ADJUSTABLE OPTO-ACOUSTICAL LOW
PASS FILTER AND TECHNIQUE
The present application claims priority from U.S. Provi- 5 sional Patent Application No. 60/111,343 filed Dec. 7,1998, and said Provisional Patent Application is incorporated herein by reference.
FIELD OF THE INVENTION 10
This invention relates to the filtering of light containing image-representative information and, more particularly, to an adjustable opto-acoustical low pass filter and technique.
BACKGROUND OF THE INVENTION 15
When a solid state sensor, for example in a video camera or a still image digital camera, senses images containing spatial frequencies beyond the Nyquist limit, the electronic video signals from the solid state sensor may produce a 20 displayed picture containing artifacts such as aliasing, moire patterns, and phantom color patterns. An optical low pass filter can be used to suppress high frequency components of the image received by the sensor. However, existing approaches to low pass optical filtering have various disad- 25 vantages.
One type of conventional optical low-pass filter is based on birefringent crystal plates. A disadvantage of this type of filter is the requirement for a precise orientation axis of the stacked birefringent crystal plates, making it unsuitable for 30 mass production. Also the number of crystal plates in this design increases the overall length of the optical system. The stack of crystal plates requires a large optical path to separate a passing ray on two or more exiting rays. Another disadvantage of this type of filter is the high cost of the 35 crystal raw material. Still another disadvantage of the birefringent filter is that it depends on the configuration and pitch of the pixels. The birefringent filter is inflexible in that it can be used only with a certain type of solid state sensor.
Another commonly used filter is a phase noise type of optical low-pass filter, which has a periodic structure of phase diffraction grating that can be two dimensional. This filter is thinner than the birefringent filter. The diffraction grating structure depends on the pixel structure. The position of this type of filter relates to the pitch of the diffraction grating and the pitch of the pixels in the solid state sensor. The optical characteristics of the diffraction grating low pass filter are low in quality as compared with the birefringent filter but are significantly less expensive.
It is among the objects of the present invention to provide an improved optical low pass filter and technique that overcomes problems and limitations of prior art approaches.
The present invention is directed to an optical low pass filter and technique that provides excellent performance in conjunction with simplicity and low cost. The filter of the invention has the advantage of flexibility; that is, it can be used with any solid state sensor. Also, the filter is adjustable, go
In accordance with an embodiment of the invention, there is disclosed an apparatus for receiving light containing image-representative information, and low-pass filtering said light, the apparatus comprising: a pellicle positionable in the path of said light; at least one transducer coupled with 65 the pellicle; and means for applying an AC electrical signal to the transducer to establish waves in the pellicle; whereby
said waves are operative to low-pass filter the imagerepresentative information in light reflected from the pellicle.
In a preferred embodiment of the invention, the waves are ultrasonic waves and the pellicle is a plastic sheet. The pellicle can alternatively be a thin glass sheet.
In a described embodiment, the light is also transmitted through the pellicle, and the pellicle is thin enough to avoid low-pass filtering of the image-representative information in the portion of the light transmitted through the pellicle.
Further features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of a portion of a camera system in which the invention can be utilized.
FIG. 2 is a diagram of an embodiment of an optical low pass filter in accordance with an embodiment of the invention, as employed in a camera system.
FIG. 3 is a diagram of a further embodiment of an optical low pass filter in accordance with the invention.
FIG. 4, which includes FIGS. 4A, 4B, 4C, 4D and 4E, shows oscillograms of a video line signal in a color (red) channel of a standard burst resolution chart at 30 kHz and at respectively different amplitudes of 0 volts, 5 volts, 7.5 volts, 10 volts, and 15 volts.
FIG. 5 is a graph showing Modulation Transfer Function (MTF) as a function of line/screen width for each of the different amplitudes of FIG. 4.
FIG. 6 is a diagram of an embodiment of an optical low pass filter in accordance with an embodiment of the invention, as employed in a camera system with a single solid state sensor.
FIG. 1 shows part of a camera system of the general type set forth in Published PCT Application WO 98/160068, which can produce a high definition video signal, and which can be advantageously modified to employ an optical low pass filter of the present invention. It will be understood that this application of the invention is illustrative and nonlimiting. In the FIG. 1 example, light received from a scene being viewed is focused by a camera lens system, represented at 105, through a splitter, e.g. a half-silvered mirror 110, onto CCD image sensors 120 and 150, respectively. The image sensor 120 is used as a luminance sensor, and the image sensor 150 is provided with color stripes and is operated as a color sensor. As described in the referenced Published PCT Application, the image sensors 120 and 150 can be operated at different rates. The outputs of the image sensors 120 and 150 are coupled to respective processing circuitry (not shown), and can be ultimately combined and utilized to produce output progressive and/or interlaced scanned high definition video signals. In a prior art version of FIG. 1, an optical low-pass spatial filter, represented at 148 (and which could be of a type described in the Background portion hereof) could be interposed before the image sensor 150 to prevent alias frequencies from being generated by low spatial frequency sampling in the color channel of the camera system.
An example which illustrates an optical low-pass spatial filter in accordance with an embodiment of the invention, employed in the FIG. 1 type of system, is shown in FIG. 2,