Optical: systems and elements – Diffraction – From grating
Reexamination Certificate
1999-12-21
2003-01-14
Chang, Audrey (Department: 2872)
Optical: systems and elements
Diffraction
From grating
C359S569000, C359S566000
Reexamination Certificate
active
06507437
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a diffractive optical element to be used either at a plurality of wavelengths, or with light in a predetermined band, and a, photographic optical system having the diffractive optical element and, more particularly, to a diffractive optical element suited to be used as a part of a photographic optical system using three or more light beams of different original colors in forming a color image.
2. Description of Related Art
Conventionally, as one of the methods for correcting chromatic aberrations of an optical system, there is known a method of combining two glass materials (lenses) which differ in dispersion from each other.
Unlike this method o: reducing the chromatic aberrations by selectively using two glass materials, it has been known to provide the optical system with a diffractive optical element (hereinafter also called the “diffraction grating”) as made up in either one of the lens surfaces thereof or somewhere else. Such a method of reducing the chromatic aberrations is disclosed in, for example, “International Lens Design Conference” in SPIE Vol. 1354 (1990), Japanese Laid-Open Patent Applications No. Hei 4-21342and No. Hei 6-324262 and U.S. Pat. No. 5,044,706. This method is attained by utilizing such a physical phenomenon that, for a refractive surface and a diffractive surface in an optical system, if their refractive powers are of the same sign, chromatic aberrations for the rays of light of a certain reference wavelength occur in the opposite directions. Further, with such a diffractive optical element, when its periodical structure is changed in pitch as it can be done freely, an effect similar to an aspherical lens is produced. Therefore, the diffractive optical element has an additional great advantage of reducing even mono-chromatic aberrations.
Here, it is in refraction that one ray, even after having refracted, remains one ray. In diffraction, on the other hand, one ray brakes up to a plurality of rays in different orders of diffraction. Therefore, for a case of using the diffractive optical element in an optical system, there is a need to make determination of the grating structure so that a light beam of the useful wavelength region concentrates on a particular order (hereinafter also referred to as the “design order”). In a situation when light concentrates on the particular order, the intensities of the diffracted rays in the other orders become low. If the intensity is “0”, the corresponding diffracted ray becomes non-existent.
In order to make useful the above-described advantage of the diffractive optical element, it becomes necessary throughout the entire range of predetermined wavelengths including design wavelenghts that the diffraction efficiency for the rays in the design order is sufficiently high. It should also be pointed out that the rays having other orders than the design order focus themselves at different places than the rays of the design order do, becoming flare (light). In an optical system employing the diffractive optical element, therefore, it is of great importance to consider the spectral distribution, too, of diffraction efficiencies of the rays in the design order fully and, further, the behavior of even more rays which are in the orders other than the design order (or the useless diffracted rays).
FIG. 19
shows a diffractive optical element
1
in which a diffraction grating
3
is made up in one layer on a substrate
2
. With such a diffractive optical element
1
formed on a surface of an optical system, the rays in particular orders diffract with diffraction efficiencies shown in FIG.
20
. The values of the diffraction efficiency are in percentage of the diffracted amount of light at every wavelength to the transmitted amount of light. The reflected light from the grating boundary or the like is not taken into account in the evaluation, because the explanation becomes complicated. In
FIG. 20
, the abscissa represents the wavelength and the ordinate represents the diffraction efficiency. This diffractive optical element
1
is so designed that the diffraction efficiency in the first order (a solid line curve in
FIG. 20
) is highest in the predetermined wavelength region. That is, the design order is the first one. Furthermore, the diffraction efficiencies in the orders near to the first one (or (1±1)st orders, namely, zero order and second order) are also depicted for comparison. As shown in
FIG. 20
, it is in the design order that the diffraction efficiency has the highest value at a certain wavelength (hereinafter referred to as the “design wavelength”) and becomes gradually lower toward the ends of the whole spectrum. This decrease of the diffraction efficiency in the design order is translated into an increase of the amount of diffracted rays in the other orders, becoming flare. In addition, in a case where two or more diffraction gratings are used, in particular, the lowering of the diffraction efficiency at the other wavelengths than the design wavelength leads to reduction of the transmittance.
To diminish the lowering of the diffraction efficiency, many previous proposals have been made.
For example, Japanese Laid-Open patent Application No. Hei 9-127322 discloses a diffractive optical element made up in such a way that, as shown in
FIG. 21
, three different materials of different kinds (for three layers
4
,
8
and
5
of diffraction gratings) and two different grating thicknesses d
1
and d
2
(for the bottom and top gratings
4
and
5
) are appropriately selected and that the bottom and top diffraction gratings of an equal pitch distribution are juxtaposed. By this construction and arrangement, a high diffraction efficiency in the design order is realized over the entire visible region, as shown in FIG.
23
.
Also, a diffractive optical element capable (if diminishing the lowering of the diffraction efficiency has been proposed in Japanese Laid-Open Patent Application No. Hei 10-133149. As shown in
FIG. 22
, this diffractive optical element has two layers superimposed one upon another. For the stratification of the layers
4
and
5
in cross-section, the refractive indices and dispersions of their materials and the thicknesses of the gratings in them are made optimum, thus realizing a high diffraction efficiency in the design order over the entire range of visible spectrum.
In another Japanese Laid-Open Patent Application No. Hei 10-104411, with the use of a diffractive optical element of the kinoform type shown in
FIG. 19
, the grating thickness is adjusted to shift the design wavelength as desired, thus reducing the amount of needless diffracted light in the orders near to the design order.
Of the prior known techniques described above, the one proposed in Japanese Laid-Open patent Application No. Hei 9-127322 has greatly improved the diffraction efficiency in the design order. Therefore, the proportion of the diffracted rays in the orders other than the design order, or the needless diffracted rays, too, is improved. So, the diffractive optical element produces less flare. However, color flare is appreciable in the obtained image. Also, there is no detailed description about the color appearance of flare and the amount of flare.
Meanwhile, Japanese Laid-Open Patent Application No. Hei 10-104411 is concerned with the grating having one diffractive surface like that shown in
FIG. 19
(hereinafter called the “mono-layer DOE” for Diffractive Optical Element). With this regard, it suggests the influence of the color flare due to the light in the needless orders. However, as far as the diffractive optical element in the stratified form of two or more layers (hereinafter called the “stratified multilayer DOE”) is concerned, nothing is said about the flare.
Using the stratified multilayer DOE described above, the optical system has succeeded in greatly reducing the flare from that when the mono-layer DOE is in use. However, this does not mean that the useless diffracted light is not present at all. So, it is, t
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