Optical fiber bundle with field stop

An image transmitting optical fiber bundle has at an end of it a field stop for defining the size of image viewed thereby. The field stop is sandwiched between a pair of transparent thin plastic films. The pair of films which sandwiches the field stop is fixed to an end of the optical fiber bundle. A face plate is further fixed on the pair of films on the end face of the fiber bundle. Since the surfaces adhered are all flat, the formation of bubbles in the adhesive is easily precluded.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical fiber bundle, and more particularly to 
an improvement in the structure of an end face of an image transmitting 
optical fiber bundle (hereinafter referred to simply as "image guide"). 
2. Description of the Prior Art 
In the image guide composed of a number of thin optical fibers stacked in a 
close-packed structure, the outer fibers on the periphery of the image 
guide are liable to slack and the arrangement of the fibers is apt to be 
disordered. Particularly, in image guides which are combined with an 
optical system used for enlarging an image, the disordered arrangement of 
the fibers is also enlarged, which results in degradation in image 
quality. In order to evade this problem, it has been generally known to 
provide a field stop between the end of the image guide and an eyepiece. 
In the image guide practically manufactured, the end face of the image 
guide is precise finished to make an optically flat end face. In order to 
prevent the finished end from staining, weathering or dimming, a face 
plate is generally cemented to the finished end of the image guide. 
Therefore, in the conventional image guide, the face plate is fixed 
directly to the end of the image guide and thereon is mounted a field stop 
to provide a mask on the image obtained by the image guide. An eyepiece is 
then provided on the field stop. 
The above described conventional image guide has a defect in that the 
effective aperture size of the field stop is changed as the position of 
the pupil is moved along the optical axis of the eyepiece, since the field 
stop is separated from the end face of the image guide. When a picture of 
the image viewed through the image guide is taken, the marginal boundary 
of the image defined by the field stop is apt to be blurred. 
If the field stop would be inserted between the eyepiece and the end face 
of the image guide in order to preclude the above described defect caused 
by the distance made therebetween, there would occur the following 
problems. When the field stop is fixed to the end face of the image guide, 
an adhesive is used. In this case, it is very difficult to remove bubbles 
contained in the adhesive. The bubbles in the adhesive will accelerate the 
staining, weathering or dimming of the end face of the image guide. Since 
the inner edge of the field stop which has a substantial thickness works 
to accelerate the formation of the bubbles when the field stop is pressed 
against the end face of the image guide with the adhesive sandwiched 
therebetween, the removal of the bubbles is considerably difficult. When 
the bubbles are fixedly formed in the adhesive, it would become necessary 
to remove the field stop from the end face to again polish the end face. 
SUMMARY OF THE INVENTION 
In view of the above defects, it is the primary object of the present 
invention to provide an image guide in which a field stop is fixed to the 
end face thereof at substantially the same position on the optical axis of 
the eyepiece as that of the end face, whereby the change of the effective 
aperture size of the field stop is prevented. 
It is another object of the present invention to provide an image guide in 
which the formation of bubbles in the adhesive when a field stop is fixed 
to the end face of the image guide is effectively prevented. 
It is still another object of the present invention to provide an image 
guide in which a thick face plate can be used at the end thereof, whereby 
the degradation of image quality caused by dusts or stains on the surface 
of the face plate is prevented. 
The above objects are accomplished by providing a field stop sandwiching 
means which sandwiches a field stop therebetween between the end face of 
the image guide and the face plate. The sandwiching means is composed a 
pair of thin plastic films. Since the field stop is first sandwiched 
between the pair of thin plastic films and then the plastic films are 
attached to the end face of the image guide, there is no fear of forming 
bubbles in the adhesive used between the plastic film and the end face of 
the image guide. 
The present invention will be described in more detail with reference to 
the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 shows a field stop 1 employed in the image guide in accordance with 
the present invention. The field stop 1 has a square aperture 1a. The 
field stop is sandwiched between a pair of thin transparent plastic films 
2a and 2b as shown in FIG. 2 and fixed thereto by means of an adhesive. 
Since the field stop 1 is fixed to the films 2a and 2b separately from the 
image guide 5, the bubbles if formed in the adhesive can be removed by 
vacuum deforming. Further, if the bubbles are not completely removed in 
some assemblies consisting of the field stop 1 and the films 2a and 2b, 
the image guides may be manufactured without using those assemblies 
containing bubbles. 
Referring to FIG. 2, a stack of optical fibers 4 is sealed in a sleeve 6. 
The end of the sleeve 6 and the end of the fibers 4 are polished precisely 
to provide a flat face 5a to which said assembly is attached. The assembly 
consisting of the pair of plastic films 2a and 2b and the field stop 1 
fixed therebetween is adhered to the end face 5a of the image guide 5. 
Since the surface of the film 2b is flat, the assembly can easily be fixed 
to the end face 5a of the image guide by means of an adhesive without 
forming bubbles therein. On the assembly fixed to the end face 5a of the 
image guide, a face plate 3 is adhered to the surface of the film 2a by an 
adhesive. Since the surfaces of the film 2a and the face plate 3 are also 
flat, the face plate 3 can easily be fixed to the assembly without forming 
bubbles in the adhesive. Thus, an image guide free from the fear of 
staining, weathering or dimming on the end face thereof can be obtained.