Endoscopic zoom lens

Apparatus (1) for use as an endoscope or borescope comprising an outer tube (2) having a distal end (3) which is insertable into an inaccessible location to be inspected and a proximal end (4) which is accessible to the user. A zoom lens is provided within the outer tube adjacent the distal end and is operable to focus an image with variable magnification at a focal plane within the outer tube. The zoom lens comprises an objective lens (7) which is fixed relative to the outer tube and two independently movable lenses (8,13). An elongate viewing means (43) is provided for viewing the image and extends through the outer tube from a location at the focal plane to a location proximal of the proximal end. A cam operated actuating means (29,21) is operable to move the movable lenses to change magnification and to synchronously move the viewing means relative to the objective lens to compensate for movement of the focal plane. Refocussing of the apparatus following adjustment of magnification is thereby carried out automatically.

BACKGROUND OF THE INVENTION 
This invention relates to an optical apparatus for use as an endoscope or 
borescope for inspection of inaccessible objects. 
It is desirable for such apparatus to be provided with variable 
magnification in the form of a zoom lens which ideally should be located 
at the distal end of the instrument and may incorporate the objective 
lens. Endoscopes and borescopes however typically comprise a long tube of 
the order of 10 mm or less in diameter. Consequently any mechanism for 
providing independent movement of lenses within the zoom lens must be 
miniature in form and must be actuated by a mechanism which does not 
significantly obstruct the available space for a viewing means such as an 
optical relay which must extend through the tube. 
A particular difficulty encountered in the incorporation of such zoom 
lenses in such apparatus is that of providing compensation for movement of 
the focal plane of the zoom lens when magnification is varied. In the 
absence of compensation the viewed image may be completely out of focus 
and indistinguishable after each adjustment of magnification and it is 
undesirable for the user to have to carry out anything other than a minor 
re-focusing operation after each adjustment. 
OBJECTS OF THE PRESENT INVENTION 
It is an object of the invention to provide an apparatus which avoids the 
need for the user to carry out refocusing operations after each adjustment 
of magnification. 
It is a further object of the present invention to provide an apparatus in 
which focus adjustment is available to the user to accommodate variation 
in object range or eye performance. 
SUMMARY OF THE INVENTION 
According to the present invention there is provided apparatus for use as 
an endoscope or borescope for inspection of an inaccessible object 
comprising an outer tube having a distal end which is insertable in use 
into an inaccessible location and a proximal end which is accessible to 
the user, a zoom lens within the outer tube adjacent the distal end and 
operable to focus an image of the object with variable magnification at a 
focal plane within the outer tube, the zoom lens comprising an objective 
lens which is fixed relative to the outer tube and a plurality of 
independently movable lenses, the apparatus further comprising an elongate 
viewing means for viewing the image and extending through the outer tube 
from a location at or adjacent to the focal plane to a location proximal 
of the proximal end and actuating means operable to move the movable 
lenses to change magnification and to synchronously move the viewing means 
relative to the objective lens to compensate for movement of the focal 
plane. 
An advantage of such apparatus is that (provided the object distance 
remains unchanged) it is not necessary to re-focus the viewing means 
following actuation of the zoom lens to change magnification. 
Preferably each of the movable lenses is provided with a respective axially 
movable control tube to which they are fixedly mounted, the control tubes 
extending coaxially within the outer tube and being engaged at their 
respective proximal end portions by the actuator means. 
An advantage of such arrangement is that the control tubes provide robust 
support for the movable lenses in a simple and space efficient manner. 
Preferably the actuating means comprises a cam means providing movement of 
the movable lenses and the viewing means in response to movement of an 
actuator. 
Conveniently the cam means comprises a cylindrical member, the cylindrical 
member being provided with circumferentially extending cam slots engaged 
by respective radial projections of the control tubes, and the cam slots 
being profiled such that rotation of the cylindrical member relative to 
the outer tube provides axial displacement of respective movable lenses in 
accordance with a predetermined characteristic. 
Preferably the cylindrical member is provided with a further cam slot 
engaged by a cam follower connected to the viewing means to effect axial 
movement of the viewing means relative to the objective lens by rotation 
of the cylindrical member. 
In a preferred embodiment the zoom lens comprises two movable lenses. 
The viewing means may comprise a housing operatively connected to the 
actuating means to provide axial movement synchronised with movement of 
the movable lenses, an image relaying means operable to relay an image 
from the focal plane of the zoom lens to the user and an adjustable 
coupling operatively connecting the housing to the image relaying means so 
as to provide focus adjustment accommodating variation in object range or 
eye performance of the user. 
In the absence of such adjustment the apparatus would remain focused at 
only one particular object range of interest. 
An advantage of the particular form of image relaying means of this 
embodiment is that no adjustment is made to the relative position of the 
optical components of the image relaying means but instead the entire 
image relaying means is moved relative to the zoom lens. Such "distal 
focusing" results in the eye relief for the user remaining constant 
thereby allowing easy and comfortable use throughout the zoom and focusing 
ranges of the apparatus. 
Conveniently the image relaying means includes an optical relay. 
The apparatus may further comprise an object range sensor operable to 
produce an electrical signal representative of the range of the object 
from the objective lens and an electromechanical focus actuator operable 
in response to the signal to focus on the object by movement of the image 
relaying means relative to the housing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In FIG. 1 apparatus 1 comprises an optical system for an endoscope having 
an elongate outer tube 2 with a distal end portion 3 which is insertable 
into an inaccessible area for inspection of an object (not shown). 
The outer tube 2 has a proximal end portion 4 which is accessible to the 
user and is connected to a casing (5) incorporating a handle (not shown) 
for manipulating the apparatus. 
The outer tube 2 has at the distal end portion 3 a window 6 arranged at 
right angles to the cylindrical axis of the outer tube and in proximity 
with the window 6 an objective lens 7 of positive power is fixedly 
connected to the outer tube. 
A first movable lens 8 comprising first and second lens elements 10 and 11 
is mounted within a first control tube 12 which is axially slidable within 
the outer tube 2 so as to control the position of the first movable lens. 
The first and second lens elements 10 and 11 are fixedly mounted within the 
first control tube 12 and are arranged so as to provide in combination the 
first movable lens 8 with negative power. 
A second movable lens 13 comprising third and fourth lens elements 14 and 
15 is fixedly mounted in a second control tube 16 which is slidable within 
the first control tube 12 so as to be able to control axial location of 
the second movable lens 13. 
The third and fourth lens elements 14 and 15 are fixedly connected to the 
second control tube 16 and in combination provide the second movable lens 
13 with positive power. 
Within the second control tube 16 is slidably mounted an image relaying 
means 17 which is generally cylindrical in external shape so as to extend 
between a distal end portion 18 adjacent to the second movable lens 13 and 
a proximal end portion 19 to which is mounted an eyepiece 20. 
The image relaying means 17 contains an optical relay consisting of a 
plurality of axially spaced lenses (not shown). 
The apparatus 1 includes a cam means 21 located adjacent the proximal end 
portion 4 of the outer tube 2 and including a generally cylindrical member 
22 mounted coaxially with the outer tube 2 so as to be rotatable relative 
to the casing 5. 
The first and second control tubes 12 and 16 extend coaxially through the 
cylindrical member 22 and are provided adjacent their respective proximal 
ends with respective first and second cam follower pins 23 and 24 which 
project radially through respective cam slots 25 and 26 extending 
generally circumferentially of the cylindrical member 22. 
The first control tube 12 projects into the cylindrical member 22 to a 
lesser extent than the second control tube 16 so that clearance is 
provided between the second cam follower pin 24 and the first control tube 
12. 
The second control tube 16 projects into the cylindrical member 22 to 
approximately the mid-point of the cylindrical member. A third cam 
follower pin 27 projects radially to engage a third circumferentially 
extending slot 28 formed in the cylindrical member, the pin 27 being 
operatively connected to the image relaying means 17 in a manner described 
below. 
An annular magnification control actuator 29 projects radially from the 
cylindrical member 22 and is manually accessible so that an operator can 
manually rotate the cylindrical member 22 so as to provide axial motion of 
the first, second and third cam follower pins 23, 24 and 27 in a manner 
dependent on the respective profiles of the first, second and third slots 
25, 26 and 28. 
The third cam follower pin 27 is connected by a longitudinally extending 
cylindrical linkage 31 to a generally cylindrical housing 32 which is 
axially slidable relative to the casing 5. 
The image relaying means 17 extends coaxially through both the cylindrical 
linkage 31 and the housing 32 and is adjustably coupled to the housing by 
means of a screw coupling 34. The housing 32 is rotatable relative to the 
casing 5 so as to constitute a focus control actuator as described below. 
The screw coupling 34 is arranged such that any longitudinal movement of 
the housing 32 due to movement of the linkage 31 is transmitted to the 
image relaying means 17 so that it will move in unison with the linkage 31 
and hence in unison with the third cam follower pin 27. The longitudinal 
position of the image relaying means 17 can however be adjusted relative 
to the linkage 31 by a user manually turning to the left or right the 
housing 32 which then functions as a focus control actuator. The housing 
32 is movable through a limited angular displacement which is delimited by 
engagement between a detent 37 connected to the housing 32 and first and 
second stop surfaces 38 and 39 of a focus limiting member 40. The focus 
limiting member 40 is part annular and fixedly connected to the casing 5 
so as to remain static at a location intermediate the housing 32 and the 
cam means 21. 
The first and second stop surfaces 38 and 39 limit the angular displacement 
of the housing 32 to an extent which is dependent upon the axial position 
of the housing 32 relative to the casing 5, the first and second stop 
surfaces 38 and 39 being profiled so as to serve as cam surfaces engaged 
by the detent 37. 
The image relaying means 17 includes a radially outward projection 41 
received in an axially extending groove 42 of the casing 5, the groove and 
projection cooperating to allow relative axial movement but preventing 
rotation of the image relaying means relative to the casing 5 and relative 
to the outer tube 2. The first and second control tubes 12 and 16 are 
provided with similar mechanisms (not shown) operable to prevent rotation 
relative to the outer tube 2. 
The image relaying means 17, the housing 32 and the eyepiece 20 together 
constitute a viewing means 43 which is axially movable relative to the 
outer tube 2 and hence relative to the objective lens 7 by actuation of 
the cam means 21. 
Operation of the cam means 21 is illustrated in FIGS. 2 and 3. In FIG. 2 
the displacements of the first and second movable lenses 8 and 13 relative 
to the fixed objective lens 7 are represented by displacements X1 and X2. 
The displacement of the distal end portion 18 of the image relaying means 
17 relative to the fixed objective lens 7 is represented by displacement 
X3. An image 44 is formed in the focal plane of a zoom lens 45 constituted 
by the combination of the objective lens 7, the first movable lens 8 and 
the second movable lens 13. In order to vary the magnification and field 
of view of the zoom lens 45 the movable lenses 8 and 13 are moved towards 
or away from the fixed objective lens 7 in a predetermined manner as 
illustrated in FIG. 3 in which values of displacement X are shown against 
various values of field of view F. As is well-known in the art of zoom 
lens construction, it is generally desirable to compensate for movement of 
the focal plane of the zoom lens in some way. In the apparatus 1 this 
compensation is provided by movement of the image relaying means 17 as 
illustrated in FIG. 3. 
In order to focus the apparatus 1 onto objects at varying distance from the 
objective lens 7, rotation of the housing 32 provides axial adjustment of 
the position of the image relaying means 17 so that to view a distant 
object X3 is decreased and to view a near object X3 is increased. The 
upper and lower limits of this adjustment are indicated by the upper and 
lower broken lines 47 and 48 in FIG. 3. The range of focus adjustment for 
smaller fields of view is necessarily more than that required for larger 
fields of view. By action of the focus limiting member 40 co-operating 
with the first and second stop surfaces 38 and 39 the apparatus 1 is 
provided with means which limit the range of focus adjustment according to 
the magnification/field of view setting of the zoom lens 45. This prevents 
the user from inadvertently focusing outside of the desired range which 
could result for example in the user focusing on components within the 
image relaying means 17 itself. 
The focus limiting member 40 and the stop surfaces 38,39 together 
constitute a focus limiting means 46 which under some circumstances will 
respond to adjustment of the magnification/field of view by automatically 
moving the image relaying means 17 relative to the housing 32 until the 
focus adjustment is within acceptable limits. If for example the 
cylindrical member 22 has been adjusted so that the field of view is at 
its smallest (producing maximum image size) and the object being viewed is 
at either the closest possible position or at infinity, the housing 32 
will have been fully rotated such that detent 37 is in contact with one of 
the stop surfaces 38 and 39. If the cylindrical member 22 is then adjusted 
in position to increase the field of view the housing 32 will be urged 
axially by action of the linkage 31 driven by the third cam follower pin 
27 and this axial movement can only be accommodated if accompanied by 
rotation of the housing due to cam action between the detent 37 and stop 
surface 38 or 39. 
An alternative apparatus 60 is shown in FIGS. 4, 5 and 6 where 
corresponding reference numerals to those of preceding figures are used 
where appropriate for corresponding elements. 
Apparatus 60 includes an outer tube 2 having a distal end which is not 
shown in the Figures but which corresponds to that of the apparatus 1 in 
that it contains an end window and a zoom lens. Outer tube 2 has a 
proximal end portion 4 fixedly connected to a collar 61 which is connected 
to a casing 5. 
A handle 62 is connected to the casing 5 and incorporates a light guide 
connector 63 through which light is directed for illuminating the object 
via the distal end portion 3. 
The apparatus 60 is viewed in side elevation so that the cam follower pins 
23, 24 and 27 are shown projecting vertically through the cylindrical 
member 22. The pins 23 and 24 are connected to their respective control 
tubes 12 and 16 by brackets 72 and 73. 
The apparatus 60 functions essentially in the same way as the apparatus 1 
but differs in some constructional detail. The screw coupling 34 of 
apparatus 1 is replaced in the apparatus 60 by a radially outwardly 
projecting thread engaging pin 64 which is received in a helical thread 65 
formed in the housing 32. The pin 64 is connected to an eyepiece assembly 
66 which is connected to both the eyepiece 20 and to the image relaying 
means 17. The pin 64 also projects through an axially extending slot 67 
provided in the casing 5. The slot 67 prevents the eyepiece assembly 66 
from rotating relative to the casing 5 whilst permitting relative axial 
movement. 
The housing 32.is rotatable relative to the casing 5 by manual engagement 
with a knurled focus actuating ring 68. Rotation of the focus actuating 
ring 68 produces axial movement of the image relaying means 17 which is 
thereby moved closer to or away from the zoom lens (not shown). 
Compensating movement of the image relay means 17 during adjustment of 
magnification is provided by means of cylindrical linkage 31 which engages 
the housing 32 by means of a radially outwardly projecting pin 69 which is 
received in an annular groove 70 defined by the housing 32. The annular 
groove 70 co-operates with the pin 69 to allow relative rotational 
movement of the housing whilst constraining the linkage 31 and the housing 
32 to move axially in unison. The pin 69 projects through an axially 
elongate slot 74 formed in the casing 5 which acts to prevent rotation of 
the linkage 31. 
Axial movement of the linkage 31 is provided during magnification 
adjustment by means of the third cam follower pin 27 engaging the third 
cam slot 28 provided in cylindrical member 22. The cylindrical member 22 
is rotated relative to the casing 5 by manually engaging a knurled 
magnification actuating ring 71 which is fixedly connected to the 
cylindrical member 22 and overlays the pins and slots 23 to 28. 
The apparatus 1 is provided with a five times variation in magnification by 
operation of the zoom lens 45 corresponding to a field of view varying 
between 10.degree. to 50.degree.. 
The apparatus 1 may alternatively be provided with an automatic zoom 
control and focusing system 50 as illustrated in FIG. 7. In such an 
alternative arrangement the apparatus would be provided with an 
electromechanical focus actuator 53 operable to move the image relaying 
means axially to compensate for movement of the focal plane of the zoom 
lens 45 during a change of magnification. A manual control is provided to 
select the required field of view and an object range sensor 51 produces 
an electrical signal representative of the range of the object from the 
objective lens 7. The output of the object range sensor is input to a 
processor 52 controlling actuation of the electromechanical focus actuator 
53 which is operable to move the image relaying means 17 to the required 
value of displacement X3 from the objective lens 7. 
A focus position sensor 54 provides feedback to the processor 52 as to the 
position of the image relaying means 17. 
Movement of the movable lenses 8 and 13 of the zoom lens 45 is similarly 
controlled by electromagnetic actuators 55 and sensors 56 provide feedback 
to the processor as to the position of the lenses 8 and 13. 
The required field of view is input to the processor 52 from an input 57 
and the processor derives the required values of X1, X2 and X3 by means of 
a parameter lookup table 58. 
Compensation for temperature effects is provided by a temperature 
responsive input 59. 
Apparatus in accordance with the present invention may alternatively 
include a zoom lens having more than two movable lenses provided that 
corresponding means are provided for moving the movable lenses. 
The viewing direction relative to the outer tube may be varied from the 
axial direction by the inclusion of suitable mirrors or prisms. The 
objective lens may for example be a lenticular prism. The mirror or prism 
may be movably mounted to provide scanning of the viewing direction. Also, 
for apparatus with a direction of view other than in the axial direction, 
the outer tube 2 together with the zoom lens 45 and the image relaying 
means 17 may be rotated in unison by up to 370.degree. to facilitate 
orbital scanning of an inspection site. 
The first and second control tubes 12 and 16 may alternatively be axially 
moved by actuators other than cam means. For example electromagnetic 
actuators could be utilised. The tubes could alternatively be replaced by 
suitable cables or control wires. 
Alternative forms of image relaying means may be used such as an optical 
fibre bundle or an electronic camera device. Where an electronic camera 
device is utilised the apparatus may incorporate electronic automatic 
focusing of a type known in the art of video recording cameras.