflected back to the position of the 
light-emitting section 1. 
The elliptical condensing mirror 2 in the present invention satisfies Eq. 
(1) already mentioned. Moreover, it is desirable that the elliptical 
condensing mirror 2 satisfies the following conditions: 
EQU 1.9&lt;F/X&lt;8.0 (1') 
EQU 0.09&lt;F/.phi.&lt;1.35 (1") 
where X is the size of the light-emitting section 1 of the lamp along the 
optical axis and .phi. is the aperture diameter of the elliptical 
condensing mirror 2. 
Since the first embodiment is constructed with the elliptical condensing 
mirror 2 satisfying Eq. (1), the light emitted from the light-emitting 
section 1 of a certain size can be efficiently condensed at the entrance 
end of the light guide LG, and the entire light source optical system can 
be designed to be compact. Specifically, if the focal length F of the 
elliptical condensing mirror 2 is set below the lower limit of Eq. (1), 
the major axis of the ellipse becomes relatively long compared with the 
minor axis to make a large difference in curvature between different 
portions of the ellipse. In this way, rays of light originating from a 
point shifted from the primary focal point F.sub.1 will be collected at a 
position considerably shifted from the secondary focal point F.sub.2. 
Consequently, the projected image of the light-emitting section 1 is 
markedly distorted, and rays capable of being incident through the relay 
optical system 4 on the entrance end face of the light guide LG are 
limited to those emitted from a part of the light-emitting section. On the 
other hand, if the focal length F exceeds the upper limit of Eq. (1), 
there will be little difference in length between the major and minor axes 
of the ellipse and the curvature of the ellipse becomes moderate. When 
such an ellipse is used for the elliptical condensing mirror, the aperture 
diameter of the elliptical condensing mirror 2 must be enlarged in order 
to efficiently condense the light emitted from the light-emitting section 
1, and thus compactness of the light source optical system cannot be 
maintained. 
Furthermore, the elliptical condensing mirror 2 satisfies Eqs. (1') and 
(1"). Eq. (1') defines the condition that the projected image of the 
light-emitting section 1 formed by the elliptical condensing mirror 2 is 
not distorted with respect to the size X of the light-emitting section 1 
along the optical axis. If the lamp is combined with the elliptical 
condensing mirror 2 so that the value of F/X is set below the lower limit 
of Eq. (1'), the projected image of the light-emitting section 1 by the 
elliptical condensing mirror 2 will be distorted and the efficiency of 
incidence of light on the entrance end face of the light guide LG will be 
impaired. This indicates that, for example, when the size X of the 
light-emitting section, along the optical axis, of the lamp, which is 
located close to the primary focal point of the elliptical condensing 
mirror with a certain focal length, is such as to exceed the lower limit 
of Eq. (1'), rays emitted from such a portion as to exceed the lower limit 
of Eq. (1'), of the light-emitting section, are collected at a position 
considerably shifted from the secondary focal point of the elliptical 
condensing mirror 2. As a result, the projected image of the 
light-emitting section is distorted. Conversely, if the lamp is combined 
with the elliptical condensing mirror 2 so that the value of F/X is set 
beyond the upper limit of Eq. (1'), the elliptical condensing mirror 
becomes much larger than is necessary and compactness of the light source 
optical system cannot be held. 
Eq. (1") defines the condition for determining an effective diameter most 
suitable for the elliptical condensing mirror with the focal length 
determined by Eq. (1'). Specifically, in view of the relationship between 
the angle of incidence of a ray on the entrance end face of the light 
guide LG and the numerical aperture of the light guide, if the effective 
diameter .phi. of the elliptical condensing mirror becomes so large as to 
pass the lower limit of Eq. (1") with respect to the focal length F of the 
elliptical condensing mirror 2, the angle of incidence of the ray on the 
entrance end face of the light guide LG becomes larger than that 
corresponding to the numerical aperture of the light guide. Thus, the area 
of the elliptical condensing mirror reflecting rays which cannot be 
substantially transmitted by the light guide is merely added and the 
amount of light incident on the light guide is not increased, with the 
result that only the elliptical condensing mirror becomes large-sized. On 
the other hand, if the effective diameter .phi. of the elliptical 
condensing mirror becomes so small as to exceed the upper limit of Eq. 
(1"), the area of the elliptical condensing mirror reflecting rays at the 
angles of incidence at which the rays can be substantially transmitted by 
the light guide will be eliminated and the amount of light incident on the 
light guide will be decreased. 
In the elliptical condensing mirror 2 of the first embodiment, as mentioned 
above, the focal length of the elliptical condensing mirror 2 is 
determined with respect to the light-emitting section having a certain 
size along the optical axis in such a way as to satisfy Eq. (1') as well 
as Eq. (1). Furthermore, the effective diameter is determined in such a 
way as to satisfy Eq. (1") with respect to the elliptical condensing 
mirror, and thereby the amount of light incident on the light guide and 
the size of the elliptical condensing mirror can be optimized. Also, the 
values of respective parameters relative to the elliptical condensing 
mirror 2 of the first embodiment are as shown in Table 1. 
TABLE 1 
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X .alpha. 
.beta. .phi. 
F 
(mm) (mm) (mm) (mm) (mm) F/X F/.phi. 
______________________________________ 
3.0 35.8 28.4 50.0 11.3 3.77 0.23 
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The relay optical system 4 is composed of at least two lens units, each 
having a positive refracting power, and has the function of transmitting 
the image of the light-emitting section 1 projected by the elliptical 
condensing mirror 2 to the entrance end face of the light guide LG. The 
first embodiment is constructed so that rays incident on the relay optical 
system 4 are converted into a parallel beam by the first plano-convex lens 
4a with the positive refracting power and the beam is condensed on the 
entrance end face of the light guide by the second plano-convex lens 4b. 
The convex side of each of the plano-convex lenses 4a and 4b is configured 
as an aspherical surface whose curvature reduces progressively in 
separating from the axis of the optical system. This makes it possible to 
correct for axial aberration produced in the relay optical system 4 and 
obviate the defect that the efficiency of incidence of rays on the light 
guide LG is impaired by the distortion of the image of the lighte-mitting 
section 1 transmitted to the entrance end face of the light guide LG. When 
f denotes the focal length of the lens unit located closest to the light 
guide LG, of the lens units constituting the relay optical system 4, 
namely that of the plano-convex lens 4b, and D denotes the diameter of the 
axial light beam incident on the lens unit, the relay optical system 4 is 
designed to satisfy the following condition: 
EQU 0.556&lt;.vertline.f/D.vertline.&lt;1.462 (1'") 
The relay magnification of the relay optical system 4 used in the present 
invention is governed by the focal length of the lens unit located closest 
to the light guide LG. In order to determine the focal length of the lens 
unit, that is, the relay magnification of the relay optical system 4, it 
is desirable that the efficiency of incidence of the ray on the light 
guide LG is optimized in view of the relationship between the angle of 
incidence of the ray on the entrance end face of the light guide LG and 
the numerical aperture of the light guide LG. Eq. (1'") defines the 
condition for determining the relay magnification so that the ray is 
efficiently incident on the light guide. 
In the first embodiment, if the value of .vertline.f/D.vertline. is below 
the lower limit of Eq. (1'") in the relay optical system 4, a ray with the 
angle of incidence larger than that corresponding to the numerical 
aperture of the light guide will be produced, and the efficiency of use of 
the amount of light will be impaired. Conversely, if the value of 
.vertline.f/D.vertline. exceeds the upper limit of Eq. (1'"), the image of 
the light-emitting section transmitted to the entrance end face of the 
light guide will be enlarged, and similarly the efficiency of use of the 
amount of light will be impaired. In the first embodiment, the light 
source optical system is thus designed to satisfy Eqs. (1)-(1'") with 
respect to the elliptical condensing mirror 2 and the relay optical system 
4. In this way, even when the light-emitting section has a certain length 
along the optical axis as in the discharge lamp, axial light is 
efficiently collected and rendered incident on the entrance end face of a 
relatively fine light guide for endoscopes and can be transmitted to the 
exit end side thereof, and the entire optical system can be constructed to 
be compact. 
According to the first embodiment, since the infrared removing filter 5 is 
used at a place where the light beam is parallel with the optical axis, 
the effect of the filter 5 for removing infrared rays can be optimized, 
and the entrance end face of the light guide LG can be positively 
protected from burning damage caused by infrared rays. 
Also, in the first embodiment, when an infrared cutoff coating, instead of 
the infrared removing filter 5, is applied to the flat side of at least 
one of the plano-convex lenses 4a and 4b, the same effect can be achieved, 
and thereby the entire optical system can be designed to be more compact. 
Second embodiment 
FIG. 3 shows the arrangement of the second embodiment of the light source 
optical system for endoscopes according to the present invention. In this 
figure, like numerals and symbols are used in like or similar members with 
reference to the first embodiment. The second embodiment has the same 
arrangement as the first embodiment with the exception that a plane mirror 
7 is interposed on the optical path between the elliptical condensing 
mirror 2 and the relay optical system 4 so that the optical path is bent 
at right angles, and the stop 6 is placed on the entrance side of the 
relay optical system 4. 
In the second embodiment, the light source optical system satisfies Eqs. 
(2) and (3) previously mentioned. Specifically, in these equations, P 
denotes an angle between the optical axis of the light beam which travels 
from the light-emitting section 1 to the plane mirror 7 and the optical 
axis of the light beam which travels from the plane mirror 7, through the 
relay optical system 4, to the entrance end face of the light guide LG; L 
denotes a distance from the secondary focal point F.sub.2 of the 
elliptical condensing mirror 2 to the plane mirror 7; and .theta. denotes 
the maximum angle of incidence of a ray reflected by the elliptical 
condensing mirror 2 on the secondary focal point F.sub.2. 
Eq. (2) defines the angle at which the optical path of the light source 
optical system is bent by the plane mirror 7, while Eq. (3) specifies the 
location of the plane mirror 7 in the light source optical system. 
In this case, if the angle P is smaller than the value of the lower limit 
of Eq. (2), a positional problem will arise that the relay optical system 
4 may penetrate into the light beam reaching the plane mirror 7 from the 
elliptical condensing mirror 2 to block a part of the light beam. On the 
other hand, if the angle P becomes larger than the value of the upper 
limit of Eq. (2), the problem will be raised that the optical system 
occupies a comparatively large space and compactness of the optical system 
cannot be achieved. Further, if the value of .vertline.L/tan 
.theta..vertline. is beyond the limit of Eq. (3), the section of the light 
beam will be increased and the optical system must be enlarged. The result 
is that the compactness cannot be achieved. 
In this way, the plane mirror 7 is placed in the light source optical 
system so as to satisfy Eqs. (2) and (3) and bends the optical path, and 
thereby a space for incorporating the light source optical system can be 
made extremely small. In the second embodiment, the elliptical condensing 
mirror 2 and the relay optical system 4 are designed as in the first 
embodiment, and the plane mirror 7 is placed in the light source optical 
system so as to satisfy Eqs. (2) and (3) and bends the optical path at 
right angles. Hence, even when the light-emitting section has a certain 
length along the optical axis as in the discharge lamp, source light can 
be efficiently condensed and rendered incident on the entrance end face of 
a relatively fine light guide for endoscopes and can be transmitted to the 
exit end side. Moreover, the second embodiment, in contrast with the first 
embodiment, is capable of constructing a further compact light source 
optical system. Also, the values of respective parameters relative to the 
elliptical condensing mirror 2 of the second embodiment are as shown in 
Table 2. 
TABLE 2 
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X .alpha. 
.beta. .phi. 
F 
(mm) (mm) (mm) (mm) (mm) F/X F/.phi. 
______________________________________ 
0.64 33.0 13.0 25.0 2.6 4.08 0.10 
______________________________________ 
FIG. 4 shows an example where the light source optical system for 
endoscopes of the second embodiment is used. According to this example, 
the elliptical condensing mirror 2 and a discharge lamp 1' are integrally 
incorporated in a block B.sub.1, and the plane mirror 7 and the relay 
optical system 4 are integrally incorporated in a block B.sub.2. The block 
B.sub.2 is designed to be rotatable around an optical axis x of the 
elliptical condensing mirror 2 or an axis parallel with the optical axis 
x, for example, around the optical axis x, with a point of intersection 0 
of x, y, and z axes in FIG. 4 as a center, thereby facilitating the use of 
a light source apparatus for endoscopes. 
FIG. 5 shows the appearance of the light source apparatus, in which a belt 
is used to be portable. In observation through an endoscope, cases 
frequently occur in which the endoscope must be inserted in a fine curved 
tube as in the inspection of the inner wall of the tube. In such cases, an 
observer repeats such behavior as to bend the endoscope or vary his 
position so that the endoscope can be easily inserted in the fine tube. In 
a conventional light source apparatus for endoscopes, since a connection 
with a light guide connector is fixed, a light guide cable is twisted and 
the observer's work efficiency is considerably impaired. This is 
responsible for undue fatigue of the observer. According to the example 
where the light source optical system of the second embodiment is used, 
the block B.sub.2 is designed to be rotatable around the optical axis x, 
with the point of intersection 0 of x, y, and z axes in FIG. 4 as a 
center, and hence the direction of the light guide connector can be 
changed in such a way that the light guide LG is not twisted. Thus, the 
second embodiment has the great advantage that the observer can easily 
insert the endoscope without assuming an uncomfortable position. Moreover, 
if the block B.sub.2 is made rotatable not only around the optical axis x 
but also around the y and z axes, with the point of intersection 0 as a 
center, to such an extent that the plane mirror does not block the light 
beam, the facilitation of use of the apparatus will be further improved. 
The appearances of the light source apparatuses for endoscopes in which the 
elliptical condensing mirror 2, the plane mirror 7, and the relay optical 
system 4 are fixedly arranged in a state where the optical axis x makes a 
right angle with the z axis are shown in FIG. 6 (shoulder belt type) and 
FIG. 7 (a type that the light source optical system can be incorporated in 
a housing rack for endoscopes). The light source optical system of the 
second embodiment, because its housing space can thus be made very small, 
is used as a portable light source apparatus integrated with a battery as 
depicted in FIG. 6, or as a systematic light source incorporated, together 
with a video system, in the housing rack as depicted in FIG. 7. In this 
way, a light source apparatus for endoscopes which has exceptional 
versatility can be designed. 
Third embodiment 
FIG. 8 shows the arrangement of the third embodiment of the light source 
optical system for endoscopes according to the present invention. FIGS. 
9A, 9B, and 9C show the diagrams of spectral transmittance characteristics 
of an RGB rotary filter used in the third embodiment. In FIG. 8, like 
numerals and symbols are used in like or similar members with reference to 
the first embodiment. The third embodiment has the same arrangement as the 
first embodiment with the exception that the stop 6 is located at the 
position of the secondary focal point F.sub.2 of the elliptical condensing 
mirror 2, and an RGB rotary filter 8 is used instead of the infrared 
removing filter 5. The RGB rotary filter 8 comprises a B (blue light) 
transmission filter, a G (green light) transmission filter, and an R (red 
light) transmission filter, arranged to be concentric and equidistant, 
having spectral transmittance characteristics such as those shown in FIGS. 
9A, 9B, and 9C, respectively, so that an object to be observed can be 
viewed as a colored image. Since the fundamental function and effect of 
the third embodiment are the same as those of the first embodiment, their 
detailed explanation is omitted. Also, the values of respective parameters 
relative to the elliptical condensing mirror 2 of the third embodiment are 
as shown in Table 3. 
TABLE 3 
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X .alpha. 
.beta. .phi. 
F 
(mm) (mm) (mm) (mm) (mm) F/X F/.phi. 
______________________________________ 
1.50 50.0 33.6 66.0 11.3 7.52 0.17 
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