Finite conjugate projection lens system

A disclosed lens system is an atypical Gaussian-type lens comprised of two lens groups located at opposite sides of an aperture stop. Each of the groups is comprised of a doublet and a singlet. The doublets and singlets are located in sequential asymmetry with respect to the aperture stop. The front group has a cemented doublet as its foremost object-side lens component. This doublet consists of a lens element of negative power followed by a lens element of a positive power.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to lenses which are suitable for use as a 
finite conjugate projection lens system, and, more particularly, to a 
small-size, high-performance, wide-angle reader-printer lens for use in a 
facsimile machine, an image scanner, a microfilm machine, etc. 
In order to attain a high resolution of the order of 7 to 10 .mu.m on the 
image side, lenses used in facsimile machines, image scanners, microfilm 
machines, etc., are required to provide a high contrast ratio at high 
spatial frequencies. In addition, such lenses must have very little or no 
vignetting in order to minimize the decrease in the quantity of marginal 
light. It is also necessary to compensate various aberrations to a 
satisfactorily small level. 
In order to meet all of these requirements, Gaussian-type lenses have been 
traditionally employed for these applications. Examples of such lenses are 
disclosed in U.S. Pat. Nos. 4,784,480 and 4,426,137. A typical 
Gaussian-type lens system is composed of two groups wherein one group is 
located preceding the stop and the other group is located behind the stop. 
Each group has two sub-groups (inner and outer) and the entire lens 
usually contains 6 to 8 lens elements. Typical Gaussian-type lenses have 
two doublets or a doublet and a singlet in each group and are symmetrical 
with respect to the sequence of the doublets and singlets about an 
aperture stop and also in terms of the signs of the powers of the lens 
elements. For example, this lens type often has a positive lens element as 
a first or outer sub-group of the front group, and a cemented doublet as a 
second or inner sub-group of the first group. Because the most typical 
type of Gaussian-type lenses is generally symmetrical about an aperture 
stop, the first or inner sub-group of the rear group is usually also a 
cemented doublet. The second or outer sub-group of the rear group 
typically comprises one positive lens element. Sometimes the second 
doublet is uncemented to improve the lens performance, but this results in 
greater sensitivity to tolerance errors. Sometimes more than one lens 
element follows the second doublet. For example, the last positive lens 
element may be split into two or more single lens elements, or may be 
converted into a cemented doublet, resulting in a better performance at 
the expense of having more lens elements. Examples of such lenses are 
described in U.S. Pat. Nos. 4,773,746 and 3,815,974. 
The typical Gaussian-type lens system has a large amount of comatic flare 
at middle-angle positions and very large amount of oblique spherical 
aberration at the maximum angles. In addition, a major problem with using 
the conventional Gaussian type lens such as disclosed in U.S. Pat. No. 
4,784,480 in a reader printer is that small changes in the element 
thicknesses of the first doublet produces relatively large changes in 
image distance and field curvature. Because magnification as well as 
object-to-image distance are fixed with little or no adjustment tolerance 
in most microfilm reader printers, this image distance sensitivity causes 
the first doublet of a conventional design to have either very tight 
tolerances on the thicknesses of both elements or the thicknesses of both 
elements comprising the first doublet must be "matched" so that the total 
thickness of the doublet is held to tight tolerances. Also, typical 
Gaussian-type lenses tend to suffer from a large amount of inward field 
curvature, causing additional image quality deterioration. 
U.S. Pat. No. 4,671,627 discloses a two-group lens configuration of the 
same general type described above that has an overall symmetrical shape 
with respect to a diaphragm but which contains 8 lens elements. 
U.S. Pat. No. 2,171,640 discloses an atypical variation of the Gaussian 
type lens which can be characterized in terms of its elements as (+-)- 
stop (-+)+ configuration. For analysis purposes, the disclosed embodiment 
of this lens was scaled to have the same magnification, object-to-image 
distance, coverage and an F-number as one of the illustrative embodiments 
of the present invention. The analysis showed that this prior art lens has 
an unacceptable amount of astigmatism at the field coverage achievable by 
lenses according to our invention. In addition, this prior art lens 
suffered from much larger amount of vignetting than that of the present 
invention. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a small-size, 
high-performance reader-printer lens projection system which has a 
relatively flat field. 
Another object of this invention is to provide a microfilmer lens that has 
high resolving power and that is relatively insensitive to thickness 
changes and that at the same time has relatively few lens elements. 
In the present invention, as taught by the above-cited U.S. Pat. No. 
2,171,640, we depart from typical symmetry about an aperture stop in the 
sense of signs of the power of elements of the sub-groups involved, and 
also in the sense of the sequence of doublets and singlets. However, in 
addition, the foremost lens element has negative power. For example, the 
lens of U.S. Pat. No. 2,171,640 can be characterized in terms of elements 
as (+-)- stop (-+)+, whereas a representative lens according to the 
present invention might be characterized in the same way as (-+)- stop 
(-+)+. Because the first lens component is a doublet composed of a 
negative lens element followed by a positive lens element, not only is the 
color corrected, but, more importantly, the field curvature is less inward 
curving and the astigmatism is reduced. Hence, the general image quality 
throughout the field is enhanced.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following illustrative embodiment of the present invention shown in 
FIG. 1 is a lens system comprised of lens elements or lens components 
disposed in sequential asymmetry relative to the aperture stop location. 
In order from the object side, the illustrative lens comprises a first 
lens element (E.sub.1a) which is a negative meniscus lens element having 
its convex surface directed toward the object side, a second lens element 
(E.sub.2a) which is a positive meniscus lens element having its convex 
surface directed toward the object side, a third lens element (E.sub.3a) 
which is a negative meniscus lens element having its convex surface 
directed toward the object side, an aperture stop (AS.sub.a), a fourth 
lens element (E.sub.4a) which is a negative meniscus lens element having 
its concave surface directed toward the object side, a fifth lens element 
(E.sub.5a) having its concave surface directed toward the object side and 
a sixth lens element (E.sub.6a) which is positive. The first and second 
lens elements (E.sub.1a and E.sub.2a) are cemented and so are the fourth 
and fifth (E.sub.4a and E.sub.5a) lens elements to make the overall system 
a four-component six-element lens configuration, wherein a component 
consists of one or more lens elements. Because the first subgroup of the 
front group is a cemented doublet, the sensitivity of both the tangential 
field and the image distance to changes in the thickness of the elements 
in a cemented doublet is significantly reduced when this lens is compared 
to a typical Gaussian lens. Data for this illustrative embodiment is given 
below in Table 1. 
TABLE 1 
______________________________________ 
THICK- REFRACTIVE Abbe V 
SURFACE NESS INDEX NUMBER 
______________________________________ 
RADIUS 
S1a PLANO 4.762 1.523 58.8 
S2a PLANO A 
S3a 19.6416 1.860 1.648 33.9 
S4a 10.6482 4.500 1.713 53.8 
S5a 38.6019 0.200 
S6a 10.1979 2.660 1.755 27.6 
S7a 7.7827 4.662 
DIAPHRAGM 6.145 
S8a -10.3662 1.650 1.625 35.6 
S9a -108.480 3.210 1.713 53.8 
S10a -15.1402 0.200 
S11a -207.643 3.875 1.734 51.7 
S12a -25.5152 
______________________________________ 
where A = 998.044 when l/magnification = 24.823 and half angle coverage = 
20.3 
A = 1018.243 when l/magnification = 25.332 and half angle coverage = 19.9 
Table 1 shows numerical data of the first illustrative embodiment of the 
present invention. The first illustrative embodiment has a half-field 
angle of 20.3 to 19.94 degrees for magnification variance of 
1/24.82.times. and 1/25.33.times., with an effective focal length of about 
39.59. The change in magnification is achieved by variation of space A, 
between a plano plate and the front of the foremost element (E.sub.1a) 
with optical power. There is no vignetting, and the relative illumination 
is 76% at the maximum obliquity. This configuration results in a 20% to 
40% modulation at 150 line pairs per mm. 
FIG. 2 illustrates a second embodiment of the present invention, which is 
generally similar in construction to the first embodiment described above 
and illustrated in FIG. 1. In FIG. 2, the six optical elements are given 
the same reference letters as those of FIG. 1, but with suffix "b" instead 
of "a". Table 2 below gives the values for the various parameters. 
TABLE 2 
______________________________________ 
REFRACTIVE 
THICK- Abbe V 
SURFACE NESS INDEX NUMBER 
______________________________________ 
RADIUS 
S1b 16.0838 2.244 1.648 33.9 
S2b 8.49960 3.858 1.713 53.8 
S3b 29.7291 0.100 
S4b 8.4996 2.186 1.620 36.4 
S5b 6.4240 3.911 
DIAPHRAGM 4.483 
S6b -7.91850 1.092 1.625 35.6 
S7b -99.5818 2.630 1.713 53.8 
S8b -11.9576 0.100 
S9b -111.548 3.164 1.734 51.7 
S10b -18.9943 
______________________________________ 
Table 2 shows data of the second illustrative embodiment of the present 
invention. The lens system in the second embodiment is constructed from 
six lens elements E.sub.1b through E.sub.6b. However, the second 
embodiment does not have a plane parallel plate in front of the foremost 
lens element (E.sub.1b). The second illustrative embodiment of the present 
invention has a half-field angle of 20.degree., the effective focal length 
of 33 mm., and an F-number of 5.6. 
FIG. 3 illustrates a third embodiment of the present invention, which is 
similar in construction to the second embodiment described above and 
illustrated in FIG. 2. In FIG. 3, the six optical elements are given the 
same reference letters as those of FIG. 2, but with suffix "c" instead of 
"b". Because the third illustrative embodiment, just like a second 
illustrative embodiment, does not have a plane parallel plate in front of 
the foremost optical element (E.sub.1c), the same reference letters are 
used for identification of various surfaces, as in the second embodiment, 
but with a "c" suffix. Table 3 below gives the values for the various 
parameters. 
TABLE 3 
______________________________________ 
REFRACRIVE 
THICK- Abbe V 
SURFACE NESS INDEX NUMBER 
______________________________________ 
RADIUS 
Sic 12.5174 1.582 1.648 33.9 
S2c 6.38860 2.991 1.713 53.8 
S3c 24.7777 0.100 
S4c 5.92150 1.639 1.620 36.4 
S5c 4.49644 2.293 
DIAPHRAGM 2.902 
S6c -7.44050 1.017 1.625 35.6 
S7c 146.044 1.978 1.713 53.8 
S8c -12.9945 0.100 
S9c -111.548 2.387 1.734 51.7 
S10c -13.4750 
______________________________________ 
Table 3 shows numerical data of the third illustrative embodiment of the 
present invention. This embodiment is very similar to the second 
embodiment, except for the differences shown in Table 3. The third 
illustrative embodiment of the present invention has a half-field angle of 
22.73.degree., the effective focal length of 24.35 mm. and an F-number of 
5.6. 
While in the above embodiments, the first and third lens components are 
comprised of cemented doublets, it is, of course, possible to separate the 
cemented surfaces of either of the above two components. 
The present invention is not limited to the aforesaid illustrative 
embodiments, but could be, of course, variously modified within the 
technical scope of the present invention. 
While in the embodiments specifically described above all of the surfaces 
are spherical, it is to be understood that other embodiments of this 
invention may have nonspherical surfaces. 
Similarly, it should be understood that the focal lengths and other system 
parameters of the illustrative embodiments can be scaled up or down for 
different applications. It should also be understood that the lens of the 
present invention can be turned around. For example it can form a +(+-) 
stop -(+-) configuration, with a cemented doublet as a last component, and 
a negative lens element as a rear lens element of the cemented doublet 
component. 
The two group lens according to the present invention is not restricted to 
reader printer lenses, but could be usable in other applications, such as 
a copying lens. Similarly, it is not restricted to finite conjugate 
lenses, but could be adapted for use in other applications, such as a 
camera objective lens. It should be understood that principles of this 
invention apply to zoom lenses that have one or more moving elements or 
components. That is, it is possible to independently move sub-groups of 
the present invention to achieve a zoom lens. It is also possible to 
axially move the lens of the present invention in order to achieve other 
magnifications and to add one or more focusing components in the front or 
rear space to achieve a constant object-to-image distance. 
The invention has been described in detail with particular reference to a 
presently preferred embodiment, but it will be understood that variations 
and modifications can be effected within the spirit and scope of the 
invention.