Eye bottom camera

The present invention relates to an eye bottom camera which is so designed that in order to eliminate the undesired light beam reflected by the objective lens opposed to the eye to be inspected the objective lens as one group of biconvex type whereby by making the undesired light beams reflected on both surface of the objective lens as if they were produced by a light beam emitted from a same plane of the illumination optical system the small black spot provided on the same plane can be made small and further by so designing that the image formed in the neighborhood of the objective lens, of the small black spot appears at the position closer to the eye to be inspected than the surface of the objective lens it is made difficult that the small black spot be formed on the eye bottom to be inspected.

FIELD OF THE INVENTION 
The present invention relates to an eye bottom camera. 
DISCLOSURE OF PRIOR ART 
In case of an eye bottom camera of such a type in which the illumination 
optical system and the observation photographic optical system presents a 
common objective lens facing to the eye to be inspected, a part of the 
light beam coming from the above mentioned illumination optical system is 
reflected by the cornea of the eye to be inspected or the surfaces of the 
objective lens. If this reflected light beam enters into the observation 
photographic optical system a ghost or a flare is produced, whereby the 
eye bottom can not be observed nor photographed clearly. Until now the 
undesired light beam reflected on the surface of the cornea of the eye to 
be inspected has effectively been eliminated by the ring illumination 
method. On the other hand, so far as the undesired light beam reflected on 
the surfaces of the objective lens is concerned, there are such methods 
that the reflecting mirror for leading the light beam of the illumination 
optical system to the observation photographic optical system is 
constructed as concave mirror or that as objective lens a group of 
meniscus lenses are used whereby in the neighborhood of the optical axis a 
small black spot for shading the light is provided. However, in the former 
case the optical axis is declined so that the aberration is increased and 
the resolving power is decreased, while in the latter case there is a 
danger that the observation photographic light beam in the neighborhood of 
the optical axis of the objective lens be shaded and furthermore the 
photographic angle can not be made more than 30.degree. because the 
meniscus lenses are used. 
In case of an eye bottom camera so designed that a biconvex lens is used as 
an objective lens facing to the eye to be inspected in order to obtain a 
wider photographic angle quite recently it is proposed that a black spot 
is provided in the illumination optical system in such a manner that the 
undesired illumination light beam reflected by the objective lens and 
entering into the observation photographic optical system is shaded by the 
above mentioned black spot. The U.S. Pat. No. 3,594,071 relates to a 
camera in accordance with this method, whose one embodiment is shown in 
FIG. 1. In FIG. 1, reference character 1 represents an eye to be 
inspected, 2 a biconvex objective lens, 3 a light source, 4, 5 and 6 a 
condenser lens, 7 a reflecting mirror having an aperture or boring at the 
center, 8 a slit, 9 a photographic lens and 10 a photographic plane. The 
undesired light beam reflected by the surfaces S.sub.1 and S.sub.2 of the 
objective lens 2 is eliminated as follows in accordance with the present 
method. The dotted line d.sub.1 shows an undesired light beam coming from 
the illumination optical system. It is reflected on the S.sub.1 surfaces 
facing to the eye to be inspected, of the objective lens 2. It reaches the 
photographic plane 10 through the mirror 7 and slit 8 causes a flare or a 
ghost. This undesired light beam (d.sub.1) is radiated from a point 
A.sub.1 on the condenser lens 5, passes through the condenser lens 6, and 
the reflecting mirror 7 and is condensed in the neighborhood of the top of 
the S.sub.2 surface of the objective lens 2. The undesired light beam 
passes the S.sub.2 surface, is reflected on the S.sub.1 surface, passes 
again the S.sub.2 surface and condensed at the point A.sub.2 on the 
opening of the slit 8 through the boring of the mirror 7. Namely in case 
of the camera constructed as mentioned above, out of the image of the slit 
8 that by means of the light beam reflected on the S.sub.1 surface is 
formed in the neighborhood of the S.sub.2 surface. On the other hand, the 
undesired light beam reflected on the S.sub.2 surface of the objective 
lens and passing through the opening of the slit 8 is that which is 
reflected in a small area in the neighborhood of the S.sub.2 surface. Thus 
by making the black spot at the point A.sub.1 large, an illumination light 
beam reaching the range in which the undesired light beam is reflected on 
the S.sub.2 surface is shaded in order to eliminate the undesired light 
beam reflected on the S.sub.2 surface. Thus in case of the conventional 
eye bottom camera shown in FIG. 1, the position of the small black spot in 
the illumination system is almost conjugate with that of the top of the 
S.sub.2 surface. 
In case of the eye bottom camera so designed that a small black spot is 
provided in the illumination optical system so as to eliminate the 
undesired light beam reflected on both surfaces of the biconvex objective 
lens facing to the eye to be inspected, care must be taken so that an 
image of the small black spot should not be formed on the eye bottom and 
cover a part of the eye bottom. 
SUMMARY OF THE INVENTION 
The purpose of the present invention is to offer an eye bottom camera which 
is so designed that by means of a small black spot provided in the 
illumination optical system the undesired light beam reflected on both 
surfaces of the biconvex objective lens is eliminated, whereby it is made 
difficult for the black image of the above mentioned small black spot to 
be formed on the eye bottom to be inspected. 
In case of the eye bottom camera in accordance with the present invention, 
by so designing that the objective lens facing to the eye to be inspected 
is composed as one group of biconvex type in such a manner that the 
undesired light beams reflected on both surfaces of the objective lens are 
made as if radiated from the same plane of the illumination optical 
system, the small black spot for shading the light, provided in the above 
mentioned same plane can be made small and further the image of the small 
black spot is formed at a position closer to the eye to be inspected than 
the objective lens.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
In order to avoid the apparition of the image of a small black spot on the 
eye bottom to be inspected, the following two points must be taken into 
consideration. The first condition is that the small black spot provided 
in the illumination optical system must be as small as possible. The 
second condition is that the position of the image of the small black spot 
in the neighborhood of the objective lens must be as far as possible from 
the position at which the image of the eye bottom to be inspected is 
formed by the objective lens. FIG. 2 is the enlarged sketch of the 
neighborhood of the objective lens 2 for explaining the above mentioned 
two conditions, whereby the same elements as in FIG. 1 bear the same 
figures. In FIG. 2, the dotted line d.sub.2 shows the illumination light 
beam coming from the illumination light source, while the full line 
d.sub.3 shows the light beam to be used for the observation and the 
photographing out of the light beam reflected from the eye bottom. 
At first, the above mentioned first condition will be explained. Generally 
the F-number of the illumination light beam for illuminating the eye 
bottom is almost the same as that of the pupil of the person to be 
inspected so that it is almost constant. In consequence the converging 
angle .theta. of the illumination light beam is almost constant. 
In case as is shown in FIG. 2, the converging angle of the illumination 
light beam is .theta. while the black spot in the illumination light beam 
optical system is formed at A.sub.3 , for example a screen is placed at 
A.sub.3 so as to be moved along the optical axis 0-0'. When the small 
black spot is small enough, the range E.sub.1 in which the image of the 
black spot appears around A.sub.3 as center is comparatively larger, while 
the small black spot is large the range E.sub.2 becomes comparatively 
small. When the image of the eye bottom of the person with normal 
visibility is formed at A.sub.4 by the objective lens, the image of the 
eye bottom of the person with short sightedness is formed at the point 
closer to the eye to be inspected than the point A.sub.4. This point 
A.sub.4 is optically conjugate with the eye bottom to be inspected with 
regard to the objective lens 2, so that when the point A.sub.4 is supposed 
to be at the eye bottom to be inspected the image of the small black spot 
is actually formed on the eye bottom to be inspected in case the point 
A.sub.4 is in the above mentioned ranges E.sub.1 and E.sub.2. The sharper 
the image of this small black spot is, the closer the point A.sub.4 is to 
the point A.sub.3. Thus the larger the ranges (E.sub.1, E.sub.2) are, 
namely the larger the small black spot is, more likely the image of the 
small black spot is formed on the eye bottom to be inspected. 
The second condition means that the image forming position A.sub.3 of the 
above mentioned black spot is distant from the above mentioned point 
A.sub.4, because the sight of men assumes almost normal distribution 
around those whose sight is standard as center. Therefore, in case the 
point A.sub.3 is sufficiently distant from the point A.sub.4, the small 
black spot is never formed on the eye bottom unless the person to be 
inspected possesses extremely abnormal sight. 
In accordance with the present invention in order to meet the first 
condition it is so designed that the undesired light beams reflected from 
both surfaces of the objective lens and misted into the photographic 
optical system be radiated from the same surface of the illumination 
optical system, while in order to meet the second condition the eye bottom 
camera is so designed that the image of the black spot provided in the 
above mentioned illumination system be formed closer to the eye to be 
inspected than the face S.sub.2 of the objective lens. 
FIG. 3 is the sketch for explaining the method to meet the above mentioned 
first condition. In FIG. 3, 31 is the eye to be inspected while 32 is a 
biconvex objective lens whereby the radius of curvature of the surface 
opposed to the eye to be inspected is r.sub.2 (hereinafter this surface is 
called R.sub.2 surface), that of the other surface is r.sub.1 (hereinafter 
this surface is called R.sub.1 surface), the thickness D and the index of 
refraction N. 33 is the diaphragm plate intended to limit the light beam 
entering into the photographic optical system. Hereby 0-0' is the optical 
axis. 
The above mentioned first condition means that the position V.sub.1 of the 
imaginary image formed by the light beam reflected by the R.sub.1 surface 
of the diaphragm plate 33 for limiting the light beam entering into the 
photographic optical system coincides with that V.sub.2 of the imaginary 
image of the diaphragm plate 33 formed by the light beam 35 coming from 
the diaphragm plate 33, entering through the R.sub.1 surface, reflected by 
the R.sub.2 surface and again going out through the R.sub.1 surface. 
Namely the R.sub.1 surface and the R.sub.2 surface are designed in such a 
manner that the position V.sub.1 coincides with the position V.sub.2. 
Now let us consider about the light beam reflected on the R.sub.1 surface. 
Let the angle between the light beam 36 radiated from the center of the 
diaphragm plate 33 and directed to H.sub.1 on the R.sub.1 surface and the 
optical axis (0-0') be U.sub.1 and the angle of the light beam 34 
reflected on the R.sub.1 surface and the optical axis (0-0') be U'.sub.2, 
so the following relation is established. 
EQU U'.sub.2 = U.sub.1 - (2/r.sub.1)H.sub. 1 (1) 
hereby the index of refraction of the air is taken as 1. 
On the other hand with reference to the light beam reflected on the R.sub.2 
surface, the following relations are established. 
##EQU1## 
whereby U.sub.1 is the angle between the light beam 36 radiated from the 
center C of the diaphragm plate and directed to H.sub.1 of the R.sub.1 
surface and the optical axis (0-0'), U.sub.2 the angle between the light 
beam refracted on the R.sub.1 surface and directed to the R.sub.2 surface 
and the optical axis (0-0'), U.sub.3 the angle between the light beam 
reflected on the R.sub.2 surface and directed to the R.sub.1 surface and 
the optical axis (0-0') and U.sub.4 the angle between the light beam going 
out again from the R.sub.1 surface and the optical axis (0-0'). 
From the above mentioned equations (1) to (6), the position V.sub.1 of the 
imaginary image of the point C by means of the light beam reflected by the 
R.sub.1 surface and directed to the diaphragm plate is represented as 
follows with reference to the R.sub.1 surface, 
EQU (H.sub.1 /U'.sub.2) 
on the other hand, the position V.sub.2 of the point C by means of the 
light beam refracted on the R.sub.1 surface, reflected on the R.sub.2 
surface and going out again from the R.sub.1 surface, 
EQU (H.sub.3 /U.sub.4) 
the combination of the parameters U.sub.1, r.sub.1, r.sub.2, D and N of the 
above mentioned objective lens is chosen in such a manner (H.sub.1 
/U.sub.2) is equal or almost equal to (H.sub.3 /U.sub.4), when the 
position V.sub.1 coincides with the position V.sub.2. Hereby the 
undesired reflected light beam can be eliminated only if the small black 
spot is provided at a point in the illumination optical system which is a 
conjugate point with that at which the position V.sub.1 coincides with the 
position V.sub.2. 
Consequently in accordance with the method shown in FIG. 1, the reflection 
on the R.sub.1 surface is covered by the black point with dimension 
necessary on the R.sub.1 surface so that, as is shown in FIG. 4, the range 
E.sub.3 is shaded. Namely between the R.sub.1 surface and the position of 
the imaginary image by the light beam reflected on the R.sub.1 surface a 
black spot appears in a wide range before and behind the R.sub.1 surface 
whereby further the black spot is large. On the other hand in case of the 
optical system in accordance with the present invention, as is shown in 
FIG. 4(B), the reflections at the R.sub.1 surface and the R.sub.2 surface 
can be eliminated at the one point A.sub.5 so that the position at which 
the black spot appears is only one and further the black spot may be 
small. In the above mentioned explanation only the light beam from the 
center C of the diaphragm plate is taken in consideration, while the 
actual diaphragm plate presents a certain area so that the black spot also 
presents a certain area and therefore the range in which the black spot 
appears at the eye bottom becomes larger. 
With reference to the second condition as is clear in case of the present 
invention, it is so designed that the image of the small shading black 
spot for eliminating the undesired reflection light bema is formed at the 
position closer to the eye to be inspected than the above mentioned 
R.sub.1 surface so that the image of the black spot is hard to be formed 
at the eye bottom. 
FIG. 5 shows an embodiment in accordance with the present invention. In 
FIG. 5, 51 is the light source, 52 the condenser lens, 53 the ring slit; 
54, 56 the relay lenses and 55 the glass plate presenting a small shading 
black spot 61 in the neighborhood of the optical axis. 57 is an apertured 
inclined mirror, for example, a mirror presenting a boring 62 at the 
center. 58 is a biconvex objective lens, 59 the eye to be inspected, 60 
the cornea, 63 the relay lens and 64 the photographic plane. 
The light beam radiated from the light source 51 is condensed at the ring 
slit 53 by means of the condenser lens 52. The ring shaped light beam 
coming from the opening of the ring slit 53 is condensed again on the 
cornea of the eye to be inspected, by means of the relay lenses (54, 56), 
the mirror 57 with a boring and the objective lens 58 so as to illuminate 
the eye bottom. In order to eliminate the undesired light beams (L.sub.1, 
L.sub.2) reflected on the R.sub.1 surface and the R.sub.2 surface of the 
objective lens 58 a plane glass is provided between the relay lenses 56 
and 54. On the plane glass a small black spot for shading the light 
whereby the small black spot 61 is provided on the optical axis of the 
illumination optical system. As explained above, the small black spot 61 
for shading the light on the plane glass plate 55 is determined in such a 
manner that among the light beam from the light source 51, passing through 
the small black spot 61 and directed to the objective lens 58 by means of 
the mirror 57 with boring, the images of both the light beam L.sub.1 
reflected on the R.sub.1 surface of the objective lens and the light beam 
L.sub.2 passing through the R.sub.1 surface, reflected on the R.sub.2 
surface and again passing through the R.sub.1 surface are formed at the 
opening of the mirror 57 with opening. In other words, in case of the 
light beam directed from the opening 62 of the mirror with a boring to the 
eye to be inspected, the light beam L.sub.1 reflected on the R.sub.1 
surface is condensed on the above mentioned small black spot by means of 
the mirror 57 with boring and the relay lens 56, while the light beam 
L.sub.2 passing the R.sub.1 surface, reflected on the R.sub.2 surface and 
again passing the R.sub.1 surface is condensed on the above mentioned 
small black spot 61 by means of the mirror 57 with a boring and the relay 
lens 56. 
In the foregoing description, for better understanding of the present 
invention, the black spot and the inclined mirror's aperture have been 
explained as if rays of light emanate therefrom. However, this is no more 
than a convenient means of explaining the features of the present 
invention. In practice, the black spot functions to shield some fraction 
of the light beam from the light source, and therefore no light emanates 
from the black spot. Further, the light beam emanating from the aperture 
of the inclined mirror is shown as a means for better understanding the 
effects of the invention, and such emanation of the light beam from the 
aperture also does not occur in practice. Therefore, with respect to the 
optical systems including respective surfaces R1 and R2 of the objective 
lens as the reflection surfaces, as is also evident from the description 
made by use of the above mentioned light rays, there are arranged the 
black spot and the inclined mirror's aperture in optically conjugate 
positions to each other, so that as a matter of course the undesired 
reflecting light rays produced from the R.sub.1 and R.sub.2 surfaces of 
the objective lens do not pass through the aperture of the inclined 
mirror. 
Below serve concrete embodiments of the shapes of the objective lens for 
eliminating the undesired reflected light beam are given. 
______________________________________ 
(Embodiment 1) 
______________________________________ 
r.sub.1 = non-spherical surface basing upon -31.674 
r.sub.2 = 53.769 
D = 39.2 n = 1.568 S.sub.1 = 114.1 
f = 42.06 .vertline.m.sup.1 .vertline. = 0.12 
.vertline.m.sup.2 .vertline. = 0.2 
______________________________________ 
______________________________________ 
(Embodiment 2) 
______________________________________ 
r.sub.1 = -51.146 
r.sub.2 = non-spherical surface basing upon 35.525 
D = 43.85 n = 1.568 S.sub.1 = 114.1 
f = 45.02 .vertline.m.sup.1 .vertline. = 0.18 
.vertline.m.sup.2 .vertline. = 0.13 
______________________________________ 
Hereby; 
r.sub.1 : radius of curvature of the surface at the side of the mirror 
with boring 
r.sub.2 : radius of curvature of the surface at the side of the eye to be 
inspected 
D : Thickness of lens 
n : Index of lens 
S.sub.1 : Distance between the mirror with boring and the r.sub.2 surface 
f : Focal distance of the objective lens 
m.sup.1 : Magnification factor of the imaginary image of the light beam 
from the small black spot reflected on the R.sub.1 surface 
m.sup.2 : Magnification factor of the imaginary image of the light beam 
from the small black spot reflected on the R.sub.2 surface. 
Hereby in case of the embodiment either of the R.sub.1 surface and R.sub.2 
surface is made non-spherical in order to eliminate the light reflected on 
the cornea, whereby the non-spherical surfaces can be exchanged for each 
other or both surfaces can be made spherical. 
Further when in case of the present optical system, the radius of 
curvatures of r.sub.2 surface and r.sub.1 surface are made equal to each 
other and the thickness D of the lens is also made equal to this radius of 
curvature, the magnification factors of the imaginary image can also be 
made equal to each other as follows in such a manner that the small black 
spot for shading light can be minimized 
EQU .vertline.m1.vertline. = .vertline.m2.vertline.