Re-imaging optical system

A re-imaging objective optical system provides superior optical performance while having a short overall length and a relatively simple composition. The system has, in order from the object side of the optical system, a front lens group (G1) composed of a first lens group having a positive lens component (L1), a second lens group having a biconcave negative lens component (L2) and a third lens group having a positive lens component (L3); and a back lens group G2 composed of a cemented double lens element with components (L4 and L5) and a positive lens component (L6). The spatial or primary image I.sub.1 from the objective optical system is formed on the optical path between the first lens group and the second lens group.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a re-imaging optical system which causes 
the spatial image from an objective optical system to be re-composed under 
a specified magnification. The system is suitable for a device which 
composes a spatial image with a microscope objective lens. 
2. Description of Related Art 
In recent years, a system has been known where the composing of images 
formed by microscope objective lenses is achieved using imaging devices 
such as CCDs and the like. The imaging device is positioned at the primary 
imaging plane of the microscope objective lens. This type of system offers 
the advantage that the system itself can be easily manufactured and 
designed. However, because it is preferable to have a small imaging 
device, the system encounters a problem that the visual field ratio 
becomes smaller even though the visual field number for the microscope 
tends to expand. 
Therefore, in recent years re-imaging optical systems have been used which 
re-compose the spatial image formed by the microscope objective lens under 
a specified magnification on an imaging device. However, with the above 
re-imaging optical system, the problem exists that the object image on the 
imaging device is inferior compared to systems where the imaging device is 
positioned on the primary image plane of the microscope objective lens. 
This inferiority is due to aberrations in the re-imaging optical system. 
Increasing the number of lens component pieces in order to improve the 
optical performance of the re-imaging optical has been attempted. However, 
when performance is enhanced, the re-imaging optical system becomes 
longer. Thus, re-imaging optical systems have problems associated with 
increasing of the overall size of the system. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a re-imaging optical 
system that has superior optical performance and a short overall length. 
It is another object of this invention to provide a re-imaging optical 
system that has a relatively simple construction. 
In order to achieve the above and other objects, the re-imaging optical 
system of the present invention is provided with a construction that forms 
a secondary image of a primary or spatial image under a specific 
magnification. The secondary image is positioned on the side of the 
objective optical system facing the spatial image formed by the objective 
optical system. The re-imaging optical system comprising: a front lens 
group comprising, in order from the objective optical system, a first lens 
group having a positive lens component, a second lens group having a 
biconcave negative lens component with a concave surface facing the 
secondary image, and a third lens group having a positive lens component; 
and a back lens group having a cemented double lens element which, 
includes a negative lens component facing the front lens group and a 
positive lens component, and the back lens group further includes a 
positive lens component. Furthermore, the spatial image is formed on the 
optical path between the first lens group and the second lens group. 
With the present invention having the above-described design, it is 
possible for the principal point distance to be negative. This is due to 
the arrangement of the refracting powers in a positive-negative-positive 
order in the front lens group. The principle point distance is the 
distance between the primary or front side principal point and the 
secondary or back side principal point of the front lens group. It is 
possible for the primary or front side principal point to be positioned on 
the image side of the secondary or back side principal point. By this 
arrangement, it is possible to reduce the overall length of the re-imaging 
optical system. 
In addition, the present invention offers the advantage that the diameter 
of the re-imaging optical system does not increase because the first lens 
group serves as the field lens. 
Furthermore to achieve its objects, the present invention offers the 
advantage that it is possible for the secondary or back side principal 
point of the back lens group to be positioned on the secondary image side. 
Back focus can be obtained to a specified degree, because the double lens 
element in the back lens group is positioned so that the negative lens 
component is positioned toward the front lens group. 
In addition to the above arrangement, it is preferable for the re-imaging 
optical system of the present invention to satisfy the following condition 
: 
EQU 3.0.ltoreq.(f11 /f1).ltoreq.0.85 (1) 
Here, f1 is the composite focal length of the front lens group, and f11 is 
the focal length of the first lens group. 
Condition (1) is a condition which specifies the optimum range for the 
focal length of the first lens group relative to the composite focal 
length of the front lens group. Meeting the range of condition (1) keeps 
the overall length of the lens system short. When the lower limit in 
condition (1) is breached, the exit pupil position in the re-imaging 
optical system moves toward the front side (the side from which light is 
incident). It is possible for the distance between the front lens group 
and the back lens group to be shortened, however, this is not desirable 
since distortions and comas tend to occur. In addition, when the upper 
limit in condition (1) is exceeded, the distance between the front lens 
group and the back lens group increases. This increase is not desirable 
because shortening of the overall length of the system then becomes 
impossible. 
It is desirable for the lower limit in condition (1) to be 0.4 and for the 
upper limit in condition (1) to be 0.7, in order to suppress the creation 
of aberrations and to shorten the overall length. It is more advantageous 
for the lower limit to be 0.5 to further suppress aberrations. It is also 
desirable for the upper limit to be 0.65 to further shorten the overall 
length. 
In addition, it is desirable to have a arrangement which shortens the 
composite focal length of the front lens group to shorten overall length. 
However, when this occurs, Petzval's sum tends to increase. Therefore, 
calling f1 the composite focal length of the front lens group and ra the 
radius of curvature of the lens surface on the side of the negative lens 
component toward the secondary image in the second lens group, it is 
desirable for the re-imaging optical system of the present invention to 
satisfy the following condition: 
EQU 0.07.ltoreq.(.vertline.ra.vertline./f1).ltoreq.0.55 (2) 
Condition (2) is a condition relating to the correction of Petzval's sum. 
Here, breaching the lower limit of condition (2) is not desirable because 
Petzval's sum cannot be corrected and field of curvature occurs. In 
addition, exceeding the upper limit of condition (2) is not desirable 
because although Petzval's sum can be corrected, other aberrations, 
especially comas, result. 
It is desirable for the lower limit in condition (2) to be 0.1 and for the 
upper limit to be 0.4 in order to correct the various aberrations and 
obtain a good imaging performance. To achieve an even better imaging 
performance, it is desirable for the lower limit in condition (2) to be 
0.13 and for the upper limit to be 0.25. 
With the re-imaging optical system of the present invention, it is 
desirable to have a short distance between the second lens group and the 
third lens group of the front lens group. The front lens group, as noted 
above has a positive-negative-positive arrangement of refracting powers. 
It is thus possible to make the principal point distance of the front lens 
group even more negative, and thereby reduce the overall length. 
Therefore, it is desirable for the above-described group distance to 
satisfy the following condition: 
EQU 0.ltoreq.(D/f1) .ltoreq.0.25 (3) 
Here, f1 is the composite focal length of the front lens group. D is the 
group distance between the second lens group and the third lens group. 
When the lower limit of condition (3) is breached, the second lens group 
and the third lens group interfere with each other. This interference is 
not desirable. When the upper limit of condition (3) is exceeded, the 
secondary or back side principal point of the front lens group moves 
toward the secondary image side. The overall length of the re-imaging 
optical system becomes longer. Both occurrences are undesirable. 
Considering the simplicity of the mechanism of the lens barrel which 
supports the second and third lens group, it is desirable for the lower 
limit in condition (3) to be 0.05. It is particularly desirable for the 
lower limit in condition (3) to be 0.1, in order to correct distortions. 
In addition, it is desirable for the upper limit in condition (3) to be 
0.18 in order to shorten overall length. It is particularly desirable to 
have an upper limit of 0.15 in order to further shorten overall length. 
Condition (4) specifies the optimum range of the radius of curvature of the 
composition surface of the cemented lens component in the back lens group 
to correct for comas and chromatic aberrations. 
Calling f2 the composite focal length of the back lens group and rb the 
radius of curvature of the composition surface of the cemented double 
element lens in the back lens group, it is desirable for the re-imaging 
optical system of the present invention to satisfy the following condition 
(4). 
EQU 0.17.ltoreq.(.vertline.rb.vertline./f2).ltoreq.0.95 (4) 
When the lower limit in condition (4) is breached, chromatic aberrations 
are excessively corrected which is undesirable. Comas, and in particular 
upper comas, tend to worsen when the upper limit in condition (4) is 
exceeded. This result is also undesirable. In order to have good 
correction of chromatic aberration, it is desirable for the lower limit in 
condition (4) to be 0.3. In order to have good coma correction, it is 
desirable for the upper limit in condition (4) to be 0.8. Furthermore, to 
achieve even better chromatic aberration correction, it is desirable for 
the lower limit in condition (4) to be 0.4. To further improve correction 
of comas, it is desirable for the upper limit in condition (4) to be 0.65. 
It is also desirable for the present invention to be configured so as to 
satisfy the following condition: 
EQU 0.65.ltoreq.(f22/f2).ltoreq.1.8 (5) 
f22 is the focal length of the positive lens component in the back lens 
group and f2 is the composite focal length of the back lens group. 
When the lower limit in condition (5) is breached, the refracting power of 
the positive lens component becomes stronger, relative to the refracting 
power of the back lens group. As a result undesirable distortions are 
created. In addition, when the upper limit in condition (5) is exceeded, 
the secondary or back side principal point of the back lens group is 
positioned closer to the front side and the back focus becomes too short. 
This is undesirable. 
In order to achieve good correction of the various aberrations, it is 
desirable for the lower limit in condition (5) to be 0.78. In particular, 
to achieve good correction of distortion, it is desirable for the lower 
limit in condition (5) to be 0.9. In addition, in order to maintain a 
prescribed back focus, it is desirable for the upper limit in condition 
(5) to be 1.6.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the present invention is described hereafter, 
with reference to the drawings. 
FIG. 1 is a drawing of the lens configuration of a first embodiment of the 
present invention. The re-imaging objective optical lens system of the 
first embodiment is composed of front lens group G1 and back lens group 
G2, in the order from the object side of the optical system. Light from 
the subject that passes through the objective optical system and forms a 
primary or spatial image I.sub.1 in front lens group G1. Light from this 
primary image I.sub.1 then passes through front lens group G1 and back 
lens group G2. The light then forms a secondary image I.sub.2 which is a 
specified magnification of the primary image I.sub.1. 
The front lens group G1 has, in order from the object side of the optical 
system: a positive plano-convex lens component L1, having a convex surface 
facing the object side of the optical system; a negative biconcave lens 
component L2, with a strong concave surface facing the secondary image 
I.sub.2 ; and a positive biconvex lens component L3, with a strong convex 
surface facing the secondary image I.sub.2. 
The second lens group G2 has, in order from the object side of the optical 
system side, a cemented double lens element comprising a negative meniscus 
lens component L4, having a convex surface facing the object side of the 
optical system and a positive biconvex lens component L5, having a strong 
convex surface facing the object side of the optical system; and a 
positive biconvex lens component L6, having a strong convex surface facing 
the object side of the optical system. Negative lens component L4 is fixed 
to the positive lens component L5. 
The exit pupil position P.sub.1 of the front lens group G1 and the front 
side focal point position F.sub.2 of the back lens group G2 coincide, as 
shown in FIG. 2. Accordingly, primary light rays which pass through the 
exit pupil position P.sub.0 of the objective optical system (shown in this 
drawing as a broken line), intersect the optical axis at the exit pupil 
position P.sub.1 of the front lens group G1. After passing through the 
back lens group G2, the light rays are emitted parallel to the optical 
axis. At this time, the exit pupil position P.sub.2 of the back lens group 
G2 is infinitely distant. Therefore, the secondary image I.sub.2 side is 
telecentric. 
The front lens group G1 has refractory powers arranged in a 
positive-negative-positive order, as shown in FIG. 1. Thus, the front side 
principal point H.sub.1 of the front lens group G1 is positioned on the 
object side of the secondary image I.sub.2, while the back side principal 
point H.sub.I ' is positioned on the side toward the object optical 
system. It is possible, therefore, to shorten the overall length of the 
front lens group G1. 
The cemented double lens components L4 and L5 of the back lens group G2 
have refractory powers arranged in a negative-positive order. Positive 
lens component L6, which follows the cemented lens element, has a 
relatively strong refractory power. The back side principal point H.sub.2 
' of the back lens group G2 is positioned on the side toward the secondary 
image because of this design. Thus, it becomes possible for the back focus 
to become longer to secure a flange back FB, which is the distance between 
the mounting surface M and the secondary image I.sub.2. 
The values of the various dimensions of the primary embodiment are shown 
hereafter, in Table 1. In Table 1, f is the focal length of the entire 
system, .beta. is the magnification, and NA is the numerical aperture. In 
addition, the number in the leftmost column indicates the lens in order 
from the object side toward the subject, r is the radius of curvature of 
each lens surface, d is the distance between lens surfaces, n.sub.d is the 
index of refraction relative to d line (.nu.=587.6 nm), and .nu..sub.d is 
the Abbe number for d lines. In Table 1, d.sub.o is the distance from the 
primary image I.sub.1 to the positive lens component L.sub.1, and Y is the 
height of the secondary image I.sub.2. 
TABLE 1 
______________________________________ 
f = -105, .beta. = -0.6, NA = 0.04, do = -14, Y = 5.5 
NO. r d n.sub.d .nu..sub.d 
______________________________________ 
1 25.2 4.0 1.78797 47.5 L.sub.1 
2 0.0 20.0 
3 -281.0 2.0 1.75520 27.6 L.sub.2 
4 8.7 7.0 
5 75.3 5.0 1.74810 52.3 L.sub.3 
6 -13.4 43.5 
7 442.3 2.0 1.78470 26.1 L.sub.4 
8 14.7 9.0 1.51454 61.1 L.sub.5 
9 -59.1 0.5 
10 26.7 5.5 1.69680 55.6 L.sub.6 
11 -61.7 30.8 
______________________________________ 
FIGS. 3A-3E show various aberrations in the re-imaging optical system 
according to first embodiment. Here, in the field curve drawing of 
spherical aberrations, FIG. 3A, the dashed line indicates a sine 
condition. In the field curve drawing of astigmatism, FIG. 3B, the dotted 
line indicates the meridional image surface, while the solid line 
indicates the sagittal image surface. Furthermore, the representation of 
coma in FIG. 3D illustrates situations where the image heights are 100% 
and 70%. The drawings of the various aberrations illustrate situations 
where light rays have been traced from a hypothetical exit pupil for the 
object optical system. 
As shown in FIGS. 3A-3E, the re-imaging optical system of the present 
embodiment has superior re-imaging performance while the lens 
configuration has an overall short length. 
A second embodiment of the present invention is shown in FIG. 4. In FIG. 4, 
the re-imaging optical system of the second embodiment has, in order from 
the objective optical system side, a front lens group G1 and a back lens 
group G2. The front lens group G1 has the same composition as in the first 
embodiment. 
The back lens group G2 has, in order from the object side of the optical 
system, a cemented double lens element comprising a negative biconcave 
lens component L4, having a strong concave surface facing the secondary 
image I.sub.2, and a positive biconvex lens component L5, having a strong 
convex surface facing the object side of the optical system, and a 
positive meniscus lens component L6 having convex surface facing the 
object side of the optical system. With the arrangement of this 
embodiment, the primary or front side principal point position of the back 
lens group G.sub.2 coincides with the exit pupil position of the front 
lens group G.sub.1, so that telecentricity is achieved on the secondary 
image I.sub.2 side. 
Hereafter, the values of the various dimensions of the second embodiment 
are shown in Table 2. In Table 2, f is the focal length of the entire 
system, .beta. is the magnification, and NA is the numerical aperture N.A. 
In addition, the number in the leftmost column indicates the lens in order 
from the object side toward the subject, r is the radius of curvature of 
each lens surface, d is the distance between lens surfaces, n.sub.d is the 
index of refraction relative to d lines (.lambda.=587.6 nm), and 
.nu..sub.d is the Abbe number of d lines. In Table 2, d.sub.o is the 
distance from the primary image I.sub.1 to the positive lens component L1, 
and Y is the height of the secondary image I.sub.2. 
TABLE 2 
______________________________________ 
f = -86, .beta. = -0.45, NA = 0.04, d.sub.o = -14, Y = 4.3 
No. r d n.sub.d .nu..sub.d 
______________________________________ 
1 25.2 4.0 1.78797 47.5 L.sub.1 
2 0.0 20.0 
3 -281.0 2.0 1.75520 27.6 L.sub.2 
4 8.7 7.0 
5 75.3 5.0 1.74810 52.3 L.sub.3 
6 -13.4 38.0 
7 -182.8 2.0 1.78470 26.1 L.sub.4 
8 14.1 8.0 1.62280 57.0 L.sub.5 
9 -29.2 0.5 
10 18.6 5.0 1.71300 53.9 L.sub.6 
11 160.8 21.1 
______________________________________ 
FIGS. 5A-5E illustrate various aberrations in the re-imaging optical system 
according to the second embodiment. In the field curve drawing of 
spherical aberrations, FIG. 5A, the dashed line indicates a sine 
condition. In the field curve drawing of astigmatism, FIG. 5B, the dotted 
line indicates the meridional image surface, while the solid line 
indicates the sagittal image surface. Furthermore, the representation of 
coma in FIG. 5D illustrates situations where the image heights are 100% 
and 70%. The drawings of the various aberrations illustrate situations 
where light rays have been traced from a hypothetical exit pupil for the 
optical system. 
As shown in FIGS. 5A-5E, the re-imaging optical system of the present 
embodiment has superior re-imaging performance while having a lens 
configuration with short overall length. 
With the first and second embodiments, telecentricity is achieved on the 
secondary image side. In the case of a single tip imaging device, complete 
image-side telecentricity is not required and it is not necessary for the 
side facing the secondary image I.sub.2 to be telecentric. 
In addition, it is possible to change the re-imaging magnification by 
altering the configuration of the back lens group G.sub.2, as can be seen 
from the first and second embodiments. This is advantageous because only 
back lens group G.sub.2 needs be altered, even when the re-imaging 
magnification is changed for changing the size of the imaging device. It 
is also acceptable to change the re-imaging magnification by making the 
back lens group G2 in the first embodiment and the back lens group G2 in 
the second embodiment interchangeable. 
A third embodiment of the present invention is described in FIG. 6. The 
re-imaging optical system according to the third embodiment is an example 
wherein a color separation prism P is provided on the side of the 
secondary image I.sub.2. This corresponds to color photography with a 
three tip camera. The color separation prism P is shown in FIG. 7. The 
color separation prism P is positioned in the optical path between the 
re-imaging optical system 10 and imaging devices 11B, 11G and 11R, which 
are comprised of CCDs or the like. The prism is composed of three prism 
blocks P.sub.B, P.sub.G and P.sub.R. A dichroic membrane DM.sub.B reflects 
blue light and is positioned at the composition surface of prism block 
P.sub.B and prism block P.sub.R. 
Dichroic membrane DM.sub.R reflects red light and allows green light to 
pass. Dichroic membrane DM.sub.R provided at the composition surface of 
prism block P.sub.G and prism block P.sub.R. In addition, trimming filters 
12B, 12G and 12R which prevent moire patterns on the imaging devices are 
provided in the optical path between each prism block P.sub.B, P.sub.G and 
P.sub.R and the imaging devices 11B, 11G and 11R. 
In FIG. 6, the re-imaging optical system of the third embodiment is 
comprised of, in order from the object side of the optical system, a front 
lens group G1, a back lens group G2 and a color separation prism P. The 
color separation prism P is shown in its opened state. 
In FIG. 6, the front lens group G1 has, in order from the objective optical 
system side, a positive plano-convex lens component L1 having the convex 
surface facing the object side of the optical system, a negative biconcave 
negative lens component L2 having a strong concave surface facing the 
secondary image I.sub.2, and a positive biconvex lens component L3 having 
a strong convex surface facing the secondary image I.sub.2. The back lens 
group G2 has, in order from the object side of the optical system side, a 
cemented double lens element comprises of a negative biconcave lens 
component L4 having a strong concave surface facing the secondary image 
I.sub.2 and a positive biconvex lens component L5 having a strong convex 
surface facing the object side of the optical system, and a positive 
meniscus lens component L6 having a convex surface on the side toward the 
object side of the optical system. 
In the third embodiment, the exit pupil position of the front lens group G1 
and the primary or front side focus position of the back lens group G2 
coincide. Thus, telecentricity is achieved on the side toward the 
secondary image I2. 
The third embodiment is arranged so that light rays gradually disperse as 
they move from the front lens group G1 toward the back lens group G2. It 
is possible to shorten the overall length of the system by shortening the 
group distance between the front lens group G1 and the back lens group G2. 
Hereafter, the values of the various dimensions of the third embodiment are 
shown in Table 3. In Table 3, f is the focal length of the entire system, 
.beta. is the magnification, and NA is the numerical aperture. In 
addition, the number in the leftmost column indicates the lens in order 
from the object side toward the subject, r is the radius of curvature of 
the lens surface, d is the distance between lens surfaces, n.sub.d is the 
index of refraction relative to d line (.lambda.=587.6 nm), and .nu..sub.d 
is the Abbe number of d line. In table 3, d.sub.o is the distance from the 
primary image I.sub.1 to the positive lens component L1, and Y is the 
height of the secondary image I.sub.2. In Table 3, the back focus is 
indicated by a value for the length of the color separation prism P 
converted to atmospheric conditions. The color separation prism P has the 
dimensions shown in below-described table 5. 
TABLE 3 
______________________________________ 
f = -922, .beta. = 0.6, NA = 0.03 d0 = -4, Y = -5.5 
NO. r d n.sub.d .nu..sub.d 
______________________________________ 
1 29.5 4.0 1.78797 47.5 L.sub.1 
2 0.0 26.0 
3 -50.80 2.0 1.74950 35.2 L.sub.2 
4 13.0 7.0 
5 93.3 5.0 1.74400 45.0 L.sub.3 
6 -17.3 38.3 
7 -197.0 2.0 1.75520 27.6 L.sub.4 
8 20.5 8.0 1.62280 57.0 L.sub.5 
9 -35.9 0.5 
10 35.3 5.0 1.71300 53.9 L.sub.6 
11 313.5 44.3 
______________________________________ 
FIGS. 8A-8E illustrate various aberrations in the re-imaging optical system 
according to the third embodiment. In the field curve drawing of spherical 
aberrations, FIG. 8A, the dashed line indicates a sine condition. In the 
field curve drawing of astigmatism, FIG. 8B, the dotted line indicates the 
meridional image surface, while the solid line indicates the sagittal 
image surface. Furthermore, the representation of coma in FIG. 8D 
illustrates situations where the image heights are 100% and 70%. The 
drawings of the various aberrations illustrate situations where light rays 
have been traced from a hypothetical exit pupil for the optical system. 
The drawings also illustrate the situation where the color separation 
prism P is inserted in the optical path between the re-imaging optical 
system and the secondary image I.sub.2. 
As shown in FIGS. 8A-8E, the re-imaging optical system of the third 
embodiment has superior re-imaging performance while having a lens 
configuration with short overall length. The back focus is maintained 
where it is possible to provide a color separation prism. 
A fourth embodiment of the invention is described with reference to FIG. 9. 
The lens configuration of the fourth embodiment is the same as in FIG. 6 
of the third embodiment. 
The values of the various dimensions of the fourth embodiment are shown in 
Table 4. In Table 4, f is the focal length of the entire system, .beta. is 
the magnification, and NA is the numerical aperture. In addition, the 
number in the leftmost column indicates the lens in order from the object 
side toward the subject, r is the radius of curvature of the lens surface, 
d is the distance between lens surfaces, nd is the index of refraction 
relative to d lines (.lambda.=587.6 nm), and .nu..sub.d is the Abbe number 
of d lines. In Table 4, d.sub.o is the distance from the primary image 
I.sub.1 to the positive lens component L1. Y is the height of the 
secondary image I.sub.2. In Table 4, the back focus is indicated by a 
value for the length of the color separation prism P converted to 
atmospheric conditions. The color separation prism P has the dimensions 
shown in below-described table 5. 
TABLE 4 
______________________________________ 
f = -235, .beta. = -0.6, NA = 0.03, d.sub.o = -14, Y = 5.5 
NO. r d n.sub.d .nu..sub.d 
______________________________________ 
1 28.7 4.0 1.78797 47.5 L.sub.1 
2 0.0 26.0 
3 -33.5 2.0 1.74000 28.2 L.sub.2 
4 9.0 7.0 
5 67.0 5.0 1.74810 52.3 L.sub.3 
6 -14.1 51.8 
7 -138.6 2.0 1.75692 31.6 L.sub.4 
8 16.8 8.0 1.62041 60.1 L.sub.5 
9 -45.0 0.5 
10 26.1 5.0 1.71300 53.9 L.sub.6 
11 129.71 39.4 
______________________________________ 
FIGS. 10A-10E illustrate various aberrations in the re-imaging optical 
system of the fourth embodiment. In the field curve drawing of spherical 
aberrations, FIG. 10A, the dashed line indicates a sinusoidal condition. 
In the field curve drawing of astigmatism, FIG. 10B, the dotted line 
indicates the meridional image surface, while the solid line indicates the 
sagittal image surface. Furthermore, the representation of coma in FIG. 
10D illustrates situations where the image heights are 100% and 70%. The 
drawings of the various aberrations illustrate situations wherein light 
rays have been traced from a hypothetical exit pupil for the object 
optical system. The drawings also illustrate the situation where the color 
separation prism P is inserted on the optical path between the re-imaging 
optical system and the secondary image I2. 
As shown in FIG. 10, the re-imaging optical system of the fourth embodiment 
has superior re-imaging performance while having a lens configuration with 
short overall length. The back focus is maintained where it is possible to 
provide a color separation prism. 
Light rays are parallel between the front lens group G1 and the back lens 
group G2 in the re-imaging optical system according to the fourth 
embodiment. This offers the advantage that the entire system need not be 
optically redesigned, even when the re-imaging magnification is changed. 
Redesigning either the front lens group G1 or the back lens group G2 of 
the fourth embodiment will be sufficient to change the re-imaging 
magnification, for example by changing the size of the imaging devices. 
The dimensions of the color separation prism P of the above-described third 
and fourth embodiments are shown below in Table 5. 
TABLE 5 
______________________________________ 
NO. r d n.sub.d 
.nu..sub.d 
______________________________________ 
1 0.0 30.0 1.60342 
38.0 
2 0.0 16.2 1.51680 
64.1 
3 0.0 
______________________________________ 
The color separation prism P in Table 5 does not change optically 
regardless of where it is between the lens surface farthest to the 
secondary image side and the secondary image plane I.sub.2 on the optical 
path. 
The numerical values for conditions for each of the above-described 
embodiments are shown in Table 6. 
TABLE 6 
______________________________________ 
1 2 3 4 
______________________________________ 
(1) 0.64 0.64 0.62 0.55 
(2) 0.17 0.17 0.22 0.14 
(3) 0.14 0.14 0.12 0.11 
(4) 0.49 0.63 0.55 0.42 
(5) 0.91 1.30 1.50 1.13 
______________________________________ 
Each of the embodiments of the present invention are configured so as to 
satisfy above-described conditions (1) to (5). 
In each of the above-described embodiments, the positive lens component L1 
is provided closer to the object side of the optical system than the 
spatial image I1 is from the object side of the optical system. This 
results in the overall length of the re-imaging optical system being 
shortened. Further, dust-prevention of the system can also be readily 
achieved by providing the system with a short length and simple structure. 
By this arrangement, with the present invention it is possible to provide 
a re-imaging optical system having a superior optical performance while 
having short overall length and a relatively simple structure. 
While this invention has been described in conjunction with specific 
embodiments thereof, it is evident that many alternatives, modifications 
and variations will be apparent to those skilled in the art. Accordingly, 
the preferred embodiments of the invention as set forth herein are 
intended to be illustrative, not limiting. Various changes may be made 
without departing from the spirit and scope of the invention as defined in 
the following claims.