Optical: systems and elements – Lens – Reverse telephoto
Reexamination Certificate
2000-06-21
2002-11-05
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
Reverse telephoto
C359S749000, C359S649000
Reexamination Certificate
active
06476981
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a retrofocus lens system that is suitable for a projection optical system which requires a long back focal length in comparison with a focal distance and a projection display apparatus incorporating the retrofocus lens system.
FIG. 49
is a schematic diagram showing a configuration of an optical system of a conventional projection display apparatus (i.e., a liquid crystal projector). As shown in
FIG. 49
, the projection display apparatus
300
comprises a light source
1
which includes a lamp
120
and a reflecting mirror
130
and emits approximately parallel illuminating light
2
, dichroic mirrors
3
B and
3
G, and light reflection mirrors
4
a
,
4
b
, and
4
c
. The projection display apparatus
300
further comprises a transmissive liquid crystal panel
5
R for displaying a red image, a transmissive liquid crystal panel
5
G for displaying a green image, a transmissive liquid crystal panel
5
B for displaying a blue image, a dichroic prism
6
which outputs combined light
20
of red (R), green (G), and blue (B) by reflecting the red light
2
R and blue light
2
B and passing the green light
2
G, and a projection lens
7
for projecting incident light
20
onto a screen
8
at magnification. In the figure, a reference numeral
200
denotes a housing.
The dichroic mirror
3
B reflects the blue light
2
B from among the primary colors of the light
2
emitted from the light source
1
and passes the red light
2
R and green light
2
G. The blue light
2
B reflected from the dichroic mirror
3
B is reflected by the mirror
4
b
, passes the liquid crystal panel
5
B, and then enters the dichroic prism
6
. The dichroic mirror
3
G reflects the green light
2
G coming through the dichroic mirror
3
B and passes the red light
2
R. The green light
2
G reflected from the dichroic mirror
3
G passes the liquid crystal panel
5
G and enters the dichroic prism
6
. The red light
2
R passing the dichroic mirror
3
B is reflected by the mirrors
4
a
and
4
c
, passes the liquid crystal panel
5
R, and enters the dichroic prism
6
. The dichroic prism
6
sends out the combined light
20
of the incident red light
2
R, green light
2
G, and blue light
2
B toward the projection lens
7
. The projection lens
7
projects the combined light
20
onto the screen
8
at magnification.
In the projection display apparatus described above, the thick dichroic prism
6
must be disposed between the projection lens
7
and the liquid crystal panels
5
R,
5
G, and
5
B functioning as light valve components, which are picture sources, so that the projection lens
7
requires a long back focal length.
If the projection display apparatus described above is used in a rear projector (a rear projection display apparatus), it is preferable that the distance between the projection lens
7
and the screen
8
is short (that is, the projection lens
7
has a wide angle of view), in order to reduce the outer dimensions of the apparatus.
Because the spectral transmittance, polarization generation characteristics, and reflectivity of the dichroic prism
6
greatly vary with the incident angle of the light, the design is provided so that the illuminating light striking the liquid crystal panels
5
R,
5
G, and
5
B become approximately parallel light (that is, telecentric illumination is provided). In this case, the light striking the projection lens
7
are approximately parallel light. If this type of optical system uses a conventional wide-angle projection lens having a short back focal length, the light that passes the perimeter of the liquid crystal panels
5
R,
5
G, and
5
B and then strike the projection lens
7
are extremely reduced, making the projected picture dark in the perimeter of the screen
8
. Accordingly, it is desired that the apparatus is configured to make the principal ray of the light coming from the individual points of the picture source approximately parallel to the optical axis of the projection lens
7
(telecentric configuration). This configuration requires such a projection lens that the distance between the projection lens
7
and the position of the pupil is sufficiently greater than the focal distance.
As has been described above, a projection lens used in a projection display apparatus is required to satisfy the basic specifications associated with (1) a wide angle of view, (2) a great back focal length, and (3) telecentric characteristics on the image display component side. The projection lens of the projection display apparatus is also required to have basic aberration characteristics as described under (4) to (7) below.
(4) Low chromatic aberration: The magnification chromatic aberration must be representatively kept around the pixel pitch or preferably suppressed below a half of the pixel pitch, so that the projection magnification difference in primary-color pixels of the projected image is sufficiently reduced. The vertical chromatic aberration must be held low so that the focal points of the individual primary colors match.
(5) Low distortion: Since a wide-angle lens for rear projector projects a rectangular projection image inside the frame of the projection screen, the distortion around the perimeter of the screen often stands out. Accordingly, the deviation of a pixel from its ideal point because of distortion must be kept representatively around the pixel pitch. In rear projectors for use in CAD, multi-surface projectors that increase the number of pixels by arranging unit screens formed by rear projection, and the like, it is required to control the distortion so that the absolute deviation from an ideal point is kept to or below a half of the pixel pitch.
(6) Wide operating temperature range: The projection lens should be designed to maintain desired optical characteristics over a wide temperature range, so that the lens can be used in a wide temperature environment in which the projector is placed and can endure the heat generated by the illumination lamp. To provide the wide operating temperature range, the projection lens maybe configured only by glass lenses. In comparison with plastic materials, glass lenses generally exhibit small variations in expansion and refractive index with temperature variations, which favors the maintenance of stable optical characteristics. However, if an aspheric surface is used to correct aberrations, glass lenses have a cost disadvantage. The lens system of the present invention corrects aberrations with plastic aspheric-surface lenses and implements a projection lens with small defocusing due to temperature variations.
(7) High resolution: To project an original image produced by a light valve component having many pixels on the order of one million pixels at a high density, which has been increasingly developed in recent years, at a magnification, a projection lens having a high resolution matching the fine pixel structure of the light valve is needed. To ensure the high resolution of the projection lens, axial aberrations including chromatic aberration and distortion described above and off-axis aberrations must be sufficiently corrected.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a retrofocus lens system which has a long back focal length in comparison with the focal distance and telecentric characteristics on the picture source side and allows wide-angle projection, and a projection display apparatus utilizing the retrofocus lens system.
According to the present invention, a retrofocus lens system comprises, in order from a large conjugate side: a first lens group having a negative refracting power; a second lens group having a positive refracting power; and a third lens group having a positive refracting power. The first lens group includes, in order from the large conjugate side, a first lens from which projection light is sent out, a meniscus-shaped second lens which has a negative refracting power and a convex surface facing the large conjugate side, a third lens having a negative refracting power, and a fourth
Mistubishi Denki Kabushiki Kaisha
Schwartz Jordan M.
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