Focusable, color corrected, high performance projection lens systems

Focusable, color corrected, high performance projection lens systems are provided which include three lens units (U1, U2, U3), the first lens unit being composed of two subunits (U.sub.S1, U.sub.S2). The lens systems can have f/190 's less than 1.0, total fields of as much as 90.degree., low vignetting losses, and MTF values greater than 0.5 at 10 cycles/mm through 0.85 of the full field. By varying the space between the first and second lens units, these performance levels can be maintained as the lens system is focused over a range of conjugates of approximately .+-.7.5% from a center value of about 4 meters. The lens systems can be used in such demanding applications as flight simulators.

FIELD OF THE INVENTION 
This invention relates to projection lens systems for use in projection 
television systems and, in particular, to focusable, color corrected, high 
performance projection lens systems for use in such applications as flight 
simulators. 
BACKGROUND OF THE INVENTION 
Projection lens systems for cathode ray tube (CRT) projection televisions 
have undergone continuing development during the past fifteen years or so. 
Examples of such systems can be found in Betensky, U.S. Pat. Nos. 
4,300,817, 4,348,081, 4,526,442, 4,697,892, and 4,801,196; Moskovich, U.S. 
Pat. Nos. 4,682,862, 4,755,028, and 4,776,681; and Toide, U.S. Pat. No. 
5,148,320. 
Color images for projection televisions are normally obtained by combining 
images from three color CRTs, i.e., a red CRT, a green CRT, and a blue 
CRT. The phosphors used in commercially available CRTs do not emit light 
at a single wavelength. In particular, green phosphors have significant 
sidebands in blue and red. Similar polychromaticity exists for red and 
blue phosphors, but to a lesser extent. 
For many consumer applications, lens systems uncorrected for color can be 
used, notwithstanding the color spread of the CRTs. For more demanding 
applications, however, such as high definition television, data displays, 
or systems which operate at a high magnification, color correction is 
needed to avoid visible color fringing and/or a loss of image contrast. 
Examples of projection lens systems which provide at least some color 
correction include Betensky, U.S. Pat. No. 4,815,831; Kaneko et al., U.S. 
Pat. No. 5,404,246; Kreitzer, U.S. Pat. Nos. 4,900,139, 5,309,283, and 
5,455,713; Moskovich, U.S. Pat. No. 4,963,007; Toide, U.S. Pat. No. 
5,130,850; and Yoshioka, U.S. Pat. No. 5,237,456. 
A particularly demanding application for projection televisions is in the 
area of simulators, e.g., flight simulators, where the goal is to produce 
an image which mimics real life as closely as possible. Performance 
requirements for such systems can include an f/# of less than 1.0, a total 
field in the direction of the image (screen) of as much as 90.degree., 
full color correction over the 465 to 610 nanometer range, and a 
modulation transfer function (MTF) greater than 50% at 10 
cycles/millimeter through 0.85 of the total field. 
In addition, to enhance the brightness of the image, it is desirable to 
achieve these performance characteristics for relatively large CRTs, e.g., 
CRTs which have a diagonal on the order of 160 millimeters. Along these 
same lines, it is desirable to minimize vignetting of the light passing 
through the projection lens system so as to increase the amount of light 
which reaches the viewing screen, e.g., it is desirable to keep vignetting 
losses below 30% at full field. As known in the art, vignetting can be 
used in the design of an optical system to remove off-axis rays which if 
allowed to reach the screen would degrade the quality of the image. 
Minimizing vignetting thus puts even higher demands on the basic 
performance of the lens system since with vignetting minimized, more rays 
reach the screen and thus must be corrected. 
In addition to these considerations, the projection lens system should be 
focusable over a range of conjugates, e.g., a range of approximately 
.+-.7.5% from a center value of about 4 meters. Such focusability provides 
flexibility in the types of applications in which the system can be used 
and in the set-up procedure for any particular installation. The lens 
system, of course, must continue to meet the above performance 
characteristics as it is focused over such a conjugate range. 
Although of high quality, the existing projection lens systems are not able 
to meet all of the above criteria. There thus exists a need in the art for 
an improved projection lens system which is capable of satisfying these 
criteria. It is an object of the present invention to provide such a lens 
system. 
SUMMARY OF THE INVENTION 
To achieve the above and other objects, the invention provides a projection 
lens system for use with a cathode ray tube, said lens system having an 
overall optical power .PHI..sub.0 and comprising in order from the 
system's image side (long conjugate side): 
(a) a first lens unit having an optical power .PHI..sub.1 and comprising in 
order from the system's image side a first lens subunit having an optical 
power .PHI..sub.S1 and a second lens subunit having an optical power 
.PHI..sub.S2 ; 
(b) a second lens unit having an optical power .PHI..sub.2 ; and 
(c) a third lens unit which has an optical power .PHI..sub.3, is associated 
with the cathode ray tube during use of the lens system, and provides 
correction to the field curvature of the lens system; 
wherein each of the first, second, and third lens units has at least one 
aspherical surface and wherein .PHI..sub.1, .PHI..sub.S1, .PHI..sub.S2, 
.PHI..sub.2, and .PHI..sub.3 satisfy the following conditions: 
(i) .PHI..sub.1 &gt;0; 
(ii) .PHI..sub.1 /.PHI..sub.0 &lt;0.3; 
(iii) .PHI..sub.S1 &lt;0; 
(iv) .vertline..PHI..sub.S1 .vertline..PHI..sub.0 &gt;0.4; 
(v) .PHI..sub.S2 &gt;0; 
(vi) .PHI..sub.S2 /.PHI..sub.0 &gt;0.3; 
(vii) .PHI..sub.2 &gt;0; 
(viii) .PHI..sub.2 /.PHI..sub.0 &gt;0.4; and 
(ix) .PHI..sub.3 &lt;0. 
In certain preferred embodiments, the first lens subunit comprises a 
bi-concave lens element made of glass. In other preferred embodiments, the 
first and second lens units are separated by an axial space which is 
varied during focusing of the lens system to stabilize the quality of the 
image.

The foregoing drawings, which are incorporated in and constitute part of 
the specification, illustrate preferred embodiments of the invention, and 
together with the description, serve to explain the principles of the 
invention. It is to be understood, of course, that both the drawings and 
the description are explanatory only and are not restrictive of the 
invention. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The lens systems of the invention preferably include a first lens unit 
(U1), a second lens unit (U2), and a third lens unit (U3) wherein: 1) the 
first lens unit includes a first lens subunit (U.sub.S1) having a 
relatively strong negative optical power and a second lens subunit having 
a relatively strong positive optical power (U.sub.S2); 2) the second lens 
unit provides axial color correction and has a strong positive optical 
power; and 3) the third lens unit corrects for the field curvature of the 
lens system and has a relatively strong negative optical power. 
Each lens unit and subunit can be composed of one or more lens elements. 
For example, as shown in FIGS. 1 and 2, the second lens subunit and the 
third lens unit each consist of a single lens element, while the first 
lens subunit and the second lens unit comprise multiple lens elements 
having both positive and negative powers. The positive and negative lens 
elements of the second lens unit have appropriate optical dispersions and 
powers to provide axial color correction for the lens system. 
As discussed above the first lens subunit preferably includes a biconcave 
lens element composed of glass. The use of such an element facilitates 
thermal compensation for the lens system (see below). 
Each lens unit includes at least one lens element which has at least one 
aspherical surface. Such aspherical lens elements are preferably composed 
of plastic materials, e.g., acrylic polymers. If desired, one of the 
aspherical plastic lens elements can include an absorptive color filter 
material in accordance with Wessling, U.S. Pat. No. 5,055,922. 
The use of plastic lens elements has the drawback that the refractive index 
of plastic optical materials changes significantly with temperature. 
Another effect is the change in shape, i.e., expansion or contraction, of 
such materials with temperature. This latter effect is usually less 
significant than the change in index of refraction. 
To address this problem and to also compensate for the thermal changes in 
the plastic or aluminum mechanical components of the system, e.g., the 
lens barrel which is usually the major mechanical source of 
thermally-caused focus changes, the lens systems of the invention are 
preferably athermalized so as to take into account the location and power 
of the plastic lens elements, as well as the marginal ray heights at those 
elements. 
The location of the plastic lens elements is significant in terms of the 
amount of temperature change the element will undergo and thus the amount 
of change which will occur in the element's index of refraction. In 
general, elements closer to the CRT undergo greater temperature changes. 
In practice, a temperature distribution in the region where the projection 
lens is to be located is measured with the CRT operating and those 
measured values are used in the design of the projection lens. The 
marginal ray height at a particular plastic lens element determines, for a 
given thermal change, whether changes in the element's index of refraction 
will be significant with regard to the overall thermal stability of the 
lens. Elements for which the marginal ray height is small will in general 
have less effect on the overall thermal stability of the system than 
elements for which the marginal ray height is large. 
Based on the foregoing considerations, athermalization of the projection 
lenses of the invention is achieved by: (1) using low powered plastic lens 
elements at locations where the marginal ray height is large and the 
temperature change is large, i.e., in the second lens unit; and (2) using 
high powered plastic lens elements at locations where either the marginal 
ray height or the temperature change is small, i.e., in the first lens 
subunit and the third lens unit. It is also desirable for the plastic 
elements to have a net positive power. 
The level of athermalization achieved is preferably optimized using a 
computerized lens design program as follows. First, a ray trace is 
performed at a first temperature distribution and a performance parameter, 
e.g., image distance, is calculated. The ray trace can be a paraxial ray 
trace for the marginal ray. Second, the same ray trace is performed at a 
second temperature distribution and the performance parameter is again 
calculated. Neither the first nor the second temperature distribution need 
be constant over the entire lens but can, and in the typical case does, 
vary from lens element to lens element. The calculated values of the 
performance parameter are then constrained to a constant value as the 
design of the system is optimized using the lens design program. 
It should be noted that the foregoing approach assumes that the mechanical 
mounts for the projection lens and the CRT hold the distance between the 
last lens surface and the CRT substantially constant as the temperature of 
the system changes. If such an assumption is not warranted, other 
provisions can be made for performing the athermalization, e.g., a 
measured value for the relative movement of the mechanical mounts can be 
included in the process or an alternate distance, e.g., the distance 
between the front lens surface and the CRT, can be assumed to be 
mechanically fixed. 
Focusing of the lens system for different conjugates is preferably achieved 
by moving the entire lens system relative to the CRT. The axial space 
between the first and second lens units is then adjusted to maintain the 
high performance levels of the lens. As can be seen in FIGS. 1 and 2, the 
marginal rays are substantially horizontal between the first and second 
lens units so that small variations in the length of this space will not 
substantially change the overall focus of the system. In this way, 
aberration fine tuning, e.g., astigmatism reduction, can be achieved by 
varying this space while leaving the overall focus of the system 
essentially unchanged. 
FIGS. 1 and 2 illustrate projection lenses constructed in accordance with 
the invention. Corresponding prescriptions appear in Tables 1 and 2, 
respectively. HOYA or SCHOTT designations are used for the glasses 
employed in the lens systems. Equivalent glasses made by other 
manufacturers can be used in the practice of the invention. Industry 
acceptable materials are used for the plastic elements. 
The aspheric coefficients set forth in the tables are for use in the 
following equation: 
##EQU1## 
where z is the surface sag at a distance y from the optical axis of the 
system, c is the curvature of the lens at the optical axis, and k is a 
conic constant, which is zero except where indicated in the prescriptions 
of Tables 1 and 2. 
The designation "a" associated with various surfaces in the tables 
represents an aspheric surface, i.e., a surface for which at least one of 
D, E, F, G, H, or I in the above equation is not zero. The designation "c" 
represents a conic surface, i.e., a surface for which k in the above 
equation is not zero. All dimensions given in the tables are in 
millimeters. The tables are constructed on the assumption that light 
travels from left to right in the figures. In actual practice, the viewing 
screen will be on the left and the CRT will be on the right, and light 
will travel from right to left. 
In Tables 1 and 2, the first lens unit comprises surfaces 1-9, the second 
lens unit comprises surfaces 10-18, the third lens unit comprises surfaces 
19-20, and the CRT comprises surfaces 21-24, with the object which is 
projected onto the screen being the inner surface of the CRT faceplate. 
The first and second subunits of the first lens unit comprise surfaces 1-6 
and 8-9, respectively, in both tables. The material designations 420550 
and 539570 set forth in the tables represent the index of refraction and 
dispersion characteristics of a cover plate and fluid layer applied to the 
CRT's faceplate. Specifically, a N.sub.e value for the material is 
obtained by adding 1,000 to the first three digits of the designation, and 
a V.sub.e value is obtained from the last three digits by placing a 
decimal point before the last digit. 
Table 3 summarizes the powers of the various lens units and subunits of the 
lens systems of FIGS. 1 and 2, and Table 4 summarizes their performance 
properties. As can be seen in these tables, lens systems have the 
structure defined by conditions (i) through (ix) above have the f/#, total 
field, vignetting, and MTF properties needed for use in simulator 
applications. Note that the f/# values given in Table 4 are for finite 
conjugates; for a long conjugate of infinite length, the f/# of both the 
lens system of Table 1 and that of Table 2 is less than 1.0. 
In addition to the properties listed in Table 4, the lens systems of FIGS. 
1 and 2 are also fully color corrected over the 465-610 nanometer range. 
Moreover, these lens systems can be focused over a range of conjugates of 
approximately .+-.7.5% from a center value of about 4 meters while 
maintaining the required high performance levels for simulator 
applications (see Tables 1 and 2 for focus ranges). 
FIG. 3 is a schematic diagram of a front projection television system 10 
constructed in accordance with the invention. In this figure, module 13 
schematically illustrates a projection lens system constructed in 
accordance with the invention and module 16 schematically illustrates its 
associated CRT tube. In practice, three lens systems 13 and three CRT 
tubes 16 are used to project red, green, and blue images onto screen 14. 
Although specific embodiments of the invention have been described and 
illustrated, it is to be understood that a variety of modifications which 
do not depart from the scope and spirit of the invention will be evident 
to persons of ordinary skill in the art from the foregoing disclosure. 
TABLE 1 
__________________________________________________________________________ 
Surf. Clear Aperture 
No. Type Radius Thickness Glass Diameter 
__________________________________________________________________________ 
1 a 2901.1860 15.00000 ACRYLIC 238.00 
2 ac 209.1157 60.46000 184.25 
3 -296.4704 13.00000 FC5 174.26 
4 296.4704 10.00000 161.54 
5 ac 392.6675 16.00000 ACRYLIC 161.29 
6 a 540.9802 67.72000 161.88 
7 Aperture stop 1.91000 209.12 
8 3148.5892 41.00000 BACD18 213.21 
9 -189.1711 Space 1 216.25 
10 287.1786 42.50000 BACD18 226.61 
11 -473.3888 0.39000 225.40 
12 233.4301 44.00000 BACD5 206.27 
13 -400.1245 0.05000 201.84 
14 -398.7020 9.00000 FD6 201.83 
15 135.8890 53.87000 BSC7 178.59 
16 -287.2871 11.20000 177.52 
17 a -408.5138 17.50000 ACRYLIC 174.84 
18 ac -233.0149 64.47000 178.30 
19 a -119.5886 6.79000 ACRYLIC 152.00 
20 -29999.9997 Space 2 157.00 
21 .infin. 7.00000 BAC1 161.00 
22 .infin. 4.30000 420550 161.00 
23 .infin. 7.00000 539570 161.00 
24 .infin. 0.0 161.00 
__________________________________________________________________________ 
Symbol Description 
__________________________________________________________________________ 
a 
Polynomial asphere 
c 
Conic section 
__________________________________________________________________________ 
Object and Image Surface 
Surface 
Radius 
__________________________________________________________________________ 
Image 3657.00 
__________________________________________________________________________ 
Conics 
Surface 
Number Constant 
__________________________________________________________________________ 
2 4.0000E+00 
5 -2.5000E+01 
18 -6.1068E+00 
__________________________________________________________________________ 
Even Polynomial Aspheres 
Surf. 
No. D E F 0 H I 
__________________________________________________________________________ 
1 1.5357E-07 -1.1470E-11 5.4733E-16 1.8080E-20 -2.4692E-24 7.0415E-2 
9 
2 1.0916E-07 -9.1896E-12 -4.4716E-16 7.3390E-20 4.1308E-24 -6.1243E-2 
8 
5 -5.5549E-08 -2.0289E-11 1.1519E-15 2.4383E-19 -1.9504E-23 1.3099E-2 
7 
6 6.9500E-08 -1.1849E-11 1.1142E-15 1.0660E-19 -3.1801E-24 -3.9466E-2 
8 
17 -8.0884E-08 -1.2768E-11 -5.5910E-16 -1.8983E-20 4.4793E-24 
5.0401E-28 
18 -5.2904E-08 -1.2418E-11 -9.2265E-16 6.0394E-20 5.7518E-24 -9.2069E- 
29 
19 7.6271E-08 4.4580E-11 -3.4805E-14 9.6328E-18 -1.1505E-21 
5.2675E-26 
__________________________________________________________________________ 
Variable Spaces 
Space 1 
Space 2 
Pos. T(9) T(20) 
__________________________________________________________________________ 
1 29.300 2.520 
2 30.300 3.000 
__________________________________________________________________________ 
First-Order Data Pos. 1 
Pos. 2 
__________________________________________________________________________ 
f/number 0.99 0.99 
Magnification -0.0270 -0.0320 
Object Height -2664.0 -2345.0 
Object Distance -3876.02 -3264.13 
Effective Focal Length 105.815 105.858 
Overall Length 4401.00 3790.58 
Forward Vertex Distance 524.985 526.455 
Barrel Length 524.980 526.460 
Stop Surface Number 7 7 
Distance to Stop 0.00 0.00 
Stop Diameter 207.93 209.05 
Entrance Pupil Distance 116.310 116.310 
Exit Pupil Distance -168.605 -169.832 
__________________________________________________________________________ 
First Order Properties of Elements 
Element Surface 
Number Numbers Power f' 
__________________________________________________________________________ 
1 1 2 -0.21870E-02 
-457.24 
2 3 4 -0.33235E-02 -300.88 
3 5 6 0.35704E-03 2800.8 
4 8 9 0.35764E-02 279.61 
5 10 11 0.35094E-02 284.95 
6 12 13 0.39082E-02 255.87 
7 14 15 -0.80789E-02 -123.78 
8 15 16 0.53783E-02 185.93 
9 17 18 0.94037E-03 1063.4 
10 19 20 -0.41122E-02 -243.18 
__________________________________________________________________________ 
First-Order Properties of Doublets 
Element 
Surface 
Numbers Numbers Power f' 
__________________________________________________________________________ 
7 8 14 16 -0.21287E-02 -469.76 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
Surf. Clear Aperture 
No. Type Radius Thickness Glass Diameter 
__________________________________________________________________________ 
1 a 23000.0007 16.00000 ACRYLIC 221.00 
2 ac 244.4418 43.00000 186.00 
3 -489.7119 14.00000 FC5 183.82 
4 489.7119 8.90000 180.18 
5 ac 341.6551 16.00000 ACRYLIC 180.27 
6 a 460.3655 62.55000 176.99 
7 Aperture stop 1.70000 188.27 
8 1215.3080 32.00000 BACD18 193.65 
9 -225.2786 Space 1 196.47 
10 232.9944 42.00000 BACD18 214.80 
11 -571.8435 0.48000 214.60 
12 347.0329 32.80000 BACD5 204.60 
13 -429.4207 0.04000 202.10 
14 -428.1570 11.00000 FD6 202.09 
15 163.6727 48.00000 BSC7 184.47 
16 -314.1584 7.00000 183.55 
17 a -468.2473 17.00000 ACRYLIC 179.73 
18 ac -322.7600 79.09586 180.15 
19 a -101.3095 5.00000 ACRYLIC 147.00 
20 -2731.2480 Space 2 156.00 
21 .infin. 7.00000 BAC1 161.00 
22 .infin. 4.30000 420550 161.00 
23 .infin. 7.00000 539570 161.00 
24 .infin. 0.0 162.00 
__________________________________________________________________________ 
Symbol Description 
__________________________________________________________________________ 
a 
Polynomial asphere 
c 
Conic section 
__________________________________________________________________________ 
Object and Image Surface 
Surface 
Radius 
__________________________________________________________________________ 
Image 3657.00 
__________________________________________________________________________ 
Conics 
Surface 
Number Constant 
__________________________________________________________________________ 
2 4.0000E+00 
5 -2.5000E+01 
18 -1.2000E+01 
__________________________________________________________________________ 
Even Polynomial Aspheres 
Surf. 
No. D E F G H I 
__________________________________________________________________________ 
1 9.6035E-08 -5.4951E-12 1.7300E-16 4.2606E-21 -1.9314E-24 7.5004E-2 
9 
2 8.9656E-08 5.5927E-13 -2.4936E-16 4.5669E-20 4.1155E-24 -5.9595E-2 
8 
5 -2.2151E-08 -3.6015E-12 8.1074E-16 6.5546E-20 -6.6674E-24 3.4103E-3 
0 
6 1.4294E-08 1.3153E-12 4.7500E-16 5.7685E-21 -1.1578E-25 -2.1265E-2 
8 
17 1.0488E-08 -1.0919E-11 -2.5443E-16 -3.5170E-21 1.4947E-24 
7.7832E-29 
18 1.6872E-08 -8.6191E-12 -6.0153E-16 1.5494E-20 4.4859E-24 -1.7395E- 
28 
19 9.1806E-08 8.9085E-12 -7.4104E-15 1.5734E-18 -8.9131E-23 -3.3942E- 
29 
__________________________________________________________________________ 
Variable Spaces 
Space 1 
Space 2 
Pos. T(9) T(20) 
__________________________________________________________________________ 
1 22.000 3.520 
2 20.400 2.940 
__________________________________________________________________________ 
First-Order Data Pos. 1 
Pos. 2 
__________________________________________________________________________ 
f/number 1.13 1.13 
Magnification -0.0367 -0.0320 
Object Height -2100.0 -2345.0 
Object Distance -3603.7 -4120.6 
Effective Focal Length 131.64 131.51 
Overall Length 4084.1 4598.8 
Forward Vertex Distance 480.39 478.21 
Barrel Length 480.39 478.21 
Stop Surface Number 7 7 
Distance to Stop 0.00 0.00 
Stop Diameter 188.100 186.904 
Entrance Pupil Distance 110.36 110.36 
Exit Pupil Distance -140.79 -139.46 
__________________________________________________________________________ 
First Order Properties of Elements 
Element Surface 
Number Numbers Power f' 
__________________________________________________________________________ 
1 1 2 -0.19981E-02 
-500.48 
2 3 4 -0.20071E-02 -498.24 
3 5 6 0.38928E-03 2568.9 
4 8 9 0.33450E-02 298.95 
5 10 11 0.37948E-02 263.52 
6 12 13 0.30331E-02 329.70 
7 14 15 -0.69202E-02 -144.50 
8 15 16 0.46550E-02 214.82 
9 17 18 0.49370E-03 2025.5 
10 19 20 -0.46902E-02 -213.21 
__________________________________________________________________________ 
First-Order Properties of Doublets 
Element 
Surface 
Numbers Numbers Power f' 
__________________________________________________________________________ 
7 8 14 16 -0.18505E-02 -540.41 
__________________________________________________________________________ 
TABLE 3 
______________________________________ 
Example No. 
.PHI..sub.0 
.PHI..sub.1 
.PHI..sub.2 
.PHI..sub.3 
.PHI..sub.S1 
.PHI..sub.S2 
______________________________________ 
1 0.0095 0.0008 0.0053 
-0.0041 
-0.0056 
0.0036 
2 0.0076 0.0012 0.0049 -0.0047 -0.0038 0.0033 
______________________________________ 
TABLE 4 
______________________________________ 
Example 1 Example 2 
______________________________________ 
f/number.sup.1 0.99 1.13 
Total Field 90.degree. 70.degree. 
Vignetting Losses 29% 22% 
MTF.sup.2 &gt;0.5 &gt;0.6 
______________________________________ 
.sup.1 at the CRT, finite conjugates 
.sup.2 at 10 cycles/mm through 0.85 of full field