(a) Field of the Invention
The present invention relates to an illumination optical system having a very uniform light distribution characteristic to be used for illuminating a surface with light emitted from a light source of a certain size having a two-dimensional extent across the optical axis, and more specifically an illumination optical system suited for use with non-flexible endoscopes, fiber scopes, video endoscope and so on.
(b) Description of the Prior Art
In a case where rays are led from a light source to a desired location by using a light guide composed of an optical fiber bundle, the rays incident on the light guide at large angles with regard to the axis of the light guide, out of all the rays incident on the light guide, are remarkably attenuated as compared with the rays in parallel to the axis of the light guide while passing through the light guide. Accordingly, out of the rays emerging from the light guide, the rays emerging from the light guide at large angles with regard to the axis of the light guide are low in the intensities thereof as compared with the rays emerging in parallel with the axis of the light guide. When a surface is illuminated directly with these rays, luminance is remarkably different between the center and marginal portion of an illumination field or light distribution characteristic is degraded, thereby producing inconvenience for observation.
In order to correct this defect, attempts have been made to improve the light distribution characteristic by using lenses. As a conventional example to improve such a light distribution characteristic by using lenses, there is known the illumination optical system disclosed by U.S. Pat. No. 4,294,511.
This illumination optical system comprises, as shown in FIG. 1, a non-flexible endoscope equipped with an observation optical system consisting of an objective lens 0, relay lenses R.sub.1 and R.sub.2, an eyepiece lens E and an object side light guide (hereinafter referred to a second light guide) 2 arranged so as to surround the observation optical system, a light source 4, a light source side light guide (hereinafter referred to as first light guide) 1 and connecting lenses 7, and is so adapted as to lead the light from the light source 4 to the surface of incidence 2a of the object side light guide through the light source light guide 1 and the connecting lenses 7, and illuminate an object through the object side light guide.
The curve A shown in FIG. 2 visualizes a light distribution characteristic determined experimentally on the end surface of emergence of the light source side light guide. When the angle of emergence from the light source side light guide is represented by .theta., the curve can be approximated by a quadratic curve B using as a variable sin.theta.given by I (sin .theta.)=-asin.sup.2 .theta.+b.
In order to flatten the light distribution characteristic of the light incident on the object side light guide 2, to widen the range of light distribution and further to minimize loss of light in the coupling section, this conventional illumination optical system is so designed as to locate the end surface of emergence of the light source side light guide (the first light guide) in the vicinity of the front focal point of the coupling lens system 7 and locate the end surface of incidence of the object side light guide (the second light guide) in the vicinity of the rear focal point of the coupling lens system.
Since this illumination optical system uses a small number of spherical lenses having strong refractive powers as the coupling lens system, remarkable aberrations are produced at the marginal portion of illumination field.
FIG. 3 shows an enlarged view of the coupling lenses and the surroundings thereof illustrated in FIG. 1. When the focal length of the coupling lens system is represented by f, the angle formed by the ray emerging from each optical fiber of the first light guide with regard to the optical axis is designated by .theta..sub.1, the angle formed by the ray incident on the second light guide on the object side with regard to the optical axis is denoted by .theta..sub.2, and the distances from optional points on the end surfaces of the respective light guides to the centers thereof are represented by r.sub.1 and r.sub.2 respectively, the following relations are established in the positional relationship described above: EQU f sin .theta..sub.1 =r.sub.2 EQU r.sub.1 =f sin .theta..sub.2
When sin .theta. is taken as the abscissa and intensity I of the illumination light is taken as the ordinate, under the condition to satisfy these formulae, the light distribution characteristic is uniform as illustrated in FIG. 4.
However, when the light distribution characteristic illustrated in FIG. 4 is retraced taking .theta. as the abscissa, the light intensity is lowered at the marginal portion and the light distribution characteristic is degraded as shown in FIG. 5, but no practical problem is posed for visual observation or photographing.
In the recent days, however, it is often practised to observe images on a TV monitor with a TV camera connected to the eyepiece of endoscopes. Further, there have been developed a number of video endoscopes equipped with solid-state image sensors built in the distal ends. Furthermore, it is desired to widen field angles of endoscopes.
In cases where images are observed on TV monitors by using the solid-state image sensors as described above, it is almost impossible to obtain sufficient light quantities or sufficient light distribution at the marginal portions of visual fields with the conventional light guide coupling lens system satisfying r.sub.1 =f sin .theta..sub.2 and r.sub.2 =f sin .theta..sub.1 since an increasing number of endoscopes are now used which employ solid-state image sensors having latitude narrower than that of film for a camera and are designed for observation at field angles wider than 100.degree..
Further, as an illumination optical system designed for the purpose of obtaining favorable light distribution even at the marginal portion of a visual field, there is known the optical system disclosed by Japanese Unexamined Published Patent Application No. 178207/62.
In this illumination optical system, a light source is arranged distantly from an illumination lens. When height of incidence, on the illumination lens, of the light from the light source is represented by h and angle of emergence of the light from the illumination lens is designated by .theta., the illumination optical system is so designed as to satisfy the following condition: EQU h.apprxeq.k tan .theta. (k: constant)
An illumination optical system to be used in endoscopes must provide favorable light distribution and sufficient light intensity within a limited space. However, the illumination optical system disclosed by the above-mentioned Japanese Unexamined Published Patent Application No. 178207/62 can satisfy the above-mentioned condition only within a very narrow range of about 10.degree. of a field angle of the light source, but cannot satisfy the condition for the offaxial ray at a wider field angle of the light source, thereby degrading light distribution. Further, since the light source is arranged distantly from the illumination lens, the lens is enlarged in the diameter thereof and requires a very wide space when the field angle of the light source is widened.
On the other hand, if first light guide 1 and the coupling lenses 7 are freely attachable and detachable to and from the non-flexible endoscope 5 in FIG. 1, an economical advantage will be obtained since a single light source and a single coupling lens system are usable in combination with various types of endoscopes. When a various endoscopes are used selectively with a single light source, however, it is necessary to determine NA (Numerical Aperture) of the light incident on second light guide through the lens, i.e., illumination angle .theta. in accordance with the field angle of the endoscope having the widest field angle.
In the recent days where there have been developed endoscopes having wider field angles for coping with broader application and narrower field angles for observation of magnified images at short distances, field angles are largely different depending on types of endoscopes. Accordingly, even the above-described illumination optical system cannot provide light to the marginal portion of visual field when it is used in combination with an endoscope having further wider field angle. When the illumination optical system is combined with an endoscope having a narrow field angle for observing magnified images at short distances, the illumination optical system illuminates even outside the visual field, thereby making illumination light intensity insufficient within the visual field.
As is understood from the foregoing description, the conventional illumination optical system cannot perform effective illumination matched with endoscopes used in combination therewith.