Abstract:
An illuminating optical device includes a light source including two light emitting points which are symmetric with respect to an optical axis of the illuminating optical device, a first concave reflection mirror plane arranged behind the light source and symmetrically to the optical axis in order to form an image of the light source at a predetermined position of an illuminated area, and a second concave reflection mirror plane arranged more closely to the illuminated area than the first concave reflection plane and symmetrically to the optical axis in order to direct the light beam to the light emitting points of the light source.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an illuminating optical device used for photographing an eye fundus. 
     2. Related Background Art 
     A prior art illuminating optical device is built in an eye photographing apparatus, for example, and utilized to illuminate an eye fundus. When the eye fundus is photographed, light reflected by the eye fundus is directed to an imaging plane. Since the reflective index of a cornea is usually larger than that of the eye fundus, the eye to be checked is illuminated in a ring shape as shown in Japanese Patent Publication No. 53-43277 in order to suppress the reflection by the cornea and a light beam reflected by the eye fundus is taken out of a center area which is separate from the ring-shaped illumination area. This is the so-called ring shape illumination method. A stroboscope light source such as, a discharge tube is used to instantly irradiate with a strong illumination light the eye fundus. 
     However, in the prior art, only one or two, at most, forward light beams of the light beams emitted in every direction from the light emitting device such as the stroboscope light source are utilized and the light beams emitted in other directions do not contribute to the illumination of the eye fundus. As a result, in photographing the eye fundus, the photographed image may be not clear because of insufficient light. However, because the power supplied to the stroboscope light source is limited, an illumination light of a larger intensity is demanded. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an illuminating optical device which solves the problems of the prior art and efficiently utilizes a light beam emitted from a light source. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a configuration of an embodiment of an illuminating optical device of the present invention, 
     FIG. 2 shows a front view of a ring-shaped light source, 
     FIG. 3 shows a front view of a concave mirror, 
     FIGS. 4A to 4E illustrate the direction of reflection of light beams, 
     FIG. 5 shows the configuration of major portions of a second embodiment. 
     FIGS. 6A to 6C illustrate the direction of reflection of light beams, 
     FIGS. 7, 8, 9 and 10 show configurations of other embodiments, and 
     FIGS. 11, 12 and 13 show modifications of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows the configuration of one embodiment in which the present invention is applied to an eye fundus camera. An objective lens 1, an apertured mirror 2, a photographing lens 3 and a film 4 are sequentially arranged on an eye axis 01 of an eye E to be checked, and a relay lens 5 and an apertured iris 6 having a ring-shaped opening 6a are arranged on an optical axis 02 along the direction of reflection of the apertured mirror 2. A transparent stroboscope light source 7 having a ring shape of the same size as the opening 6a of the apertured iris 6, and a first concave mirror 8 which is larger than the stroboscope light source 7 and has a mirror plane facing the stroboscope light source 7 are arranged behind the apertured iris 6 as shown in a sectional view of FIG. 2 taken along a plane normal to the optical axis 02. A second concave mirror 9 having a light transmitting area 9a and a mirror plane 9b facing the stroboscope light source 7 is arranged on a side of the stroboscope light source 7 as shown in a front view of FIG. 3 which is viewed from the stroboscope light source 7. The mirror plane of the first concave mirror 8 is a spherical plane centered at the center of the ring (which is on the optical axis) of the stroboscope light source 7 and is constructed to focus the stroboscope light source 7 to the mirror plane as shown. The mirror plane of the second concave mirror 9 is a cylindrical mirror plane centered at the center of light emission of the stroboscope light source 7 in each sectional plane which contains the optical axis as shown in FIGS. 4A-4C, that is, a cylindrical mirror plane which reflects light beams emitted from the stroboscope light source 7 back to the emitting points. 
     In photographing, the stroboscope light source 7 emits a light beam, which travels along the optical axis 02, through the opening 6a of the apertured iris 6 and the relay lens 5, is reflected by the apertured mirror 2, through the objective lens 1 and reaches the eye E to illuminate the eye fundus from a periphery of the pupil. The light reflected by the eye fundus goes through the same path, through the aperture of the apertured mirror 2 and the photographing lens 3 and reaches the film 4 so that the eye fundus image is recorded on the film 4. 
     The effects of the first concave mirror 8 and the second concave mirror 9 are now explained. FIG. 4A shows an enlarged view of a section which contains the optical axis 02. In this plane, the light beams of the stroboscope light source 7 are emitted to all directions in the plane. If the first and second concave mirrors 8 and 9 are not properly arranged, only the light beam emitted in the direction A passes through the opening 6a of the apertured iris 6 as shown in FIG. 4B and other light beams are not utilized to illuminate the object. 
     When the first concave mirror 8 and the second concave mirror 9 are arranged as they are in the present embodiment, the light beams emitted in the directions B, C and D shown in FIG. 4A travel as shown by arrows in FIGS. 4C, 4D and 4E, respectively, and reflected by the first concave mirror 8 and the second concave mirror 9 so that they pass through the opening 6a. The above explanation is for one sectional plane. Since the stroboscope light source 7 is of generally ring shape, the same is generally true for the light beams emitted by the stroboscope light, source 7. As a result, the intensity of light of the stroboscope light source 7 which reaches the eye fundus of the eye E is increased and a sharp image of the eye fundus is obtained. 
     FIG. 5 shows a sectional view of a second embodiment which uses a combination of mirrors 10 and 11 instead of the mirror 9 of FIG. 4. The reflection mirror 10 having an opening which is of the same size and shape as that of the opening of the apertured iris 6 and has a mirror plane facing the stroboscope light source 7 is arranged behind the ring-shaped apertured iris 6, a first concave mirror 8 which is similar to that of the first embodiment is arranged behind the stroboscope light source 7, and the second concave mirror 11 which is of a ring shape and has a mirror plane facing the stroboscope light source 7 is arranged around the stroboscope light source 7. Other elements are same as those of the first embodiment. The second concave mirror 11 focuses a reflected image 7R of the stroboscope light source 7 by the reflection mirror 10 to a ring center 7R&#39; of the stroboscope light source 7, for the light beam which is in the plane of the drawing, as shown by broken lines in FIG. 6A. 
     Accordingly, as shown by arrows in FIG. 6B, the light beams emitted from the stroboscope light source 7 to the directions E and F which are normal to the optical axis 02 are reflected by the second concave mirrors 11 as the light beams emitted from the ring center 7R&#39; in the directions E and F are reflected, and they are directed to the point 7R, reflected by the reflection mirror 10 and return to the position of the stroboscope light source 7. They are further reflected by the first concave mirror 8 and pass through the opening of the apertured iris 6. As shown by arrows in FIG. 6C, the light beams emitted between the directions G and H are also reflected by the second concave mirror 11, reach the ring center 7R&#39; shown in FIG. 6A, are successively reflected by the concave mirror 11, the reflection mirror 10 and the first concave mirror 8, and pass through the opening 6a of the apertured iris 6. Since the light beams emitted in the directions A to D shown in FIG. 4 are also utilized, it is more effective to increase the photographing light intensity. 
     As shown in FIG. 7, instead of the first concave mirror 8 and the second concave mirror 11, a reflection mirror 12 having a combined mirror plane may be used. 
     In the above embodiment, the first concave mirror 8 has the mirror plane which is shaped to focus the ring-shaped light source 7 to the concave mirror 8. However, as shown in FIG. 11, the light of the light source can be more efficiently utilized for the illumination when the ring-shaped light source 7 is focused to the opening 6a of the apertured iris 6. 
     As shown in FIG. 12, the second concave mirror 9 may have a spherical plane (radius of curvature r in FIG. 12) centered in a vicinity of a point P at which the light beam emitted to the direction B or D in FIG. 4A crosses the optical axis. 
     In the present embodiment, the stroboscope light source 7 is of a ring shape. However, as shown in FIG. 13, a linear tube type stroboscope light source 16 may be used instead of the ring-shaped stroboscope light source. 
     FIG. 8 shows a modification of the embodiment of FIG. 6. In the embodiment of FIG. 6, when the optical system is cut along a sectional plane which contains the optical axis and the light beams in the sectional plane are considered, the second reflection mirror 11 is constructed to focus the reflected image 7R of the light source 7 by the reflection mirror 10 on the optical axis as shown by 7R&#39; in FIG. 6A. In the embodiment of FIG. 8, the shape and the position of the second reflection mirror 13 are selected such that the light beam emitted from the light source 7 and reflected by the reflection mirror 10 is collimated and passes through two areas of the light source 7 (which are symmetrical about the optical axis). 
     In an embodiment shown in FIG. 9, the shape and the position of the second reflection mirror 14 are selected such that the light beam emitted from the light source 7 and reflected by the reflection mirror 10 passes through the light source 7 and is focused to a cross-point of the optical axis and the reflection mirror 10. 
     In a modification shown in FIG. 10, the shape and the position of the second reflection mirror 15 are selected such that the light beam emitted from the light source 7 and reflected by the reflection mirrors 10 and 15 is collimated, passes through the light source 7 and is reflected by a third reflection mirror in a vicinity of the optical axis. 
     In each of the above embodiments, the light beam emitted by the light source forms a light path which is symmetric about the optical axis, and the light path passes through the light source a plurality of times. Accordingly, the light beams emitted from the light source in a plurality of directions are efficiently directed in one direction and the object can be efficiently illuminated.