Fiber Bundle Illumination System

This invention relates to an illumination system for visually observing a selected object plane, particularly for use with microscopes, e.g., medical operations microscopes, and other viewing systems.

There are a number of systems for illuminating areas or objects for visual observation. The simplest system, i.e., critical illumination in microscopy, or searchlight optics on a larger scale, merely produces an image of its primary light source on the target area or object plane. If the primary light source does not have a uniform brightness, neither does its image, and consequently, the illumination is non-uniform, which is undesirable for many applications.

In order to obtain both well-defined and uniform illumination, two more complex systems have been used. One of these is a Kohler illumination system in which a non-uniform light source is imaged on a second lens by a first lensed of an image of the first lens formed of the second lens on the object plane. When used with a microscope, however, the Kohler system has a long fixed length, e.g., of about 12 cm. Due to its length, the Kohler system provides an obstruction to movements of the surgeon using the microscope in performing eye and other surgery.

The other system is a projection condenser system, which is similar to the Kohler illumination system and used with larger devices. As with the Kohler system, the distance between the primary light source and the object plane is fixed and is not practical, e.g., for medical microscopes.

I have discovered an improved system for projecting uniform illumination for visual observation which combines with a flexible fiber light guide a clad cane element which, mixes light rays to provide uniform brightness across the output face of the clad cane element. The fiber light guide is of conventional construction, e.g., comprising a flexible, elongated bundle of relatively small, discrete clad fibers, the bundle having an input end adapted to receive light from a primary light source and an output end from which the light is transmitted. The fibers of the bundle are independent from one another along their length to permit flexing of the bundle as a whole to desired configurations, and the output end portions of the fibers are joined together into an array to cooperatively define an output face of the fiber bundle. The ends of the individual clad fibers are adapted to transmit light through the output face, but, as has long been known, the regions between the fiber ends do not transmit light so that the light leaving the output face is of a non-uniform brightness across this face on the micro scale. Typically, the brightness across the output face is also nonuniform on the macroscale. The clad cane element has an input face which corresponds to the output face of the fiber bundle in size and position, and the clad cane element is adapted to direct light from its output face through the remaining part of the illumination system. The clad cane element has a transparent, homogeneous core of a relatively high refractive index sized to receive substantially all the light transmitted through the output face of the flexible fiber bundle. The exterior of the core is surrounded with a relatively thin clad layer of a substance having a relatively low refractive index, whereby light as it proceeds through the core is subject to total reflection from the clad. The length of the cane element is predetermined (e.g., of the order of the diameter of the bundle or greater) such that the light rays entering the core are and mixed to provide uniform brightness across the output face of the clad cane element. A lens is located downstream from the output face of the cane element at a distance at least equal to the focal length of the lens. The lens is disposed to receive the projected light of uniform brightness from the output face of the cane element and adapted to focus the image of the output face to provide a well-defined, uniform projected beam for illumination of an object to be viewed with essentially total conservation of brightness.

In one aspect of the invention, there is a hyperhemispherical lens attached to the output face of the cane element to reduce the angle of the uniform light directed to the downstream lens. In another aspect, the clad cane element is tapered to accomplish the same function.

In another aspect, the downstream lens focuses the light at infinity. In another aspect, the system is a homogeneous illuminator and the downstream lens is a doublet lens which is movable along its central axis so as to adjust the focal plane of the uniform light therefrom.

The system may be connected to an operations microscope in a compact, easily used arrangement, to direct a well-defined uniform beam of light on an object plane to be observed. It may also be used with a compound microscope having an adjustable field iris disposed adjacent to the output face of the cane, and an adjustable aperture iris disposed between the field iris and the downstream lens. The field iris is increased in size and the aperture iris reduced in size as the microscope magnification decreases.

In a further aspect, the system comprises a microfilm projection system in which the expanded focused beam from the downstream lens is directed onto a projection screen, and the film to be displayed thereon is placed adjacent to the output face of the clad cane element.

I now turn to a description of preferred embodiments of the invention, after first briefly describing the drawings.

FIG. 1 is a diagram of this invention with a portion of a light guide broken away, and also showing an object to be illuminated;

FIG. 2 is an enlarged view of the output end of the light guide and the clad cane element, shown swung apart;

FIG. 3 is an enlarged cross-sectional view of a portion of the fiber bundle of this invention;

FIG. 4 is an enlarged cross-sectional view of the clad cane element and output end of the light guide, also showing the paths of some light rays;

FIG. 5 is a diagrammatic view of the illumination system of this invention mounted in a microscope;

FIG. 6 is a view of another illumination system of this invention;

FIG. 7 is a view of the homogeneous illuminator embodiment of this invention;

FIG. 8 is a view of a microfilm projection system of this invention; and

FIG. 9 is a diagram of a compound microscope using this invention.

As shown in FIG. 1, an illumination system 10 of this invention generally comprises a flexible light guide bundle 20 made up of small discrete optical fibers 21, a clad cane (clad rod) element 30 and a downstream lens 40. In addition, the illumination system 10 uses a primary light source 60, and may use a second lens 37, the function of which will be explained below.

The flexible fiber bundle 20 has an input end 22 and an output end 24. As best shown in FIG. 2, (which shows the clad cane element 30 and the bundle 20 swung apart for purposes of illustrating the construction; not as they would be in operation), the flexible bundle 20 is made up of several thousand individual optical light guide fibers 21. Each fiber 21 is about 0.056 mm in diameter, and the diameter of the bundle 20 is about 8 mm. As indicated in FIG. 3, each fiber 21 comprises a transparent central core 26 surrounded by a clad 28 of lower index of refraction of about 2.5 microns thick (relative thickness shown exaggerated in the figure). The clad reflects light, which would otherwise leave the fiber core, back into the core. The core 26 is Schott F2 glass, and the clad 28 is Owens-Illinois EN-1 glass. The individual fibers 21 are packed together in a hexagonal arrangement with triangular spaces 29 between each three. These spaces 29, along with the clad 28, do not carry any light. The ends of the fibers 21 are joined together at the output end 24 of the bundle 20 to form an output face 23. The output face 28, however, also includes the spaces 29 between the fibers 21 and the fiber clads 28. Therefore, the light over the output face 23 of the bundle 20 is non-uniform in intensity and brightness.

Another source of nonuniform brightness arises because of nonuniform illumination of the fiber bundle input end 22 by the light source 60.

The output face 23 of the fiber bundle 20 contacts input face 23 of the clad cane element 30. The clad cane element 30 is cylindrical and has a core 34 which is homogeneous, transparent and in the preferred embodiment, comprising a single piece. The core 34 is made of Schott F2 glass and has a diameter of 8 mm and a length of 10 mm. The core 34 has a relatively high refractive index, and light passes through it essentially without loss. Importantly, the core 34 contains no internal element which might become apparent if the core's output face is imaged. The core 34 is surrounded by a thin clad 36 of a relatively low refractive index, such as Owens-Illinois EN-1 glass. The clad 36 is about 0.3 mm thick. Clad cane elements such as this are inexpensive and available from American Optical Corporation.

As is shown in FIG. 4, rays of light tend to leave the fibers 21 at the end of the bundle 20 at an expanding angle so that a cone of light exits from each fiber 21 (only a few shown for illustrative purposes). This light enters the clad cane core 34, and proceeds through the core 34 to an output face 35 of the clad cane element 30 at its opposite end virtually without loss. As the light proceeds through the cane element, the expanding light canes, which have different intensities from individual fibers intersect and light striking the clad 36 is reflected internally back into the core 34, so that by the time the light reaches the output face 35, the light beams are so mixed together and homogenized that the light can be of uniform intensity over the entire output face 35, as diagrammatically illustrated in FIG. 4.

The homogenizing function of the clad cane element 30 is increased with increase in its length.

A short length, e.g. 1/8 of the diameter D of the fiber bundle, can provide correction of uniformity on the micro scale. This length can generally provide sufficient mixing of rays from adjacent fibers 21 to avoid nonuniformities attributable to non-light transmitting regions 28,29 between the fiber cores. At the same time a sharply defined beam of light can be achieved by focusing the output end of the core element with the downstream optical system.

In conventional fiber bundles used for illumination there is an additional problem of nonuniformity of light also on the macro scale. That, too, can be dealt with by proper choice of the length of cane element 20. This problem can be due to nonuniformity of the illumination of the input end of the fiber bundle, and imperfect scattering or "randomization" of the position in the respective faces of the bundle, of the individual fiber output ends relative to their input ends.

In general, the length of the cane element for such conventional illumination bundles is sufficient to provide a sensibly uniform brightness at the output face if it is such that a limiting ray from each fiber reaches a point on the reflecting surface of the clad cane element, as shown in FIG. 4. Note that limiting light cone C.sub.c of central fiber 21.sub.c just reaches the reflective surface of clad 36 at the downstream end of the clad element 30, and that rays of the cones of outer fibers reach the reflective surface increasingly sooner with increasing radius from the center.

Again, with this longer length cane element, by focusing the output end of the element with the downstream system, a sharply defined beam of light can be achieved, with essentially total conservation of brightness.

In cases of still greater nonuniformity of illumination of the bundle output end, still greater length of the cane element can improve the uniformity of illumination of the system to any desired value, still obtaining a sharply defined beam with conservation of brightness.

As shown in FIG. 1, the output face 35 of the clad cane element 30 contacts a plane surface 38 of lens 37. To avoid light loss, surface 38 is cemented to output face 35. Lens 37 is a hyperhemispherical lens of Schott K5 glass having a 25 mm diameter and thickness. Lens 37 also has a convex surface 39 opposite plane surface 38. Lens 37 functions only to narrow the light that exits from the clad cane element 30.

The downstream lens 40, which is rectangular in the preferred embodiment in order to fit into the body of the microscope, is positioned away from the lens 37. Downstream lens 40 is large enough to capture as much of the light as possible from lens 37 (if a circular lens could be used all the light could be captured), and lens 40 is positioned at a distance downstream from the output face 35 of the clad cane element 30 so that the output face 35 is at the effective focal plane of the combination of the hyperhemispherical lens 37 and the downstream lens 40. This effective focal length is 30 mm. The actual focal length of lens 40 is 74 mm, and it is 52.5 mm from the output face 35 of the clad cane element 30. Of course, a single lens could be used (the downstream one corresponding to lens 40), and in that case, the clad cane element output face should be placed at the actual focal plane of the lens. In the single lens case, however, the downstream lens would have to be large enough to capture all of the light from the clad cane element.

A preferred use for the illumination system 10 of this invention is as part of an operation microscope 50, as shown in FIG. 5. The microscope 50 (a modified Zeiss binocular surgical microscope is shown) basically comprises an eyepiece assembly 52, a prismatic binocular assembly 54, a magnification changer 56 and an objective lens 58. Normally, this type of microscope also carries its own primary light source as well, and that source requires a substantial amount of space. Also, because of the heat generated by the light source, the source must be positioned away from the remainder of the instrument. As such, the microscope is somewhat bulky and difficult to use in some applications.

Here, the illumination system 10 of this invention replaces the primary light source (and the associated bulky part in which it is mounted). As shown, a mirror 59, positioned at a 45.degree. angle above a portion of the microscope's objective lens 58, directs the uniform light from the lens 40 through the objective lens 58. The overall length of the system from the input face of the clad cane element 30 to the downstream lens 40 is only about 7 cm. When the illumination system 10 is connected to the microscope 50 as shown, a primary light source (not shown here) is a high intensity, a tungsten-halogen lamp with a quartz envelope, which is positioned to direct its light into the input end 22 of the fiber bundle 20 (as shown in FIG. 1). In order to direct as much of the light as possible into the fiber bundle, the source 60 is placed at the focal point of an ellipsoidal reflective surface 62 which directs the light into input end 22 of the fiber bundle 20. As the bundle 20 is flexible because it is made up of a large number of separate and flexible fibers 21 and as there is little light loss over its length, the primary light source can be positioned remotely (about 2 meters maximum preferably), wherever desired with respect to the microscope 50.

As shown in FIG. 5, the light from the downstream lens 40 overlaps the viewing path to the binocular assembly 52 so that the uniform light from the illumination system 10 is nearly coaxial with the direction of view, and a well-defined light area with a minimum of light outside the area is projected onto the object plane. The objective lens 58 of the microscope thus provides the dual effect of having its focal plane determine the object plane of the microscope as well as defining the image of the output face 38 of the clad cane element 30.

In the presently preferred embodiment, the primary light source 60 is turned on, and the reflective surface 62 directs the light from the source 60 into the input end 22 of the fiber bundle 20.

The light then passes through the cores 26 of the fibers 21 to the output end 24 of the bundle 20. The clads 28 reflect back into their respective cores 26 any light which would otherwise pass through the sides of the cores, and therefore, almost all the light which enters the fibers 21 at the input end 22 of the bundle 20 exits from the fibers 21 at the output face 23. Of course, the length of the bundle will vary from application to application, and as the bundle can be twisted into almost any shape without affecting its light transmission abilities, the heat-generating light source 60 may be positioned at a distance from the microscope 50.

The output face 23 of the bundle acts as a secondary light source. This secondary light source will be non-uniform, as previously explained. The non-uniform light then passes into the clad cane element 30 through its input face 32. As the light exits from each individual fiber 21 of the bundle 20 in a cone-shaped beam, the various rays of light overlap, and some of them reflect from the clad 36 of the clad cane element 30. The individual boundaries of the fibers cores 26 and the dark areas 29 between the fibers disappear, and the light becomes of uniform intensity across the entire output face 35.

This uniform light from the clad cane element 30 then enters the hyperhemispherical lens 37. Lens 37 narrows the cone of light to about a 16.degree. half angle. This reduced numerical aperture permits all of the light to be captured by the downstream lens 40 (if the downstream line is circular), which capture could otherwise only be accomplished with a large lens. The hyperhemispherical lens 37, however, may be omitted if the downstream lens is of sufficient numerical aperture to capture all the light from the clad cane element.

The optical distance between the lens 40 and output face 35 of the clad cane element 30 is equal to the focal length of the lens 40, the lens 40 projects the image of the output face 35, which image is uniformly bright, as previously explained.

The beam of light from the rectangular lens 40 is directed by the mirror 59 through the microscope lens 58 onto a plane to be illuminated. The beam is essentially coaxial with the light to the viewing assembly.

As shown in FIG. 6, a circular downstream lens 70 (replacing lens 40) may be used in the illumination system 10. Here, a fiber bundle 72 is attached to a clad cane element 74 having a core 73 and a surrounding clad 75. The clad cane element 74 is attached to a hyperhemispherical lens 76, as in the previous embodiment. The circular lens 70 is positioned at a distance downstream so that an output face 77 of the clad cane element 74 is at the effective focal plane of the lenses 70, 76, and an expanded, collimated beam from the lens 70 is focused at infinity (i.e., "infinity" being used in the normal optical sense where a greater distance would not make any difference in imaging). Therefore, any object placed in the beam is illuminated by a focused beam of light.

In another embodiment, as shown in FIG. 7, a doublet lens 80 may be used as the downstream lens in the illumination system. In this case, the system forms a homogeneous illuminator 82, which is very useful in illuminating deep orifices, i.e., deep surgical wounds. The doublet lens 80 is made of Bausch and Lomb opthalmic crown 523580 and is about 40 mm in diameter. The lens 80 is movable along the central axis of the light beam, but the output face of the homogenizer is optically always at or farther than the effective focal plane of the doublet lens 80 and the hyperhemispherical lens. Movement of the lens 80 allows the expanded beam to be focused on any downstream object plane so that the light may be focused, for example, at any desired depth in the wound.

In another embodiment, a microfilm projection system 100 is shown in FIG. 8. There, the output end of a fiber bundle 102 is connected to a tapered homogenizer 104. A downstream lens 106 receives and focuses the expanded uniform light from the homogenizer 104, which light is then reflected from a mirror 108 onto a back projection screen 110. In operation, the system 100 provides uniform backlighting for a transparency (e.g., microfilm). The microfilm 112 to be projected is placed adjacent to the output face of the homogenizer 104. The lens, which is placed a distance downstream from the film greater than its focal length, then projects the image of the film through the mirror to the screen. Because of the length of the fiber bundle, the heat-generating light source is positioned away from the film to be displayed. The taper of the homogenizer 104 reduces the numerical aperture or the angle of the light leaving it, and this reduces the size of the downstream lens needed to capture all the light. The tapered homogenizer may also be substituted for the cylindrical homogenizer and attached hyperhemispherical lens of the preferred embodiment.

A compound microscope embodiment is shown in FIG. 9. The illumination system 122 for a compound microscope 120 has a fiber bundle 124 with attached homogenizer 126. An adjustable field iris 128 is disposed adjacent to the output face of the homogenizer. The microscope 120 has a mirror 130 angled to direct light from the homogenizer 126 through an aperture iris 132 located below a downstream lens 134 and the object plane 136. In operation, the cone of light falling on the object plane can be adjusted with the magnifying power of the microscope. A low magnification, there is a large field of view of the object plane, and a narrow cone angle of light is desirable. This is accomplished by decreasing the size of the aperture iris 132 and increasing the size of the field iris 128. At high magnification, it is desirable to decrease the area illuminated without decreasing the brightness. This is accomplished by opening the aperture iris 132 and closing the field iris 128.

Other applications and variations will be apparent to those skilled in the art.