Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2001-04-03
2003-09-16
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S569000
Reexamination Certificate
active
06621640
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system having a diffractive optical element used in a broad wavelength region such as a visible light region and, more particularly, to an optical system for projecting and displaying an image on an image display element and a projection optical apparatus using the same.
2. Related Background Art
In recent years, in order to achieve video display with presence or effective presentation, a large-scale, high-resolution screen display apparatus is demanded, and an optical system of a projection type image display apparatus is required to have still higher performance. The projection type image display apparatus includes a so-called three-plate type apparatus which uses three image display elements such as liquid crystal panels in correspondence with the red, blue, and green wavelength regions, and a so-called single-plate apparatus which displays a color image using a single image display element.
The single-plate apparatus has a simpler arrangement than the three-plate apparatus, and can attain size and weight reductions. The optical system of the single-plate apparatus includes an optical system in which color filters corresponding to light rays of the red, blue, and green wavelength regions on pixels of a single image display element such as a liquid crystal panel or the like are provided, and an optical system in which light rays having different wavelength regions such as red, blue, and green are caused to be incident on predetermined pixels on an image display element with different angles of incidence with one another.
When color filters are used, since each pixel transmits only the wavelength of a specific wavelength region, and absorbs other wavelengths, the incident light suffers a large loss, and it is difficult to realize a bright projection type display apparatus.
An outline of the arrangement in which light rays having different wavelength regions such as red, blue, and green are caused to be incident on respective pixels on an image display element with different angles of incidence with one another will be explained below.
FIGS. 1A and 1B
show an outline of the arrangement of the aforementioned apparatus. A light source P
101
emits white light. Light emitted by the light source is uniformed by an illumination light intensity distribution uniforming means P
103
, which is set on an optical axis P
102
, and emerges from the means P
103
as a nearly collimated light beam. An optical unit P
103
includes a so-called optical integrator.
A color separation means P
104
comprises dichroic mirrors P
104
a
, P
104
b
, and P
104
c
each having wavelength selectivity, and the light from the light source is deflected by the color separation means.
In the color separation means P
104
, the dichroic mirrors are arranged to make different angles with one another, so that an image display element P
105
is illuminated with rays with wavelengths selected by the dichroic mirrors with different angles. The image display element P
105
comprises, e.g., a transmission type liquid crystal element. Light rays transmitted through the image display element form images of the image display element P
105
on a screen P
107
.
As described above, light rays ray_a, ray_b, and ray_c of the wavelength regions selected by the dichroic mirrors of the color separation optical system P
104
illuminate the image display element P
105
at given angles. The light rays ray_a, ray_b, and ray_c correspond to, e.g., those of the green, red, and blue visible light regions.
An array-like focusing means such as a microlens array P
109
is formed on the image display element P
105
, and focuses the respective color light rays at different positions. An image display portion P
110
of the image display element P
105
has pixels P
112
, the amounts of light transmitted therethrough can be controlled by a control means (not shown) that controls the image display element P
105
, and pixels P
112
a
, P
112
b
, and P
112
c
are arranged in correspondence with the focusing positions of the microlenses P
109
.
A projection optical system P
106
projects those pixels P
112
onto the screen.
With this arrangement, a projection optical system that can assure higher use efficiency of light than the system using the color filters can be realized. In such optical system, an optical system that projects an image is required to have higher performance to attain still higher resolution of the image on the screen. Since the solid angle of light from the liquid crystal panel increases due to use of the microlenses, a brighter projection lens is demanded.
A technique that improves optical performance using a diffractive optical element to meet such performance improvement requirements is disclosed in papers such as SPIE vol. 1354 International Lens Design Conference (1990), Japanese Patent Application Laid-Open Nos. 10-115777, 11-064726, and the like. These techniques exploit a physical phenomenon: chromatic aberrations with respect to light rays of a given reference wavelength appear in opposite ways on refraction and diffraction surfaces in the optical system. That is, this means that the diffractive optical element has negative dispersion (Abbe number &ngr;d=−3.453) while typical optical glass has positive dispersion. Also, the diffractive optical element has strong anomalous dispersion (&THgr;g, F=0.2956).
In addition, since an aspherical lens effect can be utilized by changing the periodic structure of gratings of the diffractive optical element, a great improvement of optical performance can be expected. Furthermore, since such characteristics of the diffractive optical element are obtainable by a microscopic shape, the space factor is very low, and weight and size reductions can be easily achieved. Exploiting such characteristics of the diffractive optical element disclosed by Japanese Patent Application Laid-Open Nos. 10-115777 and 11-064726, an improvement of performance and a size reduction of the optical system are attained.
As described above, when the diffractive optical element is used, effective features that cannot be realized by a refractive optical system alone can be provided. But since the diffraction efficiency of the diffractive optical element largely depends on the wavelength and angle of incidence, the diffraction efficiency of the element must be sufficiently taken into consideration.
FIG. 2
shows an example of the diffraction efficiency of a single-layered diffractive optical element formed of a given material. As can be seen from
FIG. 2
, the first-order diffraction efficiency at a specific wavelength (design wavelength) around 520 nm is high, but the diffraction efficiency of a wavelength separate from the design wavelength drops considerably. In the wavelength region that suffers such diffraction efficiency drop, diffraction efficiencies other than the design order such as the 0th order, second order, and the like increase, and cause image deterioration such as flare or the like.
In consideration of such diffraction efficiency characteristics of the diffractive optical element, an arrangement that reduces flare resulting from light rays of unnecessary orders is disclosed in Japanese Patent Application Laid-Open No. 08-220482.
FIG. 3
shows an outline of Japanese Patent Application Laid-Open No. 08-220482. The arrangement shown in
FIG. 3
has an imaging lens system
3
including a relief diffractive optical element, and an illumination optical system
1
. A diffractive optical element
11
has an effect of a single lens as a whole, while its relief pattern surface
10
is divided into a plurality of regions with different groove depths, which can maximize diffraction efficiencies with respect to a plurality of light rays of different wavelengths. The illumination optical system
1
comprises a wavelength selection element
9
(e.g., a band-pass filter) having a plurality of transmission regions, which respectively have, as center wavelengths of transmission, wavelengths that maxi
Canon Kabushiki Kaisha
Morgan & Finnegan , LLP
Schwartz Jordan M.
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