Optical: systems and elements – Polarization without modulation – By relatively adjustable superimposed or in series polarizers
Reexamination Certificate
1999-03-30
2001-12-04
Shafer, Ricky D. (Department: 2872)
Optical: systems and elements
Polarization without modulation
By relatively adjustable superimposed or in series polarizers
C359S490020, C359S490020, C359S352000, C348S342000
Reexamination Certificate
active
06327085
ABSTRACT:
INCORPORATION BY REFERENCE
The disclosures of the following priority applications are herein incorporated by reference:
Japanese Patent Application No. 10-101822, filed Mar. 31, 1998
Japanese Patent Application No. 10-197610, filed Jul. 13, 1998
Japanese Patent Application No. 11-18596, filed Jan. 27, 1999.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical filter and an optical device provided with this optical filter.
2. Description of the Related Art
In a digital still camera employing an imaging device such as a CCD (hereafter a digital still camera is simply referred to as a “DSC” in this specification), “beat” interference may occur as a result of a certain relationship between the spatial frequency of the subject image and the repetitive pitch of dot-type on-chip color separation filters provided at the front surface of the imaging device. In order to prevent any false color signals from being generated by the beat, i.e., in order to prevent the so-called “color moire,” an optical low-pass filter is provided between the taking lens and the imaging device. The optical low-pass filter, which is constituted by employing a birefringent plate achieving birefringence, reduces the generation of the beat through the birefringent effect provided by the birefringent plate. Normally, quartz is employed to constitute the birefringent plate.
Japanese Examined Patent Publication No. 1994-20316 proposes an optical low-pass filter employing two birefringent plates such as that described above, which is suited for application in an imaging device provided with dot-type on-chip color separation filters. This optical low-pass filter is constituted by enclosing a quarter-wave plate between two birefringent plates with the directions in which the image becomes shifted through the birefringence offset by approximately 90° from each other.
Now, the so-called direct image forming system, in which the imaging device is directly provided at the primary image forming plane of the taking lens without employing a reduction lens system or the like is becoming the mainstay in single lens reflex type DSCs that allow interchange of the taking lens among DSCs in recent years. The advent of the direct image forming system has been realized through the utilization of imaging devices having a large image area of approximately 15.5 mm×22.8 mm that have been manufactured in recent years to replace ⅔″ size (approximately 6.8 mm×8.8 mm) and 1″ size (approximately 9.3 mm×14 mm) imaging devices that have been conventionally used in television cameras and the like. With this size of image area available, an image plane having a size (approximate aspect ratio 2:3=15.6 mm×22.3 mm), which is comparable to the image plane size of the C-type silver halide film IX240 system (APS), is achieved. By employing an imaging device achieving a relatively large image area, it becomes possible to adopt a camera system that employs the 135-type photographic film in a DSC. To explain this point, providing a ⅔″ size or 1″ size imaging device at the field of a camera using the 135-type film only achieves a small image plane size for the imaging device compared to the image plane size of the 135-type film (24 mm×36 mm). As a result, a large difference will manifest in the angle of field achieved by a taking lens having a specific focal length, to cause the photographer to feel restricted. This problem becomes eliminated as the image area of the imaging device increases and becomes closer to the image plane size of the 135-type film.
However, as the image area in a single lens reflex type DSC, which forms the primary image with the taking lens at an image device directly, increases, the problems explained below arise to a degree to which they cannot be neglected.
Imaging devices in DSCs in recent years have evolved in two directions, i.e., toward a higher concentration of pixels and toward a larger image plane. When the number of pixels is increased to exceed 1 million pixels while maintaining the size of the image plane at approximately ⅓″ to ½″ as in the prior art, the pixel pitch becomes reduced. For instance, in an imaging device having approximately 1,300,000 pixels, with its image plane size at approximately ⅓″, the pixel pitch is approximately 4 &mgr;m. Generally speaking, the pixel pitch “p” at an imaging device and the thickness “t” of the birefringent plates constituting the optical low-pass filter which is employed to support the pixel pitch “p” achieve the relationship expressed through the following equation (1)
p=t
(
ne
2
−no
2
)/(2
ne×no
) (1)
with
t: birefringent plate thickness
ne: extraordinary ray refractive index at birefringent plate
no: ordinary ray refractive index at birefringent plate
When a quartz plate, which is most commonly employed to constitute a birefringent plate, is used in an imaging device with a pixel pitch of approximately 4 &mgr;m, the thickness “t” required of the quartz plate is concluded to be approximately 0.7 mm by working backward with “p” in equation (1) set at 4 &mgr;m, since the refractive indices of quartz for light having a wavelength of 589 nm are ne=1.55336 and no=1.54425. Since the thickness of the quarter-wave plate needs to be approximately 0.5 mm regardless of the pixel pitch p, the entire thickness achieved when constituting an optical low-pass filter by pasting together three plates, i.e., two quartz plates (birefringent plates) and one quarter-wave plate, will be approximately 2 mm.
However, when the area of the photosensitive surface of an imaging device increases, as in the case of, in particular, an imaging device employed in a single lens reflex type DSC, it becomes necessary to increase the thickness of the optical low-pass filter for the reasons detailed below.
While the size of the image plane of a ⅓″ imaging device is approximately 3.6 mm×4.8 mm, let us now consider an imaging device having an image plane size equivalent to that of the C-type (aspect ratio 2:3=16 mm×24 mm) in an IX240 system (advanced photo system (APS)) with silver halide film. When pixels are arrayed at a pixel pitch of approximately 4 &mgr;m on this imaging device, the total number of pixels for the entire image plane will exceed 20 million by simple calculation, and it is considered that the current technical level is not high enough to realize such a large number of pixels for practical use from the viewpoints of the yield in imaging device production, the scale and processing speed of the image information processing circuit and the like. As a result, it is assumed that it is appropriate to set the number of pixels at approximately two million and several hundreds of thousands in an imaging device having a large image plane equivalent to that of the APS-C type, which sets the pixel pitch at 10 and several &mgr;m.
For instance, when an APS-C size imaging device (16 mm×24 mm) is prepared at a pixel pitch set to 12 &mgr;m, the number of pixels in the imaging device will be approximately 2,670,000. When constituting the birefringent plates of the optical low-pass filter employed in combination with the imaging device having the pixel pitch of 12 &mgr;m with quartz, the thickness of a single quartz plate is calculated to be “t”=2.04 mm by incorporating “p”=12 &mgr;m in equation (1). By adding the thicknesses of two such quartz plates and a quarter-wave plate (0.5 mm), the thickness of the optical low-pass filter is calculated to be 4.58 mm, which is more than twice as large as the thickness of an optical low-pass filter (thickness: 2 mm) with the pixel pitch set at 4 &mgr;m.
In addition, since the spectral sensitivity of an imaging device is different from the spectral sensitivity of the human eye, an IR blocking filter is normally provided to cut off infrared light within the imaging optical path in a DSC employing an imaging device. This IR block
Osawa Keiji
Shibazaki Kiyoshige
Nikon Corporation
Oliff & Berridg,e PLC
Shafer Ricky D.
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