Television – Camera – system and detail – Optics
Reexamination Certificate
1999-06-10
2003-08-12
Garber, Wendy R. (Department: 2612)
Television
Camera, system and detail
Optics
C348S341000
Reexamination Certificate
active
06606125
ABSTRACT:
BACKGROUND OF THE INVENTION
a) Field of the Invention
The invention is directed to a shutter, in particular for an objective, for improving the depth of focus in a camera, wherein the shutter has, in a central area, a radially uniform transmission function averaged over the circumferential angle. The invention is further directed to a CCD-camera, especially for endoscopes, in which the shutter can advantageously be used together with the objective.
b) Description of the Related Art
Objectives are known from photographic cameras, for example. In order to limit the light intensity in the exposure of a photographic film, a shutter is normally provided in objectives of this type. While the shutter is usually adjustable in this application and, for that reason, does not appear completely round, this shutter still essentially constitutes a round hole optically, that is, it has a radially independent transmission function in the area of a hole.
However, this shutter has additional functions. First, by limiting the light to rays in the vicinity of the optical axis, the imaging characteristics of the objective are improved by stopping out the outer lens areas. On the other hand, an increased depth of focus is achieved as the shutter decreases in size.
However, the design of this shutter can be problematic particularly in small cameras, for example, CCD-cameras in endoscopes which often require small shutters because of miniaturized construction. A shutter which is too small also has worsened resolving capability for image presentation because, in view of the wavy nature of the light at the edge of the shutter, there is always a noticeable diffraction which limits the possible image resolution and predominates when the shutter is reduced in size below a certain lower limit.
Particularly in the case of CCD-cameras in which the individual elements for image presentation are usually in the order of magnitude of micrometers and with which diffraction effects can also be detected in the micrometer range, a clearly visible diffraction limitation occurs already in shutters in the order of magnitude of millimeters. Naturally, the shutters could be made larger; but there would be a loss in the depth of focus, which is completely unacceptable in the case of endoscopes when the image recording is carried out by means of a CCD-camera.
OBJECT AND SUMMARY OF THE INVENTION
It is the primary object of the invention to improve known shutters, especially in connection with objectives, so that they are suitable particularly for CCD-cameras and so that the depth of focus as well as the resolution for image presentation can be increased by means of them.
This object is met in a shutter of the type described above in that the transmission function determining the shutter first drops off toward zero in a radial transition area which is a continuation of the central area mentioned above and which has a quantity n of cutouts which extend toward the radial outside and are spaced apart from one another at equal angles 360°
in the circumferential direction, and the quantity n of cutouts has the value 5, 6 or 7. It is possible to use a shutter of this kind, according to the invention, in any type of optical beam path, for example, in the beam path of a laser, etc., also without an objective. However, the preferred use of the shutter in connection with an objective is referred to hereinafter.
First of all, it is surprising in terms of physics that this object can be met at all, since the shutter diameter was previously regarded as an insurmountable barrier to improvement of resolution. Heretofore, there has been no suggestion whatsoever for straying from circular shutters with a radial rectangular transmission function, since these were believed to have the best depth of focus and the loss of resolution was always taken for granted with the smaller diameter of the shutter because all of the familiar physics textbooks attributed the maximum attainable resolving capability exclusively to the very small lens diameter or shutter diameter.
Shutters of the type mentioned above always have a sharp edge, so that the diffraction pattern limiting the resolution can be represented mathematically essentially by a zero-order Bessel function. In contrast to this, however, the diffraction pattern of the shutter, according to the invention, for the objective is still a sum of different Bessel functions as a result of the transition area provided according to the invention. Through the selection of the shape of the shutter and/or the configuration of the transmission function, for the purpose of displaying the diffraction pattern, contributions of higher Bessel functions can be added which, because of a different phase relationship, can narrow the broad maximum of the zero-order Bessel function in the neighborhood of small radii. Particularly also due to the area which is provided according to the invention for the diffraction and which is widened by the transition area, the zero-order Bessel function contribution dominating the resolution is also scaled to smaller radii, so that, as a result, the diffraction pattern limiting the resolution is even substantially sharper in the neighborhood of zero. However, because of the higher light intensity in the central area, the depth of focus is essentially entirely determined thereby. Accordingly, with an improved depth of focus, an improved resolution is also possible in particular.
Surprisingly, it has been shown that a camera which is outfitted with a shutter of this type for the objective of the camera is also substantially more sensitive to light. In fact, this could be expected because the central area is somewhat enlarged by the transition area with respect to the transmissivity of the shutter. But at the same time it would have to have been expected that the depth of focus would decrease. However, with a suitable design of the shutter according to the invention, it was observed that the depth of focus did not decrease in spite of increased-light sensitivity.
Due to the cutouts which extend on the radial outside and are spaced at equal angles 360°
in the circumferential direction and because of the resulting symmetry, the Bessel functions which additionally contribute to the diffraction and whose solution, as is well known, runs in periodicity with the circumferential angle are selected. Accordingly, this further development forces the occurrence of higher-order Bessel functions than the zero order, especially of an order of the whole number n of 5, 6 or 7, which also enables the selection of a suitable phase position of these contributions based on a suitable shape for a particularly favorable increase in the resolving capability.
Surprisingly, it has proven particularly advantageous when the number n has the value 5, 6 or 7. Since these findings were made through experimentation, a plausible explanation was even found. These Bessel functions have a maximum in the neighborhood of the drop-off of the zero-order Bessel function. When it is made possible, as a result of appropriate shaping of the shutter, to generate these contributions in reverse phase position to the zero-order Bessel function, the total drop-off of the diffraction patterns in the area of the diffraction maximum is substantially steeper and the possible resolution is thus improved in a substantially more effective manner. A shape of this kind is shown later in an embodiment example.
The desired transmission function of the shutter could also be achieved through arrangements of holes in the shutter material in the radial transition area. However, such holes can cause new diffraction effects, so that the resolution by means of the provided transition area remains far from optimum. Accordingly, based on these considerations, a preferable shutter will have no closed cutouts and, above all, no circular cutouts in the transition area forming a continuation of the central area.
Based on these considerations, the shutter was further optimized. According to an advantageous further development in this respect, it has prov
Haeckl Norbert
Schulz Dieter
Garber Wendy R.
Henke-Sass Wolf GmbH
Reed Smith LLP
Wilson Jacqueline
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