Projection lens and method of using same

A new and improved projector lens arrangement which can automatically adjusted for focusing purposes in an easily and convenient manner according to a novel focusing method of the present invention. The projection lens arrangement generally includes three groups of optical elements aligned along a common optical axis with a variable vertex length and wide field coverage angle. One element group near the object is a doublet having a negative element with a concave surface and a positive element being bi-convex having one surface near the image complementarily shaped to the concave surface of the negative element.

TECHNICAL FIELD 
The present invention relates in general to an improved lens arrangement 
and method of using it. The invention more particularly relates to a 
projection lens arrangement which may be used to facilitate focusing a 
projected image on a remote viewing surface. 
BACKGROUND ART 
Projection lens arrangements for focusing a projected image on a remote 
viewing surface are well known in the prior art. Such lens arrangements 
include those utilized with front and overhead projectors, and still and 
motion picture video projectors. 
For example, consider the projection lens arrangement in a conventional 
overhead projector. In such a projector, the lens is mounted above and 
spaced-apart from the stage of the projector. A transparency or computer 
controlled liquid crystal panel for providing an image to be projected is 
positioned on the stage. The distance between the transparency or object 
and the entranceway to the projection lens is referred to as the object 
length and is about 15 inches in length in some overhead projectors. A 
Fresnel lens arrangement causes light, emitted from a high intensity lamp 
disposed below the stage, to be directed upwardly into the projection lens 
at an angle. This angle is called the field coverage angle and is about 18 
degrees. For the purpose of focusing the image to be projected onto a 
remote viewing surface, the overall length of the projection lens 
arrangement is adjustable. This overall length is referred to as the 
vertex length of the lens arrangement. 
While the above-described projection lens arrangement has proven 
satisfactory in large bulky overhead projectors, such an arrangement can 
not be readily used in a small compact projector system, such as a compact 
projector system disclosed in copending U.S. patent application Ser. No. 
08/059,550. 
In the case of a small compact projector, the object length must be 
substantially shorter and thus, the field coverage angle must be 
substantially greater. However, by increasing the field coverage angle 
various aberrations can be introduced, such as field curvature aberrations 
and other types of known aberrations. 
Therefore, it would be highly desirable to have a new and improved 
projection lens arrangement and method of using the arrangement which can 
be used readily in a small compact projector system. Such a new and 
improved projection lens arrangement would have a relatively short object 
length but yet a sufficiently narrow field coverage angle to enable 
optical compensation for eliminating or at least substantially reducing 
the effect of optical aberrations such as field curvature aberrations. 
In order to focus a variety of different sized images to be projected onto 
a remote viewing surface, a projection lens arrangement must be variable 
for focusing purposes. In this regard, the vertex length of the lens 
arrangement must be variable but yet sufficiently small to enable the lens 
arrangement to be utilized in a small compact projector system. 
However, shortening the vertex length introduces other problems. For 
example, by shortening the vertex length it is difficult, if not 
impossible to have sufficient variations to reach substantially all 
anticipated field coverage angles when the arrangement employs a 
relatively short object length. 
Therefore, it would be highly desirable to have a new and improved 
projection lens arrangement that has both a relatively small variable 
vertex length and object length to enable the lens to be utilized in a 
small compact projector but yet a sufficiently long vertex length to 
permit focusing for substantially all anticipated field coverage angles. 
Another problem associated with a lens arrangement having a short vertex 
length is that the spacing between the optical elements within the lens 
arrangement must necessarily be very short in distance. Thus, in order to 
reach substantially all anticipated field coverage angles in a relatively 
convenient manner, the focusing adjustments must be very precise and 
accurate. 
Therefore, it would be highly desirable to have a new and improved 
projection lens arrangement which can be easily and automatically adjusted 
to focus an image on a remote viewing surface. Such a lens arrangement 
should be easily adjusted for focusing purposes, and relatively 
inexpensive to manufacture. 
DISCLOSURE OF INVENTION 
Therefore, the principal object of the present invention is to provide a 
new and improved projection lens arrangement and method of using the 
arrangement which can be used readily in a small compact projector that is 
easily transportable. 
Another object of the present invention is to provide such a new and 
improved projection lens arrangement that has a relatively short effective 
focal length but yet a sufficiently narrow field coverage angle to enable 
optical compensation for eliminating or at least substantially reducing 
the effect of optical aberrations, such as field curvature aberrations and 
other known aberrations. 
Yet another object of the present invention is to provide such a new and 
improved projection lens arrangement that has both a relatively small 
variable vertex length and object length to enable the lens to be utilized 
in a small compact projector but yet a sufficiently long vertex length to 
permit focusing for substantially all anticipated field coverage angles. 
A further object of the present invention is to provide a new and improved 
projection lens arrangement which can be easily and automatically adjusted 
to focus an image on a remote viewing surface. Such a lens arrangement 
should be easily adjusted for focusing purposes, and relatively 
inexpensive to manufacture. 
Briefly, the above and further objects of the present invention are 
realized by providing a new and improved projector lens arrangement which 
has a relatively short object length, a sufficiently wide field coverage 
angle, and which can automatically be adjusted for focusing purposes in an 
easily and convenient manner according to a novel focusing method of the 
present invention. 
The projection lens arrangement is configured in a Tessar configuration 
having generally three groups of optical elements aligned along a common 
optical axis with a variable vertex length and field coverage angle of up 
to about 22.1 degrees. At least one of the element surfaces are aspheric. 
One element group near the object is a doublet having a negative element 
with a concave surface and having a positive element, which is bi-convex 
and has one surface near the image, the surface being complementary shaped 
to the concave surface of the negative element.

BEST MODE FOR CARRYING OUT THE INVENTION 
Referring now to the drawings and more particularly to FIG. 1 thereof, 
there is shown a projection lens system 10 which is constructed in 
accordance with the present invention. The projection lens system 10 is 
illustrated with a liquid crystal projector 12 and in accordance with the 
method of the present invention can cause a liquid crystal image to be 
focused on a remote viewing surface, such as a remote viewing surface 16. 
The projection lens system 10 generally comprises a projection lens 
arrangement 20 having a Tessar configuration, variable vertex length and a 
wide field coverage angle. The lens arrangement 20 is coupled mechanically 
to a servo system 22 for adjusting the focal length of the lens 
arrangement 20 to cause a projected liquid crystal image to be focused on 
the remote viewing surface 16. 
The projection lens arrangement 20 generally includes three groups G1, G2 
and G3 (FIG. 1) of lens elements arranged along a common optical path P 
from an object end .phi. to an image end I of the lens arrangement 20. The 
lens arrangement 20 is disposed between an object surface S9 via a mirror 
surface S1A and an image surface S10A. The first group, said second group 
and said third group having respective optical powers K1, K2 and K3, with 
an overall optical power of about 0.0037 inverse millimeter. The optical 
power K1 is about 0.00825 inverse millimeter. The optical power K2 is 
about -0.01365 inverse millimeter. The optical power K3 is about 0.00783 
inverse millimeter. 
The back focal length between the back vertex of the lens arrangement 20 
and the object surface S1A is about twelve inches or about 254.6 
millimeters. The object surface S1A is generally rectangular in shape 
having a corner to corner diagonal length of about 8.4 inches or about 
106.68 millimeters. Based on the foregoing, those skilled in the art will 
understand the effective focal length of the lens arrangement is between 
about 10.24 inches or about 260.86 millimeter and about 11.00 inches or 
about 280.01 millimeters. 
In order to reach full field coverage of the object with good resolution, 
the lens arrangement 20 has a field coverage angle of up to about 22.1 
degrees. In this regard, the resolution of the projection lens arrangement 
20 is about 6 line pairs per millimeter. 
The vertex length of the projection lens arrangement 20 is about 1.81 
inches or about 46.22 millimeters. The vertex length is adjustable and has 
an adjustment range between a short length of about 1.497 inches or about 
38.02 millimeters and a full length of about 1.81 inches or about 46.22 
millimeters. The aperture or speed of the projection lens arrangement 20 
is about f/5. 
In order to identify the sequence positioning of groups G1, G2 and G3 from 
the object end .phi. to the image end I, the lens elements are designated 
in their sequential position as L1-L4. Groups G1 and G2 comprise the 
inventive projection lens. Lens L4 is a Fresnel lens. Also, in order to 
identify the sequence positioning of the lens element surfaces, the 
surfaces are designated in their sequential positions as S8-S2 from the 
object end .phi. to the image end I of the lens arrangement 20. 
Considering now group G1 in greater detail with reference to FIG. 1, group 
G1 is configured in a doublet arrangement including the lens elements L1 
and L2 respectively. Lens elements L1 and L2 cooperate together to provide 
positive optical power where lens element L2 counter corrects lens 
aberrations introduced by lens element L1. 
Considering now lens element L1 in greater detail with reference to FIG. 1, 
surface S7A is complementary to surface S7 of lens element L2 to permit 
the two lens elements L1 and L2 to be contiguous along their respective 
surfaces S7A, S7. The radius of curvature of surface S7A of lens L1 is 
identical to surface S2 of lens L4. In this regard, only a single test 
plate (not shown) is required to verify the curvature of lens L1 and L4. 
Lens L1 and L3 introduce undercorrected spherical aberration and 
astigmatism, as well as positive field curvature. 
Considering now lens element L2 in greater detail with reference to FIG. 1, 
surface S5 of lens element L2 is generally plano while surface S4 of lens 
element L2 is generally concave. As noted earlier, surface S4 is 
complementary to surface S3 of lens element L1. The function of lens 
element L2 is to balance the aberration of lens L1 and L3 by introducing 
overcorrected spherical aberration and astigmatism, as well as negative 
field curvature. 
Considering now group G2 in greater detail with reference to FIG. 1, group 
G2 includes a single lens element L3, having a lens stop LS. Lens element 
L3 is a bi-concave element of negative optical power for counter 
correcting lens aberration introduced by lens elements L1 and L2. 
Lens element L3 includes two surfaces S5 and S4 respectively, where each of 
the surfaces S5 and S4 are generally concave. The distance between surface 
S4 of lens element L3 and surface S3 of lens group G3 is variable. 
Considering now group G3 in greater detail with reference to FIG. 1, group 
G3 includes a single lens element L4 of positive optical power. The 
function of lens element L4 is to relay the height output from the 
projection lens groups G1 and G2. 
As best seen in FIG. 1, lens element L4 includes two surfaces S8 and S9. 
Lens surface S3 of lens element L4 is generally plano. The distance 
between surface S3 of lens element L4 and surface S4 of lens element L3 is 
variable as lens element L4 is mounted movably relative to lens element 
L3. In this regard, the servo system 22 enables the lens element L4 to be 
moved rectilinearly along a track 26 by about 0.313 inches or about 8.20 
millimeters. 
As will be made apparent from the examples that follow in Table I, the 
aspherical surface S10 may be defined by the following equation: 
##EQU1## 
Those skilled in the art will understand that X is a surface sag from the 
semi-aperture distance y from the axis or optical path P; that C is the 
curvature of a lens surface of the optical axis P equal to the reciprocal 
of the radius of the optical axis P; and that K is a conic constant (cc) 
or other surface of revolution. 
The following example in Tables I is an exemplary of the lens arrangement 
20 embodying the present invention and which is useful primarily for 
projecting a full color liquid crystal image color corrected. The lens 
arrangement of Table I has aspheric surface S10 defined by the foregoing 
aspheric equation. In the table, the surface radius for each surface, such 
as surface S2, is the radius at the optical axis P, N.sub.d is the index 
of refraction, and V.sub.d is the Abbe number. Positive surface radii are 
struck from the right and negative radii are struck from the left. The 
object is to the left at surface S1 of a liquid crystal display panel 24. 
TABLE I 
______________________________________ 
A lens as shown in FIG. 1 scaled for a 5.6 foot conjugate; 
object distance of 1706.00000 mm; object height of - 
700.000000; and entrance pupil radius of 17.66231. 
Axial 
Distance 
Lens Between Aperture 
Ele. Surf. Radius Surfaces 
Radius Element 
No. Desig. (mm) (mm) (mm) Comp. 
______________________________________ 
En- -17.09756 
17.66231 K 
AIR 
trance 
Pupil 
S2 73.82133 7.50184 26.00000 K 
BAK1 
L4 
S3 -- 10.27072 V 
26.00000 K 
AIR 
1112.99810 
S4 -99.73322 2.69314 24.50000 A 
LF5 
L3 
S5 75.04693 8.70928 24.50000 
AIR 
S6 -274.05990 
2.81867 24.50000 K 
KF6 
L2 
S7 62.88152 9.99902 24.50000 K 
SK2 
L1 
S8 -73.82133 289.33000 
24.50000 K 
AIR 
24 
S9 -- 3.98780 124.71569 S 
ACRYL- 
IC 
S10 -46.72718 10.49020 
132.00000 
AIR 
______________________________________ 
Lens Ele- 
Ele- ment Refractive Indices (N.sub.d) 
ment Comp. RN1/RN4 RN2/RN5 RN3/RN6 VNBR 
______________________________________ 
AIR -- -- -- -- 
L4 BAK1 1.57250 1.57943 1.56949 57.54848 
AIR -- -- -- -- 
L3 LF5 1.58144 1.59146 1.57723 40.85149 
1.59964 -- -- -- 
AIR -- -- -- -- 
KF6 1.51742 1.52434 1.51443 52.19566 
1.52984 -- -- -- 
L2 SK2 1.60738 1.61486 1.60414 56.65632 
1.62073 -- -- -- 
AIR -- -- -- -- 
24 ACRYL- 1.49177 1.49799 1.48901 56.01934 
IC 1.50377 -- -- -- 
AIR -- -- -- -- 
______________________________________ 
Aspheric parameters of S10 
______________________________________ 
CC -1.01435 
P1 0.00711 
P2 -2.6576 .times. 10.sup.-8 
P3 4.1592 .times. 10.sup.-14 
P4 1.5503 .times. 10.sup.-17 
______________________________________ 
Referring now to FIGS. 2A-2C there is illustrated the ray displacement 
caused by the lens arrangement 20. FIG. 2A illustrates ray displacement 
where the FOB is about 1.0 and a 5.6 foot conjugate. In this regard, a 
pair of displacement curves 302 and 303 illustrates the ray displacement 
when the image wavelength is about 0.588 microns. Other pairs of ray 
displacement curves are illustrated for different image wavelengths such 
as a pair of displacement curves 304 and 305 illustrate the ray 
displacement when the image wavelength is about 0.486 microns; a pair of 
displacement curves 306 and 307 illustrate the ray displacement when the 
image wavelength is about 0.656 microns; and a pair of displacement curves 
308 and 309 illustrate the ray displacement when the image wavelength is 
about 0.436 microns. 
FIG. 2B is similar to FIG. 2A except the FOB is about 0.7. The pairs of ray 
displacement curves for wavelengths of 0.588; 0.486; 0.656; and 0.436 are 
312,313; 314,315; 316,317; and 318,319, respectively. 
FIG. 2C is similar to FIGS. 2A and 2B except the FOB is about 0.0. The 
pairs of ray displacement curves for wavelengths of 0.588; 0.486; 09.656; 
and 0.436 are 322,323; 324,325; 326,327; and 328,329 respectively. 
FIGS. 3A-3C and 4A-4C are similar to FIGS. 2A-2C and illustrate pairs of 
displacement curves for wavelengths of 0.588; 0.486; 0.656 and 0.436 
relative to different FOB of 1.0, 0.7 and 0 respectively. In order to 
identify curve pairs in FIGS. 3A-3C and 4A-4C as described in FIGS. 2A-2C 
the first character reference number identifying the curves in FIGS. 3A-3C 
and 4A-4C have been sequentially increased. For example, a curve pair 402 
and 403 correspond in description to the curve pair 302 and 303. Based on 
the foregoing, no further description will be provided for the 4.0 fast 
conjugate curves 402-409; 412-429; 422-429; and the 10.0 foot conjugate 
curves 502-509; 512-519; and 522-529. 
Referring now to FIGS. 5A-5C; FIGS. 6A-6C and FIG. 7A-7C there is 
illustrated astigmatism, distortion and lateral color curves for the lens 
arrangement examples having the 4.0 foot conjugate, 5.6 foot conjugate and 
10 foot conjugate respectively. The respective astigmatism, distortion and 
lateral color curves are identified as 601; 602; 603; 604 and 605 for the 
4.0 foot conjugate, 701; 702; 703; 704 and 705 for the 5.6 foot conjugate, 
and 801; 802; 803; 804 and 805 for the 10.0 foot conjugate. 
Referring now to FIG. 8 there is illustrated a series of modulation 
transfer function curves 901-905 of the lens arrangement example having 
the 4.0 foot conjugate. Each curve depicted illustrates the modulation as 
a function of frequency (cycles per millimeter). 
FIGS. 9 and 10 are similar to FIG. 8 and illustrate a series of modulation 
transfer function curves 1001-1005 and 1100-1105 respective for the lens 
arrangement examples having 5.6 and 10.0 foot conjugates respectively. 
While particular embodiments of the present invention have been disclosed, 
it is to be understood that various different modifications are possible 
and are contemplated within the true spirit and scope of the appended 
claims. There is no intention, therefore, of limitations to the exact 
abstract or disclosure herein presented.