Television – Camera – system and detail – Optics
Reexamination Certificate
1999-10-26
2004-01-06
Moe, Aung S. (Department: 2612)
Television
Camera, system and detail
Optics
C348S335000, C348S272000, C359S637000, C359S689000
Reexamination Certificate
active
06674473
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an image pickup apparatus. More particularly, the present invention relates to a compact and low-cost image pickup apparatus that is capable of photographing high-quality images comparable to silver halide photographs and particularly capable of suppressing the image quality degradation due to color shift caused by the lateral chromatic aberration of a lens.
There are digital cameras designed for business use, e.g. printing, which are capable of obtaining images comparable in quality to photographs. These digital cameras have a large number of pixels, which exceeds 2 million pixels. However, the pixel pitch of the image pickup device is as large as about 7.5 to 10 micrometers or more. Accordingly, there is a limit to the number of devices obtained from one wafer. Therefore, the cost cannot be reduced in excess of a certain limit.
The most effective way of reducing the production cost of image pickup devices is to reduce the device size so that an increased number of devices are obtained from one wafer. However, it is necessary in order to reduce the device size to reduce the number of pixels or the pixel pitch.
Many of image pickup apparatus that have been commercially manufactured as relatively low-cost apparatus use image pickup devices of ⅓ to ⅔ inch size and with about 1 to 2.5 million pixels. The pixel pitch of these image pickup devices is about 3 to 5 micrometers, which is markedly small in comparison to image pickup devices used in the above-described digital cameras for business use.
To obtain a high-quality image with an image pickup device having a large number of pixels, it is essential to use an image pickup device with a smaller pixel pitch.
Major factors in determining high image quality include color shift in addition to resolution. It is known that even if the image-forming performance for a single wavelength is high, when there are large chromatic aberrations, loss of color definition and color shift occur, causing the image quality to be degraded. In the case of an image pickup apparatus using a solid-state image pickup device, e.g. a digital camera, in particular, color shift caused by lateral chromatic aberration rather than axial chromatic aberration becomes a serious problem.
FIG. 12
shows the relationship between the input light intensity (analog image) and the output intensity signal from each pixel. As shown in the figure, in a device in which images formed in a certain fixed area are averaged or added together to form an image signal, such as a solid-state image pickup device, even when an image with gradient light intensity, the resulting image gives a “stairstep” appearance.
In silver halide photography, on the other hand, images are composed of dye units varying in dye size from several micrometers to several tens of micrometers. Thus, the image intensity approximately reproduces the gradient of the light intensity.
In comparison of the above two, there is only a small difference at a position where the gradient of the light intensity is small. However, there is a large difference at a position where the gradient of the light intensity within one pixel is large (e.g. the position of the second pixel from the left). That is, in the case of using an image pickup device, an area where the light intensity is low in the actual image (e.g. the right-hand end in the second pixel from the left) undesirably becomes an image having an average light intensity in the pixel.
This is not a serious problem in a monochrome image or an image in a range where the hues are similar to each other. That is, in such an image, the gradation of the image merely becomes stairstepped. However, in a case where only a specific color is displaced, as shown in
FIG. 13
, the color difference gives a “stairstep” appearance. For the sake of simplicity, only two colors, i.e. R (red) and B (blue), are considered in FIG.
13
. In a case where the image of B is displaced with respect to the image of R as shown in part (a) of
FIG. 13
, the difference between the colors R and B is as shown in part (b) of FIG.
13
. Thus, the color difference signal intensity R-B becomes stairstepped.
It is known that human beings are more sensitive to a color difference than to the absolute value of a color. Therefore, if a human being observes such an image, the color shift appears to be larger than the actual shift. Accordingly, it has been demanded that a lens used in an image pickup apparatus using an image pickup device should be highly corrected for lateral chromatic aberration that causes color shift.
Various studies have heretofore been conducted to determine a size of lateral chromatic aberration necessary for the above-described intensity ratio between two colors displaced relative to each other to appear to be no large color difference to the observer. As a result, it is known that a lateral chromatic aberration not larger than ½ of the pixel pitch allows the color shift of an image to be substantially ignorable.
However, if it is intended to construct an image pickup apparatus capable of providing high-quality images at reduced cost by using a small-sized image pickup device, the pixel pitch becomes 3 to 5 micrometers or less, as stated above. If such an image pickup device is corrected for lateral chromatic aberration on the basis of the conventional concept, the allowable lateral chromatic aberration is 1.5 to 2.5 micrometers or less. Accordingly, an extremely high level of chromatic aberration correction is required.
In the case of an image pickup apparatus using an image pickup device, filters, e.g. an optical low-pass filter and an infrared cutoff filter, are placed between an optical lens and the image pickup device. In the case of a multi-chip type apparatus, an optical path branching prism or the like is frequency placed therebetween. Accordingly, although the physical size is small, a long back focus is required from the optical point of view. Therefore, the common practice is to use a lens type that allows the back focus to be lengthened. Consequently, in many cases, the power distribution is asymmetrical with respect to the stop as viewed from the object side toward the image side.
Accordingly, it is difficult to correct aberrations depending on the field angle, such as distortion and lateral chromatic aberration. There has heretofore been a known method of correcting chromatic aberrations by using anomalous dispersion glass.
To correct lateral chromatic aberration to a value within the above-described range, it is necessary to further increase the number of lens elements using anomalous dispersion glass.
However, anomalous dispersion glass generally has a large coefficient of expansion or has such properties that it is soft and readily scratched. Therefore, a large number of man-hours are needed to process anomalous dispersion glass. Thus, using a large number of lens elements of anomalous dispersion glass causes the processing cost to increase unfavorably.
To realize a high level of chromatic aberration correction with a reduced number of lens elements of anomalous dispersion glass, it is necessary to weaken the power of each lens element by increasing the number of lens elements used or increasing the size of the lens system. This interferes greatly with the achievement of a reduction in size of the apparatus.
Meanwhile, an image obtained by using an image pickup device can be handled spatially as digital data.
Therefore, if the intensity of the image is digitized, it becomes easy to subject the image to electrical processing, e.g. image processing. Accordingly, a method wherein chromatic aberrations produced in a lens system are corrected by image processing has been proposed [Japanese Patent Application Unexamined Publication (KOKAI) No. 8-205181]. However, if image processing is carried out, it becomes necessary to perform processing for preparing data at image points where there is no data in the original image. Consequently, the image quality is unavoidably degraded. Moreover, a
Moe Aung S.
Olympus Optical Co,. Ltd.
Pillsbury & Winthrop LLP
Ye Lin
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