Process For Recording Acoustic, Synthetic And Microwave Holograms

Abstract

The process of recording acoustic, synthetic and microwave holograms in which the object wave field emanating from an object is scanned point by point and row by row by a receiving transducer which produces electrical output signals which are transformed into light signals that are recorded as holograms on a light sensitive medium in rows characterized by displacing every second row with respect to every first row in a direction parallel to the rows of the light signals being recorded to improve the signal to noise ratio of an image reconstructed from the hologram. Preferably, the displacement between adjacent rows is less than the resolving power of the reconstructed image of the hologram.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a process for recording acoustic, synthetic and microwave holograms in which a receiving transducer scans an object field emanating from an object to produce electrical output signals which are subsequently transformed into light signals and are recorded on a light sensitive medium to form the hologram.

2. Prior Art

In the reconstruction of holograms, in order to spatially separate the image of the object from non-diffracted light and the conjugated image, conventional holography processes utilize a reference wave which strikes at an angle to the object wave during the recording of the hologram. In acoustic and microwave holography since linear receivers are employed for the reception of the microwaves or sound waves, the reference wave is generally added electrically. There are two possibilities of electronically producing the reference wave. The first possibility consists in scanning the sound field point by point and at every scanned point adding a sound or microwave frequency reference signal to the received signal. This results in a reference wave being simulated which runs normal to the hologram plane. In this possibility, the object must be displaced perpendicular to the hologram normal in order to obtain the necessary angle between the object and reference wave. In the second possibility, a reference wave, which falls obliquely to the hologram plane, is simulated by changing the phase of the reference signal, which has the same frequency, at every scanned point.

Since the maxim angle between the object and reference waves is limited in many cases, there are regions of the object which occur at an angle to the hologram normal which angle lies in the vicinity of the angle of incidence of the reference wave. Therefore, in the reconstruction of holograms, these object regions are either separated only slightly from the non-diffracted light or partly superimposed by the non-diffracted light. The non-diffused light is then filtered out in the Fourier transformation plane of the hologram but in the particular case of the small angle, a considerable impairment of the signal/noise ratio may occur.

SUMMARY OF THE INVENTION

The present invention is directed to a process for the recording of acoustic, synthetic or microwave holograms which during reconstruction of the hologram enable the complete separation of the reconstruction image from the non-diffracted light so that a considerable improved signal to noise ratio can be obtained. The process records the hologram by scanning point by point and row by row the object wave field emanating from an object by a receiving transducer which produces an electrical output signal which is transformed into light signals that are recorded as holograms on a light sensitive medium in rows with the improvement comprising displacing every second row with respect to every first row in a direction parallel to the rows as the light signal is being recorded row by row on the light sensitive medium. Preferably, the degree of row displacement is less than the resolving power of the reconstructed image. In one embodiment of the process, the electrical signals are transformed into light signals by being presented on an oscilloscope screen and the step of displacing is accomplished by adding to the deflecting voltage of the oscilloscope a rectangular wave-form voltage whose frequency is equal to half of the frequency of the deflecting voltage of the oscilloscope. In a second embodiment, the row displacement of every second row is accomplished by applying a time delay to the electrical output signals from the receiving transducer when scanning the second rows. In a third embodiment of the process of the invention, the receiving transducer scans the first rows in one direction and the second rows in an opposite direction and the electrical signals are transformed by an incandescent lamp or bulb into the light signals and the step of displacing utilizes the inertia of the incandescent bulb to produce the desired displacement between the rows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an arrangement for executing the recording process according to the present invention;

FIG. 2 illustrates a hologram with displaced rows produced by the process of the present invention;

FIG. 3 diagrammatically shows an arrangement for a reconstruction of the holograms with displaced rows;

FIG. 4 schematically shows an embodiment of the arrangement for executing the recording process according to the present invention; and

FIG. 5 schematically shows another embodiment of the arrangement for executing the recording process according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The principles of the present invention are particularly useful in the method of recording an acoustic, synthetic or microwave hologram. For example, for recording of acoustic holograms, the amplitude and phase distribution of a sound field produced by a coherently irradiated object is scanned on a receiving plane (x.sub.O, y.sub.O) by a receiver transducer such as a sound receiver. A coherent reference signals is attached to the output of the sound receiver and the interference signal which has thus been formed is visibly represented on electro-optical transducer and is then stored by being recorded on film. The sound receiver can be a matrix of sound transducers which are electrically scanned or a single sound transducer which is mechanically moved over the receiving plane and thus scans the sound field in rows having predetermined spaced intervals.

In FIG. 1, an example of a device for performing the process is illustrated. An object which is to be recorded for example a metal plate 1, which has holes, is arranged in a tank 2 containing liquid. To irradiate the object 1, an oscillator 3 produces a signal which is amplified by an amplifier 4 which drives a sound transmitter 5 which is disposed in the tank 2 and can be a transducer which irradiates the object 1 with coherent sound waves. The sound waves from the transducer 5 are diffracted by the object 1 to produce a sound field having an amplitude and phase distribution in a receiving plane 8' which is on the liquid surface of the tank 2. The sound field in plane 8' is scanned by an ultrasonic receiver 8.

The ultrasonic receiver 8 is a receiving transducer which as illustrated is moved to scan the sound field in the receiving plane 8' point by point along rows. The movement of the receiver 8 is controlled in two dimensions by a mechanical shifting device identified as X-Y. The receiver 8 converts the ultrasonic signal into an electrical signal which is amplified by amplifier 9 and then conducted to a mixer stage 7. In the mixer stage 7, a signal, which is obtained from the oscillator 3, is added to the electrical signal obtained from the ultrasonic receiver 8. A resultant interference signal formed by the mixing of the coherent reference signal with the electrical signal from the receiver 8 is used to control the brightness of an incandescent bulb 10 which is moved synchronously with the ultrasonic receiver 8. To record the hologram point by point and row by row, a camera 11 is focused on the bulb 10.

The camera 11 records the optical signals from the incandescent bulb 10 on a storage medium, such as a light sensitive medium 14, in rows 12 and 13 which have a spacing of .DELTA.x between rows as illustrated in FIG. 2. The improvement of the process is the displacing of every alternative row so that every second row 13 is displaced a distance .DELTA.y with respect to every first row 12 in a direction parallel to the rows.

There are several embodiments of the improved recording process for obtaining the desired displacement .DELTA.y. In one embodiment, (FIG. 4) the output signal of the mixer stage 7 is conducted to an oscilloscope 6 which converts the electrical signal to an optical signal by displaying the signal on a screen. The screen display is then recorded by the camera 11. To obtain the desired displacing of every second row, a constant rectangular wave-form voltage from means 25 is applied to the deflecting voltage of the oscilloscope while the signal of the every second row are being displayed. The constant rectangular wave-form has a frequency which amounts to half the frequency of the deflecting voltage and cause the displacement .DELTA.y for every second row 13.

A second embodiment (FIG. 5) of the process for producing the displacement is by delaying the output of the mixer stage 7 to the bulb 10 by means 26 of an electric delay line when the stage 7 is producing the signals for the second row. The time delay causes the desired spatially displacement .DELTA.y for the recording of each of the second rows 13 with regard to the first rows 12.

A third embodiment of the process for producing the displacement is a process in which the receiving transducer 8 scans the rows 12 in one direction and the rows 13 in the opposite direction. The electrical signal from the mixer stage 7 is applied to the incandescent bulb 10 which has a time delay due to inertia. Since row 12 is scanned in one direction and row 13 in scanned in opposite directions, the inertia of the bulb 10 produces the desired displacement .DELTA.y.

During reconstruction, the row displacement causes a scanning frequency of the amplitude and phase information of the object wave field to be effectively halved while maintaining a uniform band width and while the row displacement does not affect the constant to slowly varying component of the hologram transmission which produces a non-diffracted light during reconstruction. Thus the scanning frequency of the non-diffracted component amounts to double the recorded scanning frequency of the image information. In the Fourier transformation plane of the hologram these components are spatially divided by this frequency difference so that the non-diffracted light can be filtered out without impairing the signal to noise ratio.

A reconstruction of the hologram with the displaced rows is illustrated in FIG. 3. The row displacements run parallel to the image plane of the figures. For reconstruction, the hologram 14 is illuminated with a convergent laser beam 15 which has a focal plane 16. Due to the illumination by the laser beam 15, the reconstructed image 17 of the object is produced shortly before the focal plane 16 and a conjugated image 18 together with higher order diffractions is formed behind the focal plane 16. In the focal plane 16 of the laser beam 15 as a result of the row displacement, the first orders of diffraction of the reconstructed image 17 and 18 are spatially separated from the non-diffracted light and the first order of diffraction of the constant component of the hologram transmission (the latter components lie outside of the image plane of FIG. 3).

The non-diffracted light as well as the higher orders of diffraction which do not contribute to the image reconstruction can be filtered out due to the spatial separation by providing a mask with a light transmitting area or aperture on the focal plane 16. By providing an enlargement lens 19, an enlarged image 20 of the image 18 can be created.

Although minor modifications might be suggested by those versed in the art, it should be understood that we wish to embody in the scope of the patent granted hereon all such modifications as reasonably and properly come within the scope of our contribution to the art.

Claims

We claim:

1. In a process of recording acoustic, synthetic and microwave holograms in which process an object wave field emanating from an object is scanned point by point and row by row by a receiving transducer which produces an electrical output signal which are transformed into light signals that are recorded as holograms on a light sensitive medium point by point in rows, the improvement comprising displacing every second row with respect to every first row in a direction parallel to the rows as the light signals are being recorded row by row on the light sensitive medium.

2. In a process according to claim 1, wherein the degree of row displacement is less than the resolving power of the reconstructed image of the recorded hologram.

3. In a process according to claim 1, wherein the electric output signals are transformed into light signals by being applied to an oscilloscope screen to produce an optical image which image is recorded by a light sensitive medium and wherein the step of displacing every second row with respect to every first row is accomplished by adding a constant rectangular wave-form voltage to the deflection voltage of the oscilloscope screen during transforming of the electric signals of every second row, said rectangular wave-form voltage having a frequency equal to half the frequency of the deflecting voltage of the oscilloscope.

4. In a process according to claim 3, wherein the degree of the row displacement is less than the resolving power of the reconstructed image of the recorded hologram.

5. In a process according to claim 1, wherein the step of displacing every second row with respect to every first row is accomplished by applying time delay to the electrical signal output of the receiving transducer for every second row of the hologram which is to be recorded.

6. In a process according to claim 5, wherein the degree of row displacement is less than the resolving power of the reconstructed image of the recorded hologram.

7. In a process according to claim 1, wherein the direction of scanning for every second row by the receiving transducer is in an opposite direction to the direction of scanning for every first row, and wherein the electrical signals are transformed into the light signals by being applied to an incandescent bulb with the displacing of the second rows with respect to the first rows utilizing the inertia of the incandescent bulb.

8. In a process according to claim 7, wherein the degree of row displacement is less than the resolving power of the reconstructed image of the recorded hologram.