Illuminating device having optical light guide formed as fibre bundle

An illuminating device including an optical light guide in the form of a fibre bundle, and a light source having a condenser lens to incident the incidence end surface of the fibre bundle is disclosed. An optical reflecting body having a peripheral reflecting portion is attached to the incidence end surface of the fibre bundle. Parameters of the lens and the optical reflecting body are determined to decrease light loss and to uniformly distribute light on the incidence end surface. The relation between the parameters is represented as formulae which in one case is applicable when the light intensity distribution of the bright spot image is peaked and in another case is applicable when the distribution is substantially flat.

BACKGROUND OF THE INVENTION 
The present invention relates to an illuminating device having optical 
light guide in the form of a fibre bundle which may be used as an 
endoscope. 
Generally, optical light guide fibres are made from glass or plastic 
material by crucible, acid melting method, rod, or thermal fusion method. 
These fibres are bundled, end rings are fitted at both ends of the bundle, 
and both ends are suitably ground to form a fibre bundle of flexible, 
semi-rigid or rigid conduit. 
Such fibre bundles are used in light communication systems, illuminating 
devices sensor heads, light image transportation systems, illuminating 
devices light and safety anti-explosion devices. In any case, it is 
important to obtain a low loss and good light transporting fibre bundle. 
One proposal to achieve this result is described in Japanese Laid Open 
Patent Application No. 19761/1979. This application attempts to decrease 
Fresnel reflection on both input and output end surfaces of the fibre 
bundle. To this end, a reflection decreasing layer e.g. MgF.sub.2 is 
applied on a glass base plate having substantially the same refractive 
index as that of the fibre bundle and the glass base plates are adhered on 
the input and output end surfaces of the fibre bundle by a transparent 
adhesive having substantially the same refractive index as that of the 
fibre bundle. 
While this structure decreases Fresnel reflection loss on both the end 
surfaces of the fibre bundle, it does not uniformly distribute input light 
to the input surface of the fibre bundle. 
In the field of endoscopy, high accuracy and high capacity are desired and 
especially a high level of light transmission is desired. In conventional 
endoscopes, to improve light transmission, a strong light source, e.g. 
small xenon lamp of high luminance, is used and also a condenser lens is 
inserted to concentrate the high output of the lamp in narrow end surface 
of the fibre bundle efficiently. 
However, this structure may cause other problems such as a shortened life 
of the lamp and a burning of the end surface of the fibre bundle by highly 
concentrated high luminance light. Consequently, the most intimate problem 
of such illuminating device is to concentrate the illuminating light from 
the light source uniformly and with least light loss to the incidence end 
surface of the fibre bundle. 
SUMMARY OF THE INVENTION 
Accordingly, the primary object of the present invention is to provide an 
illuminating device having an optical light guide formed as a fibre bundle 
in which illuminating light from light source is distributed substantially 
uninformly to the entire incidence end surface of the fibre bundle. 
Another object of the present invention is to provide an illuminating 
device having optical light guide in the form of a fibre bundle in which 
illuminating light from the light source is applied substantially 
uniformly and with the least light loss to substantially the entire 
incidence end surface of the fibre bundle. 
The illuminating device having the fibre bundle according to the present 
invention comprises an optical reflecting body having a peripheral 
reflecting surface attached to the incidence end surface of the fibre 
bundle. Parameters, i.e. radius, length and refractive index of the 
optical reflecting body, effective radius of the fibre bundle, air 
equivalent length between the bright spot image of illuminating light and 
incidence end surface of the optical reflecting body, height of the upper 
ray on the incidence end surface of the optical reflecting body, height 
and angle to the optical axis of the marginal ray are determined in 
relation to specific formulae. The illuminating device according to the 
present invention distributes illuminating light from the light source 
substantially uniformly to substantially the entire incidence end surface 
of fibre bundle with the least light loss. Thus, light transmission 
efficiency is improved and too narrow a concentration of light on the 
incidence end surface of the fibre bundle is avoided.

EXPLANATION OF PRIOR ART 
Before explaining the embodiments of the present invention, a conventional 
illuminating device utilizing an optical light guide formed as fibre 
bundle will be explained referring to FIG. 1. 
In FIG. 1, 1 designates a fibre bundle formed of a bundle of glass fibres 2 
of e.g. tens to thousands of fibres. Rings 3 and 3' are fitted at opposite 
ends of the bundle, and a thermo-setting resin is adhered between the 
fibres and also between the rings 3 and 3' and the bundle 2. The incidence 
end surface 4 and emerging end surface 5 of the fibre bundle 2 are ground 
flat. The flexible portion 6, located between the both ends 4 and 5 of the 
bundle 2, is covered by a cover tube 7 of e.g. silicone. 
A light source device 8 comprises a special lamp 9 e.g. halogen lamp or 
xenon lamp, a condenser lens 10 and a connector 11. When the lamp 9 is 
lighted, bright spot image 12 produced by the lens 10 is formed on the 
incidence end surface 4 of the fibre bundle 1 to perform effective 
incidence of illuminating light from the light source. On the emerging end 
surface 5 of the fibre bundle 1, an illuminating lens 13 is mounted to 
diverge illuminating light which is transmitted through the bundle of 
glass fibres 2. 
In the above-described illuminating device utilizing optical light guide 
formed as fibre bundle, the incidence end surface 4 of the fibre bundle 1 
is inserted into the connector 11 of the light source device 8, and the 
lamp 9 is lighted so that the bright spot image 12 (located at the focal 
point of lens 10) of the filament 14 of the lamp 9 is formed on the 
incidence end surface 4 of the fibre bundle 1. When the dimension of the 
bright spot image 12 of the filament 14 is smaller than the effective 
diameter (.phi.D) of the incidence end surface 4 of the fibre bundle 1, a 
portion of the incidence end surface 4 is illuminated stronger than other 
portions. The fibre bundle 1 for illuminating purpose, arrangement of each 
fibre at the incidence end surface 4 and the emerging end surface 5 does 
not perfectly correspond compared with image transporting fibre bundle. 
However, fibres tend to gather to form multi-fibres 15 shown in FIG. 1, so 
that correspondence between the incidence end surface and the emerging end 
surface can not be denied. Thus, in generally speaking, the arrangement of 
each fibre at the incidence end surface 4 and the emerging end surface 5 
corresponds with each other evenly for illuminating purposes. Thus, when 
the bright spot image 12 of the light source 9 is formed on a portion of 
the incidence end surface 4 of the fibre bundle 1, e.g. on the 
multi-fibres 15, the bright spot image 12 is also formed on the emerging 
end surface 5 without scattering in wide range. The illuminating light 
from the end surface 5 is diverged within narrow range of a body 16 to be 
observed. Such illuminating light distribution is undesirable when 
observing a body cavity using an endoscope. 
To overcome this problem, the incidence end surface of the fibre bundle 1 
is separated a few millimeter from the imaging point of the bright spot 
image 12 as shown in FIG. 2. Thus, bright spot image on the incidence end 
surface 4 fades to distribute illuminating light all through the end 
surface 4. However, as the bright spot is not a sharp spot, light 12' on 
the periphery of the bright spot illuminates out of the incidence end 
surface 4 of the fibre bundle 1. Thus, uneven distribution at incidence 
end point is avoided, however, the amount of light utilized is decreased 
and light transport efficiency is decreased. 
DETAILED DESCRIPTION OF THE EMBODIMENTS 
In all the drawings the same reference numeral shows the same or similar 
part or portion for the sake of clarity. 
A first embodiment to explain basic construction of the present invention 
is shown in FIG. 3. A single light guide 17 is made of a core 17" and a 
clad 17' which is secured with outside surface of the core 17". The single 
light guide 17 acts as an optical reflecting body and is mounted in front 
of the incidence end surface 4 of the fibre bundle 1. When the single 
light guide 17 is connected with the fibre bundle 1, the bright spot image 
12 of the light source 9 focuses in the single light guide 17 and diverges 
and then incidents into the end surface 4 of the fibre 1. Thus, peripheral 
light of the bright spot also incidents onto the end surface 4 of the 
fibre bundle 1 after being reflected off the peripheral wall surface of 
the single light guide 17. 
By connecting the single light guide 17 on the incidence end surface 4 of 
the fibre bundle 1, illuminating light from the light source 9 can be 
applied to the incidence end surface 4 without light loss. However, to 
improve light transport efficiency, the dimension and refractive index of 
the single light guide 17 and imaging point of the bright spot image or 
air equivalent length of the imaging point from the incidence end surface 
18 of the single light guide 17, must be accurately selected and 
determined. 
The selection of the parameters of the single light guide 17 will be 
explained. In this case, the set condition is different when the light 
intensity distribution of the bright spot image 12 has high peak value or 
when the distribution is relatively uniform. Thus, each case must be 
considered separately. 
First, it will be assumed that the light intensity distribution of the 
bright spot image 12 of the light source has high peak value as shown in 
FIG. 4. 
In a fibre bundle connecting with a single light guide 17, the parameters 
which must be examined to ensure that incident illuminating light will be 
uniformly distributed on the incidence end surface without causing light 
quantity loss are defined as follows: 
d: axial length of the single light guide 17, 
r: radius of the core 17" of the light guide 17, 
n: refractive index of the core 17", 
r: effective radius of the fibre bundle 1, 
I: height of the bright spot image, 
S: equivalent air length from the incidence end surface of the single light 
guide to the bright spot image, and is plus in the direction from the end 
surface 18 to the fibre bundle, 
h: height of upper light at the incidence end surface of the single light 
guide 17, 
k: height of marginal ray 19, 
.alpha.: angle of marginal ray 19 to optical axis, 
C: radius of diverged bright spot on the incidence end surface of the fibre 
bundle. 
At first, the randomness of the fibre bundle 1 is considered. The radius C 
of the diverged bright spot on the incidence end surface 4 of the fibre 
bundle 1 can be used practically when the radius C is more than 1/3 of the 
effective radius r of the fibre bundle 1. Thus, the condition to incident 
illuminating light on the incidence end surface 4 of the fibre bundle 1 
substantially uniformly without causing uneven distribution of light is: 
EQU C.gtoreq.1/3r (1) 
As, 
EQU C=(-S+d/n) tan .alpha. (2) 
From formulae (1) and (2), 
EQU (-S+d/n) tan .alpha..gtoreq.1/3r (3) 
The formula (3) describes the condition which is necessary to cause 
incident illuminating light to be substantially uniformly distributed on 
the incidence end surface of the fibre bundle 1. 
To avoid light quantity loss on the incidence end surface 18 of the single 
light guide 17, upper side ray 20 shown in FIG. 5 must be incident in the 
single light guide 17. Thus: 
EQU h&lt;r (4) 
As a practical matter, the outer portion of the divergence of the bright 
spot is rather dark, so that the formula (4) may be rewritten as: 
EQU (h+k)/2&lt;r (5) 
Accordingly, when parameters S, d, n and .alpha. are selected to satisfy 
formula (3), an illuminating device utilizing a fibre bundle is obtained 
which causes incident illuminating light to be substantially uniformly 
distributed on the incidence end surface of the fibre bundle. Also, when 
parameters S, d, n, .alpha., h, k and r are selected to satisfy formulae 
(3) and (5), an illuminating device utilizing a fibre bundle is obtained 
which causes incident illuminating light to be substantially uniformly 
distributed on the incidence end surface 4 of the fibre bundle 1 without 
causing light loss. 
Next, it is assumed that the light intensity distribution of the bright 
spot image 12 is substantially uniform as shown in FIG. 6. 
In this case, the light transport path is shown in FIG. 7. Generally, the 
height I of the bright spot image is small compared with distance l 
between the light source lens 10 and the imaging point of the bright spot 
image 12, it can be considered that the principal ray 21 is substantially 
parallel to the optical axis. In this case, light stop is considered as 
lens frame. Now, the height of the upper ray 20 on the incidence end 
surface 4 of the fibre bundle 1 is represented as Q. To improve 
illumination light distribution on the incidence end surface 4, regarding 
the randomness of the fibre bundle 1, practically Q may be wider than 1/3 
of effective radius r of the fibre bundle 1. Thus, 
EQU Q.gtoreq.1/3r (6) 
As, 
EQU Q=I+(-S+d/n) tan .alpha. (7) 
From the formulae (6) and (7), the condition which must be obtained to 
improve the illumination light distribution on the incidence end surface 
is: 
EQU I+(-S+d/n) tan .alpha..gtoreq.1/3r (8) 
The condition to avoid light loss on the incidence end surface 18 of the 
single light guide 17 is that the upper ray 20 is within the single light 
guide 17. Thus, as in the first case, the light intensity distribution of 
the bright spot image has peak value, 
EQU h&lt;r (9) 
Accordingly, in the second case where the light intensity distribution of 
the bright spot image is substantially uniform as shown in FIG. 6, when 
parameters I, S, d, n and .alpha. relating to the dimension of the single 
light guide 17 and imaging point of the bright spot image, are selected to 
satisfy the formula (8), the illuminating device utilizing a fibre bundle 
causes incident illuminating light to be substantially uniformly 
distributed on the incidence end surface of the fibre bundle. Also, when 
the parameters I, S, d, n, .alpha., h and r are selected to satisfy 
formulae (8) and (9), the illuminating device utilizing a fibre bundle 
connected with the single light guide causes the incident illuminating 
light to be substantially uniformly distributed on the incidence end 
surface of the fibre bundle without causing light loss. 
When light flux is not symmetrical with the optical axis, for example, when 
a light stop is used, the values of h, k and .alpha. in the equations 
(2), (3), (4), (5), (7) and (8) should adopt minimum values respectively. 
The radius r of the core 17" of the single light guide 17 may preferably be 
determined from the manufacturing stand point such that the core radius r 
of the single light guide 17 is larger than the effective radius r of the 
fibre bundle 1, so as to absorb misalignment between the single light 
guide 17 and the incidence end surface 4 of the fibre bundle 1. Thus, 
effective area of the fibre bundle 1 can be fully utilized to transport 
illuminating light. 
The numerical aperture of the single light guide 17 may preferably be the 
same or greater than that of the fiber bundle 1 so that the maximum 
angular component of light can be transported on the incidence end surface 
4 of the fibre bundle 1. 
In an illuminating device a utilizing light transporting fibre bundle 1, 
the single light guide 17 is connected with the incidence end surface 4 of 
the fibre bundle 1, and various parameters of the construction parts can 
be determined by the above described formulae according to the present 
invention. In practice, construction parts can be easily made 
corresponding to the above-mentioned formulae. However, in assembly, when 
the fiber bundle 1 is connected with the connector 11 of the light source 
device 8, some misalignment can occur. To absorb the misalignment, the 
condenser lens 10 of the light source device 8 is axially adjustable to 
adjust the imaging point and size of the bright spot image. Thus, some 
parameters can be adjusted. Consequently, illuminating light will be 
incident on the incidence end surface 4 of the fibre bundle 1 most 
efficiently with minimum loss and over the full effective area of the end 
surface. 
In the above-mentioned illuminating device, when Fresnel reflection loss is 
decreased, light transport characteristics is improved. To decrease the 
Fresnel reflection loss, both end surfaces 18 and 18' of the single light 
guide 17 and the incidence end surface 4 of the fibre bundle 1 are coated 
with thin layer of material having a similar or the same refractive index 
with that of the core 17" of the single light guide 17, e.g. magnesium 
fluoride or silicon oxide. The adhesive between the incidence end surface 
4 of the fibre bundle 1 and the single light guide 17 may also be formed 
of a material having similar or same refractive index with that of the 
core 17" of the single light guide 17. Such material may preferably be a 
thermosetting resin or organic silicone compound of transparent, 
relatively high heat resistivity, and slightly less hardness than the core 
when cured. Such coating of thin layer and applying adhesive decrease end 
surface reflection loss and also act to water proof the end surfaces. 
In the above-mentioned embodiment, the light reflection body which is 
connected with the incidence end surface 4 of the fibre bundle 1 is a 
cladded single light guide 17. However, a cylindrical reflection body or a 
conical reflection body which reflects incidence light at peripheral wall 
surface may be used in place of the single light guide 17. Off course, the 
optical reflecting body 17 may preferably made from transparent material 
to assure good transmission factor.