Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
1999-11-08
2001-10-09
Lester, Evelyn A (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S650000, C359S651000, C359S648000, C359S715000, C359S716000, C359S753000, C359S782000, C359S784000, C348S781000
Reexamination Certificate
active
06301056
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to projection lens systems for use in projection televisions.
BACKGROUND OF THE INVENTION
There exists a need in the art for a projection lens system and, in particular, a rear projection lens system, that has some and preferably all of the following properties:
(1) The system is fully color corrected.
(2) The system is a high speed system, e.g., it has an infinite conjugate f/# of 1.0.
(3) The system has a wide field of view in the direction of the screen, e.g., a half field of view in the direction of the screen of at least 25°.
(4) The system produces a high quality image, e.g., the system has an MTF above 0.5 at 10 cycles/millimeter.
(5) The quality of the image is insensitive to changes in temperature of the lens system, e.g., changes between room temperature and the system's operating temperature.
(6) The system produces an image which is bright in the corners, e.g., the pupil area in the corner is at least 50% of the pupil area on axis.
(7) The system is compact, i.e., its lens elements are spaced close together so that the overall system has a short barrel length.
(8) The system is suitable for use with small diameter cathode ray tubes (CRTs), e.g., it can be used with 3-inch CRTs which have a diagonal of about 65 millimeters.
(9) The system is economical to manufacture.
SUMMARY OF THE INVENTION
To satisfy this need in the art, it is an object of the invention to provide projection lens systems which have some and preferably all of the above nine features.
To achieve this object, the invention provides a projection lens system for use in combination with a cathode ray tube that comprises the following lens units in order from the image (i.e., screen or long conjugate) side:
(a) a negative first lens unit (U
1
) which comprises: (i) at least one aspheric surface and (ii) a negative lens element (E
1
) at the unit's image side, said negative lens element having an image side surface (S
1
) which is concave towards the image;
(b) a positive second lens unit (U
2
) which comprises the following lens subunits in order from the image side:
(i) a positive first lens subunit (U
S2
) which consists of a cemented color correcting doublet composed of glass and having spherical surfaces; and
(ii) a positive second lens subunit (U
S2
) which comprises at least one lens element of weak optical power having at least one aspheric surface; and
(c) a third lens unit (U
3
) which has a strong negative power, is associated with the CRT during use of the lens system, and contributes to the correction of the lens system's field curvature.
Compared to the typical CRT projection lens, the U
3
unit has reduced negative power, i.e., the ratio of the magnitude of the power of the U
3
unit to the overall power of the lens system is relatively small, e.g., less than 0.8 and preferably less than 0.7. This reduction is accomplished through the use of concave surface S
1
on lens element E
1
which helps correct the field curvature of the lens. As a result, the magnitude of the power of the negative third lens unit (U
3
) can be reduced. This reduction in power allows the lens to collect more light from the CRT, especially from the corners of the CRT.
The first and second lens units (U
1
and U
2
) have a retrofocus, i.e., a negative-positive, configuration. This configuration allows the combination of these units to have a short focal length and a back focal length which is sufficiently long to accommodate the third lens unit at the CRT faceplate.
In certain preferred embodiments of the invention, all of the lens elements of U
1
, U
S2
, and U
3
are composed of plastic. Thus, for these embodiments, the projection lens system is composed of a cemented glass doublet and plastic lens elements on either side of the doublet. This configuration provides a number of advantages to the lens system.
First, it makes the system inexpensive to manufacture. As illustrated in the examples presented below, the plastic elements of the lens have configurations which allow them to be readily molded in plastic. As also illustrated in the examples, the doublet has spherical surfaces and a relatively small clear aperture diameter (e.g., a clear aperture diameter of less than 65 millimeters and preferably less than 60 millimeters) which allows it to be readily made in glass. Accordingly, each of the components of the system can be readily manufactured at low cost.
Second, the most critical portion of the lens in terms of its optical performance is the second lens unit's first subunit (U
S1
), i.e., the cemented glass doublet. Because of its size and configuration, this doublet can be made with high precision, again at low cost. Accordingly, the use of this doublet provides an effective way to achieve a high level of optical performance in an inexpensive lens.
Third, the plastic-glass-plastic construction of the lenses of the invention, with the glass portion being of strong power, makes the lenses insensitive to changes in temperature. Again, this thermal stability is achieved for a lens that has both a high level of optical performance and a low cost.
In other preferred embodiments, the lens system has some or all of the following characteristics:
(1) to provide a high level of aberration correction, U
1
and U
S2
each have two aspheric surfaces;
(2) again to provide a high level of aberration correction, surface S
1
is an aspheric surface;
(3) to aid in the correction of field curvature, |&PHgr;
E1
|/&PHgr;
0
is at least 0.15, preferably at least 0.2, and most preferably at least 0.25, where |&PHgr;
E1
| is the magnitude of the power of element E
1
and &PHgr;
0
is the overall power of the lens system;
(4) to provide the foregoing power level without unduly increasing the curvature of the S
1
surface, E
1
is preferably biconcave;
(5) to provide a compact lens, D
12
/f
0
(or, equivalently, D
12
·&PHgr;
0
) is less than or equal to 0.25, where D
12
is the distance between the object side surface of E
1
and the image side surface of U
S1
and f
0
is the effective focal length of the lens system (f
0
=1/&PHgr;
0
); and
(6) to aid in thermal compensation, U
S1
is a strong lens unit and the image side lens element (E
2
) of U
S2
has a positive power.
When the above preferred embodiments of the invention are used in combination, each of the nine desired features for a CRT projection lens system, listed above, are achieved by the projection lens systems of the invention.
As used herein, the term “weak” is used to describe an element or unit whose focal length has a magnitude which is at least about 2.5 times the effective focal length of the entire lens system, and the term “strong” is used to describe an element or unit whose focal length has a magnitude which is less than about 2.5 times the effective focal length of the entire lens system. Also, the effective focal length and power of the entire lens system, as well as the effective focal length and power of the third lens unit (&PHgr;
U3
), are determined for the lens system associated with the CRT.
REFERENCES:
patent: Re. 35310 (1996-08-01), Moskovich
patent: 4755028 (1988-07-01), Moskovich
patent: 4776681 (1988-10-01), Moskovich
patent: 4801196 (1989-01-01), Betensky
patent: 4815831 (1989-03-01), Betensky
patent: 4900139 (1990-02-01), Kreitzer
patent: 4976525 (1990-12-01), Matsumura et al.
patent: 5016994 (1991-05-01), Braat
patent: 5200814 (1993-04-01), Hirata et al.
patent: 5272540 (1993-12-01), Hirata et al.
patent: 5309283 (1994-05-01), Kreitzer
patent: 5329363 (1994-07-01), Moskovich
patent: 5455713 (1995-10-01), Kreitzer
patent: 5633757 (1997-05-01), Park
patent: 5808804 (1998-09-01), Moskovich
patent: 6141154 (2000-10-01), Kreitzer
patent: 833 179 A1 (1998-04-01), None
patent: WO97/41477 (1997-11-01), None
patent: WO99/03006 (1999-01-01), None
patent: WO99/67662 (1999-12-01), None
Corning Precision Lens
Klee Maurice M.
Lester Evelyn A
LandOfFree
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